This dissertation has been 64—7070 microfilmed exactly as received

WINCHELL, Jr., Robert Eugene, 1931- X-RAY STUDY AND SYNTHESIS OF SOME COPPER- OXYCHLORIDES.

The Ohio State University, Ph.D., 1963 M ineralogy

University Microfilms, Inc., Ann Arbor, Michigan X-RAY STUDY AND SYNTHESIS OF SOME

COPPER-LEAD OXYCHLORIDES

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosopher in the Graduate School of the Ohio State University

Robert Eugene W inchell, J r ., B. S ., M. S.

The Ohio State University 1963

Approved ty

Id viser Department of Mineralogy ACKNOWLEDGMENTS

The author wishes to acknowledge the assistance, cooperation and

encouragement of a great number of people without whom this thesis

could not have been completed.

The specimens used in the study of these rare oxychlorides were

obtained from a number of sources. Dr. C. S. Hurlbut, Jr. supplied

samples of a l l the sp ecies from the c o lle c tio n s o f Harvard U n iversity.

Dr. Paul E. Desautels provided additional samples of all the

except pseudoboleite from the collections of the United States National

Museum. Dr. Raymond Hocart, of the University of Paris, supplied

several overgrowths of cumengeite on , which had been given to

him by G. Friedel, Dr. S. Grolier, of the St. Etienne School of Mines,

St. Etienne, France, provided material that had been available to G.

Friedel during his study (Friedel, 1906) of boleite, pseudoboleite, and

cumengeite. Dr. S. Caillere provided specimens of boleite, cumengeite

and pseudoboleite from the collections of the Paris Museum of Natural

History. Dr. P. G. Embrey furnished type percylite from the material

described by Brooke (1850) and d ia b o leite and ch loroxip h ite from the

Mendip H ills. The specimens furnished by Dr. Embrey were supplied from

the collections of the British Museum of History.

Mr. Karl Schwartzwalder made available to the author certain

facilities of the AC Spark Plug Division of the General Motors

Corporation. The author is also indebted to Dr. William Shulhof of

i i the AC Spark Plug Division for his cooperation during the use of these facilities.

Thanks are also due Dr. Charles Sclar and Dr. George Cocks of

Battelle Memorial Institute who made certain equipment available for use in this study. Dr. Reynolds M. Denning of the University of

Michigan was also kind enough to provide the use of equipment in the

Department of Geology and Mineralogy.

Financial assistance which made it possible for the author to devote full time to the research involved in the thesis was provided through the William J. McCaughey Fellowship and the author wishes to thank the contributors to this fellowship for their assistance.

The author wishes also to thank the faculty in the Department

of Mineralogy for their direct and indirect assistance and encouragement

in the course of this study. Most particularly he wishes to thank

Dr. Henry £. Wenden, his adviser, for assistance and encouragement

during this investigation.

Lastly, the author wishes to thank his wife Grace for her

encouragement and perserverance, since this dissertation could never

have been completed without them.

iii CONTENTS

Page

ACKNOWLEDGMENTS i i

LIST OF TABLES v i

LIST OF ILLUSTRATIONS v i i i

INTRODUCTION 1

I . HISTORICAL SURVEY 2

I I . SCOPE OF THE INVESTIGATION 19

I I I . EXPERIMENTAL METHODS 21

Specimens X-ray Analysis Heating Stage C rystallographic Measurements Specific Gravity Measurements Synthesis MLcrochemical Tests

IV. CUMENGEITE...... 29

H isto r ic a l Summary Results of Present Study

V. PSEUDOBOLEITE...... 77

H isto r ic a l Summary Results of Present Study

VI. BOLEITE ...... llJi

H isto r ic a l Summary Results of Present Study

V II. THE BOLEITE PROBLEM...... 166

iv CONTENTS (Continued)

Page

V III. FERCYLITE...... 173

H isto r ic a l Summary Results of Present Study

IX. EPITAXY IN THE BOLEITE G R O U P ...... 185

X. DIABCLEITE...... 192

H isto r ic a l Summary Results of Present Study

XI. C HLOROXIP HI T E ...... 20U

H isto r ic a l Summary Results of Present Study

X II. SYNTHESIS...... 209

Introduction Experimental Procedure R esults

X III. SUMMARY AND CONCLUSIONS...... 222

BIBLIOGRAPHY AND REFERENCES ...... 229

AUTOBIOGRAPHY...... 233

v LIST OF TABLES

Table Page

1. Minerals of the boleite group ...... 8

2 . The b o le ite g r o u p ...... 11

3. Angle table for cumengeite ...... 3U

U. Indices of refraction for cumengeite (Hadding, 1919) • 36

5. X-ray powder data for cumengeite ...... hi

6. Comparison of x-ray parameters according to the setting of Friedel (1906) ...... 53

7. Crystallographic data for cum engeite ...... Sh

8. Presence criteria for cumengeite ...... 55

9. Specific gravity values for cumengeite ...... 68

10. Chemical analyses for cumengeite ...... 69

11. X-ray data for cum engeite ...... 73

12. X-ray powder data for pseudoboleite ...... 91

13. Crystallographic data for pseudoboleite ..... 103

lU. Presence criteria for pseudoboleite . 10U

15. observed in boleite try Hadding (1919) . . • 126

16. Thermal cycle for b o le ite ...... l 5 l

17. X-ray powder data for b o leite ...... 1$6

18. X-ray and microchemical examination of percylite speci­ mens ...... 183

19. Comparison of parameters for members of the boleite group determined in this stu d y ...... 189

v i LIST OF TABLES (Continued)

Table Page

20. X-ray powder data fo r d i a b o l e i t e ...... 19U

21. Comparison of cell dimensions for . . . 198

22. Forrailas and reagents used in synthesis experiments . 211

23. Summary of diffusion experiments ...... 215

2J4. Summary of evaporation exp erim ents ...... 216

25. Summary of pyrex tube experiments ...... 217

26. Summary of pot bomb experim ents ...... 220

v i i IIST OF ILLUSTRATIONS

Figure Page

1. Morphology of natural cumengeite (Hadding, 1919) . , U2

2. Morphology of natural cumengeite U2

3. Powder diffraction pattern of natural cumengeite (Sainte- Etienne 2750) ...... U5

U. Powder d iffr a c tio n pattern of natural cumengeite (USNM 9U6U0) ...... , 16

5. Powder diffraction pattern of natural cumengeite from an overgrowth of cumengeite on boleite (Hocart specimen) ...... 16

6. Powder diffraction pattern of natural cumengeite from an overgrowth of cumengeite on boleite (OSU collections) ...... U5

7. Diagramatic representation of the space group FTnmm . . 57

8. Diagramatic representation of the twinned space group FU/m 2/m 2/m developed from ty twinning across (110) or rotation of 90° about 001 ...... 57

9. Diagramatic representation of the space group FU/m 2/m 2/m...... 61 10. Diagramatic representation of the twinned space group FU/m 2/m 2/m developed from P^/m 2/ m 2/m an n-glide in (001) ...... 61

11. Disordered form of AuCu^ (after Cullity, 1956) . . * 65

12. Ordered form of AuCu^ (after Cullity, 1956) . . 65

13 • Morphology of sy n th etic cumengeite ...... 71

lii. Morphology of synthetic cumengeite 71

15. Powder diffraction pattern of natural cumengeite from an overgrowth of cumengeite on boleite (Hocart s p e c i m e n ) ...... 7U

v i i i LIST OF ILLUSTRATIONS (Continued)

Figure Page

16. Powder diffraction pattern of aynthetic cumengeite obtained try alow diffusion at 25>°C. from stoichio­ metric proportions for pseudoboleite ...... 7k

17 . Powder d iffr a c tio n pattern o f sy n th etic cumengeite obtained in a sealed glass tube at 170°C. from stoichiometric proportions for cumengeite • . . 7U

18. Powder diffraction pattern of synthetic cumengeite obtained in a hydrothermal bomb at 220°C. from stoichiometric proportions for diaboleite .... 7h

19• Powder diffraction pattern of synthetic cumengeite obtained in a hydrothermal bomb at 270°C. from stoichiometric proportions for pseudoboleite . . . 7k

20. Morphology of pseudoboleite (Mallard, 1893) .... 88

21. Photomicrograph of pseudoboleite overgrowths on boleite viewed approximately parallel to the pseudothreefold axis of the composite crystal 21x ...... 88

22. Powder diffraction pattern of natural pseudoboleite (Sainte-Etienne 27U9) ...... 97

23. Powder diffraction pattern of natural pseudoboleite (OSU collection s) ...... 97

2k• Powder diffraction pattern of natural boleite (OSU collections) ...... 99

25. Powder d iffr a c tio n pattern of natural pseudoboleite (Sainte-Etienne 27U9) ...... 99

26. Morphology of synthetic pseudoboleite ...... 109

27. Morphology of synthetic pseudoboleite ...... 109

28. Powder d iffr a c tio n pattern of natural pseudoboleite (Sainte-Etienne 27U9) ...... 112

29. Powder diffraction pattern of synthetic pseudoboleite obtained in a sealed glass tube at 170bC. from stoichiometric proportion:* for pseudoboleite . . . 112

ix LIST OF ILLUSTRATIONS (Continued)

Figure Page

30. Powder diffraction pattern of synthetic pseudoboleite obtained in a sealed glass tube at 170°C. from stoichiometric proportions for percylite .... 112

31. Powder diffraction pattern of synthetic pseudoboleite obtained in a hydrothermal bomb at 270°C. from stoichiometric proportions for pseudoboleite . . . 112

32. Sectors in boleite due to anisotropic areas in boleite. 20x ...... II4.6

33. Anisotropic rim on boleite. Note discontinuous nature and banding in anisotropic rim. 20x ...... Iii6

3U. Interrupted and a ltern a tin g anisotrop ic bands in b o le ite . 20 x I l i 8

35. Fine banding present in thin fragments from the aniso­ tro p ic rim on b o le ite . 72x ...... H4.8

36. Powder d iffr a c tio n pattern of iso tr o p ic b o le ite (OSU collections) ...... 1$3

37. Powder diffraction pattern of anisotropic boleite (OSU collection s) ...... 153

38. Powder diffraction pattern of synthetic boleite ob­ tained in a hydrothermal bomb at 270°C. from stoichiometric proportions for boleite ...... 163

39. Powder diffraction pattern of natural boleite (OSU collections) ...... 163

UO. Type percylite (BM 89065) showing pseudoboleite over­ growths on boleite (lower right) associated with gold (lower left). 20x ...... 178

Ul. Powder diffraction pattern of natural boleite (OSU collections) ...... 181

U2. Powder diffraction pattern of percylite (USNM 7897) • l8l

U3. Powder diffraction pattern of pseudoboleite (Sainte- Etienne 2 7 U 9 ) ...... 181

x LIST OF ILLUSTRATIONS (Continued)

Figure Page

1*1*. Powder diffraction pattern of percylite (USNM R-9116) . 181

US• Powder diffraction pattern of percylite (H.N. 95812) . 181

1*6. Pseudoboleite overgrowths on boleite (modified after Friedel, 1906) 186

1*7. Cumengeite overgrowths on boleite (Hadding, 1919) . • 186

1*8. Morphology of synthetic d ia b o leite ...... 199

1*9* Powder diffraction pattern of natural diaboleite . . 202

50. Powder d iffr a c tio n pattern of sy n th e tic d ia b o le ite ob­ tained fcy slow diffusion at 25°C. from stoichiometric proportions for d iab oleite ...... 202

51. Powder d iffr a c tio n pattern of sy n th e tic d ia b o le ite ob­ tained by slow diffusion at 25°C. from stoichiometric proportions for ...... 202

52. Powder d iffr a c tio n pattern fo r natural ch loroxip h ite (BM 1957,211) ...... 207

53. Powder d iffr a c tio n pattern fo r sy n th etic ch loroxip h ite obtained in a sealed glass tube at 100°C. from stoichiometric proportions for diaboleite .... 207

xi INTRODUCTICN

Among the copper-lead oxychlorides which occur in nature the seventh edition of Dana's System of Mineralogy lists the following rare species:

Mineral Composition

B oleite Pb9Cu8Ag3Cl21(OH)l 6 -2H20 (?)

Cumengeite Pb^Cu^ClgCOHjg'HgO (?)

Pseudoboleite Pb^Cu^Cl^fOHjg^HgO (?)

P e r c y lite PbCuCl2(OH)2 (?)

D ia b o leite Pb2CuCl2 (0H)li

Chloroxiphite Pb3CuO2(0H)2Cl2 (?)

These minerals form the subject of this thesis*

Although these oxychlorides have engaged the attention of eminent crystallographers and mineralogists for over one hundred years, no modem study of most of the species has been performed. As a conse­ quence, most of the species are incompletely characterized. It is the purpose of this thesis to present the results of further study of these s p e c ie s .

For the purpose of this study the minerals w ill be divided as fo llo w s :

1. The boleite group: boleite, pseudoboleite, cumengeite, and

percylite j

2. Diaboleite;

3. Chloroxiphi te. I . HISTORICAL SURVEY

The first description of a belonging to the species

described in this study was given by Brooke (1850), who published a very incomplete description of a mineral occurring as sky-blue cubes

associated with gold and hematite on specimens from Sonora, Mexico.

Brooke named this mineral, percylite after Dr. John Percy, a mineral

chemist of the day who performed the chemical investigation of

percylite included by Brooke in his original description. Brooke

gav8 no further information on the physical properties of the species.

Percy listed eight qualitative tests and the results of a chemical

analysis for the mineral. The chemical analysis was made on impure

material and, while the analysis showed the presence of silver, the

fomula given indicated that the species was a copper-lead oxychloride.

Brooke (in P h illip s-B rooke and M iller, 1852) amended h is e a r lie r

description of percylite by giving crystallographic data and a value

for the hardness of the species. Brooke's descriptions left the

impression in the literature that percylite was a sky-blue, silver-

free, copper-lead oxychloride.

Maskelyne and Flight (1872) observed a sky-blue mineral in

association with anglesite, and cerargyrite from an un­

specified locality in South Africa. On the basis of a color change

on heating indicated by Percy (in Brooke, 1850) and a chemical analysis

2 performed by Flight on impure material, Maskelyne and Flight concluded that this sky-blue species was percylite. These authors considered the forms present on fragmentary crystals to be cubic and stated that the mineral was isotropic upon optical examination.

Gregory (1878) called attention to an occurrence of microscopic blue transparent crystals at the San Rafael Mine, Galeria al Norte,

Bolivia. Uassive specimens gave tests for copper, lead and chlorine.

From these studies, Gregory concluded that the mineral was most pro­ bably percylite.

Raimcndi (1878), in a report on the minerals of Peru, indicated the occurrence of percylite at Cerro de Challacollo, Pica District,

Tarapaca Province, Peru. A blowpipe examination indicated the presence of copper, lead, and silver as well as some antimony and silica. A chemical analysis, performed on impure material, yielded percentages for lead, copper, silver, chlorine, water, antimony, manganese oxide, and silica. Raimondi did not include a formula derived from this analysis, but did state that he believed the mineral was an argenti­ ferous percylite.

Websky (1886), in a study of alteration products of in the region of Caracoles, Chile, noted the association of small, blue cubes with caracolite. Qualitative tests were performed on an impure sample and indicated the presence of lead, copper, silver, antimony, zinc, nickel, sodium, chlorine, and the sulfate radical as well as water.

Websky considered these blue crystals to be percylite as described by

Brooke (18^0). A qualitative analysis of a mixture said to consist only of percylite and caracolite but giving an analysis for ferric u oxide, zinc oxide, and nickel oxide yielded, after subtraction of caracolite, the formula PbCuClg^H^ for the blue crystals. This forraila was considered by Websky to be in good accord with that derived by Percy for percylite.

Sandberger (1886) described an occurrence of small, blue cubes in one of several crusts on galena in the Sierra Gorda between

Mejillones and Carocoies, Chile. His samples came from the same general area as those of Websky. An investigation of these cubes in polarized light showed them to be isotropic and a qualitative chemical examination indicated that the crystals were a lead-copper oxychloride, considered by Sandberger to be identical to percylite as previously described by Brooke (1850). He found, however, that his samples when heated strongly before the blowpipe yielded no water as previously indicated by Websky. Sandberger attributed the water found by Websky to the presence of a hydrated oxide or basic salt in intimate association with the caracolite and percylite.

Fletcher (1889) studied well-crystallized, isotropic, sky-blue crystals found in association with calcite, limonite and cerargyrite from Sierra Gorda, Chile. Goniometric measurements indicated that the crystals were isometric, and blowpipe tests indicated that this sky- blue species contained lead, copper, and chlorine. Fletcher concluded that these crystals were percylite.

Mallard and Cumenge (1891 &,b), in studying the copper deposits of Boleo near Santa Rosalia, Baja California, Mexico, noted a new species to which they gave the name boleite. The mineral was found as pseudocubes or modified pseudocubes up to two centimeters on a side and as small pseudo-octahedral groups and isolated crystals in

an argillaceous gangue covering the copper beds (the jaboncillo).

Associated with the boleite were iron oxide, gypsum, anglesite, phos-

genite and . An optical, crystallographic, and chemical

study was performed on boleite. This study indicated that the species was tetragonal, pseudo cubic and analogous in composition to percylite

except for the presence of essential silver present in boleite.

Schultze (1892) reported an occurrence of boleite, referred to

as argento-percylite, at the San Augustine Mine in Huantajaya District near Iquique, Chile. The boleite was found as secondary blue crusts

associated with atacamite and iodargyrite on stromeyerite. Although

Schultze intended to give a more complete description of the boleite

at a later date, no subsequent discussion on this subject is to be found in the literature.

Charles Friedel (1892) reported the synthesis of percylite by

a slow d iffu sio n method from copper ch lorid e and lead hydroxide. The crystals obtained were tetragonal and analogous to those described by

Mallard and Cumenge (1891 a,b). Since these crystals were silver- free, Friedel concluded that they were silver-free analogs of boleite and equivalent to percylite as described by Brooke (1850). Subsequent to the recognition of cumengeite as a distinct phase, however, Friedel

(I89U) realized that his synthetic percylite was probably cumengeite.

Cumenge (1893), during further study of the deposits at Boleo, discovered isolated pseudo-octahedral crystals up to seven millimeters in their maximum dimension. Upon examination of these crystals he found them to be significantly different fror boleite with respect to color, specific gravity and composition. An analysis by Fourment

(in Cumenge, 1893) indicated that the composition of these pseudo- octahedral crystals differed most markedly from boleite in that they contained no silver. Cumenge gave no name to this Bpecies.

Mallard (1893), in a restudy of pseudocubes and pseudo-octahedrons of boleite previously described ty Mallard and Cumenge (1891 a,b), found that the two varieties were not the same and that the pseudo-octahedrons corresponded to the crystals described ty Cumenge (1893). Mallard performed a complete crystallographic, optical and chemical examination of the pseudo-octahedral species and named it cumengeite in honor of

Cumenge who provided additional specimens of the species for study.

Mallard also restudied those boleite crystals which possessed grooves parallel to the edges of the pseudocube. A crystallographic and optical examination of the outer part of the grooves convinced him th at th is outer part was d is tin c t from iso tr o p ic b o le ite and from the more birefringent cumengeite. He consequently gave the name percylite to the anisotropic rim on boleite.

Charles F ried el (I 89U) noted the work of Cumenge (1892) and

Mallard (1893), which showed that cumengeite and boleite though related in composition and occurrence differed markedly since boleite contained essential silver. Wishing to satisfy himself that the pre­ sence o f a sm all amount of s ilv e r would modify the r e s u lt he had obtained in 1892 and produce boleite, Friedel undertook the synthesis of boleite by the method used in 1892. The description of his results indicates that he obtained boleite and possibly boleite with psaudo- boleite overgrowths. Upon receipt of new specimens of boleite and cumengeite ty the

Paris Museum of Natural History, Lacroix (1895) began a restudy of

these species. During this study he showed that cumengeite and boleite

were distinct species and that the outer part of the grooves on boleite

was indeed a distinct species as had been postulated ty Mallard (1893).

Lacroix showed that this outer part of boleite could not be analogous

to percylite as indicated by Mallard (1893) and consequently gave the

name pseudoboleite to this species. Among the crystals from Boleo,

Lacroix observed cubes and cubo-octahedrons which were only very weakly

birefringent. His study of these crystals convinced him that they were

analogous to specimens of percylite which had been described in the

literature and to specimens of percylite which were available from the

Buena Esperanza Mine, C h ile. Lacroix a lso described an unnamed sp ecies

similar in appearance and distribution to pseudoboleite but differing

slightly from pseudoboleite crystallographically. The status of the

minerals of the boleite group at the conclusion of Lacroix's investi- * gations is presented in Table 1, modified after Lacroix (1895)*

A. de Gramont (1895)> in an emission spectrographic study of numerous mineral species determined that boleite and cumengeite both

gave the characteristic spectra for lead, copper, and chlorine. In

addition, boleite gave the spectral lines for 3ilver. The samples were

provided ty Cumenge from Boleo near Santa Rosalia, Lower California,

Mexico.

Lacroix (1898), in a note on the minerals found at Boleo, indi­

cated that cumengeite and pseudoboleite were nearly exhausted in the

deposit and that large boleite crystals were to be found less and TABLE 1.—Minerals of the boleite group

Sub-group Cumengeite Boleite

General PbC^^CuCOHjg PbCl2Cu(OH)2 .nAgCl Composition

Species Cumengeite Unnamed Pseudo- Boleite Percylite b o le ite

Value o f n 1/3 1/3 1/3

Axial Ratio a:c 1:3.29U 1: 2.356 1:2.026 1:2 1:2

001 Oil 73°06' 67° 63°UU’ Cubic obs.

001 012 58

Birefringence 0.061 0.03 0.010 Very small or none

Index of 0 *= 2.026 R efraction E = 1.965 2.07

D ensity 1+.71 5.08 5.08 5.25U

aModified after Lacroix (1895). less frequently. He also reported that pseudocubes of boleite had

been found in association with cerussite for the first time.

Wallerant (1898), during a study of optical anomalies, iso­

morphism and polymorphism, reviewed the work done previously ty Mallard

and Cumenge (1891 a,b) on boleite. He noted especially that nothing

in the angular measurements of the boleite crystals indicated other

than a cubic symmetry for this species but that optical studies had

shown that each cube was composed of six interpenetrating individuals,

the base of each coincident with the face of an underlying, often isotropic cube. Etch tests on these interpenetrating crystals con­ vinced Wallerant that isotropic boleite was formed ty the interpene­ tration of three tetragonal individuals in such a way that the optic

axi3 of each individual was oriented parallel to the fourfold axis

of a cube.

SchLncaglia (1899), in an investigation of fluorescence in solids,

studied the effect of fluorescent light on boleite. He found that boleite gave no noticeable fluorescence under the experimental condi­ tions employed.

Wallerant (1900) pointed out that twinning could take place in such a way as to remain undetected optically if the twinning resulted in a retention of the original optical symmetry of the species or induced a higher optical symmetry as a result of increased morphological

symmetry. One exaaple of such twinning cited ty Wallerant was the case

of cumengeite. According to Wallerant, cumengeite was slightly biaxial

and exhibited etch figures which indicated that the species was composed

of interpenetrating individuals composed of a base and dipyramids so 10 oriented that the pseudo-octahedral faces of cumengeite were coiqposed of the bases of the individuals. In such a case the individuals would be symmetrical about the planes in the optical indicatrix and lead to the optical properties and optical homogeneity exhibited ty cumengeite.

Mugge (1903) studied the regular overgrowth of minerals one on another. Among the species studied were cumengeite on boleite. He noted the work of Mallard and Cumenge (1891 &»b) which showed boleite to be partly cubic and partly tetragonal. Mugge noted that pyramids of cumengeite were so place on each cube face as to form grooves possessing an angle of l52°Iil'. He pointed out that the outer part of boleite very nearly conformed to cumengeite in its optical proper­ t i e s and th a t Lacroix (1893) had observed a decrease in double refra c­ tion from cumengeite to tetragonal boleite and percylite and a corresponding decrease in density in the same direction. Mugge con­ cluded that since the species named possessed a great chemical simi­ larity, a further investigation was warranted to show whether or not the outer part of boleite was doubly refracting as a result of the presence of submicroscopic cumengeite in the same way as in the so-called "optically mixed" crystals of ammonium and iron chloride.

This term signified a polycrystalline aggregate of unknown cause.

Qeorge Friedel (1906) pointed out that, although several studies o f the copper-lead oxychlorides o f Boleo had been made, th ese stu d ies were as yet incomplete and ought to be resumed. Accordingly, he initiated a restudy of these minerals using a series of specimens from the collections of the School of Mines at Sainte-Etienne, samples and preparations of Mallard supplied by Ternder from the collections of the Paris School of Mines, and preparations and photographs supplied 11 ty Lacroix. The study performed ty Friedel was a comprehensive crystallographic, optical, and chemical study of cumengeite, boleite and pseudoboleite and represents a definitive treatise on these species,

F r ie d e l's work i s summarized in Table 2.

TABLE 2 .—-The b o le ite groupa

Species Cumengeite Pseudoboleite Boleite

Formula UPbCl2 «UCu0.5H20 5PbCl2 .UCuO*6H2C 9PbCl2*8Cu0*9H20

L a ttice Tetragonal, Same Same Octahedral Mode

Parameters arc 1.625 2.023 3.996

001 101 58°2U' 63°U2' 75°5>7'

D ensity U.67 U.85 5.0U5

0 - E 0.100 0.032 0.020

aFriedel, 1906.

Coblentz and Emerson (1917) stu d ied the p h o to e lec tric e ffe c ts produced in various substances. Among the substances studied was boleite. The authors observed no effect in boleite when this species was subjected to a negative potential in vacuum. No description other than s iz e and shape of th e sample was given . 12

Hadding (1919), n oting the work of Mallard and Cumenge (1891a)

and Friedel (1906), decided to study some specimens of boleite and

cumengeite from Boleo, recently acquired ty the Mineralogical-

Geological Institute of Lund. His goal was twofold: 1) to show whe­

ther boleite was cubic or pseudocubic, and 2) to show what variations

in structure were demonstrated ty cumengeite and how boleite was re­

lated to cumengeite. Hadding performed optical, crystallographic, and

density measurements on boleite and cumengeite. No pseudoboleite

specimens were available for study. From his investigations, Hadding

concluded that boleite was not tetragonal but cubic and that a continuous

solid solution extended from boleite to cumengeite.

Larsen (1921) gave refractive indices for boleite and percylite

determined ty the immersion method. The boleite specimen (USNM 809U3) was from Boleo, Baja California. The percylite specimen furnished

from the collections of the University of California was from an un­

known locality. Larsen concluded that the anisotropism displayed ty

some of the percylite fragments was the result of partial alteration to boleite.

Braly (1923), in describing a new procedure for analysis of the

coatings produced during blowpipe examinations indicated results ob­ tained from the study of coatings given ty boleite. His investigation

led him to the conclusion that boleite contained appreciable amounts

of lead, copper, silver and chlorine.

Spencer and Mountain (1923) described two new minerals found at

Higher P itts Farm, Mendip ffi.1 1 3 , Somerset, Ehgland. The first of these, chloroxiphite, occurred as green, bladed crystals completely 13 embedded in m andipite. The second, d ia b o le ite , was found as film s and

small patches of square-shaped crystals in close association with

chloroxiphite, cerussite and hydrocerussite. In so far as possible,

these authors gave crystallographic, optical, and chemical data for

these two species. They also noted that the properties found for diaboleite agreed well with those of a mineral belonging to the percylite-boleite groups, particularly with pseudoboleite. They pointed out, however, that the chemical composition and higher specific gravity of diaboleite clearly distinguished it from the mem­

bers of the boleite group.

Friedel (1926) again stressed his conclusions that boleite was tetragonal, body-centered, and possessed cubic external forms due to mimetic twinning of this tetragonal lattice. He pointed out also that it was not the lattice of boleite which is pseudocubic but rather the multiple lattice with the parameters c and Ua. In a discussion of the relationships of boleite, pseudoboleite and cumengeite, Friedel con­ tended that the forms present on these species indicated a body- centered lattice for each and that the regular growths observed in these species were probably related to approximate equality of the topical parameters. He pointed out, however, that comparisons based on the relationship of such parameters remained uncertain since the molecular size of the lattice had not been determined by means of x -r a y s.

Gossner (1928) reported his own observations as well as l&issgnug's x-ray measurements on boleite. From these investigations, Gossner doncluded that boleite was cubic and possessed optical anomalies as a Hi

result of strain.

Gossner and Arm (1929) performed a chemical and x-ray study of

species possessing complex structures. Included in this study were

boleite, cumengeite and pseudoboleite. From their investigations,

these authors concluded that cumengeite and boleite were valid species while pseudoboleite was invalid and equivalent to boleite.

Friedel (1930) noted the work of Gossner and Arm (1929) and

expressed the opinion that these authors had not correctly presented

nor given due consideration to the facts which he had presented in

1906 and had not been careful in their own experiments and calculations.

Friedel reiterated his conclusions as to the axial ratio for boleite

derived from the cleavages and stated that his theory of mimetic

twinning quite adequately accounted for the so-called optical anomalies

observed in boleite. Friedel also pointed out that crystallographic

and measurements suffice to prove the validity and tetragon al symmetry o f p seu d ob oleite.

Hocart (1930) performed an x-ray study of cumengeite, boleite and

pseudoboleite. Parameters determined during the course of this study yielded axial ratios nearly identical to those determined from crystallo­

graphic measurements ty Friedel (1906).

Gossner (1930) noted the comments of F ried el (1930) and the work

of Hocart (1930). While he admitted that there was x-ray evidence

which, if real would lead to tetragonal parameters for boleite, Gossner

concluded that this evidence arose from structural imperfections in the

species, that boleite and pseudoboleite were identical, and that boleite

was cu b ic. Hocart (193li), in a study o f o p tic a l anomalies shown by a number

o f s p e c ie s, included fu rth er comments on b o le ite as w e ll as a d d itio n a l

observations concerning the validity of pseudoboleite. In a preliminary

discussion of previous work on boleite and pseudoboleite, Hocart paid

particular attention to the work of Friedel (1906) and Lacroix (1895)

which showed that boleite and pseudoboleite were chemically, optically,

and crystallographically distinct species with tetragonal symmetry.

Hocart pointed out that the variation in birefringence in anisotropic

b o le ite was y et to be explained and might be due to s o lid so lu tio n or

strain. He further noted that quasi-isotropic boleite was the result

of twinning ty reticular pseudoraerohedry and that pseudoboleite on

boleite was a case of epitaxy controlled try a quasi-identity of a

certain lattice plane in each species.

In a critique of the work of Gossner and Arm (1929), Hocart

pointed out that it was probably quite true that the cubo-octahedral

crystals of boleite possessed a cubic symmetry due to the twinning

postulated by Friedel, but that the true symmetry of boleite could

only be determined from the most anisotropic, least twinned portions

of the boleite pseudocubes. Hocart attempted no explanation of the

identity of the x-ray photographs of Gossner and Arm produced from

cubo-octahedral and supposedly anisotropic boleite, but did point out

that these authors failed to give numerical values for the bire­

fringence and cleavages displayed ty the anisotropic boleite used for

their work. Hocart concluded that their anisotropic boleite was thus

s till poorly defined. Turning next to a consideration of the work of

Gossner and Arm on pseudoboleite, Hocart pointed out that although these 16 authors did indeed present x-ray photographs said to have been produced from pseudoboleite which were identical with those obtained from boleite, the authors had neither observed nor commented on octahedral cleavages, the constant birefringence nor the chemical composition of pseudoboleite.

Hocart posed the question as to whether there had in reality been more than a trace of pseudoboleite on the samples on which they made their study. In conclusion, Hocart expressed astonishment that these authors would give preference to results obtained from x-ray analysis on ma­ terial of admittedly poor quality, especially when results previously acquired by other methods raised such doubts as to the validity of their results. Hocart subsequently showed that by a correct choice of samples and proper experimental techniques, x-ray analysis would not only differentiate between boleite and pseudoboleite but also give positive documentary evidence of the mimetic twinning present in boleite.

Bellanca (I9l|l), in a note on a determination of the symmetry of b o le ite and pseudoboleite, pointed out that Laue diagrams obtained by him from different orientations left no doubt that these two species were distinct.

Palache et a l. (19Ul), in studying mineral specimens acquired from the Mammoth Mine, Tiger, Arizona, identified a number of species not previously described from this locality. Among these minerals were diaboleite, boleite and possibly pseudoboleite. Noting that the ori­ ginal description of diaboleite by Spencer and Mountain (1923) was based on insufficient material and was incomplete, these authors hoped to complete this description by a study of the more abundant material available from Mammoth Mine. A comprehensive crystallographic, optical, x-ray and chemical study performed by these authors characterized 17

diaboleite rather completely*

Ito (1950), in x-ray studies on polymorphism, performed a struc­

ture analysis on boleite. He noted the contradictory work of previous

authors and reexamined boleite on the basis that the external symmetry

displayed by the species might differ from its real symmetry and be the

result of minute internal twinning. Ito concluded that boleite was in­

deed tetragonal and that certain structural characteristics of the spe­

cies were the result of twinning on a unit cell scale.

Seeliger (1950) who studied pseudohydrothermal ore veins in the

Ruhr District pointed out that several rare minerals formed by the

influx of salt water were present in these veins. Among these minerals

were cumengeite, percylite and diaboleite. These species were identi­

fied by optical methods.

Bystrom and Wilhelrai (1950), in investigating the position of

n egative ions in lead compounds could n o t, in some c a se s, d e fin ite ly

choose the valid structure for some species because definite positions

could not be given for the negative ions involved. As a result, they turned to analogous compounds which possessed a sm all number of atoms

per unit cell in combination with a high symmetry since the possibi­

lity of a complete structure determination was increased in such com­

pounds. Inasmuch as diaboleite seemed to be an example of a compound

p ossessin g a sm all number of atoms per u n it c e l l and a high symmetry,

these authors performed a structure analysis of this species. Their work confirmed the x-ray investigations of Palache et a l. (l9Ul) in

every respect.

Berman in Palache et al.(l95l) gave ciystallographic, optical and x-ray data for chloroxiphite. Wilson et al. (1955), in a report dealing with the geology and mineral deposits of Boleo, noted that boleite, pseudoboleite, and cumengeite, previously found in a fairly small area of ore bed number th ree in th e Amelia IfLne near the Cumenge s h a ft, had been exhausted.

Sabina and Traill (i960) published x-ray powder diffraction data for cumengeite from Boleo, Mexico and percylite from Sierra Gorda,

C h ile. I I . SCOPE OF THE INVESTIGATION

As the historical survey given above indicates, the problems associated with the minerals included in this study are largely those of the members of the boleite group. Although the members of this group have been characterized rather completely from an optical and morphological standpoint, they remain to be differentiated unambiguous­ ly with respect to their chemical, x-ray, and paragenetic relation­ ships. Chemically, the species appear to be very similar and yet various investigators have been unable to agree on their chemical composition. The impression thus remains that their true chemical composition is in doubt. In the case of percylite, the analyses re­ ported in the literature have all been made on more or less impure material so that the validity as well as the composition of this species is in doubt. With the exception of boleite, no comprehensive, modem x-ray stucfy- has been performed on these species. Furthermore, in the case of boleite, no attempt has been made to correlate the optical behaviour of this species with the results of x-ray analysis. Although syntheses of boleite and percylite have been reported, no attempt has been made to synthesize these species over a range of temperatures or to study their behavior with increasing temperature in an attempt to shed some light on their paragenetic sequence.

19 20

Diaboleite and chloroxiphite have been included in the study because they are species closely allied chemically and, in the case of diaboleite, crystallographically with thB members of the boleite group. At the initiation of the study, it did not seem wholly unlikely that some hitherto unrecognized relationship might exist between diaboleite, chloroxiphite, and the members of the boleite group. Furthermore, it was felt that synthesis experiments on dia­ boleite and chloroxiphite might shed some light on the paragenetic relationships which, in general, result in distinct occurrences for diaboleite and chloroxiphite as opposed to members of the boleite group. Neither diaboleite nor chloroxiphite had previously been synthesized. The chemical composition of chloroxiphite is also in doubt.

The principal purpose of this study is, therefore, to charac­ terize as fully as possible each of the species involved. The method of approach is that of x-ray analysis complemented ty optical, crystallo- graphic, and chemical methods. A second line of attack has been that of synthesis in an attempt to secure sufficient pure material for x-ray analysis and chemical analysis as well as some data on the stability of these species with variation in temperature. I I I . EXPERIMENTAL METHODS

Specimens

The mineral species included in this study are all extremely rare and generally occur intimately intergrown with each other and with associated minerals. Therefore, in an attempt to provide specimens of a suitable size and purity for study, a concerted effort was made initially to synthesize each of the species. It soon became apparent that synthesis of specimens of a suitable size, if possible at all, was going to be a lengthy process. Consequently, it was necessary to perform the bulk of the work on natural specimens, supplementing the data obtained ty subsequent work on synthetic material as it became a v a ila b le .

Natural specimens of each of the species from as many different localities as possible Wei's obtained. Specimens were difficult to obtain and were of small size. Since, in many cases, the specimens had to be returned in as undisturbed a condition as possible and since, in other cases, the specimens were very fine-grained and often in an impure state, it was necessary to adopt analytical techniques suited to working w ith very sm all amounts of each substance.

X-ray Analysis

The chief analytical tool used in this study was x-ray analysis.

Both powder and sin g le c r y sta l methods were employed. The generator

21 base unit used was the North American Phillips Model No. 1201*5

x-ray diffraction unit equipped with Norelco Type 32112 copper

target tube and copper radiation was used throughout the study.

The copper tube was operated at 35 kilovolts and 20 ndlliamperes and

monochromatic radiation was obtained ty means of a nickel filter with

a thickness of .00075 inches. Since long exposure times were necessary

because of sample size, composition, and superlattice effects present

in some of the species, it was necessaiy to use an x-ray film which had

a low background to line ratio. Since Ilford Industrial Type G X-ray

Film possesses a very low ratio and yields exceptionally clear photo­

graphs despite extraordinarily long exposure times, this film was

generally used throughout the study.

N orelco Type 52056 Powder Cameras w ith the Straumanis type of

specimen mount and a camera radius of 111*.6 m illim eters were employed to produce all the powder diffraction photographs used in this study.

Each powder camera was calibrated with a quartz specimen prepared from

oscillator-grade quartz. The line positions obtained for quartz were

compared with the data given by Keith (1955) and a camera diameter and

eccentricity error were derived for each camera. Because of the small

size of the specimens available for powder work, powder samples were

mounted in sealed, low-absorption, glass capillary tubes. This type

of mount also introduced a uniformity of distribution and size of sample exposed to the x-ray beam in each case. The capillary tubes used were 0.3 millimeters in diameter and were obtained through Caine

Scientific Sales Company of Chicago. Exposure times of approximately

20 hours were used as a rule throughout the powder work. 23

The powder diagrams obtained were read on a Norelco powder photo­ graph reading device, which was fitted with a millimeter scale cali­ brated to 0.0$ millimeters. A shrinkage correction, in addition to the camera diameter and eccentricity corrections mentioned above, was applied to each film . Where cell dimensions were derived from powder data, extrapolation methods were used to eliminate the remaining errors present in the measurements.

The rotation and equi-inclination Weissenberg photographs were made with an instrument manufactured by Charles Supper of Newton

Center, Massachusetts. The camera diameter of this apparatus is

$7.3 millimeters. The crystals were generally mounted on a universal goniometer head by means of a glass rod and wax. Where a more rigid mount was desired, a suitable mixture of ethyl cellulose and toluene was used to cement the specimen to the glass rod* Preliminary align­ ment of the crystal was obtained on a large two-circle reflecting goniometer with the final orientation being made with the use of successive rotation photographs and suitable adjustments of the arcs on the goniometer head. Exposure times were quite variable and de­ pended on the size of the crystal and the structure of the species for the particular photograph sought. In general, however, exposure times varied between 20 and 200 hours.

Where cell dimensions were derived from the s in g le cry stal photo­ graphs, a reading device supplied by Charles Supper was used. This device allows the zero direction on the film to be set normal to a ndlli-meter scale calibrated to 0.0$mm. No shrinkage or camera eccen­ tricity corrections were made, but extrapolation techniques were used 2k to eliminate the other errors in measurement.

For the purpose of space group determination, the equi-inclination

Weissenberg photographs were read and plotted according to the method given by Buerger (19U2).

The x-ray investigation was complemented by optical techniques in so far as it was possible to use these techniques. Refractive index measurements would have been of inestimable assistance as a routine tool in identification and distinction of these species.

Unfortunately, however, immersion techniques were useless since the appropriate high index oils immediately caused decomposition of the specimens. Minimum deviation measurements which would have been of some use were prohibited by the small size of the specimens available.

Optical techniques were therefore limited to a determination of the optical character, purity, and homogeneity of specimens. Petrographic examinations were performed with the use of a Leitz Model III M Lux

Petrographic Microscope. Initial optical examination as well as subsequent preparation was made with the use of a Spencer Model

21C Stereoscopic Microscope.

Heating Stage

The heating stage used in th is study was a Model 767 Microhot

Stage manufactured by Arthur H. Thomas Company of Philadelphia,

Pennsylvania. Power was supplied by a variable transformer and temperature was measured by a mercury thermometer calibrated in steps of 1°C. The heating rate varied considerably depending on the setting of the transformer, but in general was probably about U°C. per minute in the range 30° to 175°C. decreasing to about 1° per minute above 175°c. A cooling block was used to cool the stage once heating was discontinued. This block produced a cooling rate of about l£° per minutej this rate dropped off rapidly as the block picked up heat.

Crystallographic Ifeasurements

Crystallographic measurements as well as the preliminary orien­ tations of crystals for x-ray work were done on a two-circle reflecting goniometer of the Goldschmidt type manufactured try P. Stoe of Heidel­ berg. This goniometer is capable of reading to ♦ 1' of arc. Since the crystals available for study were, in general, quite small or possessed poorly reflecting faces, only approximate angular measure­ ments could be obtained. The measurements thus obtained were used in a send-quantitative way to verify the identity of a particular specimen, the angles adopted by Palache et a l. (I9!?l) being used as a standard of comparison. In all cases, the angular measurements were of sufficient accuracy to identify the species, an identification which was substantiated by x-ray analysis.

S p e c ific Gravity Measurements

Because of the small size of the samples available for specific gravity measurements, it was necessary to use a Berman balance for these measurements. The balance used in this study is the Precision

Balance manufactured by the Roller Smith Company and distributed through the Bethlehem Instrument Company, Bethlehem, Pennsylvania. Toluene was used as the displacement liquid during determinations and tempera­ ture corrections for the variation is specific gravity of toluene were made by means of the equation given by Berman (1939). 26

During the course of the specific gravity determination, it became apparent that only samples with a total weight greater than four milligrams gave reproducible results. Consequently, only the results obtained from samples with a weight greater than four m illi­ grams are included in t h is study.

S yn th esis

A number of different methods were used in preliminaiy synthesis attempts. Of these methods, only evaporation from hydrous solutions in open vessels, slow diffusion of an aqueous solution into a hydrous mixture and hydrothermal synthesis in closed vessels were successful.

Except in one case, stoichiometric proportions were used in this work.

Evaporation from hydrous solutions was accomplished by mixing the reagents with distilled water, placing the mixture in an open dish, and allowing complete evaporation to take place at room tenperature.

The method of slow diffusion used was that proposed by Friedel

(1892). In this method the soluble salt is allowed to diffuse slowly into a hydrous mixture of insoluble components.

The hydrothermal syntheses were carried out in conventional pot bombs and sealed pyrex test tubes. In the case of pot bombs, a Teflon liner was used to prevent reaction between the reagents and the metal walls of the bomb. At lower temperatures than those employed for the pot bomb experiments it was possible to use hydrous mixtures sealed in pyrex tubes.

The reagents used for these experiments were of chemically pure grade or, in the case of lead hydroxide, prepared from chemically pure reagents. To insure the identity and purity of reagents, they were 27 checked by x-ray diffraction and compared with standard patterns for the reagent. Further details of the experimental procedure used for the synthesis experiments are given in the chapter on synthesis.

IfLcrochemical Tests

In soma cases during the course of the investigation it was necessary to determine the silver content of some of the natural specimens and to verify the presence of silver in one of the syn­ thetic species. Short (191*0) has pointed out that, if there are any chlorine ions present when a silver-bearing species is dissolved in nitric acid, the silver w ill be precipitated immediately. Since the species included in this study contain chlorine, the presence or absence of silver can be determined by the simple process of dissolv­ ing the specimen in chemically pare concentrated nitric acid. The lower lim it of detection given by Short (19U0) for other microchami- cal tests is 0.02 to 0.03^ silver. The nitric acid test in the case of the present study is probably of a similar sensitivity, especially since no interference is to be expected from the presence of copper or lead. To the observations of Short can be added the observation that silver chloride is light sensitive and takes on a purplish cast on exposure to an intense light source, ^oth the immediate appearance of a curdy precipitate in nitric acid and the subsequent acquisition of a purplish cast by the precipitate were used in a qualitative test for the presence of silver.

Inasmuch as not only the presence of silver, but some idea of the amount present wa3 necessary, simultaneous tests were performed on a boleite specimen as a standard and on a test specimen of approximately equal size. The relative proportion of precipitate produced was esti­ mated visually. Where silver appeared to be absent, the liquid was evaporated in an attempt to observe the formation of crystals of s ilv e r c h lo r id e . The amount of s ilv e r present was then estim ated according to the qualitative standards set forth by Short (19U0).

According to Short (19U0), silver chloride w ill develop complex forms at high concentrations but at lower concentrations, simple forms such as the cube and octahedron predominate. IV . CUMENGEITE

H isto r ic a l Summary

The fact that cumengeite was a distinct species had gone unnoticed

by Mallard and Cumenge (1891 a,b). However, before cumengeite was

recognized from a study of natural specimens, Charles Friedel (1892),

thinking this species was percylite, had synthesized cumengeite try

reacting cupric chloride and lead hydroxide. Friedel obtained a blue

crystalline mass composed of dipyramidal crystals and more rarely,

small cube-like crystals. The dipyramidal crystals were found to be

highly birefringent and seemed to Friedel to possess the characteristics

of tetragonal boleite. Except for the fact that the cube-like crystals

lacked silver, he felt that these crystals were analogous to "cubic"

boleite. The combination of several individual chemical analyses led to the approximate formula PbCu^H^Clg but Friedel made no complete

analysis on one sample. Friedel concluded from his results that he

had synthesized percylite with the two habits for boleite described

by Mallard and Cumenge (1891 a,b).

Cumenge (1893), during further study of the deposits at Boleo,

discovered isolated crystals up to seven millimeters in their maximum

dimension, which differed significantly from boleite found in the same

location. These crystals were accompanied by anglesite, boleite,

atacamite and gypsum. Their color was more violet than that of boleite.

29

% 30

A measurement of their specific gravity gave 1*.675 as opposed to 5.08 for boleite, while the hardness of the two species was approximately the same. A chemical analysis made by Fourment (in Cumenge, 1893) indicated that the chemical composition of these new crystals differed from that of boleite by the absence of silver and yielded a formula which approximated PbCu(0H)2Cl2 c lo s e ly . No name was given to th is new sp ecies by Cumenge.

Mallard (1893) performed a restudy of the dipyramidal varieties of boleite which he and Cumenge (Mallard and Cumenge, 1891 a,b) had described earlier. During this study, Mallard also made a more com­ plete investigation of the dipyramidal crystals found by Cumenge (1893).

A crystallographic study of good quality dipyramidal crystals supplied by Cumenge gave an angle of * between the di pyramid and the base and an axial ratio of 1.61*69. A determination of the refrac­ tiv e in d ices by means of the prism method gave e*1.965 and o=2.026 for refracted light of a blue-green color. The birefringence, determined from the refractive indices was 0.061. A petrographic examination indicated that the crystals were uniaxial negative. A measurement of the density gave 1*.71.

Mallard noted that the quantitative analysis given for these dipyramidal c r y sta ls by Cumenge (1893) le d to the formula PbCu0Cl2#2H20.

This formula differed from the formula PbCuCl2(0H)2 obtained by Friedel

(1892) for synthetic percylite only by the presence of an additional molecule of water. At Mallard's request, Friedel repeated the chemical analysis of the dipyramidal crystals from Boleo, and determined that the percentage of water reported by Cumenge (1893) was "too high due to 31 the v o la tiliz a t io n o f some PbClg a t the temperature of a n a ly sis .

Friedel's results showed that the formula of the dipyramidal crystals, provided ty Mallard and identical to those studied by Cumenge (1893), ought to be PbCuCl2(OH)2«

In honor of Cumenge who provided specimens of the Boleo minerals for study, Mallard named the dipyramidal species cumengeite.

Lacroix (189$), in restudying the minerals of the boleite group, indicated that cumengeite was pleochroic according to the formula

0 » greenish blue and E ■ sky blue. He also found that cumengeite became atacandte green on heating, but regained its blue color again on co o lin g .

Wallerant (1900) pointed out that twinning could take place in such a way as to remain undetected optically if the twinning resulted in a retention of the original optical symmetry of the single species.

One example of such twinning cited by Wallerant was the case of cumengeite.

According to Wallerant, cumengeite was slightly biaxial and exhibited etch figures which indicated that the species was composed of inter­ penetrating individuals composed of a base and pyramids oriented such that the dipyramidal faces of cumengeite were composed of the bases of the individuals. In such a case the individuals would be symmetrical about the planes in the optical indicatrix aid lead to the optical pro­ perties and optical homogeneity exhibited by cumengeite.

G. Friedel (1906), in a complete restudy of cumengeite, found that cumengeite was a deep Prussian blue, clearer than that of boleite or p seu d ob oleite. The powder was a sky-blue c o lo r . In transparent p la tes the mineral was a deep, clear blue color with a greenish tint less mar­ 32 ked than in the case of boleite. The marked pleochroisa reported by

Lacroix was scarcely apparent to Friedel who found the pleochroic formula to be: 0 = clear bluej E = deeper, slightly greener blue.

Friedel assigned the notation to the prism, and f00l] to the base which often occurred on the cry- stals. He found that cumengeite occurred as homogeneous, well-developed crystals which were never twinned. Reflections from the dipyramidal faces were poor but there were very good, interrupted cleavages parallel to (Oil), which gave good reflections. Also present were two optically inferior cleavages parallel to (110) and a still more imperfect clea­ vage parallel to (001). As a result of measurements on the (o il) cleavages, the angle between (Oil) and (001) was found to be 58°23' from the angle 7U°03' (mean of eight measurements), between (101) and

(Oil) and 58°25' from the angle ll6°$0' between (101) and (T01) across

(001). Using 58°2U' for the interfacial angle (101) to (001), Friedel obtained the axial ratio a:c = 1:1.62$ ♦ 0.001 as opposed to a:c =

1:1.6U29 found by Mallard (1893). Friedel also pointed out that his axial ratio is near no simple multiple for the cubic axial ratio. From a calculation of the reticular area and by use of the Law of Bravais,

Friedel concluded that cumengeite possessed a centered tetragonal lattice of the dipyramidal mode.

Noting that the water found by C. Friedel (in Mallard, 1893) was too high for the formula of cumengeite given by Mallard and that the values given for lead and copper were in only mediocre agreement with the formula, F ried el repeated the an alysis using 0*2669 grains of cumengeite fo r the lea d , ch lorin e and copper and 0.9192 grams fo r the 33 determination of the percentage of water. He found that the mineral contained no trace of silver. The results of the analysis led to the formula UPbClg'UCuO'fj^O, a fornula which preserved the ratio of

1:1 for PbCl2«CuO found by Mallard (1893) and agreed very exactly with the percentage for water found by C. Friedel (in Mallard, 1893),

The specific gravity obtained on forty-four small crystals having a to ta l w eight of 0,8271 grams was U.67.

An optical examination verified Mallard's observation (1893) that cumengeite was optically negative and had indices of refraction near 2.

Friedel did not, however, repeat the index measurements but did state that the birefringence found by Mallard (1893) was too small. By use of the circular analyzer, Friedel determined the birefringence to be

0.10 but was limited to two significant places because of •uncertainty as to the correct thickness of his specimen. A measurement of the retardation of cumengeite in contact with pseudoboleite gave the value

3.10 for the ratio of the birefringence of cumengeite to that of pseudo­ boleite. Using a value of 0.032 for the birefringence of pseudoboleite, he found that of cumengeite to be very near 0.100.

The cumengeite crystals investigated by Hadding (1919) were dominated by the prism faces although the dipyramidal faces were also well-developed. The base, observed by Friedel (1906), was not noted.

The composite crystals of cumengeite on boleite observed by Hadding were all of the rare type described by Friedel (1906), i.e . cumengeite on boleite without intervening or associated pseudoboleite.

Measurement of the in te r fa c ia l and cleavage angles in d ica ted that these angles were not constant but varied within certain lim its. Hadding a ttrib u ted th ese v a ria tio n s to isomorphous su b stitu tio n but indicated that he did not possess sufficient material for a chemical analysis to substantiate his theory.

The reflections obtained from the crystal faces were multiple and weak. Only one small portion of each face reflected in a given position.

The dipyramidal faces were found to be concave. The angles obtained by

Hadding fo r cumengeite and the a x ia l r a tio s ca lcu la ted from th ese angles are given in Table 3.

TABLE 3 .—Angle table for cumengeitea

Crystal (101) to (Td) (101) to (Oil) (101) to (110) Axial Ratio

1 119°121 ♦ 0°17* 75°071 ♦ 0°22» $2°27’ ♦ 0°10' 1.7011

6 118°U3' ♦ 0°26' 7U°58'+0°l5' 52°27' * 0°22* 1.6898

3 118°32* ♦ 0°03' 7U°U6» + 0°06‘ 52°31' ♦ 0°12' 1.6820

8 118°31* ♦ 0°l8« 75°10‘ ♦ 0°36« 52°28» + 0°13' 1.6808

U 117°3U' + 0°2U* 7h°h$1 + 0°38» 1.6501

7 117°09' ♦ 0°07« 7U°l6' ■*. o°00« 52°52‘ + 0°l5' 1.6393

8 (Cleavage) 117°08* ♦ 0°0U« 1.6361

1 (Cleavage) ll6035* 0°03' 1.6186

aModified after Hadding, 1919. A measurement of the specific gravity was performed on cumengeite

broken from combinations with boleite as well as on the cumengeite-

boleite combinations themselves. Both types of measurement were made

by means of the suspension method. A determination made on 0.125 grams

of cumengeite broken from its boleite core gave the value 1*.75 0.01.

Two individual determinations made on two separate cumengeite-boleite

combinations weighing 1.81* ♦ 0.06 and 0.15U 0.01* grams gave the

values l*.7l* ♦ 0.02 and i*.88 * 0.01, respectively, for cumengeite, after

making a correction fo r the w eight and volume of the b o le ite co re. The

correction for the boleite core was made by direct measurement of the

volume of the boleite present in each case and calculation of the weight

of each core assuming a specific gravity of 1**9 ♦ 0.1 for boleite.

Hadding did not feel that his values were more accurate than those of

Mallard (l891a) and Friedel (1906) but did conclude that different

crystals of cumengeite had important differences in composition.

Noting the disagreement concerning the true value of the birefrin­

gence for cumengeite, Hadding prepared three prisms by using two polished

crystal faces and additional ground surfaces in the prism zone of a

cumengeite crystal. Though the faces of the prisms took only an in­

ferior polish, he was able to obtain weak, diffuse signals. Using

monochromatic light, he found that only the green-blue wave-length

(about 510 millimicrons) was strong enough to give a perceptible signal. The values obtained by measurement of the prisms is shown

in Table i*.

Hadding noted that there was close correspondence between his values for the birefringence and those given by Friedel (1906). 36

Although he pointed out that there seemed to be variation in the re­ fractive indices and birefringence, he felt that the scarcity and poor quality of the material did not warrant such a conclusion. He did indicate, however, that the refraction in the outer portion of the cumengeite c r y sta l was the w eakest.

TABLE 1+.—Indices of refraction for cumengeite (Hadding, 1919)

Prism 0 E 0 - E

1 2.01+15 1.921+6 0.1169

2 2.037 ♦ 0.011+ 1.923 + 0.013 0.111+ + 0.027

3 2.01+5 * °«00il 1.930 ♦ 0.005 0.115 v 0.009

Average 2.0U- 1.926 0.115

Because o f a lack of s u f f ic ie n t m aterial, Hadding did not perform a chemical analysis on cumengeite. He pointed out once again, however, that the composition can vary as was shown by the variation in inter­ facial angles and specific gravity of cumengeite.

Hadding's investigations of cumengeite and particularly his observations of the variation in cleavage angles for this species led him to the conclusion that cumengeite formed a continuous solid solution with boleite.

Gossner and Arm (1929) performed an x-ray single-crystal inves­ tigation of cumengeite, using the rotation method. The crystals used were short, prismatic crystals possessing the forms {lio}, {.Oil] and the smaller {ooi], Composite crystals with cumengeite on boleite were 37 also available. The crystals of cumengeite on boleite were dominated try [O llj but also possessed very small faces belonging to ^001j and

£llO^. Repeated measurements on both the single crystals and the cumengeite of the composite crystals gave 58°26' for the angle {00lj to flOl). The a x ia l r a tio was thus a:c = 1:1*628.

An x-ray examination of complete cumengeite crystals and the cumengeite portions of the composite crystals gave the value 15.6& f°r 2l*.7!>£ for d^^, 21.3& for d^Q, and 16.39^ for d11]L. All these values except for were the average of five measurements from rotation photographs; d-,-,^ was the average of four measurements. The determination of &iqq from (hhO) reflections gave l5*l$$ for d^QQ; dQoi from ( OOjL) r e fle c tio n s was found to be 2U.66iL The values

15.17X for d^QQ and 2U.7lX for d^^ were adopted by the authors and gave the axial ratio a:c = 1:1.629.

Gossner and Arm concluded from the ratio of the x-ray parameters th at cumengeite was body-centered, d itetragon al dipyramidal, and be- 17-20 longed to one of the space groups ' . A Laue diagram was pro­ duced normal to a {lOl^ cleavage plate. Though the resulting spots were weak, th ese spots in d icated the presence of a symmetry plane normal to [oio]. The presence of such a symmetry plane eliminated the c la sses Sj^, C^, and Cj^.

Using the overlay method previously employed in indexing the rotations of boleite, the authors were able to index a great number of spots. The resulting indices supported the choice of a body- centered lattice already deduced from the ratio of the parameters.

It was found that only those reflections with (h+k+jO equal to a whole number were p resen t. Since both even and odd orders o f (hhO) were present and (00JL) should have reflected for the first time where

X “ Un, and were elim in ated . eliminated by (Ok/) r e fle c tio n s which occurred in great numbers fo r the f i r s t order reflections.

Inasmuch as the formula for cumengeite given by Friedel (1906) gave no rational number for Z, Gossner and Arm performed a new analy­ sis on 0.2U80 grams of cumengeite. The analysis was performed in the same way as for boleite by heating the sample with soda to 600°G., collecting the water in an absorption tube and analyzing the remaining material by wet chemical methods.

The analysis yielded ratios for lead, copper, chlorine, and w ater, which led to the formula HPbCl2*UCu(0H)2*H20. This formula, while in satisfactory agreement with that derived by Friedel (1906), did not give a satisfactory value in the subsequent calculation of Z.

The authors further indicated that there appeared to be a case for isomorphous substitution as indicated by the ratios obtained in their analyses, but that this substitution was so slight as to be of un­ certain validity.

A determination of the specific gravity performed on single crystals of cumengeite weighing 0.336 grams gave the value U.73.

A similar determination performed on 0.995 grams of cumengeite broken from composite crystals yielded U.780, U.781, and U.782, or an average of U.781. The authors could not state with certainty, however, that the cumengeite from the composite crystals was entirely free of boleite. 39 Feeling that some uncertainty existed with regard to the specific gravity, Gossner and Arm used the values U.67 (Friedel, 1906), 1*.73,

and U.78 w ith th e formula l*PbCl2#UCu(0H)2#H20 fo r the ca lcu la tio n of

Z. These values for the specific gravity resulted in values of Z equal to 10.58, 10.72, and 10.83, respectively. Gossner and Arm stated that none of the values for Z approached a whole number in a satisfactory way. They indicated, however, that a mere powerful ar­ gument e x iste d th a t th e value Z, as w e ll as th e formula given fo r cumengeite, were in error, namely, that both Z and the formula would lead to an odd number of oxygen atoms in the u n it c e l l , a condition not permitted by a body-centered lattice.

A calculation of Z using the formula PbCl2*Cu(OH)2 together with the s p e c ific g r a v itie s 1*.78 and U.73 gave the values 1*3.88 and 1*3.1*1, respectively. The authors felt that the value 1*3.88 for Z was suf­ ficiently close to the whole number 1*1* to be acceptable, but they also noted that the excess water observed in the analysis argued against the formula PbCl2BCu(OH)2. No difficulty was encountered in fitting the number and kind o f atoms in to the s it e s a v a ila b le in th e un it c e l l of D^J.

The authors further examined the possibility that the formula for cumengeite could be 3PbCl2*3Cu(0H)2*H20. A calculation of Z using

1*.73 and i*.78 y ield e d ll*.23 and ll*.l*, r e s p e c tiv e ly . The value 11*.23 was nearly within the limits of error but this value necessitated three kinds of divalent points in the unit cell, a possibility not 17 permissable in Dj^. The chemical formula used in this case departed as well from their experimental determination of the formula. Uo The authors consequently concluded th at only PbCl2*0u(OH)2 was in accord with their x-ray study and that the unit cell contained UU molecules of this composition. The excess water in their analyses was attributed to the complex structure of cumengeite, a structure said to permit water to be present in non-stoichiometric proportions.

Noting the close similarity of some of the parameters for boleite and cumengeite, these authors discussed the possibility that the com­ plex crystals supposedly formed by cumengeite on boleite might actually be formed by twinning of cumengeite on (035).

Friedel (1930) noted that Gossner and Arm (1929) had done little more than repeat his conclusion with respect to cumengeite. He pointed out that the contention of these authors that cumengeite was twinned on

(035) was in poor agreement with their own angular measurements as well as the theory of twinning. Friedel stated that the core of these supposedly twinned crystals was invariably boleite and that cumengeite cm boleite was a case of an oriented growth of one species on another.

Using the oscillation method, Hocart (1930) obtained the para­ meters cQ= and a0= Ht.9$ for cumengeite. The ratio of these parameters agreed well with the axial ratio a:c - 1:1.625 derived by

Friedel (1906) from the cleavages. Using the formula UPbCl2*l*CuO»5H20

(Friedel, 1906), Hocart found the value of Z to be 10 with a precision greater than 1$.

Results of the Present Study

The specimens of cumengeite available for this stucfy included a number of complete sin g le c r y sta ls as w e ll as p a r tia l c r y sta ls which occur as epitactic overgrowths on boleite. One prismatic crystal of Ul the type studied by Hadding (1919) was available. The crystal is dominated by flio j although £ Oil^ is s till well-developed (Figure 1).

Most of the complete single crystals (Figure 2) are dominated by the dipyramid {oil^ and the prism { lio j , according to the notation of

Friedel (1906). The basal pinacoid {001^ is generally absent. The partial crystals generally possess only the faces of the dipyramid foilj hit occasionally very small faces belonging to the form {llO \ may a lso be present. The position of the basal pinacoid is generally occupied by a broken surface. While the crystal faces of cumengeite appear to be bright, goniometric observations indicate that these faces are of poor quality. The best faces are the dipyramidal faces while those of the basal pinacoid are the poorest. A rather marked mosaic structure is indicated by the fact that all the faces yield multiple reflections.

In addition, the dipyramidal faces are composed of rather large blocks as indicated by slightly different positions of reflection for different areas on a given face. The dipyramidal faces also appear to be some­ what dish-shaped. Measurements of the angle (00l)/\ (Oil), according to the setting adopted by Friedel (1906), gave 5>8°33' 0°30' (average of ten measurements). This angle is of adequate precision, however, to indicate that the species under investigation is indeed the cumengeite of Mallard (1893) and Friedel (1906).

An optical examination of cumengeite indicates that cumengeite is uniaxial negative and possesses a uniform high birefringence quite consistent with the value 0.100 given by Friedel (1906). In some cases a slight separation of the isogyres amounting to 2-3° is present. No deviation from parallel extinction is exhibited by h2

Fig. 1.—Morphology of natural cumengeite (Hadding, 1919).

Fig. 2.—-Morphology of natural cumengeite. U3

F ig . 1 .

Fig. 2.

\ Uk cumengeite •

Powder patterns were obtained from a number of d iffe r e n t sp e ci­ mens of cumengeite. Material from complete single crystals and from epitactic overgrowths of cumengeite were used for the production of these patterns. All the patterns obtained were identical with respect t o lin e p o sitio n and in te n s ity . The powder pattern of cumengeite i s much more complex than would be expected from a tetragonal species.

Approximately one hundred and eighty lines are present in the range

0-180° 29. Powder diffraction patterns obtained from several different specimens are shown in Figures 3 to 6 .

Indexed powder data, representative of the patterns obtained for cumengeite, are given in Table 5. The pattern has been indexed on the basis of the face-centered tetragonal lattice indicated for cumengeite by the single crystal work discussed below. It is noteworthy that indexing on the basis of a primitive or body-centered lattice does not account in an adequate manner for the d-spacings observed.

Rotation and Weissenberg photographs were obtained for cumengeite for the directions [00l3, jjLOCQ, and j l l o Q , according to the setting adopted by Friedel (1906), Gossner (1928), Gossner and Arm (1929), and

Hocart (1930). The rotation photographs in combination with the zero- level Weissenberg photographs yielded the parameters

2U.U9 z ° ‘o1^ 15.10 ♦ 0.02&

21.37 ♦ O.O^X fo r [OOl] , [loo], and [lio] , respectively, according to the setting adopted by Friedel (1906). These values are compared in Table 6 with Fig. 3*—Powder diffraction pattern of natural cumengeite (Sainte-Etienne 2750) .

Fig. U. —Powder diffraction pattern of natural cumengeite (USNM 9U6UO) .

Fig. 5.~Povrder diffraction pattern of natural cumengeite from an overgrowth of cumengeite on boleite (Hocart specimen) •

Fig. 6.—Powder diffraction pattern of natural cumengeite from an overgrowth of cumengeite on boleite (OSU collections) • F ig . 3.

F ig . h.

F ig . 5 .

F ig . 6.

Cr O U7 rpA^V cj a - ra y Gc/.iGGr u ca. lor eu..:ar.joitca .

Sabina ct al. 0. .0 j u j c 4- V ,,4. 4 b (H o b ) *7 -a- ■; ^ v. .. OJi. Ui.OolL Joleo, liable 0 Bo’i eo, i co

ibasurGC. ~oasurod 7 -jaaaroci Calculitodc

:l-:I I d I d a a d

12 o il Li o t'O 12.23 P 1 0 7.71 7o^0 w • '»> *V ' , ' ' j 7.37 I U -r_^‘ 7.1:1 7.36 113 7.02 7.12 / O v-’W 4' o „■ 3’ c.13 Q C’ o 1' / / ;',0 6.12 4' • j 1 1 ■c 1 p f j • 3.17 ]_ 1 3.21 331 3 > .v j 1.93 4 9 I. . p . 7c m 11.76

P *pH I^ p■ t f 6 ® / ■' ..:0a ) 9 • i -i 1 - 3 7

r ^ * /1 o_v~ I 3 o o 3 .12 I 9*6 • «^.L ^ .■■a C 6 w v 1 6 23? ( ^ a 1^ 3311 >.32 > • 9 — 117? c2.;j f a - 3.22 m C 3.11 n 62c 6 © Oo ( \ 3.10 (.3.06 3.06 6-, 711 c.; u 2.77 o Co ol. 9 ? .01! 2 Q2° < - “-'--a H i (2.93)

33?; 1 0 7: ! , n /2„o73 < j. 2.6 oO A 0 IvLf 1 22oi 12.037 313,713 2.62 o 6 2.012 2 .o3l 1*8 ^ _. i

sr.. -?c j ; y.L c ''' J

- o r - ' ; - ...rr.rv.i'c:. : ,;r.:jM rod C a l c u l a t e d

h 9 : l I Cl I d I d d

616 2 2.77 2.766 2.765 oOOl 666 | 1; 2.66 2.655 2.657 E\

t:m 2.59 o 2.595 O' 2.590 r-* 555,715 . 1? 2.563 5 0.060 66' 2.506 2 0 U2 } 3 2.503 <- 735 Wo O- ; p 2.621 u6_LJ >10 ! t-4 f 10 J-U 10 2.372

I s J \,0 12" ° 1 .-1 ) 2.32 2.530

>57, 717] 2 . >

1 : 3

:::} 3 2.257 755 2.013 2 2.210 iO O lo j 6 2.1c l^U 7 0o „i 1 o'/4

UuljS 1 j 1 2.127 10 °0 °0 J 10“2° 0 } 773 J J 2 J - i- 1 0 2 2.053 £1O O '-'"I 3°3°111 n ^ 7 j 2.029 5 2.027 Coo,'5/ J yQ ^ ’-< ^ 6 2 .01 6 2.007 8 2.010 0«12 000 775 1.975 5 1.976 6 1.576 Sabine ot a l. Ohio OT J.uu ^ 0 6' *■ 1 (1?(JUj Oc-llc cticr.o StmjiielaC Eoleo, ’22X1 CO Eoleo j lexica

' ° as area urea .'1 asured Calculated

hl-cl i d I a I a d

739 f1.956 5 1.950 5 1.952 Ou j « :1.955 CoO ) 71.569 6°Uo10 1.686 1 0 * u °6 0 1.667 J 1.685 5 1 o L>1 5 1.660 < 937 1 »C* G G

1 0 C'GLJ. n « i•3, .1.663 2 1.6 53 ]_ 1.359 2 1 ,02t> 2 1.6 25 3 1.601 3 1.7 96 1.774 2 1.776 2 1.-775 1 i 1.762 1.762 1 1 0 ?oo / ' -3 1.735 G J-' . "9) : 0-7 6 1 . i 1.719 2 1. ?2p G 1.706 b 1.701 -'i 2 1.669 G _ . -_y G 1 » 0 a>* G n 1 . a 0 s 1 . 6 6 0

i — • cu 9 1 1.636 ‘j 1.619 1.01? 2 1 0 r .10 2 1.597 1 _L • G GG G -L O > wj 3 1.350 r, 1.505 0 1.566 0 3 1.530 5 1.55c j 1.535 r-t — C y a 2 Ol 0 ~l - ' 1 1.505 G — O .y -- v- G 1 oUO / r. ~i ' 6 ’ ; s 1.5 90 2 1.1 o? G ' ' 'V 5 1.077 1 1X55 1.153 5 1.035 J_ 1X37 1 — 0 .3 0 0 1 1 oa22 1 1.026 1.126 *■> g 1 0 1 . 1 0 6 ‘) 1 1.399 i_ 1 . 3 5 9 2 1.353 6 6 ’•. 2 2 ia O '-' G 1.561 1 O 0 1.370 L. 1 o G 7 X t_ 1.377 0 1.362 <_ 1.3o0 * 0.5 1.353 J. 1 . 3 5 1 2 1.350 I ( 3 1.321 > 1 . 3 2 9 X- 1.326 t 1 . 3 1 5 1 1.3G o.5 1.30? 1 0 _vab 1 1.303 I 2 1.266 G 1.29r u 1.256 1 1.272 G 1.273 2 1.276 TAELS $ « — (0xitir.i;ed)

cscLuina nt» CciO o ~ " .Z O (I960) 0 oi_LCCT/l CliC v.a xxu: Eoleo, llx ic o lo ico , ddxico

arad Mrs a r ^ ■> x - ..i red Calculated hi-:! I d T d I a d

2 1.233 2 _L « <- O 3* tw>0 J_”> fc •(-^ E', 7 2 1.233 1 loll, 7 1 l . l l o V '3 O 1 1.223 3 j_ 4 <- ^ 2 1.233 1 1.221 i -o'-1 O <-'3 1-1

1 1.207 Jy .L • 2x’«i 2 1.211 1 ''-'Vl * 1.199 1 "j n “! 3 1. J_0 - y 0 1 1 3 * 7 /,3 0 1 * L 390 i ! i d “! . 5'. 1 -1 ' ■' 0 2 1.119 0 z 1* 1;.'- j- 1.113 1 1 , ’l ■■1 0 1 . 1 3 6 2 1.135

< 1 1.113 « 1 1 . 1 2 3 d. ^ * 1_ ' ^ 2 1.112 y 1.1-'1 0 l.C vi

-c 1 — C '- y‘ •- 1 . 0 : 9 d y. u -..v _ vj 9 1.0 7 O’ J- .L f. '-y . _ 1 „C6y 21 i . c o i O 1.059 1 , ~\ 1 r •. < 1 — * i- y. y « i 1.057 n ' ... I /-' _L _L ^ < * 1 _u 0 v.y •w' O “1 1 0 < 1 i. • dyl. -1 ■

< 1 J-0 ^ li J. 1 J»«d y

c 1 2. c ^ j. .i. < 1 i . » < I -i- •> v •> ■-•' <. 1 d oOl>.'0 fc! •1 > 5> 2 0 „ v v 5 2 1 vJol o y y . o ' 1 ' < 1 •y „ '~y.y « i W * /" O ^1 >wG■ <

y - ;•• : -'J 1 0 . 9 ’ol ”, C.. -//do 1 0.5727 ■i C y < 1 0 .9c 9 3 L * ' ' 0 x; 1 O 0 CCCO < 1 ^ - 0 1 0.5>w0 ^ i \- © 0 : « J l d a / . / y

< 1 d w ‘ ' ■;•• 2 j •i 1 d 0 y... _ y 3 C , d;>3 y d • \J: .5 ^ . > X x 1 1 o oVyy'J r\ c ' 1 1 2 0.3269 1 u . ; 0J4 ,- •*_ n 1 0 . 9 2 6 ? < 1 y «, vi'j^ 5 1 5 .— (0 -jr.tinuod)

( 1 / O 'j) id _ X :C LoGOO 1 (7ilX X C llC loleo, Ooxicc I'ioioo, 'Oxloo lOaoxrxl jajiXu.. I ccic mu Caic-ilaxcxi - G-i --

"1 VJ J .. O .oiii X 0. vCl-3 < ]_ 0.9005 <-< 1 0o9 ;6i «a l 0.0 C O' o "1 0.V- xx 3 1 o.c > 1’ 3 < 1 V 'Jy >0 C „P l° < 1 o.,.oou • ■ ' r - -J i_ X t. L ; p <; 1 0.0,ir7? c 1 «"*' . ’ '" <3 « 1 X ^ ^ ; p / /-> ■, ■ A O r\ < x 2^ O _./ ~'P <■< 1 OJ fc ■ - P f ' J < x O ^ . - • - « 1 0 O ^ .5 0-! / <. x P «. ^ P << 1 0.0326 i . 7 •■/; X o ^ -. • ■ •• ' X W : ' ■*— s' .> < i 0 e 0 1 X9 < 1 0„xI72 < i O.oilo c< X _Lr .. X

<"< 1 G . ., l c ? < i 0 . c.o?3 9 0 ,00;.;p o_o 0.0oiO J- 0.cC3? 1 X o vJ 0- [- /"\ 0 < I X » ty 03 i_ J) c 1 0 . 1 0 2 7 1 0.0010 < 1 0 a V> ^ « 1 0 .7 9 P « 1 0.7919 5 2 T.'iBlE 5 • — (Ccntinued)

Sabina et al, Ohio State (196 b) C o lle c tio n s Eoleo, hjrcico

l.eosurc d 1.2ecu v% he as urcd Cale u la te d h k l I d I -rX d d

”7 p. : >- .■55 < 1 0.7/51 1 c .73 —>1, ^ < 1 0.7/1:1 1 0 <> 1 0iy « 1 0 ,7 /2 9 2 c « 1 00 1 0 ,7 /1 3 r->- 1 0 •» / ron 1 0.7797 i 0 o77 / 0 1 0.7777 (■/ r 1 0 .77 yc 1 0.7757

y - -v-> s’ 1 ... ■■■' '"Car.-era Cia; y Ilia 2. 00 V cc peer r a d iotio n. t Cr.ly lines for Cu K. A - «i_ 4 ^ ^T r c.O . ape.e ays Ley end ri-: c - ^ O ^, l u t i cn lic it (ay.■re■••a a r-_. a) - • uv v. r* a Ci- <-'v. ai: d 0> s_ ^ line2. lilr.; cor■r 0e-... ju-- Z a fur c a r e r a c3. «,v__ 1 5 41‘ »er.d a a.riniw.^0 * 1 ire a : n -\ i. r- " T" 19.5 hours lor Coildr Z cir.cn > * — 1 ^l ! *' OUI'J 1 . c r ‘‘"V *2 c. .tj ale ob'd-oiii.Oii * Vo lta ^ e 35kV f. T' 'Pr'2. -0 ! C\*00 « b"' Lrrep:.rca ao 'CC . LX...ec r a t u r 0 r.31 Cl r;r 0 413 312' 0 L ro.re stoichior.c tric r> • r-1 ^ propo rtic r.s by s i.0'.:' diffusicn of * 2 ■; 2 0 in to ?b (01)2 and PbC 12* 0 , 0 ’•'Calculated uo c/ G0“ 21.371 c~_ .C. * 2l in - those obtained ty previous authors for a similar setting of cumengeite.

The parameters obtained in this study are in good accord with those ob­ tained previously. It must be noted, however, that the geometrical relationships present in a tetragonal lattice demand that the setting of Friedel and subsequent authors be reoriented by a rotation of U5° about [ooi] in order to conform with the x-ray parameters derived in the current study. The concomitant reorientation of the crystallo- graphic data given try Friedel (1906) is summarized in Table 7. In the discussion which follows, the crystallographic nomenclature is based on the reoriented cell discussed above: [ooi] refers to the direction with the spacing 2U.U9A and [loo] to the direction with the 21.37A spacing.

TABLE 6 .—Comparison of x-ray parameters according to the setting of Friedel (1906)

Axis of Gossner & Arm Hocart This Study Rotation (1929) (1930)

[o c a ] 2U.79A 2U.15A 2U.U9 + o . oUa

[100] 1 5 .6 I 1U.9 I 15.10 ± 0.02&

[ n o ] 2 1 .3 i 21.37 ♦ 0.05&

The Weissenberg photographs obtained for a rotation about [OOl] indicate the presence of symmetry planes Li5° apart. Cumengeite thus possesses symmetry planes normal to both the directions [lOoj and

[jioj. The Weissenberg photographs about [looj possess planes l80° apart and indicate that cumengeite also possesses a plane normal to 5U the direction [ooi] . The symmetry of cumengeite as derived from the

Weissenberg method is that of the Laue group U/m 2/m 2/m.

TABLE 7 .—Crystallographic data for cumengeite

Tetragonal or Pseudotetragonal;

Di t et rag on al-d i pyrand dal-U/m 2/m 2/m

axe = 1:1.11*9 P0 , r o E l» lU 9 :l

Forms I am A 1 c 001 0°00« 90°00' 90° 00»

a 010 0°00’ 90°00’ 90° 0 0 ' U5°oo«

p 111 1*5°00' 58°23' 52°58' 90°00«

Transformation: Friedel (1906) to Winchell (1963) - llO/TlO/OOl

Rectilinear gnomonic plots of the Weissenberg photographs about

£00l] and [lO oJ were constructed. Both sets of photographs were then

indexed and yielded the presence criteria listed in Table 8. The (hkjl)

criteria indicate that reflections are only present where h+k, and

h+Iis an even integer (2n). The (hkj?) reflections indicate that cu­

mengeite possesses a face-centered lattice. Such a lattice is, how­

ever, forbidden in the tetragonal system, since such a lattice can

ordinarily be reoriented to a body-centered lattice. A consideration

of the parameters derived for cumengeite considered from the stand­

point of the geometrical relationships inherent in a tetragonal lattice Indicates that, if these parameters are correct, the only possible lattice choice is a face-centered lattice. Consequently, other specimens of natural cumengeite were used to verify the parameters

These specimens yielded the same parameters as those already reported for the crystal which yielded the Weissenberg photographs. A synthetic specimen of cumengeite, as described below, also gave the same parameters as the natural specimens. It seems, therefore, that one mist adopt a face-centered lattice for cumengeite.

It w ill be noted that all the other presence criteria for cumengeite

are subsidiary criteria to that which determines the lattice type.

TABLE 8.—Presence criteria for cumengeite

Axis of Rotation 0 -le v e l 1 -le v e l

hOO t h=2n hO^ s l,h=2n

cQ— 2U.ib9 * O.OliA OkO : k=2n Ok/ : k, =2n

hkO s h=2n b k l : h+k«2n

k=2n k«-/“2n

hhO : h=2n h-*/=2n

hOO : h=2n hkO : h,k=2n

aQ= 21.37 z 00J? : -2n Ok/ : k, =2n

hO/ :h, =2n h k / : h*k=2n

k-*-J?«2n

h-*i=2n 56

Since convention dictates that a body-centered lattice be adopted in preference to a face-centered lattice in the tetragonal, and since considerations involved in the classical concept of a tetra­ gonal lattice indicate that one ought to be able to choose a bocty- centered lattice from a face-centered lattice, some attempt should be made to explain the lattice choice which must be made in the case of cumengeite. An explanation is suggested by Ito's (1950) work on boleite in which he derives a twinned space group for boleite. In the case of boleite, however, the lattice found by Ito is a body-centered lattice but, according to one view suggested by this author, the lattices are twinned across (001) planes which are alternately mirror and glide planes. In the case of the glide plane, the glide is a diagonal

In the case of cumengeite, it also seems that the explanation may be structural or lattice twinning. Since twinning always raises the apparent symmetry of the species and since a lattice always possesses the full symmetry of the particular system to which it belongs, it seems at first glance that the real symmetry of cumengeite must be lower than tetragonal and that cumengeite thus possesses pseudo­ symmetry. Consequently, the first attempts to explain the lattice and param iers exhibited by cumengeite were based on twinning of an orthorhombic face-centered species with pseudotetragonal parameters, since face-centering is permitted in the orthorhombic system. As illustrated in Figures 7 and 8, a face-centered lattice possessing the parameters and symmetry of a tetragonal species but incapable of re­ orientation to a body-centered lattice could be developed from an 57

Fig. 7.—Diagramatic representation of the space group .

Fig. 8.—Diagramatic representation of the twinned space group FU/m 2/m 2/m developed from F—™ by twinning across (110) or rotation or 90° abouC 001 . s*

• ''K • orthorhombic species with parameters having a tetragonal relationship by twinning across (110) or a rotation of 90° about [00l]. Because it is, in the last analysis, the symmetry of the general positions in a lattice which, when twinned, mast conform to the face-centered tetragonal symmetry, these positions have been shown in preference to the lattice nodes usually portrayed. The twinned space group in this instance is P ^ , The untwinned positions are represented by the black circles. The positions produced by twinning are represented by open circles. It should be mentioned that twinning of an orthorhombic species on a unit cell scale could not be detected optically or crystallographically and thus such an explanation conforms with the optical and crystallographj.c observations reported for cumengeite.

A lattice belonging to a particular symmetry system is characterized by the full point group symmetry of that system. It would seem, there­ fo r e , th at twinning a la t t ic e would r a ise the symmetry to th at of a higher symmetry system. Such a conclusion, when examined from the standpoint of the additional symmetry operations possible in the derivation of a space group, proves to be erroneous. Since twinning may take place w ith respect to a symmetry element by an operation which is not inherent in the symmetry of the element concerned, it would be possible for a space group symmetry operation to be a twin operation with respect to a point group symmetry operation. Thus, twinning of a lattice would not necessarily lead to a lattice belonging to a higher symmetry system, but could lead to a lattice of a different type within the same system. The twin operation introduces no new symmetry element, but changes the character of those present. As a result, a mirror plane may become a glide or vice versa.

Inasmuch as cumengeite appears optically and crystallograph!cally tetragonal, it seems possible that the species may indeed be tetra­ gonal. If such is the case, it must be possible to derive a tetragonal lattice. The most obvious lattice type for consideration is the primi­ tive lattice since other lattices may be derived from this lattice by the proper introduction of additional nodes which conform to the point group symmetry of the system to which the lattice belongs. A considera­ tion of Figures 9 and 10 w i l l show th a t a fa ce-cen tered tetragon al lattice which can not be reoriented to a body-centered lattice may be derived from a tetragonal primitive lattice by an n-glide in the (100) plane of the primitive lattice. The black circles indicate the general points of the primitive lattice while the open circles denote the po­ sitions developed as a result of twinning. The only apparent space group from which such a configuration could be developed is Pi / ? / ?/

Note that the introduction of an n-glide to the point group symmetry causes the concurrent introduction of an n-glide in (001). The space group symbol for the twin might be given

The two possibilities discussed above seem to indicate that cumengeite is tetragonal or orthorhombic, pseudotetragonal. In reality, however, the true symmetry of cumsngeite is s till very much in doubt and perhaps w ill remain so for some time to come. If the species is indeed twinned structurally, its true symmetry may be quite low. Inasmuch as one would expect any significant departure of the true symmetry from the pseudosymmetry to show up optically and 6 l

F ig . 9.—Diagramatic representation of the space group P^m 2/ m 2/m

Fig. 10.—Diagramatic representation of the twinned space group Fh/m 2/m 2/m developed from P^/m 2/ m 2/m ^ 311 n~glide ln t001) *

4 62

N °^ o /

v.| / X / I0' N'' /

y . l ' V Fig. 10

CN faO £ cryatallographically, there would have to be a progressively higher degree of pseudosyrametry present the lower the real symmetry of the s p e c ie s .

The s in g le c r y sta l p ictu res of cumengeite show an ad d ition al feature which is worthy of note, since this feature is common to all the members of the boleite group and indicates, in the light of their similar chemical composition, that the species are probably quite close­ ly related structurally. The feature here referred to is a superlattice effect shown most markedly, in cumengeite, on the rotation photograph about [p O ll• ^he e ffe c t i s le s s marked in cumengeite than in b o le ite and pseudoboleite. As a result of the superlattice effect, every (ljn) layer (where n is any integer) is noticeably more intense than are the other layers. The same type of an effect appears for rotations about IjLOO] and jjLlo] . About [lOo] i t i s both the (Un) and th e (Ltn-l) layers which are most intense while about £llo) the (2n) layers are more in te n se .

The superlattice effect, although recognized in minerals, is nnch more common in alloys where it is usually attributed to an order-disorder transformation in the alloy as a result of the tem­ perature sensitivity of the structure. The analyses of cumengeite show no indication of the diadochy present in alloys, however, so it seems that the superlattice mist be the manifestation of another phenomenon in this case. It is thus proposed that the superlattice effect in cumengeite is due to a polymorphic inversion, the low temperature form being twinned with respect to the high temperature form. The twinning is then the phenomenon which to a rearrange- ment of the structural groups such that a superlattice, analogous to that produced ty ordering in alloys, is produced.

A comparison of the mechanism of the order-disorder transfor­ mation in alloys with the polymorphic inversion proposed for cumengeite may make the above discussion somewhat clearer. The classic example of an order-disorder transformation in alloys is the alloy AuCu^ shown in Figures 11 and 12 (modified after Cullity, 1956). It w ill be noted that the disordered form of AuCu^ is face-centered but that in the ordered form, the copper atoms have moved to the face-centered positions of the original cell while the gold atoms now occupy the comers of the original cell. The copper positions are related to the gold positions ty a glide in the (100) plane. The lattice type has gone from face-centered cubic to primitive. If one refers again to

Figure 8 or 10, it becomes obvious that the arrangement in cumengeite as observed in this study and arrangement of the disordered phase of AuCu^ is the same. It would seem then that the polymorphic trans­ formation ty twinning suggested for cumengeite involves a reverse relationship between the lattice of the high temperature form and the low temperature twinned form. Cue other point suggested ty this ana­ logy between cumengeite and AuCu^ is that the high temperature form of cumengeite could possess a cubic primitive lattice, while the low temperature form possesses a face-centered lattice ty reason of a reordering of the urdts or parts of the units such as to produce a face-centered lattice and a lengthening of the parameters cQ and aQ.

In order to calculate the number of formula weights per unit cell (Z), it is necessaiy to have a good idea of both the chemical 6 5

Fig. IX.—Disordered form of AuCu^ (after Cullity, 1956).

Fig. 12.—Ordered form of AnCu^ (after Cullity, 1956). composition and the specific gravity of cumengeite. In view of the large size of the cell, furthermore, the chemical composition as well as the specific gravity must he known rather precisely in order to obtain any sort of precision from the calculation.

Specific gravities have been given for cumengeite ty Cumenge

(1892), Mallard (1893), Friedel (1906) and Gossner and Arm (1929).

These values are all within 1.5$ of one another except for the second value U*78l cited by Gossner and Arm for the epitactic overgrowths of cumengeite on boleite. Specific gravity measurements were made on two different specimens of cumengeite ty means of the Berman balance.

The first of these specimens was a single crystal of cumengeite (USNM

9U6L1O) while the second wa3 composed of single crystal fragments

(Sainte-Etienne 2750). The results of these measurements are summa­ rized and compared to those of previous authors in Table 9. The specific gravity determined as a part of this stuc^y is very near the average of all the comparable measurements listed and the value U«70

+ 0.03 is adopted for the specific gravity as a result of this study.

The results of this study seem to indicate that the second value given by Gossner and Arm (1929) is due to contamination of the sample ty b o le ite .

Although no chemical analysis was undertaken as a part of this study, an extensive survey of the literature indicates that the uncertainty indicated by Palache et a l. (1951) concerning the chemical composition of cumengeite may be more apparent than real. A summary of chemical analyses to be found in the literature is given in Table

10. These analyses have been recalculated to 100$ for the purposes of 68 TABLE 9.—Specific gravity values for cumengeite

Author Specific Gravity Remarks

Cumenge (1892) U.675 Mallard (1893) U.71 F r ie d e l (1906) U.67 UU small crystals pycnometer method

Gossner & Arm (1929) U.73 Individual crystals (U.781) (Epitactic overgrowths) This Study U.71 ♦ 0.01 USNM 9U6U0

Average of two measure­ ments U.70 ♦ 0.02 Sainte-Etienne 27^0

Average of four measure­ ments

Average U.70 z 0.03

this study. Values enclosed in parentheses represent percentages which have been derived ty subtraction from 100^, Inasmuch as the two formulas Pb^Cu^ClgfOlOg^I^O and PbCuCl2(OH)2 have been reported fo r cum engeite, the ca lcu la ted values fo r th ese compounds are reported in the last two colunns. Except for lead and water in the first analysis and copper in the second, the values given are in good agreement with one another and w ith the fornula PbjiGu^Clg(OH)g*H20. The reason fo r the low lead content of the first analysis is probably that advanced by

Friedel (in Mallard, 1893) as he notes that during his analysis for water, some lead chloride had already been vaporized. It seems, however, that, in any case, the percentage found for lead and water differ significantly from those necessary for the formula PbCu(OH)2Cl2«

It should be added that while the other authors reported no trace of silver in cumsngeite, Cumenge (1892) found 0,1$% present. A micro- chemical test made as a part of the present study indicates that there is no silver present in cumengeite, whatsoever, because the sample dissolved rapidly and conqpletely in nitric acid arid gave no silver chloride crystals on evaporation of the liquid.

TABLE 10.--Chemical analyses for cumengeite

Cumenge F riedel F ried el Gossner pbl*Cul*G18 PbCuCl2 (1892) (in Mallard, (1906) & Arm ( oh)8 * h2o ( oh)2 1893) (1929)

Pb 53.07 52.9li 51*.25 5U.17 5U.50 55.15

Cu 15.22 17.98 16.13 16.01 16.72 16.92

Cl 18.56 19.07 18.96 19.21* 18.65 18.87

HoO 9.01 5.1*5 5.88 6.23 5.92 U.80

0 (I* .1U) 1**56 U.78 i* .03 1*.21 1**26

100.00 100.00 100.00 100.00 100.00 100.00

It is now possible to make a calculation for the number of for­ mula weights per unit cell. With the specific gravity 1*.70, the for­ mula Pb|iCuj^Clg(0H)g*H20 and the c e l l dimensions 21.1* and 2l*.5, Z i s equal to 20.88 or 21 w ithin 0.$7%, With the fornula PbCuCl2(OH)2, Z is equal to 81**5; this value is equal to 81* within 0,60% and to 85 70

■within 0.5956* No d is tin c tio n between the fornulas can be made on th e basis of the value derived for Z. Indeed, the error for either is well within the experimental error considering the large parameters of the unit cell and the value of the specific gravity. On the assumption that the unit cell parameters used in the calculations are correct, the calculated density for Pb^Cu^GlgCOHjg^HgO is U*73 (Z=21), while that fo r PbCuCl2 (OH)2 i s fo r Z*85 and U.67 fo r Z«8Iu

From a consideration of th e number o f m olecules per u n it c e l l , the only difference between the two chemical fornulas which have been given for cumengeite is 21 molecules of water. The water could be interstitial or zeolitic, but no dehydration studies have yet been made on the species.

The results obtained from an x-ray investigation during the present study are summarized in Table 11. The axial ratio derived from the x-ray parameters c 01153ares well with that obtained from gcniometric measurements ty Friedel (1906).

Cumengeite has been synthesized during the course of this study over the range from room temperature (approximately 20°C.) to 270°C.

Except for a few tabular crystals obtained in the range 170°C. to

270°C., the crystals obtained are prismatic. All the synthetic cry­ stals of cumengeite possess the faces of the ditetragonal dipyramid.

In addition, the basal pinacoid is present on the tabular crystals and some of the prismatic crystals. Small prism faces are also developed on those prismatic crystals which possess the basal pinacoid. The morphology displayed ty synthetic cumengeite is shown in Figures 13 and lii. An optical examination indicates that these synthetic 71

Fig. 13•—Morphology of synthetic cumengeite*

Fig. lli.—Morphology of synthetic cumengeite . t 72

P i g . 1 3 .

P i g . lh , 73 crystals are uniaxial negative and possess a high birefringence of the order of that given ty Friedel (1906).

TABLE 11.—X-ray data for cumengeite

Space Oroup: Fu/m 2/m 2/m or Pu/n 2/n 2/m

a0= 2I1.W » o.oliA cQ= 21.37 z

aQ:c0= 1:1.11*6

Cell contents: Pbg^CugkCl^^gCOHl^g^lf^O, assuming the formula Pb^Cu^ClgCOHjg'F^O (Z*2l) or ( ° H)l6 8 or pb8$Gu85c l 1 70(°H)l7 0 assuming the formula PbCuC^COH^ (Z®81* or 8$)

Powder diffraction data are given for synthetic cumengeite in

Table $. A comparison of these data with those obtained from natural cumengeite indicates that the synthetic cumengeite prepared in the study is identical to cumengeite found in nature. Powder patterns obtained from synthetic cumengeite are compared in Figures 15 to 19 with a pattern from natural cumengeite.

Although it was not possible during the course of this study to perform a complete single crystal investigation of synthetic cumengeite, a rotation diagram was obtained for the direction Qlio) in a synthetic crystal of cumengeite. The diagram obtained indicates 7h

Fig. 15.—Powder diffraction pattern of natural cumengeite from an overgrowth of cumengeite on boleite (Hocart specimen).

Fig. 16.—Powder diffraction pattern of synthetic cumengeite obtained ty slow diffusion at 25°C. from stoichiometric proportions for pseudoboleite.

Fig. 1?.—Powder diffraction pattern of synthetic cumengeite obtained in a sealed glass tube at 170°C. from stoichiometric pro­ portions for cumengeite.

F ig . 1 8 .— Powder d iffr a c tio n pattern o f sy n th etic cumengeite obtained in a hydrothermal bomb at 220°C. from stoichiometric proportions for diaboleite.

F ig . 1 9 .—Powder d iffr a c tio n pattern o f sy n th etic cumengeite obtained in a hydrothermal bomb at 270°C. from stoichiometric proportions for pseudoboleite. Fig. 1

F ig . 16.

F ig . 17.

I

F ig . 18.

F ig . 19.

vn

.. M s a spacing of 21.3& for this direction and confirms parameters for

[]ll(3obtained from natural specimens of cumengeite. V. PSEUDOBOLEITE

H isto r ic a l Summary

Mallard and Cumenge (1891 a,b) overlooked the fact that those

crystals of boleite which appeared to possess the faces of the iso­

metric form (021^ were in reality composed of two species. Mallard

(1893), in a restudy of these crystals noted the presence of a small

reentrant angle between adjacent faces which had been described as

belonging to [02lj • The best measurements of the angle between a face

making up th e groove and an adjacent pseudocube face gave 63°UV as

compared to 63°2 6 ' for the angle (100) a (°21) for the isometric system.

An optical investigation of plates parallel to the pseudocube face from

the outer part of these grooves indicated that these plates were uni­

axial negative. Mallard found that the core of the crystals was iso­

tropic or quasi-isotropic boleite and that this core was surrounded by

broad alternating envelopes of isotropic and anisotropic material, the

anisotropic bands corresponding to the uniaxial negative species. The

optic axis of the uniaxial negative species was found to be normal to the pseudocube face of the underlying boleite. A qualitative estimate

of the birefringence of this uniaxial species indicated that it pos­

sessed a birefringence two to three times less than that of a cumengeite

plate of similar thickness. Because of insufficient material, Mallard was unable to perform a chemical analysis.

77 Mallard concluded that this uniaxial species was tetragonal and assigned the indices (Oil) to the faces at 63°Uli• to the base. Accor­ dingly, he obtained the axial ratio a:c = 1:2.026. Noting that the value for c was very near 2, i.e . pseudo-isometric, and that he and

Cumenge (1891 a,b) had postulated that boleite was pseudo-isometric,

Mallard concluded that this new species corresponded to the basic element which, when multiply twinned, gave rise to isotropic or quasi­ isotropic boleite. Since no chemical analysis was available for this new sp e cie s and sin c e some question s t i l l remained as t o i t s id e n tity with boleite, Mallard chose to give the name percylite to this species.

He considered this name to be available because it had been applied

•to a substance which is isometric and the properties of which have not yet been determined'.

Lacroix (1895) re-examined the "percylite" of Mallard and found that the species possessed a birefringence of approximately 0.038 while that of boleite was near 0.010. (H the basis of the difference in the birefringence in the two parts of the boleite, he concluded that the two parts were distinct species. As verification of this conclusion, he indicated that a measurement of the density of the external part gave lw92, a value intermediate between U.71 for cumengeite and £.08 for boleite proper.

After referring to both the original work on percy lite (Brooke,

1850) and subsequent work on this species ty various authors, Lacroix found that, although the formula PbCl2«Cu(0H)2 given for percyllte was derived from an incomplete analysis which did indicate the pre­ sence of some silver supposed to be due to an impurity, subsequent 79 authors had insisted that percylite was isotropic. Lacroix, therefore, concluded that the percylite of Brooke could not be identical with a species which had a birefringence of 0.038.

Or the assumption that the difference in density and birefringence eliminated the possibility of the identity of Mallard's percylite and boleite, Lacroix proposed the name pseudoboleite, in allusion to its relationship to boleite, for the species referred to as percylite ty

Mallard.

A complete restudy of pseudoboleite was performed ty Friedel

(1906). Pseudoboleite as observed ty Friedel possessed a marked external resemblance to boleite in that the two species possessed the same color, luster and pseudocubic form. However, those crystals can which pseudoboleite occurred always presented reentrant angles formed ty faces veiy near the tetrahexahedron {210} of the isometric system. When these reentrant angles were very deep, the faces of the pseudocube were also present.

Plates taken parallel to the external pseudocubic face of these crystals in such a way that the plate passed through the center of the crystal showed, when viewed between crossed nicols, three different zones: 1) a central quasi-isotropic zone, 2) a birefringent envelope surrounding the central zone, penetrating the central zone aL ong the diagonal of the plate, and possessing the cleavage and birefringence of the external zone of boleite, and, 3) a uniformly birefringent, homo­ geneous zone which possessed a slightly more bluish tint and a much greater birefringence than boleite. The contact between the second and third zones was sharp, planar and parallel to the external pseudocubic 80

faces. Friedel concluded that the first two zones belonged to boleite

upon which the third zone, pseudoboleite,waa emplaced.

Friedel emphasized the fact that pseudoboleite could be distingui­

shed immediately from birefringent boleite ty the fact that pseudo­

boleite possessed, in addition to a birefringence distinctly greater

than that of boleite, a pseudo-octahedral cleavage making an angle with

the adjacent pseudocubic face of nearly twice that found for boleite.

Pseudoboleite proved to be uniaxial negative with the optic axis

normal to the external pseudocubic face. The optic orientation of

pseudoboleite on boleite was found to be such that the pseudoboleite z

zone extinguished at the same time as the outer birefringent zone of

the underlying boleite. The mean of five direct measurements of the

birefringence of pseudoboleite gave the value of 0.032. The ratio of

the for anisotropic boleite (E - 0 = 0.017) and pseudo­

boleite obtained ty use of the circular analyzer was 1.90 and yielded

an approximate value of 0.032 for the birefringence of pseudoboleite.

Friedel observed no appreciable pleochroism in pseudoboleite.

Pseudoboleite presented a dominant, perfect basal cleavage

parallel to the pseudocubic face of the underlying boleite. No trace

of cleavage parallel to the other pseudocubic faces of the boleite was observed in the pseudoboleite crystals. In addition to the basal cleavage, pseudoboleite possessed a distinct, easy pseudo-octahedral

cleavage parallel to pseudotetrahexahedral faces on the crystal. The mean of fourteen measurements of the angle between the pseudo-octahedral

cleavage and the adjacent pseudocube of boleite gave the value 630U2t for this angle. The angle between the pseudo-octahedral face and the 81 base of pseudoboleite was thus found to be 63°U2' as compared to the angle 63°26' for the angle between (001) and (210) for the isometric system . pseudo-octahedral cleavage,

Friedel obtained an axial ratio of ate = 1:2.023.

Ely means of the Law of Bravais and a calculation of reticular areas, Friedel demonstrated that pseudoboleite possessed a tetragonal lattice of the dipyramidal mode. He pointed out that the basal cleavage as the principal cleavage together with the s till more per­ fect dipyramidal cleavage supported his choice of lattice mode. In addition, he observed three of the remaining four most important forms of the dipyramidal mode: { lio ] , [ll2j, and jlO o].

Friedel emphasized the fact that pseudoboleite was even less pseudo-isometric than boleite and that none of the inhomogeneities present in boleite were present in pseudoboleite. He pointed out, however, that the parameter of pseudoboleite is such that twinning of this species might well be expected to occur if single crystals existed.

Although hampered ty the small size and rarity of crystals of pseudoboleite on b o le ite , F ried el was able to separate 0.9U3 grains of pseudoboleite and 0.1829 grams of the underlying boleite for quanti­ tative chemical analysis. Inasmuch as the results of the analysis of the underlying boleite gave percentages for lead, copper, silver, and chlorine close to those obtained for boleite crystals alone, he con­ cluded that the central part of the composite crystals was indeed boleite. The same percentages obtained in the analysis of boleite indicated, however, that this species contained more lead and chlorine and significantly less silver than boleite. Friedel called attention to the fact that the silver present in boleite night well belong to boleite present as an impurity in the material analyzed. The analysis for water, made on a composite sample of boleite and pseudoboleite weighing 0.1*968 grams, gave U•(>!%• Although he had no direct way of determining the exact ratio of pseudoboleite to boleite in the sample, he estimated, by a measurement of the thickness of pseudoboleite on broken composite crystals, that approximately one third to one quarter of these crystals was composed of pseudoboleite. As the percentage of water was known for boleite, he calculated the percentage of water in pseudoboleite to be between 5*31 and 5.63 or an average of $•$%•

Assuming that pseudoboleite was silver-free, Friedel obtained

5PbCl2*UCu0»6H20 as a first approximation of the formula of pseudo­ b o le ite . He noted, however, th at the formula 8PbCl2*6Cu0«9H20 agreed s till better with the results of the analyses, but that the inexact­ ness of his measurements forced him to choose the simpler formula in or­ der not to exceed the limits of experimental error.

A measurement of the specific gravity performed on eighteen small, composite cry sta ls w ith a t o ta l weight of 1.0987 grains gave the value

1**996. Using th e r a tio previously assumed fo r pseudoboleite to b o le ite in composite crystals, Friedel concluded that the specific gravity of pseudoboleite lay between the lim its L*.83 and 1**88, with the average being 1**85*

In summing up the results obtained from the chemical analyses aid density determination for pseudoboleite, Friedel indicated that, although the work was rather inexact, the results indicated conclusive­ ly that the pseudoboleite contained little or no silver compared to boleite which contained approximately eight percent and that pseudo­ boleite had a specific gravity between that of cumengeite and of boleite.

Gossner (1928) reported that an x-ray examination indicated a o spacing of 1$.6I for pseudoboleite as described by Friedel (1906).

Gossner concluded that pseudoboleite did not differ from boleite and was the outer part of penetration twins of boleite.

Gossner and Arm (1929) performed an x-ray study on the pseudo­ boleite of Friedel (1906). They noted that the groove, said to be formed ty pseudoboleite on boleite, was similar to that formed by a penetration twin. They pointed out that, if the outer portion of this groove constituted a species different than that which formed the underlying part of the crystal, this outer portion should possess a different chemical composition and different x-ray parameters. Since they did not possess adequate material for a chemical analysis, they decided to study this outer portion by means of x-ray analysis. No optical data were given by these authors for material which formed the grooves on the crystals which they used, but they did give 63.5° for the angle (OOI)a(OII) in the notation of Friedel (1906).

A rotation photograph produced about jjoOlQ, in the notation of

Friedel, gave an identity period of l^.UoX and a film identical to that obtained from boleite which they considered to be isometric.

A rotation about [ i i o ] gave an identity period of 21.76^ and a film identical to that obtained for this direction in boleite.

Another rotation photograph for the direction (ooi] in a second . .o crystal gave the periodicity 15.35A. On this second film, however, a number of intermediate spots were observed between the principal layer lines. At first the authors believed that these extra spots belonged to an intergrowth of cumengeite but could not satisfactorily account for these spots on such a basis. They finally decided that the extraneous spots must be present as a result of ndsorientaticn of units within the crystal. From the results of their x-ray study,

Gossner and Ann apparently concluded that the crystals of boleite were penetration twins and that pseudoboleite was an invalid species equi­ valent to boleite.

Referring to the faint intermediate spots on the photographs of pseudoboleite obtained ty Gossner and Arm, and pointing out that these spots would double the periodicity given ty these authors for pseudo­ boleite in the direction [pOlj, Friedel (1930) indicated that the ro­ tation photographs were most certainly not the same as the photographs obtained by these authors for the direction [100] in pseudocubic boleite. Friedel did admit that there were resemblances, but that such resemblances were to be expected since the a parameters of both species were identical and the c parameters were very nearly a whole multiple of one another as he had pointed out p reviously (F ried el, 1906).

Friedel also noted that these authors had almost completely ignored his analysis of pseudoboleite in which he showed that pseudoboleite differed from boleite. To eliminate all doubt, Friedel repeated his analyses of pseudoboleite and the core of boleite on 0.09U3 grams of pseudoboleite filed from composite crystals and 0.1829 grams of boleite from the core of these crystals. Although he felt that the amount of pseudoboleite was too small for a precise analysis and that there might be some boleite present in the sample, he did obtain ratios which 8 5 compared favorably with those obtained earlier (Friedel, 1906) for pseudoboleite and proved that pseudoboleite differed from boleite.

The new boleite analysis compared favorably with his earlier analyses

(Friedel, 1906) and proved that the core of the composite crystals was indeed boleite.

Friedel again repeated his observations on the birefringence of pseudoboleite as compared to that of anisotropic boleite, pointing out that the birefringence of pseudoboleite was markedly higher and constant whereas that of boleite was quite variable.

Lastly, Friedel recalled his observations on the cleavage angles of boleite and pseudoboleite, pointing out that these cleavages and the axial ratios derived from these sufficed to prove the validity and tetragonal symmetry of pseudoboleite. He further noted that the ex­ ternal forms present on pseudoboleite sufficed to prove that the species was tetragonal, not cubic.

On the basis of the formula 5PbCl2»UCu0«6H20, a density of U.85 and the parameter 15.U1A (Gossner and Arm, 1930), together with the axial ratio a:c = 1:2.023, Friedel found the value 11.97 or very nearly

12 for the number of formula weights per unit cell. Friedel noted that the value found for Z did not prove the validity of his formula.

For the x-ray analysis of pseudoboleite, Hocart (1930) used a small, perfectly homogeneous, basal-cleavage plate on which he ob­ served cleavages at 63°U2* to the basal cleavage. Oscillation photographs about [00lj and [loo] gave, respectively, 31.2$ for cq and 15.Ul for Sq. The axial ratio obtained from the x-ray parameters was approximately 2, in good agreement with the value 2.023 obtained 86 ty Friedel (1906) from a stu^y of the cleavages. Using the formula

!>PbCl2»i|CuO*6^0 (Friedel, 1906), Hocart found the value of 12 for

Z. Hocart noted that the most intense spots on the rotation photo­ graphs of pseudoboleite were present on the 2n layer lines (where n ■ a whole number) and th at those layer lin e s could be shown by super-position to match exactly the iin layer lines of boleite.

Gossner (1930), in a reply to Friedel (1930) and Hocart (1930), pointed out that he and Arm had ind icated the presence of weak in te r ­ mediate spots on the rotation photograph for pseudoboleite about

[001] * He added that although they did not fully understand the cause of these intermediate layer lines, they had attributed them to struc­ tural disorder in the crystals. Although Gossner felt that the intermediate lines observed ty Hocart (1930) were real, he felt these lines arose from structural imperfections in a complex structure and concluded that boleite and pseudoboleite possessed analogous cubic l a t t i c e s .

For a further x-ray study of pseudoboleite, Hocart (193U) used homogeneous crystals which possessed cleavages at 63°1*2' to the basal cleavage. These fragments yielded the parameters cq= 31.2$ and aQ= . . o 15»hk w ith the r a tio of th ese parameters being 2.026 compared to 2.023 obtained from the cleavages. The unit cell was found to contain 12 molecules of the formula 5PbCl2*UCu0*6H20 (Friedel, 1906). A study of the x-ray photographs produced from an oscillation of 1 $° about

£bo£Jinthe same way, indicated that the layers leading to the para­ meter c0= 31.2^ were weak but could not be overlooked in even the most cursory x-ray examination. Laue photographs produced with [oOlJ and 8 7

[ioq] parallel to the beam yielded photographs which were much more markedly different than in the case of anisotropic boleite. Hocart noted that a certain structural relationship between boleite and pseudoboleite was indicated ty the fact that some intense spots were present on both the Laue and the rotation diagram of the two species.

Hocart concluded that his observations left little doubt that boleite and pseudoboleite were distinct species.

It may be well, before presenting the results of the present study, to indicate that an examination of the rotation photographs obtained by Gossner and Arm (1929) clearly indicates that there are weak but still distinct layer lines present half way between the obvious layer lines on the photographs for pseudoboleite about [ o o i] .

The criticism of the work of Gossner and Arm by Friedel and Hocart is a valid one and the results of this study indicate that pseudoboleite is a distinct species and does indeed have a 30 % parameter in the

[ooi] d ir e c tio n .

Results of the Present Study

Although it was not possible to secure type specimens of pseudo­ boleite for study, several specimens which exhibited very small, but typical overgrowths of pseudoboleite on boleite (Figures 20 and 21) were discovered in the collection of boleite of the mineralogy depart­ ment of Ohio State University. These overgrowths were studied crystal- lographically, optically and ty means of x-ray methods in order to show identity in so far as it was possible with the specimens described ty

Friedel (1906) and Hocart (1930). When a specimen of pseudoboleite which had been available to Friedel (1906), was obtained from the 88

Fig. 20.—Morphology of pseudoboleite (Mallard, 1893).

Fig. 21.—Photomicrograph of pseudoboleite overgrowths on boleite viewed approximately parallel to the pseudothreefold axis of the com­ posite crystal. 21x. %

89

F ig . 20

Fig. 2i. 90 collections of Sainte-Etienne, a goniometric examination and an x-ray powder investigation indicated that the pseudoboleite available in the collections of Ohio State University was identical to that studied ty

Friedel (1906).

The overgrowths of pseudoboleite used in this study were generally simple crystallographically, possessing only the forms {oOl^ and ^Ollj, according to the notation of Friedel (1906). Flashes of light along the edges between faces in the zone containing (001) and (Oil) were noted, but the reflections were too scattered and small to form a reflection suitable for even a rough measurement. Measurements of the interfacial angle (001) to (Oil) on three different crystals gave

63°Ul', 63°U;', and 63°U7' (Friedel, 63°Ul.5')» There seems to be no doubt that, crystallographically, the pseudoboleite used in this study is identical to that used try Friedel (1906) and Hocart (1930).

An optical examination of the overgrowths indicated that they are uniaxial negative and possess a uniform, moderately high bire­ fringence. No banding of the type observed in boleite was observed in the case of pseudoboleite.

X-ray powder photographs were obtained from a number of specimens taking care in each case to use fragments definitely from the over­ growth itself. The patterns were compared visually and found to be identical with respect to both the position and the intensity of the lines. Powder diffraction data for pseudoboleite are given in Table

12. Powder photographs obtained from two specimens of pseudoboleite are given in Figures 22 and 23.

A comparison of the powder diagrams of boleite and pseudoboleite

(Figures 2k and 25) indicates that the patterns have a very close a 91 rnALLS 1 2 .— X-ray pov,'dor d a tafcr paaucobolei to

r* ' Pseudoboleite S'•t. the ti e '-'J *■th e t i c TJ n -.-v "i - - -• ^ c xarvnrd Collections Pscudoboloito° 1 L>^UU0X0 — dx ^ O lo lo o, M:cico ,r< Me.SHaTo-CI in su rea L1/X""*0 Cl Gal e u la te d r± l x d I Cl X r5 d

110 Jx lU .73 H iH .c? U i!i. £5 i p 9/- 1M. 7 13 X l 7 13.50 7 1 3 .h i 13.66 102 3 12.37 3 12.12 3 12.03 1 2 .5h 201 < 1 10.00 i o .16 202 1 6 .?6 1 e .72 x 6 0 7 6 6 . Oh 113 « 1 6.53 6.52 9 9 r' 9 \ 07.63 ' < 7.61, 203 j U 7.53 A a ■ 7.57 1 7 .X 221] O f 7 .ho ±AM 4. 7.17 x 7.1? 1 6 .6 3 222 ] 0 6 . 6 a 2 310 J 2 c.79 1 O 0 ; O 6.61 I 6.63 ^ • 0 7 -u.U r ^ . 1 • X' * i- 1 o.xX u 312) ii> ] 5 5.72 71 Xr' « O mX_,""> ~ J\ 7 5.65 5.65 ] c- U Jh.25 U.22 h 6 ^ x 0 | !i 6.25 U.25 *X • — ^ 117 J I h.23 316] X .10 *•90 > " h .09 6 h*05 6 6.07 6 h . 0 3 20? U . C 3 iiOSJ k U - C 6 92 93 TALLl 12.— (Continued)

9 - -!• -i Pseudoboleite Syr.l’-.c-tic w» L / i o

Harvard C ollections Ct C d d O L>0_ ___ ; i_ o l Posedobolcite

l o l e o ? M xico

M e s u r e d :.;jdourM .d lb--.'.cured C a l c u l a t e d

>1:1 I d I c I d d

d O T V^i ^ 9 " 9 9 ^ 3 2.131 2 2.1): 2 2 2.9,3 2 2.112 2 2.12 3 2 2 . 11? 1 2.077 1 ill 1 2.021 3 2. CO? 3 2.C., j 3 2 997 9 6 ‘ -0 3 2.012 3 <_ & \ _ 9 » e' o'-9 1 2 .O il 3 © - 'J d 9 .0 ; *\ 1 C 7 1.915 7i C / / V/ 7 ~\ © ,/ c 0 o , 1 ' O 1 2 l .? l 9 _L » I- -! s • 5 . 10 ■, 1 1 (\ 3 1.917 if ------1 * / _ *■» *> 9 3 i d ? ? 3 o -j 1 4 0 y 2 1.662 9 _L e w ?-.2 O 1 a 03 3 1.G11 9 1 3 1 * : 10 1 < -= ■ 1 © d o /■ 1 l i.-ui — ]_ c : 1 d © : -9 -> 6 1.752 ■0 * i - ' . 0 d */ ; ; d "i ”1 1 1.7'1 - • J - .? 79 7 1.751 7 _L e , / ■ 0 i 5 1

1 0 1 i ro 1 *1 'r " 1 1.722 „ / _____ 0 / — 9 1 (- l.C','9 2 d « i L o l < 1 1.631 *1 vc ;i < 1 d • L - O O J_ 1.670 3 © 'w ' K( 1.610 ' ’d' *1 1 © 2 10 - n_ 1 . 11? I o C . 5 . l * s . 1 ; ___) •7 .9 1.626 < < d i • 5 *_ d d . 0 — J 1.556 1 * 3 .1. 1 1 u 5? y 0 ]_ 1 1 1.371 _L »9 ! *• 6'' w .5 [ e. “I 5 0 1 . ^7 o o •< 1 ■-'9 n i nV7 "f - } 2 d . l e i 0 > 1 71 1 . e i t <• d d / l *] T r. 9 6 1.521 J *> “1 H 9 u _ © . ) a . <— 9 *1 2 1.503 2 1 -A. «■9 ;23 3 1.181 3 1.112 3 1.9-1 1.172 0 2 ^ ll.;6? ',07 O 2 : 1 :-70 1.159 3 1.155 3 1.156 1 l.llo <1 loll? -1 1.11.0 2 1.127 2 1.1.03 2 1.130 1 1.118 <-1 1.115 < 1 1.116 1 l . l i o 1 1.103 1 1.103 9h / 95 /'I I—1 \ 1 ; t C-r >, hJ p o . [! j ’ »• .• 5 • p: d- '1 b- c b c.» 1 : s- ! : P cl- ci- f- ( * fi t- • O' H- 1j. i_j 1 ; i j ^ i f ; l C;* O 0. i 0 ;' i- f) p‘ OO s C-' ; f v 0- o > * C > K r > ♦ p i j o f ^ o ;' I C'J GI K /> *"0 i J O ro r ■■ ; r. p ti V O v • < o ■>J o <. i Vn c< I- ' V H l-J I-’ VO ) J I-1 I J VJ H VJ |-J OsV \ V. i-1I-1 !-JH H IV) l-J\ -> pi - \ - 3 i P o o » ■“ (. j . i » ; C ' 0 (./ c p- p i-| ; c- t j ( ■ c C' r . r v o fi- SO • 'ci e i - r • i ’• r. ■' a co> o C<- \ [ ;• o 1 •• 1 P o t" a ( ■ • * t ‘ ’ I-; r> 1 ‘j c »; 1 c: A 0 Mo

( • 0 r'. 0 i> c P- ' j o O O O O O 0 o O O o O C U' O O !b O Ci (;T c, • i- c •' ^ x. , , *• C c c e • 6 e t • ♦ » • » t ft • p- X (0 (•' i LI o ►J \ ' o O o* -0 -0 ^ __ i —J - o -o O;< -< c r ('; f;) C"! CV- C‘ ■' p- o p- O 1 o (.) ro 1 V } v . - :• -0 -.1 -•3 ■< r". c- ' ■ '-x; v ■*' O' C '■C; C > ! -»1 J M 1-' A*) o Ci- C '• H- K* O C' Ci O M r ' c.: Co P' '-■i r , i '• ---! (. *• ! prt -O c > Vi r \ r Cj Cl p* tb UJ f- p o' c;. c'* f <- c^ S* v; ■. -3 t : > o \~-o PI v\ Vv -! - OO roo V\ 1 -1O o Cl O O H* O r CO Cv I") H O t 1' i-1 V' p o O' » P ro M 1 (* C o • O H* « Cl !•’ t\ A !’ ro »; p- c: re,- ni C„ ‘ ■O 1 • i .< A A K A A A o r” ro rV to 1 , •'0 ( * 'x—^ H H H H I-' ro *-JI-.. i 1H h-: r . r/j H O c‘ I > J r» ( r JC. ! • f > C b <_ 1 i * i ; r' • / « 0- vn (ro j h ' ' ;- c' ( 1; «■ i - o C r \ 1 CO i: ' , •'V J • O' ; c to p~o h- c ' r< O r , j •; ( o' r ( c • •' O CJ l-J C b > o o 0 o o o o Cl < |V ' » . C f* C f r c 0 P c r- (j i ’• n (/ i ». 1 -O —O c c ; <: f ' c ' i ' O i' \ _ o 6 ■' 1 .*■ <■ P ! : - ' 1 1 c '■ C': i; O (' ( \ . c ' r P c " if' j Cl * * * i ' I- O V*; ^ . 'x-' 1 ; f ^ ‘ VO v_y ' • I ; r , ' • 1 o IO 7- •• - C‘b i- - 0 ' .. o- V : c 1 C - * . *. : ■ CO r I 0 ‘ ! ” A f> A A A £ A /% A A 1 0 ro 1 .5 J.. i " o £ A A A r ; ; o ( C-A I-.- r' L , • ' 1 l--J H H i-1 I j 1-' H H H t--1 i- ’ l-J !-' H « M l-J iJ } ■: • - ( /'-■ ro *.* c; t : ( p c' ’ o r) o , - V P. b C ’ t ■>t I j n ( r; - r: : f • o Is ; r 6 r • Cj i' v p CS r~ {' C- Co r :• {' *, ;. c Cj 1 ■> () \ : ; ' (* C *' (, • (. ! f . O O O O O O o c O C- OO o <•■ p O t . »* O (' K ' r< • 0 C r. o « c t O C * C t <. «■ ( P A 'b • »'• I-J! o - t -* •r _i r- ( r >) C’- rr r- G i-j o M •o I '• C'i 0 vo V) o> e'o O !• 1 i-- ’ C;- O *x- r-' e / • ( : \ ji p . r \ ■-' I ' ( J / - r-> VO VpJl P- rc i J ' . P C- r* • co o V'u--' <_• - v : v C' 0 ‘ t xJ o »!-i _l J : V. «*» P- \ V 'l : i r \ ro ' 4 « P f' O (c b- CO roro c, c. r- C' C-* v« P o . '0 v e . C; H- hJ hj to c •* f\.‘) C) r ^ CO C(U hJ- 1 i 10 ro H c ;• O A • 1 J- rj O b ' »... H P h' ■i »; » p l ’• C' O O ‘O o b b J' 0" h2 ‘r-1 1 P P r;: C Ci « p. O H- V_^ O 1 i ( + i ! ' 1 1 ro j i t; c: e Cb V5 P- O (■- VO 1 y\ P- o O o' M 91

Fig. 22.—Powder diffraction pattern of natural pseudoboleite (Sainte-Etienne 27U9).

Fig. 23.—Powder diffraction pattern of natural pseudoboleite (OSU collections). VOOO 99

Fig. 2iu—Powder diffraction pattern of natural boleite (OSU collections).

F ig . 2$.—Powder diffraction pattern of natural pseudoboleite (Sainte-Etienne 27U9). F ig . 2U.

H 8 101

resemblance to one another. The resemblance is so close, in fact, that

a cursory comparison of the patterns would lead one to the conclusion

that they are the same. Upon careful comparison, however, it becomes

evident that there are a few low 20 lines which are only present on

the pseudoboleite pattern and that there are additional lines, rever­

sals of intensity and a splitting of lines on the pseudoboleite with

respect to the boleite pattern. It is noteworthy, however, that in

general, there is an exact correspondence of the majority of the lines

in the range 0-90° 20, all the more striking as there is also an exact

correspondence of intensity between the strongest lines on both patterns.

Beyond 90° 20, a s h if t of equivalent lin e s on th e two patterns becomes

quite noticeable, as should be the case if there are small differences

at 20 values of less than 90°. The 3trong similarity of the boleite

and pseudoboleite patterns points to a close similarity of structure

in the two species.

For the purposes of a single crystal study, a typical overgrowth

of pseudoboleite was detached by means of its perfect basal cleavage

from an underlying boleite core. This single crystal possessed the

crystallographic and optical characteristics mentioned above for

pseudoboleite. A portion of the crystal which was broken during sepa­

ration from the boleite gave a powder pattern identical to those

obtained from the other specimens of pseudoboleite examined.

Rotation and equi-inclination Weissenberg photographs were

obtained from the c r y sta l mounted p a r a lle l to [ o o i] , (jLOoj, and

[ n o ] according to the setting of Friedel (1906) and Hocart (1930).

The parameters obtained for these directions were 30.8U + 0.06^, 102 o o 1^.29 * 0.03A, and 21.57 ♦ 0.03A, r e s p e c tiv e ly . The parameters ob­ tained for [OOIJ and [jLOO] are in accord with those obtained by Hocart o o (1930) who found 30.21A and 15.1»A, r e s p e c tiv e ly . I t i s obvious, however, that the orientation adopted try Friedel (1906) and Hocart (1930) mist be rotated U5° about joof] in order to conform with the geometrical relationships dictated by the x-ray parameters observed in this study.

Such a rotation necessitates a transformation of the morphological data given by Friedel (1906) for pseudoboleite. Table 13 summarizes the crystallographic data for pseudoboleite consistent with the setting adopted in the present study. The axial ratio aQsc0= 1:1.1*29 derived from the x-ray parameters obtained in this study is very close to a:c

= 1:1.1*30 obtained from goniometric measurements. In the discussion which follows, the crystallographic nomenclature is based on the re­ oriented cell discussed above: [00l] refers to the direction with the

30.81*$ spacing while Q-OO] refers to the direction with the 21.57$ sp acin g.

Weissenberg photographs obtained for a rotation about jjX)l] indicate the presence of symmetry planes 1*5° apart. Pseudoboleite thus p ossesses symmetry planes normal to both th e d irectio n £iOgT| and jjLlo}. Weissenberg photographs about [lOcTj have planes 180° apart and indicate that pseudoboleite possesses a plane normal to the direction

[oof] . The symmetry of pseudoboleite as derived from the Weissenberg method i s l*/m 2/m 2/m.

Rectilinear gnomonic plots were made for zero and first level

Weissenberg photographs about [OOlf. Indexing of these plots yielded the criteria listed in Table lit for the rotation direction [ooi]. 103

TABLE 13.—Crystallographic data for pseudoboleite

Tetragonal or Pseudotetragonal;

Ditetragonal-dipyramidal-U/m 2/m 2/ra

a:c = 1:1,1*30 ro 0 = 1.1*30:1

Forms I A 1 c 001 0°00» 90°00' 90°00'

a 010 0°00» 90°001 90°001 l*5°oo»

m 110 U5°oo' 90° 0 0 1 l*5°oo» 90°00*'

r 011 o°oo» 55°02« 90°00» 51*° 35 •

p 111 1*5° 001 63°U2' 50°39.5' 90°00*

Transformation : Friedel (1906) to Winehell (1963) - 110/I 10/001

The (hk,/) r e fle c tio n s , which should in d ica te the la t t ic e type unambi­ guously, do not do so . As (hkjO is present only when h=2n and k=2n and X. may be odd or even, the body-centered and face-centered lattices are elim inated in any ca se. Whereas th e array of th e points appears to be primitive when one layer is placed on the next, the possibility s till remains that the lattice could be a non-reducible tetragonal

C-centered lattice. Whether the lattice is primitive or end-centered one must s till account for the absence of nemerous reflections which would be permissible (hkjt) reflections. Although the (hkO) reflections could be explained easily by an a-glide in the (001) plane, the reflec­ tions for (hhO) and (hh/) are as puzzling as those which should give the lOli lattice type. The only criterion which seems to hold throughout for the r e fle c tio n s about [00l3 i s th a t h=2n and k=2n fo r a l l r e f le c tio n s .

TABLE 111.—Presence criteria for pseudoboleite

Axis of Rotation O -level 1 - le v e l

[ora] hOO r h=2n h 0£ : h»2n

3 0 .8U 0 . 06% OkO : k-2n 0k £ : k*2n

hkO : h=2n h k / : h*2n

k=2n k=2n

hhO : h=2n h h / : h=2n

D-Oo] hOO : h=2n Ok / : k=2n

21.57 ♦ 0 . 03ft OO/ : /-2 n hkO : k=2n

hO / : h=2n hk / : h+k=2n

/=2n kt-i=2n

h+Jt=2n

Similar rectilinear gnomonic plots for the zero and first level

Weissenbergs about jjLOo] gave the criteria listed in Table lU for this direction. In this case, the (hkjl.) reflections, as well as the dis­ tribution of reflections on successive levels, seem to indicate that the lattice is face-centered. The (hk/) reflections are, however, still consistent with an end-centered lattice, although here, as in the case of the rotation about [00l|, numerous (hk/) reflections permitted by the lattice type are absent. The (00/) reflections indicate that 105

the c-axis is a Ug screw axis. The remaining reflections are in accord with those obtained for rotation about [00lj.

The above discussion indicates that the only lattice type consis­

tent with the presence criteria observed is an end-centered lattice.

As such a lattice is ordinarily not permitted in the tetragonal system,

the existence of such a non-reducible tetragonal end-centered lattice

seems to indicate that twinning on a unit cell basis may be present in

p seu d o b o leite. Numerous attem pts have been made to f i t the experimen­ tal results to a possible space group and although it would be quite

simple to produce a non-reducible tetragonal end-centered lattice by twinning, no space group symmetry which is compatible with this lattice

and the experimental results has yet been found. Inasmuch as the

possibility also exists that some of the omissions are quite regular

structural omissions which simulate space group omissions, the problem

is one which may well be beyond the scope of this thesis.

Although an optical examination of, and rotation photographs from

the specimens used in this stuc^1 show no evidence of contamination of

the specim ens, th e Weissenberg photographs commonly contain ex tra ,

often doubled spots. Although these extra spots generally do not lie

at possible nodes of the pseudoboleite pattern, occasionally these

spots do disrupt the apparent periodicity of the pseudoboleite lattice.

It is the feeling of this author that these spots are the result of

a very minor contamination present in or on the pseudoboleite crystals.

The extra points are generally of limited extent and are easily dis­ tinguished from those belonging to pseudoboleite. The effect of the

contamination is more marked in successive levels so that, while the 106

zero and first levels are quite usable, the second and third layers

contain so many extra spots as to be almost useless. In view of the

perfect epitaxy between pseudoboleite and cumengeite, an the one hand,

and pseudoboleite and boleite, on the other, it is not surprising that

contamination of the specimens is present. Although the specimens are

small and contamination could have been concealed optically, any sig­

nificant contamination should have shown up as irrational layer lines

an the C o o l] rotation photographs. The single crystal data reported

for pseudoboleite has been obtained from the most reliable specimens

and photographs obtained during this study and it seems that the data

obtained, although unusual, are valid. Although pseudoboleite has been

synthesized, no suitable single crystals analogous to natural specimens

have been obtained and verification of the results given for pseudo­

boleite must await further synthesis attempts.

Pseudoboleite, like cumengeite, possesses a superlattice which

gives rise to alternating weak and strong layer lines in the rotation

photographs and to an extremely complex powder pattern. Cn the ro­

tation photographs about [boi]] the (2n) layers are noticeably stronger than the (2n + 1) layers. For photographs about [loo], the (2n)

layers are also noticeably stronger than the (2n + 1) layers. The

rotation about [n o ], does not appear to show this superlattice

effect. The sub-cell of pseudoboleite based on these relationships o would be a cube l5.UA on a side. It seems worthwhile to recall, at

this point, that Ito (1950) has found that boleite may be considered to be built up try a quadrupling of a cubic cell approximately 15.2&

on a s id e . 107

Ctaly the chemical analysis and specific gravity given ty Friedel

(1906) are to be found in the literature for pseudoboleite. Although no chemical analysis has been performed for pseudoboleite as a part of this study, enough material was available to determine the specific gravity of the species. The average of six measurements on the Berman balance gave I1.9U ♦ 0.05. As the samples used were small, the error in the measurements is probably higher than indicated. Nevertheless, the individual measurements serve to indicate that the density of pseudoboleite is distinctly lower than that of boleite and in the range indicated ty Friedel (1906).

A calculation of the number of molecules per unit cell (Z) using the cell dimensions and specific gravity obtained in this study and the formula given by Friedel (1906) for pseudoboleite gave

Z=23.5. This value, although it is not close to a whole number and is not within the ordinary limit of experimental error, does indicate that Z is equal to 23 or 2U. A similar calculation using the specific gravity U.85 given ty Friedel (1906) gives Z=23.05 and is well within the experimental limit for Z*23.

Although the density for pseudoboleite given ty Friedel gives a better calculated value for Z, this value is not divisible ty 2 or ty U. If Z were 23, it would be difficult to distribute the ions involved in a way which would satisfy the high symmetry of pseudo­ boleite. A better value for Z would be 2l* and would require that the real density be near 5«0, on the assumption that the fornula given ty Friedel is valid. As in the case of cumengeite, the possibility of occluded water exists. It has been possible during the course of this study to synthesize pseudoboleite over a temperature range from room temperature (approxi­ mately 20°C.) to 270°C. Two types of pseudoboleite, distinguished at present only on the basis of color, have been obtained. The first type of pseudoboleite, obtained between 20°C. and 100°C., forms micro- to cryptocrystalline aggregates of blue pseudocubic crystals. No modifying faces are apparent on these crystals. These crystals are definitely anisotropic and have an apparent birefringence of about

0.01. The crystals are so small and fragments so thin that the birefringence could be considerably larger than this apparent value.

Powder diffraction data for this blue form are given in Table 12.

A comparison with similar data for natural pseudoboleite leaves little doubt that this synthetic species is indeed pseudoboleite. It seems worthy of note that natural specimens of pseudoboleite are all blue in color as is this synthetic material.

The second type of pseudoboleite obtained at 1?5°C. and above forms leek-green to emerald-green pseudocubic crystals. These cry­ stals are often modified ty the faces of the pseudo-octahedron or the pseudo-octahedron and pseudo-dodecahedron (Figure 26). Occa­ sionally among the crystals synthesized at 170°C., flattened plates

(Figure 27) are observed, which have a holohedral tetragonal appearance and are composed of a large basal pinacoid and the smaller faces of a dipyramid. These flattened crystals have exactly the appearance one would expect the pseudoboleite overgrowths on boleite to have if these overgrowths were developed as single crystals. Unfortunately both the pseudocubic and the flattened plates were too small for 10?

Pig. 26.—Morphology of synthetic pseudoboleite.

Fig. 27.—Morphology of synthetic pseudoboleite • F ig . 26. I l l goniometric examination on the instrument available*

A petrographic examination of the green crystals of synthetic pseudoboleite indicates that these crystals are uniaxial negative and exhibit an apparent birefringence of about 0.010. As these crystals are highly absorbing, only small, very thin fragments were available for an estimation of the birefringence and, consequently, the b irefrin gen ce may be somewhat la rg er than th a t given a lo n e. This green form also displays anomalous interference colors in shades of red to yellow brown probably due to the strong absorption of the species.

Whereas the synthetic crystals obtained at 170°C. show no banding, the crystal produced at 270°C. exhibits alternating isotropic and aniso­ tropic bands which give the appearance of polysynthetic twinning. These bands are very similar to those observed in very thin sections of the anisotropic rims of boleite. The anisotropic bands possess parallel e x tin c tio n and ex tin g u ish as a u n it.

Powder diffraction data are given in Table 12 for the green form of pseudoboleite. These data are compared with those obtained for the blue form and for natural pseudoboleite. As may be seen from a comparison of the sets of data, there is very nearly a line for line correspondence as well as a close correlation of relative intensities among the three species. Powder photographs of synthetic pseudoboleite are compared with a powder photograph obtained from natural pseudo­ boleite in Figures 28 to 31. There seems to be little doubt that this green form is also pseudoboleite.

Although it was not possible to make a complete single crystal study of the synthetic green pseudoboleite crystals during the course 112

F ig. 2 8 .—Powder d iffr a c tio n pattern of natural pseudoboleite (Sainte-E tienne 271*9).

F ig . 2 9 .—Powder d iffr a c tio n pattern of sy n th etic pseudoboleite obtained in a sealed glass tube at 170°C. from stoichiometric proportions for pseudoboleite.

Fig. 30.—Powder diffraction pattern of synthetic pseudoboleite obtained in a sealed glass tube at 170°C. from stoichiometric proportions for percylite.

Fig. 31.-—Powder diffraction pattern of synthetic pseudoboleite obtained in a hydrothermal bomb at 270°C. from stoichiometric proportions for pseudoboleite. F ig . 28.

F ig . 29,

F ig . 30.

F ig . 31. 112

0 / 113 of this investigation, it was possible to obtain rotations normal to

each of the pseudocube faces present on these crystals. For two of

these rotations, the spacing 1^.23 + O.O^X was obtained while the

spacing 30.73 + O.O^jt was obtained for the third. Comparison of these

parameters with those determined for natural pseudoboleite in this

study indicates that these parameters agree very well with those given fo r d 1 1 0 cQ, respectively.

As the synthetic analogues of pseudoboleite were produced from

starting compositions which preclude the presence of silver in the

sy n th etic c r y sta ls and as th e id e n tity between the sy n th etic and the natural specimens of pseudoboleite has been established above, there

can be no doubt that pseudoboleite can exist as a silver-free species.

On. the other hand, microchemical tests on natural specimens which yield the pseudoboleite pattern indicate that the species may contain

some silver. The name pseudoboleite then applies to a silver-free to silver-poor species as compared to boleite which contains essential

silver. The amount of silver which the pseudoboleite structure can

tolerate has yet to be determined. V I . BOLETTE

H isto rica l Summary

Boleite, as originally described by Mallard and Cumenge (1891 a,b),

consisted of boleite as well as cumengeite and pseudoboleite, which

these authors did not recognize as distinct from boleite. Oily their

results for boleite are given in this chapter. Mallard and Cumenge

found that boleite commonly occurred as unmodified pseudocubes or

occasionally as pseudocubes modified by the faces of the pseudo­

octahedron and pseudododecahedron. They observed an easy cleavage

parallel to the pseudocube faces and a less easy cleavage parallel to the pseudo-octahedron.

An optical study of boleite indicated that it was composed of an

isotropic core surrounded by an anisotropic band or alternating iso­

tropic and anisotropic bands. The enclosing bands were broad plates

parallel to the cube-like faces of the crystal and intersected or

coalesced along the edge between adjacent faces of the pseudocube.

Anisotropic plates proved to be uniaxial negative and to have their

optic axis normal to the face of the pseudocube from which each had

been detached. Mallard and Cumenge found that the core of boleite was quasi-isotropic and that the birefringence of this core decreased gradually from the edge to the center. They concluded that boleite was tetragonal, pseudo-isometric and that the isotropic parts of boleite

11U 115 wore duo to the intimato intergrowth of small tetragonal individuals in such a way that the optic axis of each individual was parallel to the fourfold axis of a cube. Due to absorption, a precise value for the index of refraction could not be obtained but an approximate value derived from a prism bounded by pseudocubic and pseudo-octahedral cleavages gave 2.07.

A quantitative analysis made with care on a pure sample gave percentages for silver, copper, lead, chlorine and water and indi­ cated that the formula for boleite was Pb^CujAgCl^OH)^ It is not clear from their article whether or not these authors used crystals which possessed pseudoboleite overgrowths on boleite for their analysis, but it seems quite possible that they did, since they described all crystals with a cubic appearance together. There seems to have been some doubt in th e ir minds even at th is time th at the "octahedral" crystals (i.e. cumengeite) were the same as the "cubic" crystals in­ asmuch as they described the former in a separate part of the original description and gave the results of a separate study on these crystals.

The liklihood, therefore, that cumengeite was included in the analysis of boleite seems remote.

Mallard and Cumenge noted the sim ilarity between the composition derived for boleite and the formula given ty Brooke. They did not give a reference to Brooke, but probably referred to Phillips-Brooke and Miller (1852) where no mention of silver in percylite was made.

They decided that they ought to retain the name boleite for the argentiferous variety inasmuch as the analysis of percylite was imperfect, incomplete and did not indicate the presence of silver. 116

A measurement of the density performed on Bmall, very pure crystals gave 5.08. The hardness was found to be slightly greater than that of c a lc it e .

With th e recogn ition of cumengeite (Cumenge, 1893* and Mallard, 1893) as a distinct species, Mallard (1893) found that the isolated pseudo- octahedral crystals previously described as boleite were cumengeite.

He also found that the outer part of the pseudo-octahedral group was cumengeite and that the groups were formed ty the emplacement of a cumengeite individual on each of the pseudocubic faces of a boleite c r y s ta l.

Prompted try the recognition of cumengeite, Mallard (1893) a lso restudied those boleite crystals which possessed a groove parallel to the edge of the pseudocube. He found that the outer part of these grooves, though tetragonal and isotropic, was significantly different from cumengeite. Mallard applied the provisional name percylite to the anisotropic parts of boleite which were not cumengeite, and felt that isotropic boleite was the result of multiple twinning of percylite.

Since he could not definitely prove that boleite was a composite species, he indicated that it was a species which could be isotropic, had a d en sity of £.07 and a re fr a c tiv e index of 2.07, and was p o ssib ly formed try very fine-scale multiple twinning of the species to which he had given the name percylite (equal to pseudoboleite of Lacroix).

Charles Friedel (I89U) reported a synthesis of boleite by the reaction of silver hydroxide, lead hydroxide and copper chloride according t o the method of slow d iffu sio n which he had used previously

(1892). From his description of the products, it seems that he did

% 117 indeed synthesize boleite as well as pseudoboleite on boleite.

Liversidge (189U) found that boleite from Australia possessed a vitreous luster, perfect cleavage parallel to the faces of the pseudo­ cube and an imperfect pseudo-octahedral cleavage. He found that

b o le ite had a hardness o f 3.5 and a s p e c ific g ra v ity of 5*02. Under the microscope, Liversidge found that boleite possessed a ‘striated structure'. An incomplete quantitative analysis by Armstrong (in

Liversidge, I89U) gave percentages for silver, copper and lead very similar to those obtained by Mallard and Cumenge (1891 a,b). The percentage obtained for chlorine was 13.50 as compared to 19.98 ob­ tain ed by Mallard and Cumenge. C alculation o f the water content o f boleite gave 5»U$ but an independent determination gave 6,39% on

0.3253 grams of m aterial. The d ifferen ce between measured and c a l­ culated percentages obtained for water was attributed to leakage of chlorine or lead oxide during the first analysis.

During a restudy of the minerals from Boleo, Lacroix (1895) found that boleite was not pleochroic. Upon heating boleite, he found that it became green, but regained its blue color upon cooling.

Lacroix reported that the birefringence of anisotropic boleite was

0.010. He obtained the vaLue 5.25U for the specific gravity of these boleite crystals which possessed the faces of the pseudocu.be in combination with the pseudo-octahedron and a maximum birefringence of 0 . 001.

Wallerant (1896) apparently using unmodified pseudocubes of boleite performed etch tests on the anisotropic plates removed from these boleite crystals. Following Mallard and Cumenge (1891b), he supposed 118 that an anisotropic plate parallel to the pseudocube face of a boleite

crystal would give him a section through five anisotropic crystals of

boleite. The center of the plate would then be a single crystal of

boleite whose optic axis was normal to the pseudocube, whereas trape­

zoidal sections parallel to the optic axis of other anisotropic single

crystals of boleite would make up the edges of the plate. Upon

etching such a plate with dilute nitric acid, Wallerant found that the

center of the plate yielded tetragonal etch figures, the sides of which were parallel to the diagonal of the plate. Near the edge of the plate he found lenticular etch pits with the long axis of each pit parallel to the adjacent edge.

Next, Wallerant etched a section through a boleite crystal tan­ gent to the edge of the crystal. According to his interpretation, such

a section would be parallel to the optic axis of two anisotropic boleite

crystals and lie at hS° to two other optic axes. On that part of the

section parallel to the optic axes, he found lenticular etch pits

similar to those observed in the first experiment. On the part of the section at U$° to two of the optic axes, he obtained etch figures with the shape of a tetragonal pyramid the apex of which was generally truncated by a facet parallel to the base of the pyramid. The sides

of the pyramid were generally parallel to the sides of the section.

Lastly, Wallerant studied a section normal to the pseudothreefold

axis of boleite. The section was supposed to pass through three aniso­

tropic boleite crystals. Etch figures obtained on this section possessed

a trigonal form. The base of the trigonal form was an isosceles tri­

angle with a very acute angle. The apex of this acute angle pointed 119 toward the edge formed try the section and the base of the crystal to which the figure was supposed to belong. Pits with the form of an isosceles triangle are to be expected pseudocube in which the £00lj parameter of the species possessing the pseudosymmetry is an integral multiple of the isometric parameter. It is, therefore, not surprising that such etch figures were observed by Wallerant on b o le ite .

From these results, Wallerant concluded that anisotropic boleite was twinned in such a way that the edge of the twin was parallel to and coincided with the pseudothreefold axes of the cube. The dominant face of these twins would then be oriented normal to the apparent fourfold axis of the boleite crystals. Wallerant stated that boleite was an example of the fact that optical anomalies did not exist in actuality but were due to a completely arbitrary convention limiting th e degree of symmetry which could be shown by a given r e tic u la r symmetry.

The exact significance of Wallerant’s observations remains in doubt inasmuch as pseudomorphs of cerargyrite or cerargyrite and cotunnite after pseudocubes of boleite, have been obtained during a microchemical test on boleite by the use of nitric acid. Thus Waller­ ant’s etch test may reflect the symmetry of a reaction product rather than th e symmetry of b o le ite . Furthermore, as w ill be shown in th e results of the present study, fine banding in the larger anisotropic bands of boleite indicates the intergrowth of one species in different orientations or the presence of two distinct species in anisotropic boleite on a scale much smaller than that of the twinning proposed by W allerant. 120

Friedel (1906), during a complete restudy of boleite, described the color of boleite as a very deep Prussian blue modified by a

blackish, pearly luster on the crystal faces due to the easy cleavage

parallel to the faces of the pseudocube. In thin plates as well as in

the form of powder, boleite appeared more greenish than cumengeite.

No trace of pleochroism was apparent in boleite.

The crystal form of boleite was generally that of an unmodified

cube without reentrant angles. In some cases, these crystals were modified by the faces of the pseudo-octahedron and pseudododecahedran.

All crystal faces gave poor reflections and were imperfect.

In most cases, on examining a cleavage plate in reflected light with his naked eye, Friedel found it was quite easy to distinguish two types of zones in boleite; a large central zone bordered by a thin

external zone parallel to each of the faces of the pseudocube.

Examination of a cleavage plate between crossed nicols indi­ cated that the central zone was isotropic or nearly so in comparison to the external zone which was obviously birefringent. Some plates were found to be completely isotropic. More generally, however, the central zone showed feebly anisotropic spots or fibers. These fibers, confused and indiscernible near the borders of the zones, were parallel to the edges of the cleavage plate near the outer edge of the central zone but became parallel to the diagonal of the plate in the vicinity of the diagonal. In crystals possessing pseudo-octahedral and pseudo- dodecahedral faces, these quasi-isotropic diagonal fibers were found to terminate against these faces. The central zone exhibited a per­ fect cleavage parallel to the pseudocube as well as a pseudo-octahedral 121

cleavage from place to place on this pseudocubic cleavage surface. On

the basis of the quasi-isotropic fibers and spots in the central zone

and on the basis of the pseudo-octahedral cleavage observed in both

types of zones, Friedel concluded that the central zone was not an

homogeneous substance but rather a composite mixture of differently

oriented crystals of the substance which occurred as the single crystal

making up each of the external zones. He attributed the isotropic or

quasi-isotropic central zone, as had Mallard (1893), to the symmetrical

distribution in three dimensions of birefringent crystals and compared

the behavior of boleite to that of leucite.

Friedel found that the external zone did not differ in color

from the central zone. This external zone was uniaxial negative and

possessed a variable birefringence. The optic axis of each birefrin­

gent zone was oriented normal to the face of the pseudocube on which

each zone was emplaced. In some cases, the external birefringent zone

enclosed a second isotropic zone. In rare cases, the birefringent

zone was absent altogether. In those pseudocubic crystals modified by

the pseudo-octahedron, the birefringent zone was present only as

slightly birefringent, nearly homogeneous, irregular bands parallel

to the face of the pseudocube and terminating against the pseudo-

octahedral face. Generally, only a single birefringent zone was found to enclose the isotropic or quasi-isotropic zone.

The contact of the two types of zones was not gen erally d is ­ tinct or planar but possessed undulations and indentations of truly

birefringent fibers of the external zone extending into the central

zone along the diagonals of the plate. All the crystals examined by 122

Friedel possessed parallel extinction with respect to the edge of a

cleavage plate from the crystal.

Friedel found that the value 0.010 given by Lacroix (189U) fo r

the birefringence of boleite was true only for certain areas of the

external zone of the unmodified pseudocubic crystals and for the

birefringent areas of the pseudocubic-pseudo-octahedral crystals.

3y considering the less birefringent areas of the external zone to

be the result of a mixture of crystals of different orientation and by using only the part of the external zone having the maximum birefrin­ gence, Friedel found that the value for the birefringence varied from

0.015 to 0.020. Crystals possessing only the pseudocube generally gave

a value of about 0.017 but occasionally very thin bands gave a value

of 0.02U to 0.025, a value which Friedel indicated might possibly

belong to an intervening zone of pseudoboleite. He concluded, however, that the true value of the birefringence of untwinned boleite was at

least 0.020 and perhaps even attained 0.025*

The external zones exhibited a very easy, distinct cleavage parallel to the pseudocube face on which each zone was emplaced. In addition,

each external zone possessed a pseudo-octahedral cleavage parallel to the edge of the pseudocube and making an angle of 75°5 7 ' (mean of

eleven measurements) with the pseudocubic face of the adjacent exter­ nal zone. In a section through the crystal parallel to the pseudocube,

Friedel could also observe, in a particular external zone, local areas with the pseudocubic cleavage or pseudo-octahedral cleavage of an adjacent external zone.

From the study of the cleavages present in the two zones, Friedel concluded that untwinned boleite had an extremely easy, distinct basal cleavage and a dipyramidal cleavage parallel to the edges of the base. The angle from the base to the dipyramid was 75°57'. He further concluded that the central zone, dominated by three pseudocubic cleavages was composed of nearly equal proportions of three mutually perpendicular individuals and that the external zone, although domi­ nated by individuals oriented in such a way that their fourfold axes were normal to the pseudocubic face of the zone, was composed of variable proportions of the individuals having the other two orienta­ tions present in the central zone.

Friedel noted that the dipyraraidal cleavage was very nearly parallel to the form (oUl^ 811 isometric crystal but that such a face was of insignificant importance according to the Law of Bravais and that this form was scarcely known as an actual crystal face. He further pointed out that no cubic crystal known possessed such a cleavage.

Ey giving the notation {oil^ to the di pyramidal cleavages of the external zone, Friedel obtained the axial ratio a:c = 1:3.996 for untwinned boleite. Reasoning from the Law of Bravais and a calcula­ tion of the reticular areas based on his axial ratio, he concluded that boleite possessed a tetragonal lattice of the octahedral mode.

Because of the unusual length of the c-axis, however, fooi^ predominated rather than the usual pseudo-octahedron.

Friedel considered the isotropic core to be the result of twinning of the simple tetragonal species by reticular pseudomerohedry about

[ 1 0 0 ) or [ b io ] . Very fin e elements twinned in such a way and eq u ally distributed in three mutually perpendicular dimensions would, he

reasoned, lead to an isotropic to quasi-isotropic, apparently homo­

geneous species possessing a cubic form. The multiple lattice of

such a grouping would appear to be that of a cubic species of the

hexoctahedral mode in which the important forms in decreasing order of importance would be the cube, octahedron, and dodecahedron. Friedel

concluded, therefore, that, in those crystals possessing the faces of

the pseudo-octahedron and pseudododecahedron, these forms belonged to the isotropic core and were the effect of the multiple lattice pro­

duced by twinning. In support of this conclusion, he offered the fact that these pseudo-isometric forms interrupted the birefringent exter­ nal zone and obrdered the isotropic zone directly. In addition, he

pointed out that the tetragonal forms analogous to the octahedron and

dodecahedron are of subordinate importance in the tetragonal lattice

derived for untwinned boleite but are of prime importance in the nul- tip le, pseudocubic lattice produced by twinning.

A specific gravity determination made on five very pure cubic

crystals having a total weight of 1.3185 grams gave the value 5*05U.

Lacroix (in Friedel, 1906) indicated that his value 5.21*5 for the

density of quasi-isotropic boleite crystals was probably due to the

presence of an impurity such as .

Inasmuch as previous analyses given for boleite showed mediocre

agreement with the formula proposed for this species and inasmuch as

confirmation of the theory that the two zones present in boleite were

identical in composition was needed, Friedel performed new analyses

on boleite. Quantitative analyses of the external and internal zones 1 2 5

of boleite yielded the same ratios in both zones for lead, copper and

chlorine. The analyses also indicated that there was a slight silver

deficiency in the external zone as compared to the internal zone,

but Friedel considered this deficiency to be too small to be certain or

characteristic. The percentage of water present in boleite was then

obtained on a sample consisting of entire boleite crystals. The mean

calculated from the analyses of the two zones in boleite indicated

that the fonmla of boleite was 9PbCl2»8Cu0»3AgCl»9H20.

Friedel also performed a study of the composite crystals in the

boleite group. He verified the conclusion of previous authors that

boleite forms the core of the composite crystals.

In an examination of the boleite crystals at his disposal,

Hadding (1919) found that boleite occurred as simple cubes and

combinations of the cube and octahedron. The latter usually possessed

facets of the rhombic dodecahedron. He observed no twin sutures and no reentrant angles along th e cube edges of th ese c r y s ta ls . Hadding

also pointed out that nothing in the external form of boleite indicated

other than a cubic symmetry for this species.

On breaking the edges of two of the cubes, he observed small,

brightly gleaming reflections on these broken surfaces. When these

c r y sta ls were mounted on the goniometer, Hadding observed weak lig h t

bands possessing stronger, but still weak, reflections. These stronger

reflections were attributed to the small brightly gleaming planes

observed e a r lie r on th e broken cube edge. Hadding observed the angles

between the cube face and the stronger r e fle c tio n s shown in Table 15.

He pointed out, however, that, as the reflections were very weak, the

measurements were very poor. He also concluded that these small, 126 gleaming planes were not cleavage planes.

TABLE 15.—Cleavage observed in boleite by Hadding (1919)

Reflection Angle to Cube Face

(810) 83°02«

(17*U*0) 76°U2»

(Uio) 75°53•

(5 2 0 ) 68°09’

(530) 5 8 °5 l'

(SUO) i i l ° l 6 1

(870) 38°52'

On cleaving a b o le ite cube through the cen ter, Hadding found that the previously observed planes disappeared and two sets of dis­ tinct cleavages appeared. The first set was parallel to the cube face whereas the second made an angle of 75°53' + 0°3 9' w ith the cube fa c e . The la tt e r cleavages were large and gleaming but gave very weak sig n a ls on the goniometer. Hadding concluded th at none of the angular measurements supported a tetragonal symmetry for boleite

and under no circumstances were these measurements exact enough to form the basis of an axial ratio calculation.

Both the cube and octahedron faces possessed a hardness somewhat greater than three.

A specific gravity determination performed on three different 127 crystals gave the values 5.155, U.977, and U.802. E>y a comparison of these values with the corresponding absolute specific gravities,

Hadding concluded that th e ex tern a l, doubly refra ctin g part of th e boleite crystals had a greater specific gravity than the inner part.

He pointed out that this variation in specific gravity could be explained on the basis of solid solution in boleite. In an attempt to show the mechanism of this solid solution, he supposed that the core contained more silver and less lead than the external zone. As a basis for his supposition, he indicated that the more difficultly soluble member of a solid solution is the first to form.

Hadding noted th at F ried el (1906) had a ttrib u ted the o p tica l anomalies of boleite to twinning of a tetragonal species but pointed out that similar optical anomalies had been reported for isometric species which possessed solid solution. Hadding indicated that strongly refracting layers parallel to crystal faces, alternating anisotropic and isotropic bands, stronger birefringence toward the outer portion of the crystal and radiating, irregular anisotropic bands occurred in boleite in the same way as in these isometric species showing solid so lu tio n . Hadding a lso reca lled th e work of F ried el (1906) which showed that the birefringence of the anisotropic areas of boleite varied considerably.

In order to measure the refractive index of boleite, Hadding detached a cleavage prism from one of the crystals but found that th is prism was composed of three grains having three d iffe r e n t orien­ tations. He found, however, that rotation of an analyzer set up on the goniometer had no effect on the signal obtained by refraction 128 through the prism. He obtained the values n = 2.081 / 0.001 and n = 2.087 f 0.008 for two different orientations of the prism. The

refracted rays were weak but those yielding the value n = 2.087 /

0.008 were the weakest. He pointed out that the lower value could be included in the limits of the higher value. Hadding concluded that, although the refractive index may vary in different parts of the

crystal and no exact value was obtained, the values obtained may

be considered characteristic for boleite.

Although Hadding did not repeat the chemical analysis of boleite,

he did point out that the analyses performed by Friedel (1906) indi­

cated some possibility of a variation in chemical composition, the inner zone being richer in silver and poorer in lead than the outer

zone. Hadding explained the close similarity of both zones as the result of alternation of silver-rich and silver-poor bands in the

outer zone of bol9ite.

From his in v e stig a tio n s, Hadding concluded th at b o le ite was not tetragonal and that the associated optical anomalies were not the result of twinning as Mallard (1891a) and Friedel (1906) had proposed.

He pointed out that tetragonal symmetry would indeed account for the optical anomalies but would not explain the variation in chemical composition from core to shell found by Friedel (1906) in boleite.

Further, Hadding indicated that the differences in specific gravity observed in boleite could not be explained by Friedel's (1906) theory. Hadding reiterated his conclusion that the variation in chemical composition, specific gravity and optical properties was the result of a decrease in the silver to lead ratio toward the exterior of boleite. 129

He used the cumengeite-boleite combinations as support for the argu­

ment that growth of boleite does not terminate when silver is no

longer present in the parent solutions. Hadding also suggested that

cumengeite could make up the outer birefringent shell of boleite but

indicated that the birefringent shell could equally well be explained

on the basis that boleite was a zoned crystal formed as the result of

solid solution. The fact that boleite was not enclosed ty cumengeite

indicated to Hadding that the sequence of crystallization was boleite

followed by cumengeite.

As a p o stsc r ip t to h is paper, Hadding pointed out th at a Laue

diagram taken normal to a cube face of boleite yielded a fourfold

symmetry for this direction. He did not attempt to draw any conclusion

as to the symmetry of boleite on the basis of this diagram since, as

he indicated, mimetic twinning of the type ascribed to boleite ty

Friedel would produce the same symmetry for this direction.

Larsen (1921) determined new indices for boleite using a speci­

men from Boleo, Baja California (USNM 809U3)» He found that

0 = 2.0J? + 0.02 and E = 2.03 + 0.02 and confirmed F r ie d e l's (1906)

observations that boleite was uniaxial negative and non-pleochroic.

Larsen stated that boleite was not entirely homogeneous and that

there was a marked variation in the refractive indices. During a

study of percylite from an unknown locality, he concluded that part

of the percylite was altered to boleite with 0 = 2.06 4 0.01 and a

birefringence of 0.02.

Gossner (1928) reported his own observations as well as Mussgnug’s

x-ray measurements on boleite. By means of the oscillation method, 130

Mussgnug obtained 15.6$ for 21#^ for dno* 26.8$ for d-[-|1,

Each of these parameters represented the average of five measurements.

The formula PbCl2 ,Cu(OH)2 was assumed fo r b o le ite and on the b a sis of

this formula, a molecular weight of 375.8, a density of 5.051* and a

value of l5.6jj for a0, Z was found to be 31.2. A Laue diagram taken

normal to a {looj cleavage plate showed a fourfold axis and four

planes. No evidence for other than the isometric symmetry Td, 0 or

0^ was observed.

A microscopic examination of boleite indicated that the crystals

possessed isotropic and anisotropic areas, a structure not indicated

try x-rays. Gossner concluded that boleite was isometric and possessed

optical anomalies as a result of strain developed possibly from iso­

morphism.

As a preliminary examination of boleite indicated that some

boleites with an unmodified cubic habit possessed areas of marked

birefringence while those of cubo-octahedral habit were almost entire­

ly isotropic or quasi-isotropic, Gossner and Arm (1929) divided the

boleite crystals at their disposal into two groups. The first group

of boleite crystals to be examined by these authors were cubo-octa-

hedral. In thin cleavage plates nearly no optical anomalies were

observed. The s p e c ific g ravity determined on 1.(4986 grams was 5.191

and 5.186 with the average 5.188 being chosen as the value for a

subsequent calculation of Z.

Two analyses performed on 0.3119 and 0.1*335 grams o f b o le ite yielded ratios for lead, copper, silver, chlorine and water. Although

the ratio for copper as copper hydroxide exceeded the calculated ratio

in the formula 3PbCl2*3Cu(OH)2AgCl, this fornula was adopted and the 131 excess copper hydroxide was attributed to solid solution. The authors noted that this fornaila was essentially that found by Friedel (1906)

and that the ratios yielded try their analyses were those found try

Mallard (1891). An examination of the boleite from the core of com­

posite crystals confirmed the silver content of boleite.

An x-ray examination yielded the parameters 15>.l+lpA. for d^QQ,

21.72A for d , 26.62A for d , and 37.6A for d The parameters 110 111 2.12 for d and d _ were the average of four measurements whereas those 100 112 for d^]_Q and d]_]j were the average of six measurements. Gossner and

Arm considered that the relationship of <^qq to d-^ indicated that boleite was cubic. A calculation of Z gave the value 9.0lu A simi­ lar calculation based on the formula reported by Friedel gave the value

3.26 for Z.

The ratio d^QQ? d^Qj d-mindicated that the lattice was primitive.

This conclusion was supported when the rotation s around [jLOo] and Q-loJ were laid one on top of the other. Both rotations possessed the odd and even orders of the cubic form £lOoj. The authors thus concluded th a t b o le ite must belong to one of the follow in g space groups: T^,

T^, T^, 0, or 0j!j. A great number of reflections were indexed and in d icated th a t the r e fle c tio n s (hkO) w ith h and k equal to an odd number and (hh£) w ith A odd were present. The authors pointed out that the reflections supported the conclusion that the lattice was primitive. A Laue diagram normal to (lOO) indicated that this direction possessed fo u rfo ld symmetry and was not an aggregate of d iffe r e n tly oriented lamellae. This diagram at once eliminated two space groups:

T^ and T^. A consideration of the number of atoms and equivalent 132 points did not reduce the choices further but the forms exhibited ty the crystals belonged only to those minerals belonging to 0^. Thus the space group chosen was Oj!j.

The second group of boleite crystals to be studied by Gossner and

Arm was composed of unmodified cubes. These authors verified the marked anisotropy observed in unmodified boleite cubes by Friedel (1906).

They also observed a uniaxial interference figure from the anisotro­ pic portion of boleite.

Two measurements of the specific gravity performed on 1.0U82 grams of b o le ite gave 5.185 and 5.189 or an average value of 5»l87.

Two chemical analyses of cubic boleite gave ratios quite near those obtained for the cubo-octahedral variety.

A rotation photograph about [00l] of the anisotropic area gave

15.U1A, an identity period the same as that for the cubo-octahedral crystals. The position of analogous points in the layer lines was found to be the same for analogous films for both the cubic and the cubo-octahedral varieties.

The authors were unable to obtain a cleavage plate from the cubic crystals showing the angle (hOjQ to (100) of 75°56', reported by

Friedel, (1906).

Gossner and Arm thus concluded from their studies that both the cubo-octahedral and cubic varieties of boleite were identical and possessed cubic symmetry despite the anisotropy observed in some boleite crystals. The authors pointed out that other cubic species with complex structures possessed optical peculiarities and thus did not consider boleite to be strongly anomalous. They concluded that 133 whatever the future conclusions might be as to the optical anomalies, boleite was at least pseudocubic if not really cubic.

In a reconsideration of the parameter a:c for boleite, which was considered by Gossner and Arm (1929) to be equal to unity, Friedel

(1930) pointed out that these authors had erred in several ways in their determination of this parameter. According to Friedel, in order to observe the true parameter of a mLmetically twinned species by means of x-rays, one mist address himself to the untwinned portion of the species. In the case of boleite, the untwinned portion of the species was the anisotropic external zones present on crystals of boleite, not the isotropic to quasi-isotropic boleite used for this purpose by Goss­ ner and Arm. Friedel also called attention to the fact that the photo­ graphs for the cubo-octahedral crystals and for anisotropic boleite published by these authors were most assuredly not the same, as one might observe by reference to these photographs. The difference, according to Friedel, would have been even more pronounced and led to the correct parameters for boleite had not these authors underexposed their films.

Friedel again reiterated his conclusions as to the axial ratio of boleite derived from the cleavages, pointed out errors in and omission of his data as reported by Gossner and Arm, and indicated that his

(Friedel, 1926) theory of mimetic twinning quite adequately accounted for the so-called optical anomalies in boleite.

F ried el next commented on the s p e c ific g ravity found by Gossner and Arm, and the chemical fornula adopted by these authors. Friedel pointed out that although both his analyses and those of these in v e stig a to r s had in d icated a d efic ie n c y in cupric oxide as compared to the fornula for boleite derived ty Mallard (1893), Gossner and

Arm (1929) had adopted the fornula given ty Mallard, rather than that given ty Friedel (1906), for their calculations. Further, Gossner and

Arm (1929) obtained a density of 5.188 and used this density in their calculations despite the fact that Mallard (1893) had found 5*08 and he himself (Friedel, 1906) had obtained 5.051*. Friedel pointed out that Gossner and Arm (1929) had eliminated his formula on the basis of an arithmetical error in their calculation of Z. Friedel repeated the ca lcu la tio n s of Gossner and Arm using th e ir numbers and su b stitu tin g the correct value for the molecular weight of 9PbCl2*8CuCU3AgCl»H20.

Friedel obtained a value of 3.08 rather than 3.26 (Gossner and Arm,

1929). Friedel then calculated Z on the basis of the specific gravity and chemical fornula found in his earlier study (Friedel, 1906), and obtained the value 3.00. He pointed out that the true value for Z would be 12, inasmuch as the cleavages indicated that the lattice was tetragonal and the parameter c was four times that of a. At Friedel's request, Hocart (in Friedel, 1930) repeated the density measurement on 0.997 grams composed of very pure crystals and obtained the value

5.0U. Friedel was not certain that the value obtained for Z verified his chemical formula, but did conclude that his formula corresponded with the results of analyses ty both himself and Gossner and Arm, and that the value 5.188 obtained ty these authors for the density of b o le ite was in error.

Hocart (1930) performed an x-ray study of boleite, pseudoboleite and cumengeite in order to verify the conclusions obtained ty Friedel 135

(1906, 1930). For the study of boleite, Hocart detached a basal

cleavage plate from the birefringent portion of a boleite cube. He

verified the presence of pseudo-octahedral cleavages at 75°57* to the

base on this cleavage plate. Oscillation photographs produced ty a

15° oscillation about [jOOl] gave an identity period in this direction o of 62A. Hocart noted that the spots in the fourth layer line were

strong compared to those of the other three layer lines between the

zero and fourth layers and that it was from these intermediate layers o that the 62A parameter was obtained. He indicated that if only the

Iin layer lines (where n = a whole number) were observed as a result

of lack of proper exposure, the spacing obtained would be that observed

by Gossner and Arm (1929), 15.1A. For a rotation about £l00], Hocart o obtained l5.1*A, a spacing in good agreement with that obtained ty

Gossner and Arm (1929). The axial ratio obtained from the x-ray para­

meters was thus approximately equal to U. The precision of measurement,

according to Hocart was within 0.$%. Using the formula 9PbCl2*8CuO»3AgCl«

9H2O, Hocart obtained the value 12 for Z with an error of less than l£.

Gossner (1930) noted the work of Friedel (1930) and Hocart (1930)

and commented on h is and Arm's (Gossner and Arm, 1929) e a r lie r work

on boleite. Gossner recalled that his and Arm's values for aQ and c0

had been obtained from (hhO) and (OOj?) reflections, not from the layer

lines. As a check of his previous determination of the parameters,

Gossner performed new measurements using the third layer lines and

obtained th e same values he had obtained e a r lie r .

Gossner performed a new specific gravity determination on boleite

and obtained once again $ .18 when the determination was performed two 136 hours after placing the boleite in the pycnometer. If the determi­ nation was performed again at a later time, the gravity was found to have changed markedly. Gossner concluded that 5.05-5.06 was probably the correct value for the specific gravity of boleite but found it strange that a constant specific gravity appeared slowly when boleite was introduced into the water in the pycnometer while water alone showed the proper specific gravity immediately.

Gossner r e ca lled th at he and Arm (1929) had performed an exten­ sive x-ray investigation of boleite for a great number of directions as well as in a great angular arc of U2° about the directions jj-OO],

[Old], and fool]. These films were produced under the same expert- mental conditions as used ty Hocart (1930) and in some cases were exposed for a longer time and over a greater oscillation angle than were those of Hocart (1930). Although Gossner pointed out that he and Arm (1930) had observed no additional layer lines which would indicate a periodicity greater than they had reported, Gossner did indicate that some of Hocart's films possessed such intermediate layer lines which would lead to the larger parameters given ty Hocart (1930).

Hocart (1930) discussed the work of previous authors on boleite and although he did not attempt an explanation of the work of Gossner and Arm (1929) and Gossner (1930), he concluded that the boleite which they had used was poorly d efin ed .

For the purpose of determining the absolute parameters of tetra­ gonal boleite, Hocart chose cleavage plates showing the maximum bire­ fringence and octahedral cleavages at 75°57' to the base.

An oscillation photograph about [00l] produced by an oscillation 137

of 15° starting with (100) approximately parallel to the team, yielded

a value of 62A with a precision of almost 0,$%* Hocart noted that the

most intense spots on the photograph belonged to the Un layers (where n = an integer) while the spots belonging to the other layers were, in general, weaker and even absent if the exposure time was reduced*

He pointed oux- that only the spots of the Un level were visible on the photographs of Gossner and Arm (1930) and th a t i t was because of the lack of the intermediate that these authors had deduced the para­ meter l5.UX for cQ. An oscillation photograph produced in a way similar to that used for dQQ^, gave 15.UA for aQ with a precision of almost 0.53>. The ratio of the x-ray parameters a0:c0 was approxi­ mately equal to U> in good agreement with Friedel's (1906) value determined from the cleavages (a:c = 1:3*996). The unit cell thus possessed four times the volume of the cell chosen by Gossner and

Arm and contained 12 rather than 3 (Gossner and Arm, 1929) molecules of the composition 9PbCl2*8Cu0*3AgCl»9H20. Hocart sta ted th at his value for Z possessed a precision greater than 1%, In an attempt to explain the strong reflections which had led Gossner and Arm to the choice of a primitive lattice of the hexoctahedral mode for boleite,

Hocart attributed their results and choice of lattice to the fact that these authors had used finely twinned boleite for their determi­ nation. Oscillation photographs from anisotropic boleite, produced by a 15° oscillation about [OO^ and [loo] and indexed ty means of a Bernal chart, indicated that only reflections with (h+k-t^Zn were present. In addition, on the oscillation photograph produced about

[loo], Hocart found that the spots (00jt) with/=Un were strong while 138 those with,£ sljni-2 were weak. Hocart concluded that the lattice of boleite was body-centered. According to Hocart, the reflections (h0£), such as (501), eliminated the space groups and Df^, while the reflections (hhO) such as (550) eliminated and respectively.

As the la t t ic e m ist contain 12 m olecules of 9PbCl2*8Cu0«3AgCl*9H20, a n e c e ssity demanding at le a s t four groups of equivalent p o in ts, and as neither of the space groups provided sufficient equivalent sites, these two space groups were further eliminated. Ch the basis of the theory of the centered lattice, Hocart concluded that the space group must be dJJ (Ij^ mm).

In order to determine the symmetry of boleite conclusively,

Hocart used cleavage plates from anisotropic boleite to produce Laue photographs normal to [00l] and £jL00]. Using a basal cleavage plate which was 0.55 millimeters on a side and 0.08 millimeters thick, yielded a uniaxial interference figure normal to the basal plate and showed the pseudo-octahedral cleavages characteristic of boleite, he obtained a Laue diagram with the beam parallel to [00l]• This diagram indicated the presence of a fourfold axis parallel to [ooi] and planes normal to I^IOO] andj^lioj. In order to obtain a Laue diagram normal to jjL0(£|j Hocart used a cleavage p la te taken p a r a lle l to the pseudo- octahedral cleavage. The plate was 0.52 fcy 0.63 millimeters, possessed a thickness of 0.072 millimeters and a birefringence of 0.020. In convergent light, the plate exhibited an optic axis displaced lli° from the center of the field. Although the Laue diagram normal to [lO o] resembled that normal to jjDOlJ, the former possessed less spots and the distribution of these spots indicated a twofold symmetry axis 139 parallel to [lOo] and a plane in each of the axial directions.

Hocart noted that, although nearly all spots on the Laue photo­ graph normal to [lOo] conformed to the symmetry indicated, there were a sm all number of weak to very weak spots on each h a lf o f the p la te which did not occur on the other half and thus did not conform to the symmetry derived above. If these spots were taken into account, the symmetry would be reduced to that of a monoclinic species. An examination of the mount used for the crystal indicated that this mount was not the source of these spots. As the possibility existed that an error in orientation of the crystal could lead to differences in ab­ sorption for symmetrically distributed spots and a consequent apparent dis-symmetry, Hocart performed an intensity calculation to determine the effect of such an error in orientation. The results indicated that such an error in orientation would produce the spots observed, but that such spots would be confined to one half of the plate only. Hocart thus concluded that the cause of the dis-symmetry must belong to the crystal although all other results indicated that anisotropic boleite was rigorously tetragonal.

In further comments on the Laue spectra obtained with the x-ray beam parallel to the twofold axis of anisotropic boleite, Hocart noted that there were certain spots present whose repetition with respect to both intensity and position corresponded quite closely to the repetition about a fourfold axis parallel to the beam. Upon indexing these spots, he found that they all belonged to the family of planes

(hkli). He analyzed these reflections in an attempt to reduce them to a cubic rotation without disturbing the reticular density relationships lUo

of the original cell, but was unsuccessful. He concluded that perhaps

this fourfold repetition of a certain few spots was the result of the

quasi-cubic submotif of the tetragonal cell in combination with the

presence in these planes of certain atoms important in the diffraction

of the x-rays. He pointed out that in the case of kyanite, the oxygen

atoms formed a compact pseudocubic assemblage and that the most in ten se

reflections from this species corresponded to these planes inhabited by

oxygen. In the case of boleite, such a pseudocubic lattice was also

present. If the planes bounding this lattice were centered and composed Oo of chlorine atoms with an ionic radius of 1.8A, the dimension of such

a lattice parallel to the edge of the cube face in the plane of the

face would be l5.l£, in comparison to l5.lj? for the parameter [lO(f]

of boleite. He noted, however, that the formula of boleite did not

indicate the presence of enough chlorine for such a compact cubic

arrangement to be present as an uninterrupted entity.

Hocart also indicated that the sample of anisotropic boleite used for his studies was not a single crystal, but contained a mix­

ture of the three orientations and that use of a single crystal might

result in photographs even more different than he had obtained. He

added that photographs of boleite possessing a birefringence of about

O.OOl; were essentially those of quasi-isotropic boleite and explained why Gossner and Arm (1929) had been unable to obtain the correct

[oof] parameter.

A Laue photograph obtained by Hocart from quasi-isotropic boleite was comparable to that obtained by Gossner and Arm (1929). Hocart

stated that it was to quasi-isotropic boleite that the parameter llll

[ioql = [oiq]-[ooi|= 15.UX of Gossner and Arm belonged, and that this

parameter was the effect of sub-multiple cubic lattice derived from

a tetragonal lattice tjy twinning. Hocart found it remarkable that the

sub-multiple lattice in a twin of such a great size could still be

observed.

In summary, Hocart pointed out th at th e Laue sp ectra confirmed the symmetry of boleite deduced from the birefringence and cleavages

(Friedel, 1906). Boleite was thus found to belong to the dipyramidal

class of the tetragonal system. Twinning ty reticular paeudomerohedry

resulted in a pseudocubic external form which was found to possess an

apparent absolute parameter [00l] = [oioj-jioo]^ 15.UX where the twinning was nearly complete.

I to (1 9 5 0 ) performed a single crystal study o f boleite using

slices of appropriate thickness from indigo blue cubes f o u r m illi­

meters on a side. The specimens were reported to be from Boleo,

California. No optical or morphological data were given for the

specim ens.

The Weissenberg photographs obtained indicated tetragonal symmetry w ith resp ect to in te n s ity and p o sitio n of the sp o ts. The Laue symmetry was found to be U/m 2/m 2/m. The absolute parameters obtained with

an ionization spectrometer were a0= l5.2liX ■*> O.O 2X and c0- 6 0 .8 2 *

0.06X.

In order to obtain a representative chemical composition for

boleite, Ito took an average of what he considered to be the best

analyses. The analyses used were those of Gossner and Arm (1930)

and Friedel (1906). Although he obtained ratios very near those given 1U2 by Friedel (1906), Ito adopted 26 instead of 27 molecules of PbCl2 in the fornula because this formula gave better agreement between his calculated specific gravity (5.098) and the measured specific gravity.

With Friedel’s formula, the calculated specific gravity was 5.228. Ito gave Z as U on the basis of his calculated chemical formula 26PbCl2*

2liCuO*9AgCl*27H20 .

An examination of the extinction criteria indicated that boleite belonged to one of the following space groups: C]^, or D^h* How­ ever, an additional extinction criterion indicated that JL could never be a number Un+2 and was found to be tru e fo r the more than 3000 re­ flections examined. Ito explained that this additional criterion was the result of the presence of a double Bravais lattice in boleite. He showed that such a double lattice could be built up in two ways: l) by the interpenetration of body-centered tetragonal lattices in the direc­ tion [OOlj, and 2) by stacking cubic lattices in the direction [00l3 such that these lattices were alternately related to one another by a mirror plane parallel to (001) and a glide plane parallel to (001) with the glide being in [116] direction.

Reasoning from the cubic lattice which would have to be twinned to produce the double lattice, Ito derived three possible twinned space groups for boleite. Inasmuch as he could not discriminate between these, he performed a structure analysis of boleite in an attempt to determine which was the correct space group. Chly twinned space group derived by twinning the isometric space group 0^> as noted above, satisfied the structural data he obtained. He described this twinned space group 2/m 2/m|to distinguish as well as compare it to the untwinned space group I y m 2/ m 2/ m.

A consideration of Ito's structure indicates that it is composed

of sheets with the composition and sheets which contain

linked It should be noted that the structure as

described by Ito is a defect structure with respect to Pb, Ag, and Cl,

and th a t part of th e s ilv e r su b stitu te s fo r lead and part occupies

unique sites. On the basis of Ito's structure, the cleavage parallel

boleite takes place parallel to the |CuCl(0H)c sheets. ^ -i»o

Results of the Present Study

During the present study of cumengeite and pseudoboleite, it

became apparent th at th ese sp ecies had been m isoriented by previous workers, apparently cm the assumption that the morphological orienta­

tion was the correct one and was to be retained in an x-ray determi­

nation of the parameters. The possibility existed that such a mis-

orientation had been retained also in the case of boleite and that

Ito (1950) had inadvertently overlooked the possibility of a reorien­

tation in boleite. Furthermore, Ito (1950) did not give any account

of his specimens other than to state that they were cubes of boleite

from Boleo and th a t he had used th in s lic e s of th e se cubes fo r his

measurements. No optical account of the specimens he used is given and yet the parameters obtained would indicate that his specimens were

largely anisotropic boleite. It thus seemed desirable to check the

parameters of boleite once again and especially since a certain amount

of evidence indicated the possibility of a solid solution between

b o le ite and p seu d ob oleite. Hill Consequently, an anisotropic plate parallel to the cube face

of a typical boleite cube from Boleo, Mexico was mounted so as to

obtain the spacing in the direction of the base diagonal, d^o,

orientation of Ito (1950) and Hocart (1930, 193U)* The spacing ob­

tained was 21.59X, a spacing comparable to that of aQ in pseudoboleite

and cumengeite and inconsistent with the orientation of boleite adopted

Iy Ito and Hocart. In order to further correlate the specimen with

those used by I to and Hocart, the c r y sta l was mounted on the cut sur­

face so that the axis of rotation was Q)0lJ. Instead of yielding a

60X spacing, however, the plate gave a 15A spacing. A number of

anisotropic plates which gave a uniaxial negative interference figure .0 when viewed normal to the cube face also yielded 15A spacings by the use of the rotation method. To date, no anisotropic boleite specimen

has been found to yield the 60X parameter observed by Ito (1950) and o Hocart (1930, 193U)« It should be noted, however, that a 60A spacing was given by a specimen of pseudoboleite which had been cleaved from

boleite. After the cleavage surface of pseudoboleite was re-examined o and cleansed of small projections on the cleavage surface, the 60A

spacing disappeared. Presumably, therefore, there is a species . o possessing a 60A spacing present in this group of minerals but the

distribution of such a species has not been confirmed as yet.

During the selection of material for x-ray study, it was necessary to examine numerous cut plates and fragments of boleite. In general,

the cores of boleite cubes used were found to be only quasi-isotropic.

The quasi-isotropism seemed to be due to fibrous, weakly anisotropic

elements. In some cases, these fibrous areas extended along the face diagonals through the center of the crystal, thus dividing the crystal into four triangular sectors (Figure 32). No completely isotropic cores were observed. In thick section, it was found that the aniso­ tropic rims which are coplanar with the cube faces of boleite were not uniformly anisotropic but possessed lenses and elongated areas of different birefringence than the rest of the rim (Figure 33). These areas of different birefringence have their long dimension approxi­ mately parallel to the cube faces. In many cases the anisotropic rims and bands are interru pted , patchy, and vary in thickness along th e ir length (Figure 3U). In thin fragments, however, the anisotropic rims are composed of very thin, alternating anisotropic plates of boleite

(Figure 35). When these are viewed normal to the cube face under conoscopic conditions, one generally obtains a rather well-defined uniaxial negative figure. Occasionally, however, a two to three degree separation of the isogyres may be observed. Very often, furthermore, the interference figure behaves in an anomalous fashion upon rotation of the stages for example, the arms of the figure may rotate about the center of the field or parts of the figure may break up and leave the field while other parts which should also leave the field remain. In addition, one may obtain figures of differing aspects by moving the plate slightly in the plane of the stage. In thick sec­ tion, the anisotropic rim does not seem to extinguish as an entity but the difference is of such a small magnitude that one can not be sure of the non-parallelism of extinction. In thin section, however, where the banding of the anisotropic rim is clearly visible, the non-parallelism of the extinction is quite readily determined. Upon rotation of the XU6

Fig. 32.—Sectors in boleite due to anisotropic areas in boleite. 20x.

Fig. 33.—Anisotropic rim on boleite. Note discontinuous nature and banding in anisotrop ic rim . 20x. 1U7

F ig . 33. Fig* 3li.—Interrupted and alternating anisotropic bands in boleite. 20x.

Fig. 3$.—Fine banding present in thin fragments from the anisotropic rim on boleite. 72x. \o I I v ' i : 1' 1' * * \

Fig. 3U 1$Q stage, the extinction moves as a wave toward the cube edge until parallel extinction is obtained when the cube edge is parallel to the cross hair. The angular deviation appears to exist over a range of about three degrees.

Inasmuch as the optical behavior exhibited by boleite suggests that observed in species which possess a polymorphic inversion, and inasmuch as a color change upon heating had been noted in b o le ite by

Lacroix (189$)> the thermal behavior of boleite was investigated.

Three specimens rf boleite, one from the anisotropic rim and two com­ posed of both the anisotropic and quasi-isotropic portions of the crystal, were used for the heating experiments.

Table 16 gives the results of the heating cycle for the first of the three specimens mentioned above. This specimen, composed entirely of material from the anisotropic rim, was a clear blue and showed a marked but variable birefringence, due to the presence of lens-like areas of higher birefringence than the majority of the specimen. The specimen was heated over the range from room tempera­ ture to approximately 26$°C. at which temperature decomposition began to take place along cracks and edges. During the heating, there was a gradual decrease in birefringence and a concomitant gradual change from blue to green color in plane polarized light.

The second specimen used was a clear blue color also but was composed of both the anisotropic rim and a portion of the quasi- isotropic core adjoining this rim. The specimen was heated over from room temperature to approximately 180°C. The anisotropic rim behaved in much the same way as the first specimen over this range. Heating TABLE 16.—Thermal cycle for b o le ite . lSi

Approximate Observations

Tenoenature

A. Keating Cycle

25>°C. (Room) Larked birefringence

Areas of variable birefringence

Clear blue color

30°C. Distinct boundary between areas of

different birefringence disappeared

130°C. Overall birefringence has decreased

Lenses and bands of different bire­

fringence are disappearing

Grain has taken on greenish cast

lli5°C. Birefringence has decreased markedly

Grain is distinctly green

175°Q. Birefringence is disappearing

Grain is com pletely green

230°C. Isotropism is nearly complete

G rain is aark green

2o3°C. Isotropism is complete

G rain is beginning to decompose along

c r a c k s

B. Cooling Cycle

180°C. Quasi-isotropism persists

Grain has regained a bluish cast

95°C. _ Quas i-isotropism persists

G rain has become blue

25°C.(Room) Q uasi-isotropism rem ains

G rain rem ains blue 152

to 80°C. caused the anisotropism of the core to decrease. The color

in plane p olarized lig h t became an even c le a r e r blue and the t o t a l

amount of light transmitted by the specimen increased. From 80°G. to

180°C., the distinction between the rim and core gradually disappeared

and the whole specimen became green in plane p olarized lig h t . Upon

cooling, the grain again regained its blue color.

The third specimen, also composed of the anisotropic rim and

adjoining quasi-isotropic core from a cube of boleite, was somewhat

thinner than the two other specimens and, as a result, possessed a

distinct greenish blue color. Again the behavior of the anisotropic

rim was identical to the behavior of the first specimen. The core,

however, developed two sets of small, needle-like birefringent areas.

One set was parallel to the cube edge and the other sot was at U5° to the first set. Both extinguished parallel to their elongation.

These birefringent areas developed at approximately 80°C. along with

a distinct decrease in birefringence of the rest of the core and a

simultaneous deepening of the blue color. From 80°C. to 180°C», the

behavior of the core was much the same as that of the core of the

second specimen.

An x-ray powder pattern was obtained for both the anisotropic and

quasi-isotropic zones of boleite. Visual comparison of these patterns

indicates that there is an exact correspondence of both patterns with respect both to position and intensity of the lines over the whole

range 0°-l80° 20 (Figures 36 and 37). No evidence of line shift is

present in the back reflection area. 153

Fig* 3 6 .—Powder d iffr a c tio n pattern of iso tr o p ic b o leite (OSU collections).

Fig. 37.—Powder diffraction pattern of anisotropic boleite (OSU collections). V 155 An indexed powder pattern for boleite is given in Table 17. The indexing was done on the basis of the parameters and space group given by Ito (1950), since the cell dimension and space group suggested by this study are in doubt. The spacings obtained in the present study are compared against the only other pattern given in the literature for boleite (Waldo, 1935). The extra lines reported as a result of the present investigation appear to be valid lines since they can be in­ dexed on Ito's cell, which has a simple relation to all other possible c e lls considered. In the event a new c e l l i s shown to be the tru e c e l l of boleite, a simple transform w ill exist.

No verification of the synthesis of boleite reported by Friedel

(189U) was attempted in the early part of this study, as it appeared that boleite was a well-characterized species and would only be dealt with in a summary manner. When it became obvious, however, that the species might yet need considerable work and that there was some doubt as to the validity of the data given for boleite by Ito (1950), synthesis was attempted and appears to have been successful. The synthetic species produced is closely related if not identical to na­ tural boleite.

Although Friedel (1892) had reported a synthesis of boleite, the experiment required many months and apparently yielded small crystals.

Consequently, hydrothermal methods were employed in this study with the result that crystals of good quality were obtained. Data on the synthesis of boleite is given in the chapter can synthesis below.

The crystals of synthetic boleite differ markedly from those of natural boleite in color. These synthetic crystals range from a deep 156 157 j

• ( 1 3 - r> - - .* ! \ 1935) ^ ...... :.oo - C1 J do,UvC '..‘aC.v

7 r„ c-.L. u 9 Cl . . j cj u r .. 6 . ■.;.:un.d /«—. g* 0 -. _ v.1 L/ ^ '-J. j. d T c X d c

3*2-13") [3 .1 U 1 r \ i c , ! _L * O • _L J y -i ~i "'i *7 ! \, G -lu O O /- * j. 0 0 0 . l o 1 ^ * --- O 3 .1 1 2 »JLU I 3 .1 1 625 J 1 3 .1 1 o

1. O : . ' - 0 9 c,'' <- « 1 !' X o — r '. 6 0 2 . g_> J' j ^ o il « /■ # "l - <, / o 0 •' ICO 2 .6 9 2 <>/ j 10 r> ' 9 - o o <1 •’ *t CO 9 20 - 2 o670

20 r ‘ '' 7 2 „. ! ) ICO 2 .3 3 ... c j < < _ ri -> /■' ■ 7 1 0 0 ^ > i-

... i. 5 O

GG 2 .1 5 - -o 4. /% *• , 9 t 20 2 .1 2 2 .0 ? > o 6 1 -• o 53 •;< O r? T i ' ’ ' . .. J 1 .9 9 7 — © )•' ■- 0 o a ' - r 1 „ VOU 1 20 1 *20 -9 - ‘-i' X fc» . 'J‘ f , 0 i r- 9 “• -- : O J x o j Co 1 .0 0 fr 1 O' ) 2.0*/;! 2 < 2. 1.7'- 5 60 x • / g X. © ^ - .1 0 "> 9"-' “j O - ~ 9

1 lex-; O liO 1 .6 ? .0 1-677 I*iV7d’. 1 .6 7 2 ~ 1 o V O 1 1 .6 1 3 20 1 . 6 1 -i o O J / 9 Xod09 158 159 160 l6l emerald green to a clear grass green depending on the thickness of the adamantine luster on the cube face as a result of nearly complete reflection of the light from the cube face. In other positions where light passes into the crystal and is reflected, the crystals show a mottled green glassy appearance. Thin, very small crystals are leek green in transmitted light, the color becoming grass green or deep green with only a very slight increase in thickness.

The morphology of synthetic boleite is either that of a simple cube or that shown in Figure 26 for synthetic pseudoboleite and is essentially that indicated by Friedel (1906) for crystals of boleite composed entirely of quasi-isotropic boleite. As is apparent from

Figure 26, the synthetic boleite crystals are dominated by the faces of the pseudocube {l003 and modified by small faces of the pseudo­ dodecahedron £ll0J and pseudo-octahedron ^111$. No other forms were observed on any of the synthetic crystals. Although Figure 26 shows an idealized synthetic crystal, in reality one or more of the pseudo­ dodecahedron faces may be entirely absent. The pseudo-octahedral faces were, in general, too small to provide accurate measurements of the angle (100) to (111). As a result, the precision of measurement is insufficient to substantiate the measurements of Friedel for this form.

The form which Friedel denotes, -[llUf, corresponds to {lll^ 011 the synthetic crystals. Considering the accuracy of the measurements possi­ ble on the synthetic crystals, there is nothing to indicate that they are other than cubic. It should be noted that the value given by

Friedel for (111*) to (001) would appear to be significantly different from the value for the octahedron to cube were it not for the fact 162 that the cleavage used for the measurement is the poorer of the two poor cleavages of the species. An optical investigation of the synthetic crystals indicates that they are isotropic. An x-ray powder photograph obtained from synthetic boleite shows correspondence in intensity and position of the lines to those obtained from natural boleite within the precision of this method. The d-spacings of syn­ thetic boleite are tabulated with those of natural boleite in Table

17* A powder pattern for synthetic boleite is given in Figure 38*

Comparison with a pattern for natural boleite (Figure 39) suffices to indicate that synthetic boleite obtained during this stucfy is identi­ cal with natural boleite. There seems to be no doubt that these crystals are indeed analogous to natural boleite at least to the quasi-isotropic portion of natural boleite crystals.

The optical investigation of natural quasi-isotropic boleite and the thermal behavior of both quasi-isotropic and anisotropic boleite indicate that boleite undergoes a polymorphic inversion from a tetra­ gonal low temperature form to an isometric high temperature form. The morphology of some natural boleite and the color, optical character and morphology of synthetic boleite substantiate the contention that high boleite is isometric. In a dry system, the transformation to the high temperature form seems to occur in the range 20° to 265°C., the upper limit of stability of the high temperature form being approximately

265°C. Hydrothermal synthesis experiments lead to the conclusion that the stability of boleite under confining pressures of the order of

750-100 p .s.i. extends to at least 275°C. The polymorphic inversion may be an order-disorder transformation with respect to silver. More Fig. 38.-—Powder diffraction pattern of synthetic boleite obtained in a hydrothermal bomb at 270°C. from stoichiometric proportions for boleite.

F ig . 3 9 .—Powder d iffr a c tio n pattern o f natural b o le ite (OSU collections). H £ 165 w ill be said on the subject of this order-disorder transformation in the next chapter which deals with the boleite problem.

The inability to verify the 60% spacing in anisotropic boleite as a whole is somewhat puzzling inasmuch as the specimens used showed the characteristic anisotropism and optical character which one would expect to find associated with the tetragonal form. The single case o in which a 60A spacing has been observed in this study, a case in which the material yielding the spacing was in close association with pseudo- boleite, deserves further discussion and will be dealt with in the next chapter. i The banding observed in anisotropic boleite as well as the anoma­ lous behavior of the interference figures obtained from anisotropic boleite suggests parallel or sub-parallel plates of one species on another, or of one species on itself with the alternating plates having a different orientation or different optical character from one another. Chemical evidence given by Friedel (1906) indicating that the bulk composition of the anisotropic rim is the same as that of the isotropic core, is inconclusive. X-ray powder evidence tends to support the latter explanation. It is to be hoped that subsequent examination of anisotropic boleite by means of electron diffraction and the electron probe w ill shed further light on the subject. VII. THE BOLEITE PROBLEM

As has been pointed out in the discussion of boleite and pseudo- boleite, there exist quite conflicting opinions in the literature regarding the validity of pseudoboleite and the symmetry of boleite.

The results of this study verify the conclusions of Friedel (1906,

1930) and Hocart (1930, 193U) that pseudoboleite is a valid species and support the evidence advanced by these authors as well as by Ito

(1950) concerning the presence in the boleite group of a species with a spacing of 6O.8A in the direction. On the other hand, investi gations of the apparently homogeneous, anisotropic rim of boleite, which should exhibit a 6O.8A spacing along 0 » ij according to Hocart

. o and Ito, actually yields a 15.UA spacing in this direction. Gossner and Arm (1929) found l5.UA for the direction [ooij in boleite but

Friedel (1930) attritwted this observation to the fact that they had used the miroetically twinned (isotropic to quasi-isotropic) part of boleite and insufficient exposure time for their x-ray determinations.

Such a criticism is not applicable to the results of the present study inasmuch as the specimens were clearly anisotropic, gave a uniaxial negative figure and were exposed to a minimum of ll+ hours to Cu, * p C radiation from a copper tube run at 35kV. and 20mA. Hocart obtained his results ty the use of nickel-filtered copper radiation from a tube run at 35kV. and 15 mA. with an exposure time of only 25 minutes.

166 167 Furthermore, during the course of this study, a spacing of 21.6$ was

obtained in the direction [ l l c T j for plates from the apparently com- o pletely anisotropic rim of boleite. Oossner and Arm found 21.7A for

this direction but Hocart and Ito apparently did not attempt to obtain

the cell dimension in this direction. From the standpoint of the x-ray

data alone, there still seems to be a problem in the boleite group.

Optical examinations of boleite performed during the present

study as well as Friedel's (1906) and Larsen's (1921) observations on

the variable birefringence in boleite indicate that the apparently

homogeneous, anisotropic rim is a composite substance made up of two

different alternating orientations of the same species or alternating

intergrowths of one species with another. In view of the small scale

banding observed during the course of this study, a banding not

reported by previous authors, the value of the birefringence given for

boleite by Friedel (1906) is probably an average value rather than the true value for the anisotropic bands within the rim of boleite. It

is worth noting that the value for the birefringence of pseudoboleite

(0.032) given by Friedel (1906) lies well within the variation of the

birefringence of boleite as derived from the index of refraction

measurements given by Larsen (1921). If the banding is the result

of lamellar intergrowth of boleite with pseudoboleite, one would ex­

pect the presence of pseudoboleite to show up directly on the rotation

photographs obtained so far for boleite. No such evidence has yet been

obtained.

Although Friedel (1906) indicated that the cleavages shown by the

anisotropic rims on boleite were interrupted but of sufficiently good 168 q u a lity to give r e lia b le crystallograp h ic measurements, Hadding (1919) found that these cleavages were quite interrupted, generally formed bands of light rather than distinct reflections and were of poor quali­ ty. Hadding's observations have been verified during the course of this study. Indeed, interrupted cleavages and band-like reflections are exactly what one would expect to find in a composite substance in which the cleavage of one individual terminates against a second individual or a poorer cleavage of the second individual.

Thermal experiments on the rim as well as the quasi-isotropic core of boleite indicate that, at elevated temperatures boleite becomes a homogeneous, green, isotropic phase. Synthetic studies support the conclusion that high boleite is a green, isotropic phase since syn­ thetic crystals of boleite formed at 270°C. are green, possess an isometric habit and are isotropic. Thermal and synthesis experiments strongly point to a polymorphic inversion in boleite.

At this point, it i3 worthwhile to recall the green color and isometric appearance exhibited by most synthetic crystals of pseudo­ boleite at or above 170°C. The color and crystal form of this pseudo­ boleite are thu3 identical to those fcund for synthetic boleite formed at 270°C. Inasmuch as low temperature natural and synthetic pseudo­ boleite are blue and inasmuch as low temperature pseudoboleite has been

3hown to be tetragonal pseudo-isometric, there is good evidence that a polymorphic inversion to a green, isometric phase also takes place with increasing temperature in pseudoboleite. High pseudoboleite and high boleite seem, therefore, to be isomorphous, both being isotropic, green, and similar in . On cooling, high pseudoboleite inverts 169 to an aggregate of anisotropic lamellae, but the green color persists.

High boleite inverts on cooling on the microscope hot stage to a blue phase, which remains isotropic. On the other hand, high boleite synthesized under hydrothermal conditions, remains green and isotropic at room tem perature, These phenomena cannot be fu lly explained at this time. It is clear, however, that both species undergo polymorphic inversion, although the reversibility of this inversion seems to be conditioned by many factors. Further, the two species seem to become more alike at high temperature. Apparently they disorder to the 15$ cubic substructure under suitable conditions.

Unfortunately, trustworthy chemical analyses for boleite and pseudoboleite are lacking. Nevertheless, the chemical information available indicates that boleite and pseudoboleite have quite similar chemical compositions. Furthermore, qualitative tests on "percylite", which w ill be shown to be b o le ite , pseud ob oleite, or b o le ite plus pseudoboleite, and on fragments from overgrowths of pseudoboleite, indicates that pseudoboleite may contain some silver and that there may be some variation in the silver content of boleite. In view of the fact that the ionic radii for Ag* and Pb”,■■,■ (1.26X and 1.20&, respectively) are within $% of one another, it would be possible for a complete solid solution with respect to silver for lead to extend from boleite to pseudoboleite. According to Ito's structure for boleite, some of the silver occupies definite sites within the structure while the remaining silver substitutes in a random way for lead.

An examination of Friedel's fornula indicates that diadochy of

Ag+ for Pb++ would probably be compensated for by a coupled omission 170

of CX“. Therefore, any Ag containing member of the solid solution

series formed would be defective with respect to chlorine if this

assumption with respect to chlorine is correct. The ideal formula

for boleite would be x^CugC^j^OHj-^yl^O. A general formula for the

solid solution would be (Pbi 2-x^x^Cu8(^12U-x( 16*y^2°• With x=3 (and y"2), one obtains Friedel's formula for boleite. It is rather

interesting to note that, if x=l (and y=2), one obtains the formula

(PbQjAg^)CugCl23(0H)i6*2H20. If one considers the silver reported

in Friedel1s (1906) analysis of pseudoboleite to be essential rather

than an impurity, one obtains Pb^QAgiCugCXl^tOH^^KkO, a formula which is quite near that indicated for the solid solution member in which x»l. It may be worthwhile to recall the fact that the analysis fo r p seudoboleite was considered ty F ried el him self to be somewhat

inexact. Confirmation of the theory that such a defect solid solution

exists between boleite and pseudoboleite must await a more reliable

chemical analysis of this species.

The observations summarized above are conclusive in some respects but inconclusive in others. It does seem, however, that the aniso­ tropic rim on boleite is really a composite substance and that there

may be a partial or complete solid solution between boleite and pseudo­

boleite together with polymorphic inversion in each of the end members.

Whether the anisotropic rim is composed of one species in two different orientations or two distinct species has yet to be determined.

It is interesting to note, however, that a consideration of the epi- tactic relationships common in the boleite group together with the

optical orientation and concomitant structural parameters involved 171 w ill show that alternating layers of boleite with the parameters deter­ mined in this study would yield x-ray rotation photographs exactly like those of a s in g le c r y s ta l. As w ill be shown in a d iscu ssion o f ep i­ taxy, one species emplaced on another or on itself in a different orientation, w ill be oriented so that similar parameters in the over­ growth are parallel to similar parameters of the substrate. In boleite, the optical orientation for plates normal to the banding is such that the optic axis must alternately be normal to the section and lie in the plane of the section. If one assumes an epitactic relation-

.0 ship between the parameters of alternate band3, this means the 15A o spacing of one individual would be emplaced parallel to the l£A spacing of the next. Furthermore, as the direction at k $ ° to the crystallo- graphic axes in each of the axial planes is 21$ for one individual, the

211 spacings in adjacent bands w ill also be parallel. In order to account for the banding in the rim of boleite, however, the [OOl] direction of one species mist be parallel to the JjLloJ direction of the next. In such a case, the composite nature of the crystal would be obvious optically, but pass unnoticed in an x-ray investigation.

Despite the explanation just given, it would s till be possible for the rim on boleite to be made up of alternating layers of two phases if the phases possessed the correct parameter relationships.

Thus the anisotropic part of the rim might be low boleite as determined .0 in this study, if the isotropic part were high boleite with aQ= 15A.

In this case, the direction [lio] would have a d-spacing very nearly one half that of low boleite and the presence of this smaller para­ meter would be concealed by the layer lines due to the larger parameter. 172

There seems to be some evidence th a t there i s a p a r tia l or complete solid solution in boleite. It seems most probable that this occurs only at high temperatures and that the solid solution is one in which silver substitutes for lead. Such a substitution would re- quire the presence of negative-ion vacancies in the structure. At lower temperatures, however, the species undergo polymorphic inver­ sions. In the case of boleite, this inversion may involve only a re-ordering of structural units th a t the parameter in this direction is changed but the parameters in the a - and c-d ir e c tio n s remain the same as in th e high temperature form.

If, however, there is an ordering of the silver with respect to the lead in the low temperature form, the parameter in this direction of

. o ordering would increase and might give rise to a species with a 60A spacing such as has been observed in this study between boleite and pseudoboleite. In the case of pseudoboleite, the inversion seems to involve a reorientation of structural units in the high form such as to give rise to a 30.8$ spacing in the [OOlJ direction in the low form. V III . PERCYLITE

H isto rica l Summary

Percylite is the most ill-defined of the species included in the boleite group. Paucity and the exceptionally fine-grained nature of most specimens are probably the two factors most responsible for the uncertainty associated with percylite and have tended to compound the uncertainty which remained in the literature subsequent to the o r ig in a l d escrip tion try Brooke (l8 £ 0 ). The d escrip tion ly Brooke

(1850) and the accompanying analysis performed try Percy (in Brooke,

1850) on impure material left the impression that the species was a blue, isotropic, silver-free copper-lead oxychloride. Although Percy had noted the presence of silver and appears to have considered it essential, he did not include it in the formula of the species, tfhen

Brooke (in Phillips-Brooke and Miller, 18£2) published a description of the species as part of a descriptive mineralogy text, he amended the original description with crystallographic data which indicated that the isometric forms {lOO^, {lioj, {ill} and {^loj were present on percylite, but once again did not note the presence of silver in the species. It is, therefore, not surprising that percylite was accepted as a silver-free copper-lead oxychloride by subsequent investigators. It is noteworthy that among the qualitative tests given by Percy is a statement that percylite becomes emerald green

173 when, heated, becoming blue again whan the specimen is cooled. This

qualitative test was verified by Maskelyne and Flight (1872) on a

specimen of percylite from South Africa. Quantitative analyses of

percylite subsequent to the analysis made by Percy (Maskelyne and

Flight, 1872j Raimondi, 1878; Websky, 1886) were likewise made on

impure material and, since the species was generally associated with

other silver-bearing minerals, the presence of silver as an essential

constituent of percylite would have gone unnoticed. Raimondi (1878),

however, appears to have noted the presence of silver in percylite

from Peru and described the species he observed as argentiferous

percylite. Although Websky (1886) reported the presence of water in

percylite, other authors (Sandberger, 1886 and Fletcher, I889) were not able to verify this observation. Maskelyne and Flight, Sandberger,

and Fletcher reported that the percylite which they described was op­ tically isotropic. Fletcher also reported the isometric forms ^100^,

{llO j and £lll$ on the crystals which he studied. Mallard and Cumenge

(1891 a,b) noted the chemical similarity of boleite and percylite

(Brooke, 1850) but noted also that boleite possessed silver whereas

percylite had been regarded as a silver-free species. Friedel (1892)

reported a synthesis of percylite but later (Friedel, I89U) indi­

cated that the crystals obtained were probably cumengeite. Mallard's

(1893) application of the name percylite to the silver-bearing

anisotropic rim of boleite was an incorrect use of the name as

pointed out by Lacroix (1895). Lacroix (1895) applied the name

percylite to those cubo-octahedral crystals of boleite from boleite which were isotropic or nearly so because he found that these crystals possessed the crystallographic and optical characteristics of crystals of percylite from the Buena Esperanza Mine, District of Challacollo,

Province of Atacama, Chile. In view of the results obtained in this study to be discussed below, and F r ie d e l's (1906) work on b o le ite , there seems to be little doubt that the Chilean specimens used by

Lacroix were boleite. Larsen (1921) studied two specimens of percy­ lite . One of the specimens was isotropic and possessed an index of refraction of 2.06 v 0.01. The second was uniaxial negative and pos­ sessed a moderate birefringence. He considered this second specimen to be partly altered to boleite. Seeliger (1950) identified blue, isometric crystals from the Ruhr as percylite. These crystals pos­ sessed the isometric forms {lOOJ and^lioj, were isotropic and had an index of refraction of about 2.05. The only powder data given for percylite is for a specimen from an unknown locality in Chile (Sabina and T r a ill, I9 6 0 ).

Results of the Present Study

As the summary above indicates, the literature on percylite not only contains numerous discrepancies but casts a great deal of doubt on the validity of percylite in view of what has been said about boleite and pseudoboleite. Type material was unavailable at the ini­ tiation of the study, so a very fine-grained specimen of percylite from Sierra Gorda, Chile (USNM R9116) was examined optical.ly and ty means of x-rays. A petrographic examination of crushed fragments indicated that the material was distinctly anisotropic and possessed a moderate birefringence. Diligent examination of the fine-grained material produced a minute single crystal which possessed the 176

appearance of an unmodified cube. A rotation photograph produced

normal to one of the cube faces yielded a 3oX spacing, a spacing

consistent with only one member of the boleite group: pseudoboleite.

A powder photograph produced from the fine-grained material

yielded a powder pattern which, upon visual comparison, was found to

be identical with respect to line position and intensity to that of

pseudoboleite. Subsequent measurement of this powder pattern of

percylite yielded d-spacings and relative intensities equal within

the limits of error to those of pseudoboleite.

In order to determine the presence or absence of silver in this

specimen of percylite, a microchemical test for silver was performed

first on a specimen of typical boleite and then on a specimen of the

percylite of comparable size. A qualitative comparison of the results

of the test, indicated that while boleite immediately formed a massive,

curdy white precipitate of considerable bulk which gradually became

violet on exposure to light, the percylite dissolved completely and

yielded only a few scattered cubes and octahedra. From a qualitative

standpoint, this specimen of percylite seems to have not more than 1%

silver present; possibly there is considerably less than 1 % p resen t.

A type specimen of percylite (BM 89065) was kindly provided for

examination by Dr. Embrey of the British i&iseum. The specimen was

sm all but undoubtedly th e same m aterial stu d ied by Brooke and Percy

(1850) and consisted of small blue crystals and crystal aggregates in

association with gold in a predominantly hemitatic groundmass. A

binocular examination immediately revealed the fact that, while there were isolated crystals with the cubic form described by Brooke (1850), 177 the majority of the crystals formed aggregates, the individual crystals of which displayed the characteristic overgrowths of pseudoboleite on boleite (Figure 1*0). The specimen as a whole was mounted for crystallo- graphic measurement and the angle from the pinacoid to the dipyramid was measured fo r a number of th e se overgrowths. The angle between the pinacoid and dipyramid was found to be 63°32' + 0°35' (average of five measurements). Friedel (1906) gives 63°i*2• for the angle between the base and dominant dipyramid on pseudoboleite. On a crystallographic basis alone, there can be little doubt that pseudoboleite is present on the type material of percylite.

An isolated, very small, cubo-octahedral crystal was removed from th e groundmass and mounted fo r goniom etric measurement. Although i t was possible to make only crude measurements of the angle between the apparent cube and octahedron faces on this crystal, this angle was found to be IUi^ O 1 + 0°19'» On the basis of this angle, and on the appearance of these isolated crystals, it seems certain that the crystals are identical to the pseudocubic crystals described ty Friedel

(1906) and analogous to the green boleite crystals which have been syn th esized as a part o f th is stucfy-.

Inasmuch as the specimen had to be returned to the British

Museum in as undisturbed a condition as possible, it was not possible during the course of this stucty- to make a powder pattern from selec­ ted material. It was possible, however, to detach enough composite material for powder work. A visual comparison of the powder pattern from this composite sample with standard boleite and pseudoboleite patterns for which data have been given above is sufficient to 178

Fig. UO.—Type percylite (BM 89065) showing pseudoboleite over­ growths on boleite (lower right) associated with gold (lower left). 20x. 179

F ig . UO. 180 indicate that the majority of the sample is boleite but that weaker low 29 lines for pseudoboleite are also present.

As percylite has been described from several localities, speci­ mens from a number of th ese lo c a lit ie s were obtained. Powder patterns were obtained from these specimens and were found to yield either a boleite or a pseudoboleite or a composite boleite and pseudoboleite pattern (Figures Ul to U5). The results of x-ray and ndcrochemical investigations of several specimens of percylite are summarized in

Table 18. A comparison o f the powder data given by Sabina and T r a ill

(I960) for percylite with that obtained for boleite and pseudoboleite in this study indicates that the percylite specimen used ty these authors is largely boleite although some pseudoboleite is probably present also.

Except for the BM 89065, mLcrochemical tests were performed on each of the percylite specimens included in this study. Although the tests are only of a qualitative nature, there seems to be a noticeable difference between the amount of silver present in each of the specimens which give a boleite pattern and in each of the speci­ mens which yield a pseudoboleite pattern. It should be noted that cerargyrite occurs a t each o f th ese l o c a l i t i e s . Care was taken during each of the chemical tests to use only the purest fragments for these tests. The results of these tests seem to indicate that there is a variability in both boleite and pseudoboleite with respect to the amount of silver which may be present in each of the species.

Some doubt has been cast in the literature on the validity of the locality given for type percylite by Brooke (1850). According to Wilson (1963), native gold has never been observed at Boleo. It Fig. Ul. —Powder diffraction pattern of natural boleite (OSU collections).

Fig. 1*2.—Powder diffraction pattern of percylite (USNk 7897).

F ig . 1*3.—Powder d iffr a c tio n pattern o f pseudoboleite (S a in te- Etienne 27l*9).

Fig. 1*1*.—Powder diffraction pattern of percylite (USNM R-9116).

Fig. 1*5. —Powder diffraction pattern of percylite (H.N. 95812). Fig. u .

F ig . 1*2.

F ig . U3.

F ig . k h .

F ig . U5.

CD PO s

183

TABLE 18. —X-ray and mi croc he ml cal examination of percylite specimens.

Sample X-ray MLcrochemlcal Examination Examination

BM 89065 B o leite and Pseudoboleite Type Specimen

Sonora, Ifexico

USNM R9116 Pseudoboleite Scattered cubes and octahedra of AgCl Rosario Mine Estimated up to 1% AgCl present Sierra Gorde, Chile

USNM R7897 B o leite Large curds, cubes and octahedra of AgCl Mina Beatriz Maximum AgCl may be present Sierra Gorda, Chile

USNM R1332 B o leite Large cubes and octahedra of AgCl Broken H ill Mine Estimated or more AgCl New South Wales, present A ustralia

Harvard No.9^812 Pseudoboleite Small octahedra and cubes and one tetrahedron of AgCl C hile (?) Estimated up to yf> AgCl present 18U would seem, then, that the locality given by Brooke (1850) is valid and that this locality has never been adequately described.

As the results given above clearly indicate, percylite is synono- mous with boleite plus pseudoboleite and as such is clearly an invalid species as one name cannot be used for two distinct species. Further­ more, the original species to which the name percylite was given was inadequately and incorrectly described and it seems best to drop the name completely from the literature. IX. EPITAXY IN THE BOLEITE GROUP

The epitaxy which exists between members of the boleite group is exceptionally perfect and forms a classic example of this pheno­ menon in minerals. The oriented overgrowths of one species on another occur in a plane which includes the principal symmetry elements pre­ sent in each of the species involved. In the case of pseudoboleite on boleite, the composite individual has the appearance of a single crystal (Figure U6). In the case of cumengeite, however, the composite c r y sta l appears to be a penetration twin (Figure 1*7). Thus, Brooke

(1852) and Mallard and Cumenge (1891 a,b) missed the presence of pseudoboleite on boleite because of the pseudotetrahexahedral faces formed ty the dipyranndal faces of adjacent pseudoboleite overgrowths on the pseudocube faces of the underlying boleite. Because of the high symmetry possessed ty similar lattice directions in the com­ position plane of cumengeite on boleite, Mallard and Cumenge (1891 a,b) also failed to recognize cumengeite as distinct from boleite.

It seems quite possible from this study that epitaxy or related phenomena may s till be causing trouble with respect to boleite and pseudoboleite.

As a result of his investigations, Friedel (1906,1926) was able to show ty the use of topical axial ratios that the absolute para­ meters in the plane (001) were very nearly identical for the members

185 Fig. U6.—Pseudoboleite overgrowths on boleite (modified after Friedel, 1906).

Fig. 1*7.—Cumengeite overgrowths on boleite (Hadding, 1919) \

187

F ig . 1*6

F ig . 1*7. 188 of the boleite group. Friedel's further observation (1906) that, inas­ much as the topical parameters in the direction £00l] for boleite and pseudoboleite were very nearly related to one another ty a factor of two, the two species could form complete envelopes of one on another, may deserve much more consideration than has previously been given to it. With the development of Royer's (1930) theoiy of epitaxy and

Hocart's (1930) x-ray measurements, the theory advanced by Friedel for epitaxy in the boleite group was completely substantiated.

According to Royer (1930), there are three factors which control epitaxy or the formation of oriented overgrowths of one species on another: 1) l a t t ic e spacings in the plane of contact must be iden­ tical to or simple rational multiples of one another for each of the species involved; 2) ionic charge relationships must be maintained for each species; and 3) bonding in each of the species must be the same. Though th e la s t of th ese fa cto rs has been shown to be unimpor­ tant, and exceptions to the first two have been observed, particularly in vapor phase deposition, metals, and alleys, the first two rules seem to apply very well to nearly all mineral examples of the phenomenon.

Indeed, these first two rules seem to be corollaries which could have as well been derived from Pauling's rules (Hurlbut, 1959)* Royer also indicated that unless the parameters in the composition plane of the individual involved were within 12$ of one another, epitaxy would not take place. He also noted that the usual limit was 7% and most commonly the difference was considerably less than 7$. Although striking exceptions have been noted to these tolerance limits given by Rpyer, these limits seem to be realistic with respect to minerals. Observations made during the course of the present study support

Friedel's statement that epitaxy very commonly takes place in the

(001) plane among members of the boleite group. A comparison of the cell dimensions obtained in this study and summarized in Table 19 indicates that the parameters in this plane for all three of the species are very nearly the same. The difference in the parameters is well within the upper limit for this difference given by Royer.

TABLE 19.—“Comparison of parameters for members of the boleite group determined in this study*

B o leite Pseudoboleite Cumengeite

30.82# co 15.26A 2U.149A ao 2 1 .6U# 21.57A 21.37A

- . 0 d110 15.2UA 15.29A 15.10A

As is shown in Table 19, the parameter cQ of pseudoboleite is very nearly twice that of boleite as determined in this study* Inas­ much as a ll the parameters of boleite and pseudoboleite fal 1 within the requirement of the first rule given by Rcyer for epitaxy, it would not be surprising to find envelopes of one of these species on the other. Friedel (1906) actually observed, in very rare cases, such envelopes of boleite on pseudoboleite. The very rarity of such envelopes as well as other considerations make it likely that the com­ position of the solutions was such as to prevent extensive formation 190 of boleite envelopes on pseudoboleite. The relationships exhibited by both pseudoboleite and cumengeite on boleite indicate that the crystallization of boleite was essentially complete before either of the other species began to form as epitactic overgrowths.

What is of great importance, however, in the light of the results obtained during this study, is that an alternating formation of such envelopes or even a merging of separate overgrowths could lead to the banding observed in the rim of boleite. In a sense, the result would be much the same as that produced by compositional zoning, except for the fact that the orientation of the substrate would play a considerably greater role in the formation of the composite crystal. In view of the temperature relationships postulated for boleite and pseudoboleite, it is possible as well that exsolution, a special case of epitaxy, might account for the relationships observed in the rim of boleite.

It is hoped that subsequent electron diffraction and electron probe investigations -will indicate more clearly the actual relationships which exist in the boleite group as a whole.

It should be noted that even if the cell dimensions for boleite are correct, the parameters in the (001) plane with respect to pseudo­ boleite and cumengeite and in the [001] direction with respect to pseudoboleite would s till lead to the formation of the epitaxial overgrowths observed in the boleite group.

Seifert (in Gomer et al., 1952), who has given a review of the recent work on epitaxy, notes that slight but constant angular dis­ crepancies between similar crystallograph!c directions in the composi­ tion planes of the epitactically related individuals have been re- 191 ported. Such angular discrepancies were noted in the present study on a set of Weissenberg photographs for pseudoboleite taken with the axis of rotation Some of the Weissenberg photographs showed an extra central lattice line which possessed a spacing equal to that found for cumengeite. However, as all of these members of the boleite group may be made up of sheets with nearly the same dimensions in (001), this extra lattice line could as well belong to boleite. A measurement of the angle between the central lattice line for pseudoboleite and the second species gave 3°00' ♦ 0°06' which is, therefore, the angle of misorientation between the axial directions in' the composition (001) for cumengeite or boleite on pseudoboleite. As

Seifert points out, no structural explanation has yet been proposed for this type of a mLsorientation. X. DIA BOLEITE

H istorical Summary

D iab oleite was o r ig in a lly described by Sp>encer and Mountain

(1923) associated with chloroxiphite and in the Mendip H ills,

Eiigland. Only poor quality material was available for examination tut these authors were able to obtain a reliable chemical analysis which indicated that diaboleite differed significantly from percylite and members of the boleite group. They also reported incomplete physical, crystallograph!c, chemical and optical data for diaboleite. Observa­ tions on a limited number of specimens indicated that, although dia­ boleite formed individual crystals, it was commonly observed as a fin e-g ra in ed , o fter pseudomorphous a lte ra tio n product a fter chloro­ x ip h ite .

Palache et a l. (19U1) performed a complete restudy on small but excellent specimens of diaboleite from Tiger, Arizona. As a result of their study they were able to give complete crystallographic, optical, chemical and x-ray data for the species. Although these authors did not perform a detailed paragenetic study, they indicated that diaboleite had apparently formed at a late stage since it was often found in drusy quartz cavities and in association with late- stage minerals. They also noted that diaboleite was often implanted on boleite and pseudoboleite. (?)

192 193

^ystrom and Wilhelnd (1950) verified the work of Palache et a l. and proposed a structure for diaboleite. They also gave some single crystal data from which a powder pattern has been compiled (ASTM

5-0220). No complete powder pattern compiled from experimental data is to be found in the literature.

No synthesis of the species has been reported and no experimental data is available on the stability of the species.

Results of the Present Study

Three specimens of d ia b o le ite (H.N. 101*521, ROM M2l*959> and USNM

111*577) from Mammoth Mine, Tiger, Arizona were available for x-ray powder diffraction examination. The patterns obtained from these samples matched one another with respect to line position and inten­ sity. Powder diffraction data, indexed on the basis of the cell dimensions of Ejystrom and Wllhelmi, are given for diaboleite in Table

20. Extrapolated values for aQ and cQ obtained from the powder data are compared in Table 21 with the values obtained by ^ystrom and

Wilhelmi and Palache et a l .

It has been possible to synthesize diaboleite at several tempera­ tures in the range from 20°G. to 100°C. during the course of this study.

Although no synthesis experiments were performed for diaboleite in the range from 100°C. to 170°C., synthesis attempts at 170°C. and above yielded cumengeite instead of diaboleite.

Synthetic diaboleite is much simpler morphologically than the crystals described by Palache et a l. (19l*l). Although the dominant forms present on the synthetic crystals give excellent reflections, the crystals were too small to allow accurate measurement. The best TABLE 20.—X-ray powder data for diaboleite* 19k

Mammoth Mine Tiger, Arizona Synthetic** USNM 111577 Measured Measured C alculated0 hkl I d I d d

100 ' 3 5.799 3 5.770 5.870 001 10 5.507 10 5.k96 5.k9k 110 7 k .ik 3 7 1|..08U k .i5 i 101 3 3.991 3 3.967 k .O ll 111 9 3.305 9 3.282 3.312 200 8 2.929 8 2.910 2.935 002 2.737 5 2.72k 2.7k7 210 1 2.615 1 2.609 2.625 202 9 2.580 9 2.572 2.589 102 3 2.U77 3 2.k66 2.k88 211 2 2.358 1 2.351 2.369 112 10 2.283 10 2.279 2.291 220 8 2.070 8 2.063 2.075 202 7 2.001 7 1.996 2.006 300 < « 1 1.970 « 1 1.969 1.957 221 7 1.936 7 1.931 1.9k2 212 2 1.893 2 1.892 1.898 3107 fl.8 5 6 1.850 301J h 1.853 k ll.8 k 3 003 6 1.829 6 1.823 1.831 311? J1.759 103J 9 1.755 9 1.7k9 U .7k8 113 6 1.67k 6 1.671 1.675 222 U 1.653 h 1.652 1.656 320 * 1 1.627 <■ 1 1.623 1.628 302 < 1 1.590 < 1 1.590 1.59k 321 1.561 203 7 1.55k 7 1.550 1.55k 312 9 1.537 * 9 1.53k 1.538 213 1.502 Uoo 6 1.U66 6 l.k66 l.k68 k io l.k 2 k koi 5 1.U17 5 1.1*11* l .k l 6 322 < 1 1.398 < 1 1.399 l.k O l 3301 1 1.382 l 1.380 f 1.383 k ill 11.378 ook? C1.37k 7 1.372 7 1.370 223] 11.373 3311 fl.3 k 2 ioU I 5 5 1.338 i 1.337 303 J 11.337 TABLE 20.~(Continued) 195

Mammoth 15. ne Tiger, Arizona Synthetic USNM 111*577 • Measured Measured C alculated hkl I d I d d

1*20 5 1.312 5 1.312 1.313 111*) f 1.301* 313] 7 1.303 7 1.302 11.301* 1*02 1 1.293 1 1.291 1 .2 91* 1*21 5 1.271* 5 1.275 1.277 1*12 < 1 1.265 * l 1.263 1.261* 20i* 5 1.21*1* 5 1.21*2 1.2l*l* 332 5 1.235 5 1.231* 1.236 21U] (1 .2 1 7 l 1.217 l 1.21$ 323] 11.217 1*22 5 1.181* 5 1.183 1.181* $oo,l*30 1.171* 5 io 2 1.150 2 1.150 1.151 5oi,U 31 . 2 1.11*7 2 1.11*7 1.11*8 221*] (1.11*5 3 1.11*5 3 1.11*1* Jj.03 3 11.11*5 5 i i ) (1.127 301* 1* . 1.126 1* 1.126 11.121* 1*133 \ 1.121* 311*? f 1.101* 1* 1.103 1* 1.103 3331 11.101* 00$ • 1.099 $20 1.090 105 7 C l.080 < 1 1.079 < 1 1.077 $02,1*32 J 11.079 5 2 1 ) (1 .0 6 9 5 1.067 1.06$ 1*23) 5 11.067 1 1 5 ) (1 .0 6 2 6 1.062 6 1.061 512 J • 11.062 321* 1 1.051 1 1.01*9 1.050 1*1*0 3 1.037 3 1.037 1.038 20$ 3 1.029 3 1.028 1.029 1*1*1 3 1.019 3 1.019 1.020 2151 (1 .0 1 3 522) 11.013 530 1 1.006 1 1.006 1.007 l*ol* 2 1.003 2 1.003 1.003 531 1* 0.9899 1* 0.9899- 0.9903 i*il* 0.988$ 503,1*33 0.9881* 600 1 0.9779 1 0.9776 0.9781* \ TABLE 20 - (Continued) 196

Mammoth Mine Tiger, Arizona S yn th etic USNM 111*577 Ifeasured Measured C alculated hkl I d I d d

CO. 971*8 331*] 2 513 J 2 0.971*7 0.971*1 10.971*6 (0.9711 2251 2 0.9711 2 0.9710 1*1*2) 10.9708 610 0.9651 601 2 0.9628 2 0.9623 0.9632 305 0.9581 611 0.9505 620 0.91*98 1*21* 3 0.91*87 3 0.91*83 0.91*89 315 0.91*56 532 1* 0.91*51 1* 0.91*1*8 0.91*53 523 0.936? 602 2 0.9211* 2 0.9213 0.9216 51*0 ' • 0.9158 006 0.9157 621 u 0.9150 1* 0.9157 0.9152 325 0.9108 612 0.9105 106 0.901*7 51il 0.901*3 1*1*3 3 0.9025 3 0.9025 0.9028 116 3 0.891*1 3 0.8936 0.891*2 5ol*,l*3l* 0.8921* 5 i1*1 6 f 0.8823 533J 0.8818 6 0.8818 10.8822 U051 (0 .8 7 9 6 6 0.8791* 6 622) 0.8791* 10.8793 630 0.8751 206 3 0.871*1 3 0.87U1 0 . 871a 1*15 0.8699 51*2 ■0.8696 216 0.861*6 631 0.861*2 603 2 0.8628 2 0.8625 0.8630 335 3 0.8601* 3 0.8602 0.8605 521*1 C0.8538 613) I 0.8538 1*25 1* ’ 0.81*25 1* 0.81*25 0.81*26 700 0.8386 226 3 0.8378 3 0.8376 0.8377 632 0.8338 710,550 3 0.8299 3 0.8298 0.8302 197 TABLE 20.— (Oontinued)

Mammoth Mine Tiger, Arizona S yn th etic USNM 111*577 Measured Measured Calculated hkl I d I d d

306 0.8293 701 0.8290 i*i*u (0 .8 2 8 0 8 0.8277 8 0.8278 6233 I 0.8279 316 0.8212 711,551 9 0.8208 9 0.8207 0.8209 51*3 0.8198 61*0 2 0.8137 2 0.8138 0 .8 ll* l 531* 2 0.8116 2 0.8117 0.8120 720 0.8061* 61*1 5 0.8050 5 0.801*9 0.8052 505,1*35 0.8023 702 0.8021 326 0.7981 721 0.7978 601* 1 0.7968 1 0.7966 0.7969 515 7 (0.791*9 10 0.791*6 10 712,552) 0.791*5 i 0.791*7 611*7 f 0.7896 633 J « 1 0.7893 « i 0.7897 10.7896 007 2 0.781*9 2 0.781*8 0.781*9 61*2 1* 0.7801* 1* 0.7801 0.7805 107 0.7780 1*06 3 0.7768 3 0.7766 0.7769

Camera diameter, 111*.6mm. NickeJL-filtered copper radiation, (kily lines for ( A = 1.51*18) reported. Spacings beyond reso­ lution lim it (approximately 68° 20) represent average of oC j end 2 lines. Films corrected for variation in camera diameter and shrinkage. Time: 26 hours for Mammoth specimen; 20 hours for synthetic specimen. Voltage: 35kV. Amperage: 20mA. ^Prepared at room temperature and pressure from stoichiometric proportions ty sloir diffusion of CuClg^I^O into Pb(0H)2» ^Calculated using a0= 5.870 ♦ 0.003^, cQ= 5.1*91* + 0.0032, and space group P j^ (Qrstrom and WilEelmi, 1950). “ 198

measurements gave, according to the notation of Palache et al. (19U1):

(001) (Oil) * U2°52'

(001) (021) = 62°30‘

and gives arc = 1:0.928. Figure U8 shows the morphology of synthetic

diaboleite prepared during this study. The crystals are dominated by

the forms c£6oTj, a{010j, and e {Oll^ but measurements indicate that

small faces belonging to the forms ei{0lTj and s{02TJ are also present.

Development of the forms £boTj, {OlO^ and {OllJ is such as to lengthen

the crystal in the direction of the c-axLs and results in a decidedly

more equidim ensional and hendraorphic appearance than i s exh ib ited by the crystals described by Palache et al. (19lil).

TABLE 21.—Comparison of cell dimensions for diaboleite

Palache et al. Qrstrora & WilhelmL This Study (19U1) (1950) (1963)

•

ao 5.83 ♦ 0.02A 5.870 ^ 0.003A 5.869 + 0.0021

co 5.U6 ♦ 0.02A 5.U9U * 0.003A 5.U95 * 0.003A

An optical investigatiu synthetic diaboleite indicates that

these crystals are identical in optical appearance and character to

natural diaboleite. Synthetic diaboleite is a clear pale blue and yields a uniaxial negative character when viewed parallel to the

c-axis under conoscopic conditions. No evidence of subparallel growth

reported by Berman (in Palache et al. 1951) was observed in the 199

Fig. U8.—Morphology of synthetic diaboleite. 200

*

F ig . U8.

) interference figure of synthetic diaboleite.

Synthetic diaboleite yields x-ray powder patterns whose lines

are identical in position and intensity to those obtained from natural specimens of diaboleite. Table 20 lists the spacings for synthetic diaboleite and compares these spacings to those obtained from natural d ia b o le ite . Powder photographs obtained from 3yntheti diaboleite are compared with a photograph obtained from natural

diaboleite in Figures U9 to $1. 202

Fig. U9.—Powder diffraction pattern of natural diaboleite.

F ig . 5 0 .—Powder diffraction pattern of synthetic diaboleite obtained try Blow diffusion at 25°C. from stoichiom etric proportions for diaboleite.

Fig. 51.—Powder diffraction pattern of synthetic diaboleite obtained try slow diffusion at 25°C. from stoichiometric proportions for chloroxiphite. F ig . U9.

ro o V j J XI. CHLOROXIPHITE

H isto rica l Summary

C hloroxiphite was o r ig in a lly described by Spencer and Mountain

(1923) associated with diaboleite in the Mendip H ills, Ehgland.

Because only poor quality material was available, these authors gave incomplete physical, morphological, optical and chemical data. These data were sufficient to characterize the substance, however, Spencer and Mountain a lso included some data on paragenesis.

Consideration of a limited number of specimens led these authors to the conclusion that chloroxiphite altered to cerussite or hydro- cerussite plus malachite or to diaboleite and hydrocerussite. Lath­ shaped pseudomorphs of the alteration products were found to be in close proximity to the chloroxiphite or to pass along the length of the lath into unaltered chloroxiphite.

Berman (in Palache et a l., 1951) reported x-ray cell dimensions, the space group, and a new determination of the density, the value of which agreed well with the calculated density. He also gave additional optical data for chloroxiphite.

Powder data was subsequently given by Claringtull (ASTM 8-112) for type material. Claringtull's data was indexed by L. G, Berry

(ASTM 8-112) on the basis of the cell dimensions given by Berman

(1951).

20U 205 Axthough the work of previous investigators had served to charac­

terize chloroxiphite rather completely, it seemed desirable to include

chloroxiphite in the study because of its close resemblance chemically to diaboleite and to the members of the boleite group. Furthermore,

it seemed desirable to attempt to synthesize chloroxiphite as, accor­

ding to Frondel (1963), Berman's x-ray work had been done on a minute

crystal of relatively poor quality.

Results of the Present Study

It was possible during the course of this study to synthesize

a yellowish-green cryptocrystalline substance which gives a powder

pattern very similar in some respects but dissimilar in other res­

pects to that obtained from natural specimens of chloroxiphite. It

is noteworthy, however, that the powder patterns obtained in this study

from natural chloroxiphite do not agree entirely among themselves or with the published data for chloroxiphite although only very small,

apparently quite pure fragments were used for the production of each

pattern. During the selection of the material for powder analysis,

chloroxiphite was found to be intimately intergrown even on the

smallest scale with diaboleite and with a clear substance which has not as yet been identified.

On the basis of the observations of Spencer and Mountain (1923) with respect to the alteration of chloroxiphite, it seems possible that the discrepancies between th e powder patterns and perhaps the va ria b le specific gravities obtained for chloroxiphite ty Berman may be due at

least in part to incipient alteration of chloroxiphite. A powder pattern from the most reliable specimen of natural chloroxiphite is compared with the pattern of synthetic chloroxiphite

(?) obtained in this study in Figures 5 2 and 5 3 . Although the general appearance of the patterns is quite similar, there are shifts in the lines, reversals in intensity and evidence of a generally poorer degree of crystallization in the synthetic species. The similarity of the patterns suggests, however, that the synthetic material is closely related to the natural and may differ from it only on the basis of perfection of crystallization.

Information on the synthesis attempts with respect to chloro­ xiphite w ill be found in the chapter on synthesis which follows. 207

Pig. 52.—Powder diffraction pattern for natural chloroxiphite (BM 1957,211).

F ig . 5 3 . —Powder diffraction pattern for synthetic chloroxiphite obtained in a sealed glass tube at 100°C. from stoichiometric pro­ portions for diaboleite.

XII. SYNTHESIS

Introduction

Syntheses of the species involved in this study were originally undertaken in the hope of obtaining pure crystals of a size suitable for subsequent study. It seemed also that these synthesis experiments might provide some information on the stability and paragenetic relationships which exist among these species of very similar chemi­ cal composition.

It soon became apparent that synthesis of suitable specimens of each of the species was going to be a rather lengthy process and it became necessary to use natural specimens for the bulk of the study.

The synthesis experiments were continued and eventually yielded syn­ thetic analogues of cumengeite, pseudoboleite, boleite and diaboleite.

The results obtained for chloroxiphite remain inconclusive.

Experimental Procedure

The reagents used for these experiments were chemically pure or, in the case of Pb(0H)2, were prepared from chemically pure reagents.

To insure that the analyzed composition conformed to the actual composition of the reagent, the identity of each reagent was checked by x-ray diffraction. The lead hydroxide used in this study was pre­ pared by precipitation from lead nitrate and ammonium hydroxide. The identity of the precipitate was also verified by x-ray diffraction.

209 210

As doubt exists concerning the correct chemical formula for all the species except diaboleite, the formulas given by Palache et al.

(1951) were adopted in this study. Table 22 indicates the formulas and reagents used for the synthesis experiments. Stoichiometric proportions were used throughout the synthesis experiments for which results w ill be given in Tables 23 to 26. Two additional experiments, one using stoichiometric proportions and the other using non-stoicbio- metric proportions are discussed in the text.

A qualitative test was made for pH at the beginning and end of each experiment. In every case except one, one of the reagents used was CuCl2*2H20, so the pH at the beginning of each experiment was

2 to 3. In almost all cases, the pH at the end of the experiment was

7 and indicates that almost complete reaction occurred.

All weighings were performed on a Christian Becker Model AB-U chainomatic balance capable of weighing to 0.1 milligram. In some cases, the samples were then ground to insure homogeneity but, in other cases, it was found that layered samples gave better results.

Two types of furnaces were used for synthesis experiments above room temperature. At temperatures from 100°C. to 170°C., a Precision

Model I0I4. thermostated drying oven was used. For the experiments above

170°C., nichrome-wound, tube-type, muffle furnaces controlled by means of a variable transformer were used. To insure a constant voltage to the heating elements of the muffle furnaces, voltage stabilizer transformers were also used in the power-supply circuit. Temperatures were measured by means of a mercury thermometer or a chromel-alumel thermocouple. Temperatures given for the synthesis experiments are 211 average temperatures irith a maximum probable error of ♦ 5°C. As measurements were made near the top of the sample container, the temperatures cited are applicable only to the upper portion of the sample.

TABLE 22. Formulas and reagents used in synthesis experiments

Speci es Formula adopted Reagents ♦ D istilled H20

B o leite Pb9Cu8Ag3Cl21(0H)l 6*2H20 Pb(OH)2

CuCl2 .2H20

PbCl2

AgCl

Cumengeite P^Cu^ClgCOHjg-HgO Pb(OH) 2

CuC12 .2H20

Pseudoboleite Pb^Cu^C^Q (OH)g *2H20 Pb(OH)2

CuC12*2H20

PbCl2

P e rcy lite PbCuCl2 (OH)2 Pb(OH)2

CuG12 «2H20

D iab oleite Pb2CuCl2(OH)ii Pb(OH) 2

CuC12*2H20

Chloroxiphite Pb3Cu02 (0H)?Cl2 Pb(0H) 2

CuC12*2H20

PbO (yellow) 2 1 2

A number of different methods were used in preliminary synthesis attends. Of these methods, only evaporation from hydrous mixtures in open vessels, slow diffusion of hydrous solutions into one another and hydrothermal methods were su c c e ssfu l.

Evaporation from hydrous solutions was accomplished ty mixing the reagents with distilled water, placing the mixture in an open vessel and allowing complete evaporation to take place. The experi­ ments were performed at room temperature (approximately 20°C.) and at 100°C. in a drying oven. A charge of 2-3 grams was used in these experiments, 60 m illiliters of water being added to the room tempera­ ture experiments and 90 m illiliters to the drying oven experiments.

A stratification between the soluble and insoluble salts was produced and the interface thus formed seems to have been a favorable place for the formation of a salt less soluble than the insoluble reagent used.

Evaporation at room temperature required five days while the experi­ ments at 100°C. were complete in two days.

The slow d iffu sio n method used in th is study was th at proposed ty Friedel (1892). In this method a soluble salt is allowed to diffuse through a porous membrane into a hydrous mixture or solution of the other salt or salts. In the present study, the insoluble part of each charge together with 25 m illiliters of distilled water was placed in a cylindrical bottle having a capacity of 35 m illiliters and capable of being sealed with a cap. The porous membrane used was a

5 m illiliter test tube cracked along the lower third of its length.

A solution of CuClg*?!^ in U m illiliters of distilled water was placed in each test tube. Each test tube was then introduced into the 213 appropriate bottle through a hole in the cap, the bottom of the tube being placed approximately 5 millimeters from the insoluble salt at the bottom of the bottle. The tube was then sealed in the cap by means of paraffin and covered with a porous cap. The total weight of the dry charge was 1* grams. D istilled water was added to the test tube from time to time to compensate for evaporation losses. Reaction proceeded at room temperature and was terminated after approximately

10 months when reaction seemed com plete. Evaporation of the remaining liquid showed that some CuCl2*2H20 was s till present in solution.

The hydrothermal synthesis experiments were carried out in con­ ventional pot bombs and sealed pyrex tubes. Below 170°C. it was possi­ ble to use sealed pyrex tubes for the hydrothermal experiments. These tubes made it possible to determine the progress of the reaction directly but had the disadvantage that they occasionally ruptured before reaction was complete. To insure a uniform thermal environment, the tubes were heated in a water bath, the water being replenished each day from containers heated in the same oven with the samples.

Although the tubes had a capacity of 3 m illiliters, only two m illi­ liters of distilled water together with the charge of 1 to 2 grams could be placed in the tube because of the necessity of sealing the tube.

Above 170°C., metal pot bombs were used. A 15 m illiliter Teflon liner was used in all cases to prevent reaction between the charge and the walls of the bomb. The charge used varied from 2 to 5 grams w ith enough distilled water being added, in each case, to f ill the liner. 21U

R esults

The evaporation experiments yielded only micro- to cryptocry­ stalline aggregates of the species sought. Their greatest value lay in the fact that they provided an indication that some of the species could be synthesized. Such experiments are probably valuable, therefore, from a reconnaissance standpoint. The short intervals involved in such experiments do not lead to single crystal development of complex species

but w ill yield single crystals of simpler species such as cotunnite.

The slow diffusion experiments produced crystal aggregates in which the individual crystals generally exhibited a quite recognizable morphology. These crystals, though too small for measurement, were

highly perfect and exhibited brilliant, planar faces. The results of

these experiments do permit some conclusions regarding the stability

of some of the species involved.

The hydrothermal methods produced the largest crystals obtained in this study. Some rather large crystals were produced ty the hydro- thermal method where the reaction was allowed to continue over many months but, in general, the crystals obtained were small, though quite suitable for single crystal x-ray work, hydrothermal methods coupled with a long reaction time seemed to be the most advantageous for the production of large single crystals of these complex species.

Tables 23 through Table 26 summarize the results of the syn­ thesis experiments performed using the reagents and compositions given in Table 22. The results of two other experiments which were performed during this study but are not listed in Tables 23 through 26, are dis­ cussed in this and the following paragraph. The first of these experiments was an attempt to synthesize a silver-copper end member of the boleite-pseudoboleite series. In this attempt, AgCl and Cu (0H)2 in the proportions for boleite, were placed with excess distilled water in a pot bomb and heated at 2U5°C. for approximately 270 days.

This experiment produced only cuprite, cerargyrite, and some minor unidentified phases.

TABLE 23.—Summary of diffusion experiments

S tartin g Average Time Results Composition Temperature

Percylite Room 339 days Cumengeite

Pseudoboleite Room 339 days Cumengeite

Cotunnite

Minor u n id en tified phases

Diaboleite Room 339 days D iab oleite

Chloroxiphite Room 339 days D iab oleite

Chloroxiphite (?)

Unidentified phases

The second experiment not listed in the table was a preliminary attempt to prepare any of the oxychlorides included in this study. In t h is attem pt, an excess of a saturated CuClg^I^O so lu tio n was added to Pb(0H)2» This mixture was then placed in a pot bomb and heated at 2i|0°C. for approximately i*0 days. This experiment produced large, colorless crystals of nantokite, some masses of copper and a minor unidentified phase. Phases not identified were generally white or 216 black and, hence, no effort was made to identify them as they did not seem to be pertinent to the study.

TABLE 2lj.—Summary of evaporati on experiments

Starting Average Time Phases Obtained Composition Temperature

P e rcy lite Room (20°C.) 5 days Pseudoboleite (green)

Cotunniie

Pseudoboleite Room 5 days Ps eudoboleite (blue)

Cotunnite

Unidentified phase

Chloroxiphite Room 5 days Chloroxiphite (?)

Unidentified phases

Pseudoboleite 100°C. 2 days Pseudoboleite (blue)

Cotunnite

Unidentified phase

D iab oleite 100°C. 2 days D iab oleite

Chloroxiphite (?)

Unidentified phase

Chloroxiphite 100°C. 2 days Chloroxiphite (?)

D iab oleite

Unidentified phases

Although the experiments were performed only at isolated tempera­ tures, some conclusions as to the stability and paragenesis of the 217

TABLE 25.—Summary of pyrex tube experiments

S tartin g Average Time R esults Composition Temperature

Pseudoboleite 100°C. 55 days Pseudoboleite (blue)

Cotunnite

Unidentified phases

Diaboleite 100°C. 55 days Diaboleite

Unidentified phases

Chloroxiphite 100°C. 55 days D ia b o leite

Unidentified phase

Pseudoboleite 170°C. 17 days Cumengeite

Pseudoboleite (green)

Cotunnite

Percy Lite 170°C. 17 days Cumengeite

Pseudoboleite (green)

Cotunnite

D iab oleite 170°C. 16 days Cumengeite

Cotunnite

Unidentified phase

Cumengeite 170°C. 16 days Cumengeite

Pseudoboleite (green)

Cotunnite

Pseudoboleite 170°C. 15 days Cumengeite

Paratacamite (?)

Cotunnite

Minor u n id en tified phases species seems to be justified. Within the boleite group, cumengeite retains the appearance it has in nature over the whole range from

20°C. to 270°C. At 270°C., however, it coexists in minor amount with pseudoboleite. It may be that cumengeite's structure becomes unstable with respect to that of pseudoboleite with increasing temperature.

The minor role which cumengeite assumes may be the result of a more rapid increase in the solubility of cumengeite than of pseudoboleite with increasing temperature. It is rather interesting to find also that for comparatively long reaction times cumengeite is obtained rather than pseudoboleite from a pseudoboleite starting composition at room temperature. When the same composition is allowed to react over a short period of time in the range from room temperature to

100°C., only microcrystalline to cryptocrystalline pseudoboleite is obtained. There seems to be some evidence that pseudoboleite is metastable at room temperature. The short duration evaporation experi­ ments produce the green form of pseudoboleite. Experiments of a longer duration produce the blue form and very long term slow diffusion ex­ periments produce cumengeite. Thus, both forms of pseudoboleite may be metastable with respect to cumengeite at room temperature. At higher temperatures and shorter reaction times, cumengeite and pseudo­ boleite coexist. Some support to the conclusion that the sequence of formation is pseudoboleite to cumengeite is given by composite cry­ stals of these species formed in nature. Cumengeite is invariably found on pseudoboleite, never vice versa.

Except for green pseudoboleite which was obtained in the case of an evaporation experiment at room temperature from a percylite com­ 219 position, only blue pseudoboleite was obtained in the range 20°C. to

100°C. No ready explanation suggests itself except that this green form may represent a metastable step in the formation of low pseudo­ boleite. Such an explanation is lent support by the fact that this green phase had a bluish tinge and was obtained from a starting com­ position which differed significantly from the pseudoboleite composi­ tion which, under the same conditions, produced blue pseudoboleite.

At and above 170°C., pseudoboleite appears as green, pseudo-isometric crystals which have the appearance of some boleite crystals in nature and the color and appearance of boleite obtained in this study. The color and crystal form of pseudoboleite at aid above 170°G. strongly suggest a high temperature pseudoboleite polymorph isoraorphous with high boleite.

Boleite has been synthesized in this study only at 270°C.

Friedel (I89I4.) has reported a synthesis of boleite, probably in combination with pseudoboleite, at room temperature. It is quite probable, therefore, that boleite has the same thermal stability x range as pseudoboleite. Here again, however, the lower temperature form is blue (Friedel, I89IO while that prepared at 270°C,, the high temperature form, is green and probably isometric as indicated by the cubic morphology of the synthetic crystals prepared in this study and the optical properties displayed by natural boleite on heating.

Natural composite crystals of boleite and pseudoboleite indi­ cate that the paragenetic sequence with decreasing temperature is boleite — ►pseudoboleite — ►cumengeite. A consideration of the silver content as determined from previous analyses and the work on percylite seems to indicate that in the presence of an excess of silver 2 2 0 over that which can be tolerated by the pseudoboleite structure, boleite forms. Later, when the parent solutions are silver-poor, pseudoboleite separates. At an even later stage, when no silver is present and possibly when temperatures have decreased, cumengeite is formed.

TABLE 26.—Summary of pot bomb experiments

S tartin g Average Time Phases Obtained Composition T emperature Diaboleite 220°C. 9 days Cumengeite

Unidentified phases

B oleite 270°C. 193 days Boleite (green)

Cotunnite

Pseudoboleite 270°C. 193 days Pseudoboleite (green)

Cumengeite

Cotunnite

Pseudoboleite 270°C. 9 days Pseudoboleite (green)

Cumengeite

Cotunnite

Paratacandt e (?)

P e rcy lite 270°C. 7 days Pseudoboleite (green)

Cotunnite

Unidentified phase

Diaboleite 270°C. 9 days Mirdochite

Unidentified phases

Chloroxiphite 270°C. U days Metallic copper Cotunnite Unidentified phases 2 2 1

The synthesis experiments on diaboleite indicate that diaboleite is a low temperature species which does not persist to 170°C. At

170°G., the diaboleite composition yields cumengeite instead of dia­ boleite, and here the absence of diaboleite may be due purely to temperature considerations. At lower temperatures, the formation of diaboleite as opposed to cumengeite or pseudoboleite is apparently controlled by compositional factors which are more important than temperature.

The synthesis evidence with regard to chloroxiphite remains in­ conclusive inasmuch as it is not clear whether the synthetic species obtained in this study are indeed analogous to the natural specimens available. Unfortunately, the pyrex tube experiment for chloroxiphite was lost after 13 days, but it may be added that nothing among the dry products resembled chloroxiphite. It would seem from the intimate association of chloroxiphite and diaboleite in nature that chloroxiphite is a low temperature phase also. Certainly at 270°C. any vestige of a species similar to chloroxiphite has disappeared. It is rather interesting to note that in the diaboleite and chloroxiphite runs at

100°C., diaboleite and a species which may be chloroxiphite are associated just as natural diaboleite and chloroxiphite are in the

Mendip H ills .

The absence of ch loroxip h ite a t the Mammoth Mine and in the Ruhr is surprising. Possibly the iron and manganese concentration in the

Mendip deposits has some influence on the formation of chloroxiphite. H U . SUMMARY AND CONCLUSIONS

Cumengeite, pseudoboleite, boleite, diaboleite and chloro­

xiphite are the subjects of this thesis. All of these species are

lead- and copper-bearing minerals. Boleite and possibly pseudo­

b o le ite a lso contain some s ilv e r . Type p e r c y lite , which was described

as a lead- and copper-bearing, silver-free oxychloride, is in reality

equivalent to boleite plus pseudoboleite. Specimens of percylite from

other localities are boleite, pseudoboleite or boleite plus pseudo­

b o le ite .

Cumengeite, pseudoboleite and boleite are closely related

structurally as well as chemically. The cell dimensions of cumengeite o o are cQ= 21+ .1+9 v O.Ol+A and a0= 21.37 ♦ 0.0|?A and require a reorien ta­ tion of the setting chosen by previous investigators. The transfor­

mation formula for cumengeite is oldsnew - ilO/HO/OOl• The forms

present on cumengeite in the new setting are {OOl}, ^Oioj, and £ lllj.

The axial ratio derived from the cell dimensions is a:c = lsl.lU 6

and compares well with 1.1U9 derived from the cleavages ty Friedel

(1906). Friedel's value is adopted as the correct value inasmuch as

crystallographic measurements generally possess a greater precision.

Weis sen berg photographs have the Laue symmetry U/m 2/m 2/m. The (hkjfc)

reflections for cumengeite indicate that this species possesses an

F-lattice. The remaining extinction criteria are subordinate to the lattice criterion. Such a lattice is possible in the tetragonal

2 2 2 223 system by twinning on a unit cell scale and the space group observed is then a twinned apace group which may be given as^Fj^^ 2/mJor

rived from a la t t ic e w ith tetragonal or orthorhombic symmetry and p o ssi­ bly from a la t t ic e w ith even a lower symmetry. The r e a l symmetry of cumengeite remains in doubt. The density of cumengeite is it.70 +

0.03. Z is 21 with the formula Pb^Cu^ClQ^lOg^O and 8U or 85 with

PbCuCl2(0H)2» Since the formula fo r cumengeite i s in some doubt and since symmetry requirements make it difficult to accept as correct a value of Z not divisible by 2 or U, it is not possible to choose between the formulas given for cumengeite on the basis of Z. Until a better chemical analysis is available, the correct value of Z will remain uncertain. Cumengeite may possess an ordered structure. The color change on heating reported by Lacroix (1895), if such a change is real, strongly suggests a polymorphic inversion in cumengeite. There is, however, no evidence from synthesis experiments of such an inversion.

Cumengeite has been synthesized over the range from 20°C. to 270°C.

Indexed powder d iffra ctio n data are given for both natural and syn­ thetic cumengeite.

The validity of pseudoboleite as a species distinct from boleite is verified. The cell dimensions for pseudoboleite are cQ- 30.8U *

0,06% and aQ* 21.57 ♦ 0.03& and necessitate a reorientation of the setting chosen by previous authors. The transformation formula is oldsnew * llO/TlO/OOl* The forms present on pseudoboleite in the new setting are £00lj, £oioj, £lloj, £oil$,and £ l l l j . The a x ia l ra tio derived from the cell dimensions is 1.U29 whereas that derived from crystallographic measurements is 1.1*30. The value 1.U30 is adopted for the same reason given in the case of cumengeite* Weissenberg photographs obtained from pseudoboleite possess the Laue symmetry U/m 2/m 2/ra. Ex­ tinction criteria indicate a C-centered lattice for pseudoboleite, but do not lead to a suitable space group. The lattice type indicates that pseudoboleite is twinned on a unit cell scale, but no twinned space group could be found which satisfied the extinction criteria. New density measurements gave the value l*.9l* + 0.05 but a calculation of

Z indicates that this value may be too low and that the actual den­ sity is nearer 5*0. Calculation of Z using Friedel's (1906) formula and density gives a value between 23 and 21*. As the value Z=2l* would satisfy the symmetry requirements better than Z=23, and since a value for the density near the upper lim it of the value given would give Z»2U, this latter density value is probably the correct one. The blue color, tetragonal symmetry, and uniform birefringence and appearance of natural pseudoboleite and synthetic pseudoboleite pre­ pared between 20°C. and 100°C. contrast sharply with green synthetic pseudoboleite obtained at or above 170°C. which has an isometric ap­ pearance. Some crystals possess a microscopic lamellar structure.

Pseudoboleite very probably undergoes a polymorphic inversion to a high temperature isometric phase isomorphous with high temperature boleite. Pseudoboleite has been synthesized between 20°C. and 2?0°C.

Indexed powder diffraction data is given for natural and synthetic pseudoboleite.

An optical study shows that the anisotropic rim on boleite is polycrystalline and lamellar on a very fine scale. On heating, the 225 lamellar structure and anisotropism disappear, indicating that boleite undergoes a polymorphic inversion co an isometric form. The quasi- isotropism of the core of boleite also disappears on heating. The pseudo-isometric form together with the blue color of natural crystals of boleite and the isometric crystal form, isotropism and green color of synthetic boleite indicate that high temperature boleite is a green isometric phase. Boleite and pseudoboleite are isomorphous at high temperature. The cell parameter cc= 60.82 + 0.06X which is said to belong to boleite (Ito, 1950) was not obtained from the anisotropic rims of boleite that were studied. Instead, the parameter obtained o in the direction 001 was 15.26A. Furthermore, the parameters ob­ tained in the plane (001) were 21.61*1 and 15.26A, although aQ= 15.21* +

0.02X has been reported by Ito. Boleite needs a further x-ray study made on single crystal specimens.

A re-examination of the chemical data given for boleite and pseudo­ boleite indicates that a defect solid solution of Ag-*- for Pb-1"1, could exist between boleite and pseudoboleite at high temperature. The general formula for such a solid solution would be ^^12-x^x^

GugCl2|4_x (0H)^*yH20. With x*3, b o le ite would be produced and w ith x=l, a compound is obtained which is very near the formula derived for pseudoboleite, if one considers the silver reported in the original anaiysis essential.

A species related to pseudoboleite and boleite has been observed in a composite crystal of pseudoboleite on boleite between the pseudo- o boleite and boleite. This species has a 60A spacing and is probably analogous to boleite as described by previous investigators. No other 226 properties could be determined for this species since only a minute amount of it was observed on the base of a very small pseudoboleite overgrowth. This species is obviously related to pseudoboleite and boleite and probably represents an ordered structure derived by the complete ordering of silver in boleite or a composition between boleite and p seu d ob oleite.

The evidence suggests that boleite and pseudoboleite are isomor- phous and form a complete solid solution at high temperature. At

Ion temperature, each end member undergoes a polymorphic inversion the extent of which may be controlled by conditions operating at the .o time. Thus the 15A disordered cubic lattice lengthens in the direction

[lioj but remains essentially unchanged in the other directions and with respect to the disordered arrangement of the ions for low tem­ perature boleite under some or all conditions. A species with a

probably with a spacing of 21?

could then arise from inversion of boleite to a completely ordered structure or from inversion of intermediate mem- , o bers of the solid solution. These members yielding the 60A spacing would probably be s ilv e r -r ic h members. The c e l l dimensions observed in pseudoboleite would be produced due to ordering of the silver in a silver-poor member or twinning in a silver-free end member.

If Ito's (19^0) structure for boleite is correct, boleite is composed of (001) sheets which are arranged normal to [OOlj. Ito gives the spacing 15A and 10.6^ in the plane (001). The spacings observed in this study for cumengeite and pseudoboleite and in the rim on boleite strongly suggest that these sheets have the parameters 227 i S l and 21A in the plane (001). If these latter parameters are correct, cumengeite, pseudoboleite, boleite and the 6oX species may be considered to be sheet structures built up by stacking sheets in the direction

[00l|. The cell dimensions for the direction [00l[ and the perfect

£ooiy cleavage in boleite and pseudoboleite support this structural theory and analogy. Such an analogy probably holds for the 60& . o species as well and it is likely that this 60A spacing is the spacing in the direction • Inasmuch as cumengeite has a different Q)0l] parameter and possesses only a poor {oOlJ cleavage, it is obvious that the stacking would be different in this species.

All of the members of the boleite group are uniaxial negative and probably possess a sheet structure, the sheets being parallel to (001) and stacked in the direction [ooij. There seems to be a striking analogy here with the clay minerals which are in some cases very near­ ly uniaxial negative and consist of (001) sheets stacked in the direc­ tion [ooij» When one recalls that a certain amount of water may be held between the sheets of the clay minerals, it is not difficult to visualize occluded water being held between the sheets in members of the boleite group.

Diaboleite has been synthesized in the range from 20°G. to 100°C. but was not obtained under similar conditions at or above 170°C. In­ dexed powder diffraction data are given for natural and synthetic diaboleite. The cell dimensions derived from extrapolation of the powder data are c0= $.k9$ ♦ 0.003 and a0« £<689 + 0.002.

Observations on natural chloroxiphite indicate that this species is commonly found in a very impure and probably partially altered state. A substance which may be poorly crystallized chloroxiphite has 228 been synthesized in the range 20°C. to 100°C. A powder pattern ob­ tained from the most reliable specimen of chloroxiphite available is compared with the pattern obtained from this synthetic species which seems to be near chloroxiphite in structure.

Synthesis experiments indicate that chloroxiphite and diaboleite are formed only at low temperatures with respect to the members of the boleite group. Within the boleite group, stability appears to depend on temperature, composition, and solubility relationships which have not been determined. Boleite seems to be the species formed at high temperatures in the presence of a suitable amount of silver whereas pseudoboleite is formed in the absence of silver. The stability relationships which exist between cumengeite on the one hand and boleite and pseudoboleite on the other are not clear but probably depend on solubility relationships as these relationships are influenced by temperature and com position.

Although such a theory can not be proved without a thorough study of the phase relationships, there is some evidence from this study that the paragenetic sequence with decreasing temperature for the species dealt with in this study is boleite — pseudoboleite — cumengeite

— ^chloroxiphite — diaboleite. BIBLIOGRAPHY AND REFERENCES

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______. 1900. Sur une categorie de groupements cri81allins echappant aux in v e stig a tio n s op tiq u es. Compt. Rendu. , 130, IUJ4.-IU6 .

Webeky, M. 1886. Uber C aracolit und P e r c y lit. Ak. B erlin , S itz b e r ., 32, 10U5-1050. «———

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Wilson, I. 1962. Private communication. AUTOBIOGRAPHY

I, Robert Eugene Winchell, Jr., was bom in Wichita, Kansas,

September 21, 1931. I received ny secondary school education in

Roman Catholic schools in the dioceses of San Francisco and Sacra­

mento, California. I received my undergraduate training at Sacra­

mento Junior College and Stanford University, the latter granting me a Bachelor of Science degree in Geology in 1956. I received a

Master of Science degree in Geological Engineering in 1959 from

Michigan College of Mining and Technology and was awarded the

Estelle-Littlepage Macauley Fellowship for the academic year 1959-

1960 for graduate study in mineralogy at the University of Michi­ gan. After nearly two years of residence at the University of

Michigan, I transferred to the Ohio State University in March 1961.

I was awarded the William J. McCaughey Fellowship in mineralogy for two years while completing the requirements for the Doctor of

Philosophy degree in Mineralogy.

I have accepted a research position at the AC Spark Plug

D ivision of General Motors in F lin t, Michigan.

233