1-

RARE-EARTH ELEMENTS IN PYROMORPHITE-GROUP MINERALS

A thesis submitted for the award of the degree of Doctor of Philosophy of the University of London

by

Helen Pauline Newby

University of London Reactor Centre Department of Mechanical Engineering Imperial College of Science and Technology

October 1981 2-

ABSTRACT

A survey of rare-earth element (REE) concentrations in pyro-

morphite-group minerals, PbsCPOi^AsO^jVO^) 3CJI, was carried out using instrumental neutron activation analysis. 22 specimens from various locations, comprising 18 members of the pyromorphite-mimetite series,

PbsCPOi+jAsO^) 3CJI, and 4 vanadinites were examined. REE were detected in all but two specimens (both mimetites). The lowest concentrations were found in vanadinites, in which only La and Ce were detected, and in some near end-member mimetites, and the highest in phosphatian mimetite and arsenious pyromorphite. The minerals tend to be enriched in Ce-group REE with the Ce-group/Yt-group ratio being highest in mimetite and lowest in pyromorphite.

Comparison of REE concentrations in pyromorphite from the

Carrock Fell Area, Cumberland with REE concentrations in associated,

contemporary psilomelane, (Ba,H20)2M115O1 Q, and plumbogummite,

PbAfl^PO^) 2 (OH) 5H20, suggests that crystallo-chemical factors are

important in determining REE concentration profiles in minerals pre-

cipitated from hydrothermal solution.

In doping experiments using synthetic pyromorphite, which were

designed to assess the influence of various factors on the partition of

REE, enrichment of Ce-group REE pyromorphite occurred in acid solution

in the absence of complexing ions. In neutral and acid solution,

sulphate ions were effective in keeping a higher proportion of RE)£ ions

in solution, but produced fractionation of Ce-group and Yt-group REE only in acid solution. In alkaline conditions, carbonate ions were able

to hold back RE ions in solution only if the concentration of NaC£ in

solution was low, and produced no fractionation of Ce-group and Yt-group elements. 3

TABLE OF CONTENTS

Page ABSTRACT 2

TABLE OF CONTENTS 3

LIST OF TABLES 5

LIST OF DIAGRAMS 7

ACKNOWLEDGEMENTS 8

CHAPTER 1 INTRODUCTION 9

1.1 Reasons for the analysis of pyromorphite-group minerals by means of neutron-activation analysis 9

1.2 Survey of rare-earth element concentrations in pyromorphite-group minerals 10

1.3 Investigation of the factors influencing rare- earth element concentrations in pyromorphite- group minerals 11

CHAPTER 3 LITERATURE REVIEW 15

2.1 Chemistry and crystal structure of minerals belonging to the pyromorphite series 15

2.2 Previous studies of rare-earth elements in pyromorphite-group minerals

2.3 Factors which may determine rare-earth element concentrations in minerals precipitated from hydrothermal solutions

2.4 Brief description of the geology and mineralogy of the Carrock Fell area 24

2.5 Laboratory synthesis of pyromorphite 29

CHAPTER 3 EXPERIMENTAL 31

3.1 Analysis of rare-earth elements in natural pyromorphite-group minerals 31

3.2 Analysis of rare-earth elements in rocks and minerals from the Carrock Fell area 35

3.3 Doping experiments using laboratory-synthesized pyromorphite 36 4

Page

CHAPTER 4 RESULTS 45

4.1 Survey of rare-earth element concentrations in pyromorphite-group minerals 45

4.2 Study of pyromorphite and accompanying minerals from the Carrock Fell area 46

4.3 Results of the doping experiments 47

CHAPTER 5 DISCUSSION 81

5.1 Analysis of minerals and rocks from the Carrock

Fell area 81

5.2 Results of doping experiments 83.

5.3 mineralRare-earts ho f elementhe pyromorphite-grout concentrationsp in natural 87

5.4 Summary 92

CHAPTER 6 SUGGESTIONS FOR FURTHER RESEARCH 94

APPENDIX 1 CHEMICAL ANALYSIS OF PYROMORPHITE-GROUP MINERALS 97

APPENDIX 2 NON-STOICHIOMETRY IN SYNTHETIC PYROMORPHITE 101

REFERENCES 105 5

LIST OF TABLES

Page

Table No. Title

1 Unit cell dimensions of pyromorphite-group minerals 19

2 y-rays used for REE analysis 1 : Ge(Li) detector 33

3 Detection limits for the determination of REE in pyromorphite, vanadinite and mimetite 34

4 y-rays used for REE analysis 2 : Ge detector 37

5 Pyromorphite-group minerals used in general survey

of REE distribution 41

6 Mineral specimens from the Carrock Fell area 42

7 Rock specimens from the Carrock Fell area 43 8 Conditions under which precipitates of synthetic pyromorphite were left to age 44

9 REE concentrations (ppm) in pyromorphite, mimetite and vanadinite 49

10 Chondrite-normalized REE abundances in pyromorphite, mimetite and vanadinite 51

11 REE concentrations in pyromorphite and accompanying minerals from the Carrock Fell area 58

12 Chondrite-normalized REE abundances in pyromorphite and accompanying minerals from the Carrock Fell area 59

13 Distribution coefficients for the partition of REE between pyromorphite and associated psilomelane and plumbogummite 65

14 REE concentrations (ppm) in rocks from the Carrock Fell area 68

15 Chondrite-normalized REE abundances in rocks from the Carrock Fell area 69

16 Normalized REE concentrations in synthetic pyromorphite 72 6

LIST OF TABLES

Page

Table No. Title

17 Normalized concentrations of REE remaining in solution 74

18 Distribution coefficients for the partition of REE between synthetic pyromorphite and aqueous solution 76

19 Chemical analysis of minerals used in the general survey of REE distribution in the pyromorphite series 99

20 Sodium and chlorine concentrations in synthetic pyromorphite 104 7

LIST OF DIAGRAMS

Page

Fig. 1 The crystal structure of pyromorphite: projection

on to the a-b plane 18

Fig. 2 Sketch map of the geology of the Carrock Fell area 26

Fig. 3 oSketcf thhe maCarrocp showink Felgl minera Complel xvein s at the western end 27

Figs. 4-18 Chondrite-normalized REE concentrations in pyromorphite- group minerals 53-57

Figs.19-23 Chondrite-normalized REE concentrations in pyromorphite and accompanying minerals from the Carrock Fell area 60-64

Fig. 24 Partition of REE between plumbogummite and pyromorphite: distribution coefficients plotted against ionic radius 66

Fig. 25 Partition of REE between psilomelane and pyromorphite: distribution coefficients plotted against ionic radius 67

Figs.26-30 Chondrite normalized REE concentrations in rocks from the Carrock Fell area 70-71

Figs.31-35 REE distribution between synthetic pyromorphite and aqueous solution 77-80

Fig. 31 Variation of partition coefficient with time: REE added after the formation of pyromorphite 77

Fig. 32 Variation of partition coefficient with time: pyromorphite precipitated in the presence of REE 77

Fig. 33 Variation of distribution coefficient with pH 78

Fig. 34 Effect of sulphate ions on the distribution coefficient 1: in neutral saline solution 79

Fig. 35 Effect of sulphate ions on the distribution coefficient 2: in acid solution 80 8-

ACKNOWLEDGEMENTS

I would like to thank my supervisors, Dr. A. Clark, British

Museum (Natural History), and Dr. T.D. Mac Mahon, University of London

Reactor Centre, for their advice and constructive criticism. I would

also like to thank the staff of the Mineralogy Department, British

Museum (Natural History), especially those of the Rock and Mineral

Chemistry Section, for making the time I spent at the Museum so

enjoyable. I am particularly grateful to Dr. P. Henderson for his

interest, encouragement and most helpful discussion, to Dr. C. Stanley

for providing rock and mineral specimens, and to Mr. J. Easton and

Mr. V. Din for their advice and interest in the chemical analysis and doping experiments. I would also like to thank Mrs. J. Bevan for help with the electron microprobe analysis.

For instruction and assistance in the use of the Nuclear Data analyser and data-processing system, I am indebted to Dr. S. Parry

(University of London Reactor Centre) and Dr. C.T. Williams (British

Museum (Natural History)). Other staff at University of London Reactor

Centre to whom I owe special thanks are: Mr. E.A. Caesar and the reactor operators for irradiating samples, Miss M.J. Minski for support and encouragement and, not least, Mrs. J. Vine for typing this thesis. 9-

CHAPTER 1

INTRODUCTION

1.1 REASONS FOR THE ANALYSIS OF PYROMORPHITE-GROUP MINERALS BY

MEANS OF NEUTRON ACTIVATION /0_ \ rl pyomorpWite Pb^tPO^jU ,

The pyromorphite series of minerals comprises j^mimetite PbsCAsO^) 3CJI,

and vanadinite PbsCVO^)3C2,. The minerals are'found in the oxidized zone

of ore deposits containing galena and are thought to be of secondary origin. (The mineralogy and chemical properties of pyromorphite-group are. minerals discussed in Section 2.1.)

Extensive substitutional solid solution occurs between pyro- morphite, mimetite and vanadinite and variations in the ratio PO^: AsO^rVO^ are often found within a single mineral grain. Variations in composition in mineral grains are best determined using electron microprobe analysis, but with pyromorphite-group minerals there are two problems associated with microprobe analysis. The first is that the arsenic K X-ray line suffers serious interference from the lead L<* 1 X-ray line and the only other arsenic X-ray which can be used is the K X-ray which is much less PI intense. The second is that, because of the large amounts of lead and arsenic present, large corrections for absorption and atomic number differences have to be applied to the X-ray data.

One of the advantages of neutron activation analysis (INAA) is that the detection limits for some elements are very low enabling small sample weights to be used. One of the original purposes of applying instrumental neutron activation analysis to pyromorphite-group minerals was, therefore, to see whether it could be used as an alternative to, or in conjunction with electron microprobe analysis as a means of . . . t • measuring variations m composition between difference parts of a 10-

mineral grain. In addition, it was hoped to discover whether the

various colours exhibited by pyromorphite-group minerals could be

related to the presence of specific trace elements.

1.2 SURVEY OF RARE-EARTH ELEMENT CONCENTRATIONS IN PYROMORPHITE-

GROUP MINERALS

The application of INAA to a selection of pyromorphite-group minerals from the British Museum (Natural History) revealed trace

amounts of rare-earth elements (REE) in most specimens. Wide variations

in concentrations and Ce-group/Yt-group ratios were observed. It has been known for many years that trace amounts of REE occur in pyromorphite- group minerals but there is very little information on the concentations and relative abundances of REE in these minerals. It was therefore decided to concentrate research on the distribution of REE in pyro- morphite-group minerals in order to determine the extent of the variations in concentration profiles and to see whether these could be related to any other properties of the minerals or to the conditions of formation.

In the first part of the project a survey of REE concentrations in specimens of pyromorphite-group minerals from various locations was carried out. (In order to obtain as much information as possible about these specimens they were also analysed chemically and by means of an electron microprobe. The results of these analyses are given in

Appendix 1.)

In later experiments attempts were made to assess the roles of various factors in determining the concentrations of individual REE in members of the pyromorphite series. 11-

1.3 INVESTIGATION OF THE FACTORS INFLUENCING RARE-EARTH

ELEMENT CONCENTRATIONS IN PYROMORPHITE-GROUP MINERALS

Nriagu (1973, 1974) has shown that pyromorphite is one of the most stable and least soluble of secondary lead minerals under a wide

range of conditions. It can be formed in conditions which range from acid, pH <3, to basic, pH >10, and from highly oxidising to mildly reducing, and will be precipitated in preference to minerals such as anglesite and cerussite even in the presence of relatively high con- centrations of carbonate and sulphate ions. Previous attempts to relate REE concentrations in minerals precipitated from hydrothermal solutions to the geochemical environment have been complicated by the t fact that difference conditions result in the precipitation of different minerals, and it has been impossible to isolate the effects of geochemical factors from the possible effects of crystal structure (Turovsky et al.

1968, Kosterin 1959). By using pyromorphite-group minerals the effects 2_ 2_ of pH, complexation by CO3 and SO^ etc., on REE concentrations in the mineral can be studied by using a single crystal matrix. In addi- tion, the ease with which pyromorphite can be synthesized enables the influence of various factors on REE concentrations to be studied in detail in the laboratory. The information obtained from this type of study is important not only in providing a greater understanding of

REE geochemistry, but also in assessing the potential use of REE con- centration profiles in minerals as a means of obtaining information about the conditions of formation or the sequence of formation. 12-

REE concentrations in minerals precipitated from hydrothermal solutions can be influenced by a number of factors e.g.:-

(1) the concentration of individual REE in the solution

(2) the crystal structure of the mineral

(3) the oxidation states of Ce and Eu

(4) the formation of RE complexes

(5) abstraction of REE by co-precipitating minerals.

To assess the part played by each of these it is necessary to have in- formation about the mineralizing solution regarding pH, oxidation potential, concentrations of ions in solution etc., but the hydrothermal solutions are not available for analysis. There are two ways in which this problem may be partly overcome:-

(1) information can be obtained by studying accompanying minerals

(2) minerals can be synthesized in the laboratory under controlled

conditions.

Both approaches were used in the present work.

Analysis of accompanying minerals

The study of accompanying minerals was confined to specimens from the Carrock Fell area, Cumberland. This is a classic locality for minerals of the pyromorphite - mimetite series and was chosen because the British Museum (Natural History) possesses a large number of specimens from this area and because more specimens could be collected if necessary. The geology of the area is described briefly in Section

2.4 and illustrated in Figs. 2 and 3. 13-

Pyromorphite and mimetite are found in four Pb/Cu/Zn veins,

Drygill, Driggith and the two Roughten Gill veins. To the south of

these, at Carrock Mine, there are veins containing tungsten minerals and apatite, whilst to the north, at Potts Ghyll and Sandbeds, the main minerals are barytes and quartz.

Unfortunately, the origin of the REE found in the Carrock Fell pyromorphite is unknown. The possibility that REE are present in the primary minerals was investigated with the analysis of galena, sphalerite, chalcopyrite and barytes. A series of rocks in a line approaching the

Driggith vein were also analysed to see whether the REE might have been leached from the vein wall-rock. Establishing the origin of the

REE was important since this would give an indication of the relative concentrations of REE in the solution.

A search for specimens containing pyromorphite in the presence of contemporary, secondary minerals was disappointing. It had been hoped that specimens in which pyromorphite was accompanied by secondary sulphate and carbonate minerals would have been found, since sulphate and carbonate complexes are thought to play an important role in the transport and fractionation of REE in hydrothermal solutions. Although minerals such as cerus^Lte, anglesite and smithsonite are found in the

Carrock Fell area, no specimens could be found in which these minerals were associated with pyromorphite and mimetite. The only accompanying secondary minerals which were found were plumbogummite PbA^CPOi^COH) 5H2O psilomelane (Ba^I^O)2M15O10 a**d secondary quartz. 14-

Doping experiments using synthetic pyromorphite

The effects of carbonate and sulphate ions and pH on the partition of RE ions between pyromorphite and aqueous solution were therefore in- vestigated by means of doping experiments using synthetic pyromorphite.

Pyromorphite is highly insoluble and in the design of these experiments a compromise had to be made between convenience and the conditions of large solution volume and long digestion time which were ideally required for the precipitation of pyromorphite and the subsequent establishment of equilibrium between trace ions in the solution and the precipitate.

The effects of variation in the surface area of the precipitate and the risk of precipitation of other insoluble salts, such as lead carbonate and rare-earth phosphates, were also considered. Preliminary experi- ments were carried out to make sure that the relevant RE salts were soluble under the experimental conditions used.

The results of work by Nriagu (1973, 1974) on the solubility product of pyromorphite and the stability field of pyromorphite with respect to minerals such as anglesite and cerussite were of great value in these experiments. Such data are not available for mimetite and vanadinite. It was partly because of this and partly because the necessary reagents were not so readily available, that the doping experiments were confined to pyromorphite and did not include mimetite and vanadinite. 15-

CHAPTER 2

LITERATURE REVIEW

2.1 CHEMISTRY AND CRYSTAL STRUCTURE OF MINERALS BELONGING TO

THE PYROMORPHITE SERIES

Descriptions of the pyromorphite-group minerals are given in

textbooks such as Dana (1951). The group consists of a series of'

minerals having the general formula Pb5(P0i+, AsO^, VOi^CJl, of which

the three end-members are pyromorphite, Pbs(POif) 3C£, mimetite,

Pb5(As0i+) 3CJI, and vanadinite, Pbs(VOi+) 3C£. It is uncertain whether

a continuous series of solid solutions exists between the three minerals. Amounts of vanadium found in pyromorphite and mimetite

seldom exceed trace levels and this has lead Cockbain (1968 a & b) and Fortsch (1970) to the conclusion that a continuous series of

solid solutions exists only between pyromorphite and mimetite. On the other hand, workers such as Baker (1966) using synthetic minerals have found no evidence for a discontinuity between vanadinite and the remainder of the group.

Pyromorphite is the most abundant member of the group and vanadinite is the rarest, reflecting the relative abundances of P, As and V. The minerals are found in mineralized veins in regions where there are lead deposits. They occur in various forms ranging from hexagonal, prismatic crystals, through rounded, barrel-shaped crystals to globules and crusts, and are usually associated with galena, sphalerite and other sulphide minerals and with secondary minerals formed by the oxidation of these. It is generally accepted that they are of secondary 16-

origin, formed as a result of the oxidation of primary lead ores, such

as galena, by percolating ground-water containing dissolved oxygen/

(Dana 1951, Fortsch 1970), though according to Strens (1963) pyromorphite

can also be formed as a primary mineral.

The pyromorphite-group minerals exhibit a variety of colours :

vanadinite ranges from pale yellow to brownish red; pyromorphite and

mime'tite are usually green or orange but can also be yellow, brown or

colourless. Synthetic minerals prepared in the laboratory are always

white, and colour in natural specimens may be caused by impurities.

The orange colour in mimetite and pyromorphite has been attributed to

trace amounts of chromate (Carrobbi & Alfani 1931, Govorov 1955) and

the green colour is probably due to traces of Cr3+ (Carrobbi & Alfani

1931).

Substitution of lead by calcium is common and may be extensive, especially in pyromorphite where concentrations up to 15 wt % Ca are reported (Fortsch 1970). Trace amounts of Sr2+, Ag*, Fe2+, RE3+, Na+,

Cr3+, Ba2+ and Mn2+ are also found substituting for Pb2+. Traces of 2_ 2_ 3_ SiO^ , CrOit and SO^ can replace PO^ etc., and CJ£ may be replaced by 2_

OH , F and CO3 , although, in contrast to the apatite group, extensive substitution of chloride is rare (Dana 1951) .

Crystal Structure

In 1932 pyromorphite and mimetite were reported by Hendriks, a Jefferson and Mosely to have the structure of chlorgJpatite as determined by Mehmel in 1930. Vanadinite was assumed to be isostructural, and was later shown to be so by Trotter and Barnes (1958). The pyromorphite structure belongs to the hexagonal space-group P63/m and is illustrated in Fig. 1. The surroundings of the various ions in the crystal are given below:- 17-

Ion . Surroundings

Cif Octahedron of 602"

P5+/AS5+/V5+ Tetrahedron of 402"

Pbj+ (on 3-fold axis) 902~

Pb^+(on reflection plane) 602 and 2Ci

02" Tetrahedron of 1P5+ and 3Pb2+

Slight changes in the unit cell dimensions are observed between

vanadinite, mimetite and pyromorphite because of the differences in the

sizes of vanadate, arsenate and phosphate ions. The cell lengths for

the pure end-members are given in Table 1. Minerals of intermediate

composition have intermediate values of a and c, but the cell dimensions

are also affected by the substitution of C£ by F or OH and by sub-

stitution of Pb2+ by Ca2+ (Hendricks et al. 1932, Fortsch 1970).

In mimetite the length of the c axis means that, with C£ at

0,0,0 and 0,0,\9 the distance between chlorine and the lead ions on the reflection plane (z=|) is 3.19 X, which is somewhat larger than the sum of the ionic radii, 3.06 % (Hendricks et al. 1932). A Pb—C£ distance closer to the expected value is obtained by moving chlorine to the positions 0,0,Z and 0,0,Z+|. Such a change destroys the hexagonal symmetry of the structure, turning the reflection plane into a glide plane and doubling the b axis of the cell, so that it becomes mono- clinic (space group P2i/b.). This monoclinic structure is now well established for mimetite (Keppler 1968 and 1969, Brenner et al. 1970,

Fortsch 1970) and accounts for the birefringence and pleochroism which 18

Fig- 1 Hie, crystal structure, cfc ptjromorphife/. projection or\ lo ttae. a-fc> plane..

9 -CI

® Pb oJr yv _ Pb^ ^- fold coordinajib* © Pb cje z.= 3/V

® Pb oJr 2L»'/2.,0 PbX| coord;*oJ&n

o O oJt

« Oat

6 O oJr

® O «JT- Z-* "VIFE/^

• P oJr Z*VV

© P oJt- Z « 19

TABLE 1

UNIT CELL DIMENSIONS OF PYROMORPHITE GROUP MINERALS

a(£) cd)

Pyromorphite 9.95a 7.3la 9.97b 7.32b 9.990° 7.337° 9.987d 7.450d

Mimetite 10.24a 7.43a io.25k 7.46b 10.245° 7.330° 10.251d 7.442d

Vanadinite 10.31a 7.34a 10.32b 7.33b 10.323° 7.343C 10.323d 7.346d

a Hendricks et al. 1^32. b Wondralschek 1963 c Fortsch 1970 d Baker 1963 20-

are observed in natural mimetite (Fortsch and Wondratschek 1967, Fortsch

1970). X-ray crystallographic evidence for the monoclinic structure is

found more often in synthetic minerals than in natural ones, and it is

thought that impurities in the latter may stabilise the hexagonal struc-

ture (Keppler 1969). Random variations in the sign of z, which are possible if there are missing halide ions, would also result in apparent hexagonal symmetry (Keppler 1969).

Crystal structure, substitution and non-stoichiometry

Substituents commonly found in natural pyromorphite-group + minerals are listed on page 16 and include ions such as Mn^ , Na , o+ 2- 2_ CrJ , Baz , CrO^ , SO^ and SiO# ^ . The ability of the pyromorphites r to accommodate 10ns with such a range of sizes and changes is due to the extraordinary stability and flexibility of the crystal structure, and has been investigated by several workers (Hendricks et al. 1932,

Posner and Perloff 1957, McConnell 1965, Wondratschek 1963, Young and

Elliott 1966, Prener 1967, Kriedler and Hummel 1970, Merker et al. 1970).

The halide ions are not necessary to the stability of the structure and may be totally absent as in the synthetic compound

Pb5(POlf)2(SiOl+) (Wondratschek 1963, Hendricks et al. 1932, Merker et al. 1970). Vacancies in the halide site permit substitution of lead by univalent ions such as Na+ (Wondratschek 1963), though in natural systems such vacancies would probably be filled by neutral groups such as H20 or C02 (Hendricks et al. 1932). Substitution of C£ by a divalent ion, e.g. 02 , CO23_ , provides a means by which a tervalent cation such ... as RE° may replace lead though it is more likely that a tervalent cation is compensated for by a univalent cation elsexchere. 21-

Vacancies in the divalent cation sites have been reported by

Hendricks et al. (1932) who prepared the compound Cag(H20)2(P0l+)6 which

has the pyromorphite structure with vacancies in one out of every ten

Ca2+ sites. Unfortunately the lead-deficient pyromorphite prepared

by Posner and Perloff (1957), in which one lead ion per unit cell was

said to be missing statistically from the four Pb]^ sites (on the

3-fold axis)probably contained sodium, which substitutes preferentially

in the Pbj sites (Wondratschek 1963).

There is no evidence for vacancies in the tetrahedral anion

sites, but substitution by a variety of ions such as CrO^ , SiO^ 2_

and SO^ can occur with the extra positive or negative charges being

compensated by substitution or vacancies elsewhere (Wondratschek 1963,

Kriedler and Hummel 1970). In synthetic minerals precipitated from

solution substitution by the tetrahedral hydroxyl complex (OH)^ has

been suggested (McConnel 1965) and hydrogen bonding between phosphate

tetrahedra may occur in minerals precipitated from acid solutions

(Posner and Perloff 1957).

Incorporation of Na+, OH and H+ by the means described above is particularly important when dealing with synthetic pyromorphite formed by precipitation from solution and can result in non-stoichiometry

(Wondratschek 1968, McConnell 1965). In natural pyromorphite, non- stoichiometry is indicated in some of the analyses reported in the literature, e.g. Nos. 6, 8, 9 and 12 in Dana, which suggests that these types of substitution may also occur in natural minerals. 22

2.2 PREVIOUS STUDIES OF RARE-EARTH ELEMENTS IN PYROMORPHITE-GROUP

MINERALS

Although it has been known for many years that trace amounts of

rare-earth elements (REE) occur in minerals of the pyromorphite series,

there is little literature on the subject. The few studies of REE in

pyromorphite-group minerals which have been carried out were undertaken

before the development of neutron activation analysis, when the quanti-

tative determination of individual REE was difficult and time-consuming c^uod»tocTWe-

and consequently most of the information given is of a quontitative

nature only.

The presence of REE in pyromorphite was first reported in 1872

by Homer (1872a and b). Working on the mineral collection of the British

Museum, later the British Museum (Natural History), he detected 'didymium',

a mixture of Pr and Nd, in most of the specimens from Roughten Gill,

Cumberland and in specimens from Wheal Alfred, Cambourne, Cornwall and from

the Vale of Towy, Carmarthenshire.

The next major study of REE in pyromorphites was carried out more

than fifty years later by Carrobbi who, by means of spectrographic analysis,

established the presence of La, Ce, Nd, Eu, Gd, Dy and Er in pyromorphite

from Leadhills, Lanarkshire (Carrobbi 1926) and from Braubach, Nassau

(Carrobbi and Restaino 1926), and detected La, Ce, Pr, Eu and Dy in a

specimen from Germammari, Sardinia (Carrobbi and Alfani 1931).

Later, Fenoglio and Rigault (1957), working on the geochemistry of europium, detected this element in two of four mimetite specimens analysed. Both these specimens were from the Caldbeck Fells area,

Cumberland and concentrations of 0.012% and 0.008% EU2O3 are quoted.

Out of fourteen pyromorphites from various localities, Eu was detected in only one, a specimen fromMarienberg, Saxony, which contained 0.010% 23

EU2O3. Europium was not detected in any of the four vanadinite

specimens analysed. The concentrations of other REE are not given,

but the europium concentrations are reported as being anomalously high.

Goldsc^piidt (1958) also found high concentrations of europium (> 1000

ppm) in pyromorphite from the Fredericksegen Mine near Ems, Germany.

Both Goldschmidt (1958) and Fenoglio and Rigault (1957) attribute high

europium concentrations in pyromorphite group minerals to the easy

reduction of europium to the 2+ oxidation state, which results in an

ion too large to be easily incorporated into minerals formed during

the first phase of crystallization from a magma. Thus europium is

concentrated in the residual solution and is concentrated preferentially 2 + in minerals which are formed last, replacing large ions such as Sr

and Pb 2+ . There are no further reports of REE in minerals of the pyro-

morphite series.

2.3 FACTORS WHICH MAY DETERMINE REE CONCENTRATIONS IN MINERALS

PRECIPITATED FROM HYDROTHERMAL SOLUTIONS

There are several factors which might be expected to play a part

in determining REE concentrations in minerals, such as the pyromorphites, which are precipitated from hydrothermal solutions:-

1. the concentrations of REE ions in the solution

2. the mobilities of RE ions in solution, which are in

turn affected by such factors as temperature, pH,

pressure, complex formation and the solubility

products of RE salts.

3. the ability of the crystal structure of the mineral

to accommodate RE ions, and whether substitution by

light or heavy REE is preferred. 24

4. The removal of REE by co-precipitating minerals.

Kosterin (1959) suggests that carbonate, sulphate and possibly

halide complexes play an important role in the transport of RE ions,

especially in neutral or alkaline solutions where they would otherwise

be precipitated as hydroxides or carbonates. Carbonate and sulphate

complexes of Ce-group REE are less stable than those of Y-group REE

which therefore tend to remain in solution longer. Complex formation would therefore be expected to produce fractionation of light and heavy

REE, light REE being concentrated in minerals precipitated at an early

stage of the mineralizing process and heavy REE concentrated in minerals

formed at a later stage. Such fractionation has been observed by

Turovsky et al. (1968) in a series of progressively crystallizing minerals.

There is disagreement as to the part played by the crystal

structure in causing fractionation. Turovsky et al. (1968) are of the opinion that crystallochemical factors are relatively unimportant, whereas Kosterin believes that effects due to complex formation may be accentuated or diminished depending on whether the crystal structure makes substitution by light or heavy REE more favourable.

Turovsky et al. (1968) found that the solubility of RE salts was also important in determining REE concentrations in the mineral, since, in their series of progressively crystallizing minerals, the highest total REE concentrations were found in minerals precipitated during the period of precipitation of independent RE compounds. 25

2.4 BRIEF DESCRIPTION OF THE GEOLOGY AND MINERALOGY OF THE

CARROCK FELL AREA

The geology of the Carrock Fell area is shown in the Cockermouth

Sheet (No. 23) of the Geological Survey of Great Britain and described

in the accompanying memoir (Eastwood et al. 1968) and by Firman (1978).

A sketch map of the area is shown in Fig. 2.

The Carrock Fell complex is an intrusion of gabbro, granophyre

and diabase measuring approximately 1x4 miles, which occurs at the

junction of the Skiddaw slate and the Eycott volcanic series. The

granophyre and diabase are younger than the gabbro, but the relative

ages are uncertain (Firman 1978).

A region of extensive hydrothermal alteration in the western part

of the complex, continuing into the volcanic rocks to the north, is

thought to be due to an extension of the Skiddaw granite under the

complex with its axis along Brandy Gill. The Drygill shales to the north of the complex are thought to be related to the Skiddaw slate

and the felsite to the west is related to the granite (Eastwood et al.

1968).

Two types of mineralization occurred in the area. The tungsten veins running north from the Grainsgill granite into the Carrock Fell gabbro belong to the earlier mineralization and are identified with

the last stages of the Skiddaw granite intrusion (Eastwood et al. 1968,

Thimmaiah 1956). Sulphide mineralization occurred later, the ore minerals being introduced into cracks and fissures in the contact zone at the edges of the Carrock Fell complex. The principal minerals are quartz, barytes, sulphides of copper, zinc and lead and their oxidation products. The quartz and barytes are earlier than the Pb, Cu and Zn minerals, and the presence of some veins containing only quartz and 26

fS-2 Sketch map of- tfce g^eolog^ of" tfie Carrock Feil area.

miles

Carboniferous limestone FeJavte +f + -+f +t

j Grr^nopW^e

©o e >o o© Tat f (Hiajn Irebt^ c^oup^ Diabase

M G*bb ro

•*• j Ero^-olT- type lava, (Ki^V* Irebj^ G-eolocycal boundaries

FcVOklVi

Skvdclaw s\a.te Wine ral

Sk\dlc\evW o^Ceurule 27

Fici 6 Ske&h -map shotting, minefftl veins ok rfve.u/es/er n end of" tt\e. Carrock Fell Complex.

O O.S- l-O ic&le.: » i I miles

^ posUfbns Prom tolru'cla rock samples, uiere. Taken fWta . -•r— — - mineral ^ems .B& barium topper M<\ on&na^nese

Pb

Zo Z.irvc W tTvAAc^sfe^ 28

and barytes suggests that there may have been two episodes of mineral-

ization (Thimmaiah 1956).

Fig. 3 is a sketch-map showing the minerals veins at the western

end of the complex from which samples were taken. The mineralization

in these veins is described in detail by several authors (Davidson and

Thompson 1951, Eastwood et al. 1968, Marshall 1973, Postlethwaite 1913

and Thimmaiah 1956).

The pyromorphite and mimetite found at Roughten Gill, Driggith

and Drygill are thought to have been formed as a result of the oxidation

of the sulphide ores under conditions of low pressure and low temperature

(Thimmaiah 1956). At Roughten Gill brown mimetite and green, yellow

and brown pyromorphite are found. The yellow pyromorphite is sometimes coated with plumbogummite PbAl3(P0i+)2(0H)5H2O (Davidson and Thompson

1951, Foertsch 1967). Orange and green crystals of mimetite and pyro- morphite occur at Drygill, often coated with psilomelane (Ba^i^O)2Mri50iQ

(Davidson and Thompson 1951). At Driggith pale green crystals of pyro- morphite are found growing on barytes or quartz. According to Marshall

(1973) these are actually phosphatian mimetite coated with a thin layer . o of pyromorphite. The psilpmelane and plumbogummite are thought to be contemporaneous with or slightly later than the pyromorphite (Thimmaiah

1956).

REE in minerals accomapnymg pyromorphite

There are no references to the presence of REE in the primary sulphides, galena, sphalerite, and chalcopyrite. However, there are reports of studies of REE concentrations in barytes. Zaki and Schroll

(1959) were unable to find any trace of REE in 28 barytes samples from various parts of Austria and Germany using spectrographic analysis, 29

and attributed this to the difference in the ionic radii of Ba and

Later, Guichard and Jaffrezie (1972), using neutron activation analysis with post-irradiation separation, were able to detect La, Sm,

Eu and Dy in barytes. The concentrations were very low, 0.4 - 2.5 ppm

La, 0.02 - 0.07 ppm Sm, 0.01 - 0.37 ppm Eu and 0.05 - 0.09 ppm Dy.

REE have also been detected in barytes by the Russian workers, Bogdanov,

Bur'yanov et al. (1974), who reported the presence of La, Ce and Nd.

The presence of REE in plumbogummite in rocks is reported by

Bain (1969) and a rare-earth analogue of plumbogummite, florencite, is known (Dana 1951). No reference to REE in psilemelane has been found.

2.5 LABORATORY SYNTHESIS OF PYROMORPHITE

The laboratory synthesis of pyromorphite is straightforward and can be effected using common reagents and apparatus. This, and the need for homogeneous crystals of a particular composition has resulted in synthetic minerals being used extensively in X-ray diffraction work on pyromorphite and related compounds. Consequently methods for the formation of pyromorphite are widely reported in literature.

The various methods can be divided into two basic types:-

(1) those in which pyromorphite is formed by the fusion of reagents.

(2) those in which pyromorphite is precipitated from solution.

Fusion of lead orthophosphate and lead chloride in stoichiometric ratios was used by Kriedl er and Hummel (1970). Dana (1951) gives the same method, but using an excess of lead chloride. Similar methods are described by Wondratschek (1963) and Brenner et al. (1970) in which ammonium dihydrogen orthophosphate is used instead of lead orthophosphate. 30

Precipitation methods approximate more closely the formation of natural minerals, and generally involve the addition of a dilute solution of sodium orthophosphate or orthophosphoric acid to a solution of lead chloride (Fortsch 1970, Baker 1966). The disadvantage of these methods is that the precipitates are frequently non-stoichio- metric. Although precipitated pyromorphite is often finely divided, it has been shown that non-stoichiometry is not due to surface-adsorption effects (Posner and Perloff 1957). Substitution of the types discussed in Section 2.1 is thought to be responsible. 31

CHAPTER 3

EXPERIMENTAL

3.1 ANALYSIS OF REE CONCENTRATIONS IN NATURAL PYROMORPHITE-GROUP

MINERALS

Materials and sample preparation

For the study of REE concentrations in minerals belonging to la the pyromorphite group, specimens of natural pyromorphite, vanadate

and mimetite were provided by the British Museum (Natural History).

These specimens are listed in Table 5. The minerals were separated

by hand, ground in an agate mortar and dried by heating at 80°C.

Samples (10 mg) were weighed into clean polythene capsules for

irradiation.

Standards

Multielement REE standards were prepared by pipetting standard

REE solutions (Johnson & Matthey) onto filter-paper and evaporating to

dryness. The standards contained the following amounts of REE:-

M yg La 1 Tb l Ce 10 Dy 0.1 Nd 6 Mo 5 Sm 0.1 Er 2 Eu 0.1 Yb 1 Gd 9 Lu 1

These standards were checked against the NBS standard reference rock

BCR1. 32

Irradiations and counting

Samples were irradiated together with the standards in the

CONSORT II Reactor at the University of London Reactor Centre (ULRC).

Two series of irradiations were carried out. In the first, samples were irradiated for 8 hours in a mixed neutron flux of about 1.2 x 1012 neutrons cm 2s Samples were counted on a Ge(Li) detector connected to a Nuclear Data ND6600 multichannel analyser, after about 8 days

(12 days for mimetite) to detect Sm, Nd and Lu, and again after about

30 days to detect Ce, Tb and Eu

In the second series of irradiations, samples were irradiated for one minute in a flux of approximately 2.4 x 1012 neutrons cm 2s using the ICIS facility at ULRC. Pyromorphite and mimetite samples were counted after 1 minute to detect dysprosium. Vanadinite samples could not be counted until 20 mins after irradiation because of the high activity of 52V (t^ = 3.755 mins), so Dy was not determined in vanadinites. Samples were counted again after a few hours to determine

La, Sm and Eu.

The y-ray peaks used for analysis are given in Table 2. Approxi- mate detection limits are given in Table 3. Detection limits vary with composition because of the different effects of the activities of 32P,

52 76 32 V and As. In pyromorphite bremsstrahlung from P (tA = 14.3 days) 2 increases the detection limits of long-lived isotopes by increasing the level of background radiation. The activity of 76As (t, = 26.6 hrs) is a problem in specimens with arsenic concentrations of more than a few per cent and affects particularly the analysis of La and Sm in mimetite and vanadinite. 52V is short-lived and only affects the determination of Dy. 33

TABLE 2

y-RAYS USED FOR REE ANALYSIS METHOD 1 GE(LI) DETECTOR

Isotope Half-life y-ray Energy

140La 40.27h 486.8 1596.2

141Ce 33d 145.4

147Nd lid 91

153Sm 47h 103.2

152miEu 9.32h 121.8 841.4

12.4y 121.8 1407

165% 1,26m 108.2

16 0xb 72d 197 298.6 879.3

169Yb 32 d 177.2 130

177lDLu 6. 7d 208.4 34

TABLE 2

DETECTION LIMITS FOR THE DETERMINATION OF REE IN PYROMORPHITE, VANADINITE AND MIMETITE

Min. Detectable Concentration (ppm) for a 10 mg Sample Element Pyromorphite Mimetite Vanadinite

La 2.7 6.2 3.8

Ce 19.5 12.5 8.8

Nd 67 44 31

Sm 0.32 0.67 0.56

Eu 0.26 0.16 0.09

Tb 0.53 0.29 0.26

Dy 0.20 0.60 +

Yb 3.22 2.66 1.87

Lu 0.07 0.04 0.03

52V activity prevents measurement of Dy in vanadinite. 35

Pr, Gd, Er, Ho and Tm were not determined. The 80.6 keV y-ray

of Ho could not be resolved, thulium suffered serious interference from

PbKa X-rays at 85 keV and Pr, Gd and Er did not produce sufficient

activity.

3.2 ANALYSIS OF REE IN ROCKS AND MINERALS FROM THE CARROCK FELL AREA

Material

Material from the Carrock Fell area consisted of hand specimens

of pyromorphite in the presence of other secondary minerals (plumbogummite

PbAl 3CP01+) 2(0H) 5H2O, psilomelane (Ba,H20) 2Mn5010 and secondary quartz), S specimens of primary materials (barytes Ba$0^, galena PbS, chalcopyrite

CuFeS2and sphalerite (Zn,Fe)S) and a series of rock samples from a line

approaching the Driggith Vein (see Fig. 3 for locations). The mineral

specimens are described in Table 6 and the rock specimens in Table 7.

Except for specimen No. BM1964, R8104 which is from the British Museum

collection, all material was provided by Dr. C. Stanley (British Museum

(Natural History)).

Sample preparation

Minerals were separated by hand, ground in an agate mortar and

dried by heating at 80°C. 10-15 mg samples were irradiated.

Rocks were crushed in a jaw-crusher, and ground in a tungsten-

carbide mill. The powdered samples were dried at 100°C ^ 30 mg samples were used for irradiation.

Standards

Minerals : as above. 36

Rocks : A method of standard addition was used in which the standard REE solutions were pipetted onto 30 mg of powdered rock sample and evapo- rated to dryness. BCR1 was used as a reference material.

Irradiation and Counting To detect short-lived isotopes the irradiation and counting conditions were the same as for the pyromorphite-group minerals above. The duration of the longer irradiation was increased from 8 hrs to 35 hrs to improve the detection limits of Gd, Ce and Nd. Samples were counted after 5 days and 30 days and a Ge detector was used instead of a Ge(Li) detector. This enabled the low energy y-rays of Ho, Tb and Yb to be resolved. A list of y-rays used for analysis is given in Table 4.

3.3 DOPING EXPERIMENTS USING LABORATORY-SYNTHESIZED PYROMORPHITE The method used for the synthesis of pyromorphite was the one described by Baker (1966) , in which pyromorphite is precipitated from a solution of lead chloride upon the addition of trisodium orthophosphate:

5PbC£2 + 3Na3POi+ •*• Pb5(P0i+) 3Ci + 9NaC£.

Preparation of reagents 1. Lead Chloride : Lead acetate was dissolved, with heating, in the minimum amount of water. The solution was cooled to room temperature and 6N HC£ added slowly until precipitation of lead chloride was complete. The precipitate was left to stand for a few hours before filtering with a No. 1 sintered-glass filtering crucible. The precipitate was washed with cold methanol to remove excess HC£. 37

- TABLE 4

Y-RAYS USED FOR REE ANALYSIS

METHOD 2 GE-DETECTOR

Isotope Half-life Y~ray Energy (keV)

^La 40.27h 328.7

l^Ce 33d 145.45

^7Nd lid 91.10

153Sm 47h 69.67 103.18

152^ 12.4y 121.78 344.30

153Gd 241.6d 97.50

16 0xb 72d 86.80 197.04 298.57

166H0 26. 7h 80.57

16 9Yb 32 d 63.12 130.51 177.18

175Yb 4.2d 113.80 396.32

177mLu 6. 7d 112.95 208.36 , ... 38

2. Trisodium Orthophosphate : 9N H3PO4 was diluted to 10% w/v.

This solution was neutralized by titrating with 3% w/v Na2C0 3 solution, using a pH-meter to determine the end-point. The titrate was boiled occasionally to remove CO2. The resulting NasPO^ solution was diluted to 0.2% w/v.

Preparation of pyromorphite

Lead chloride (200 mg) was dissolved in water (50 ml) at 65°C.

0.2% sodium orthophosphate solution (35.5 ml) was added slowly, the solution being stirred during the addition. Pyromorphite formed as a fine, white precipitate immediately upon addition of the phosphate.

After formation of the pyromorphite, the pH was adjusted to the required value by the addition of NaOH solution. (Without the addition of NaOH the solutions are acid, pH - 2.8, due to hydrolysis + 2_ 2_ of Pbz .) Known amounts of CO3 and SO4 in the form of the sodium salts and 1 ml of a standard REE solution were then added. The con- centration of RE ions in the standard were as follows:-

Ug/ml Vg/ml La 10 Gd 90 Ce 100 Tb 10 Nd 60 Mo 50 Sm 1 Yb 10 Eu 1 Lu 10

Conditions were adjusted after the formation of pyromorphite in order to avoid precipitation and inclusion of other lead salts and RE compounds, and to keep the composition and particle size of the precipitate as con- stant as possible. 39

The solutions were made up to 100 ml with distilled water.

The precipitates were left to age at 65°C before being filtered

through a No. 4 sintered-glass filtering crucible. The precipitate was

washed several times using distilled water and acetone.alternately.

The filtrate and washings were retained for analysis. The conditions

under which pyromorphite was digested are listed in Table 8.

Deviations from this general method were made in the preparation

of specimens 14-19. With specimens 14-16 the precipitate was filtered

and transferred to distilled water to which RE ions etc., were then

added. In the case of specimen 14 extracts were taken from the pre- e. . cipitate at various stages during the ageing process. With specimens

17-19 the REE solution was added before the addition of phosphate.

Identification of the precipitate as pyromorphite was confirmed

using X-ray powder diffraction.

Preparation of samples for irradiation

1. Synthetic minerals : These were treated in the same way as the powdered natural minerals (see above).

2. Filtrates : The filtrates plus washings were made up to a constant volume (200 ml). Aliquots (10-40 ml) were taken and freeze- dried onto filter-paper. The size of the aliquot was determined by the activity which would be produced by the sodium in the filtrate. Because of the volumes of solution and the large amounts of sodium involved, it was not convenient to remove sodium from the filtrates.

After drying the filter-paper and residue were compressed into a pellet and put in a polythene capsule for irradiation. 40

Standards

Standards for the minerals were made by pipetting 0.1 ml of the

standard REE solution used in synthesis onto 10 mg of synthetic pyro-

morphite containing no REE, and evaporating to dryness.

For the filtrates, 0.1 ml of the standard REE solution was

pipetted onto filter-paper, evaporated to dryness, and the filter-paper

made into a pellet.

Irradiation and counting

Conditions were the same as for the 35 hr irradiations of the

Carrock Fell specimens (see above). TABLE 5

PYROMORPHITE-GROUP MINERALS FROM THE BRITISH MUSEUM COLLECTION USED IN ANALYSIS

Ref. No. BM. Number Mineral IR. No. Origin Description

1 BM42241 Pyromorphite 637-8 Leadhills, Lanarkshire Fluorescent pale green crust. 2 AG29 Pyromorphite 131 Rough ten Gi 11, Cumberland Bright green botryoidal massive. Light green centre with darker crust. 3 AGI Pyromorphite 118 Freiberg, Saxony Buff globular aggregates on galena. 4 BM1957, 844 Pyromorphi te 671-3 Wanlockhead, Dumfriesshire Bright yellowish-green botryoidal crust. 5 BM1964, R8100 Pyromorphite - Wheal Hope, Cornwall. Dark brown crystals on galena. 6 AG10 Pyromorphite 207 Leadhills, Lanarkshire Dark brown crystals. 7 BM1956, 227 Pyromorphite 113 Perkins Beach Mine, Salop Cavernous dark green crust on slate. 8 AG81 Pyromorphi te 112 Derbyshire Olive green crust on quartz. 9 BM1964, R8101 Pyromorphite - Leadhills, Lanarkshire Orange-brown crystals. 10 BM27542 Pyromorphite 309 Roughten Gill, Cumberland Bright green sheaths of hexagonal crysta1~ 11 BM27540 As-Pyromorphite 500 Roughten Gill, Cumberland Tapering greenish-yellow crys·tals. 12 BM733 As-Pyromorphite 329 Drygill, Cumberland Yellow-brown, rounded crystals. 13 BM1964 , R8103 P-Mimetite - Drygill, Cumberland Brown crystals. 14 BM1964, R8102 P-Himetite - Wheal Alfred, Cornwall Green hexagonal crystals. 15 BM1958, 178 Mimeti te - Wheal Gorland, Gwennap, Orange-pink crystals. Cornwall 16 BM1972 , 385 Mimetite - Mapimi, Durango, Mexico Yellow crystals. 17 BM1930, 291 Mimetite - Tsumeb, S.W. Africa Yellow prisms encrusted with globular green smithsonite on quartz. 18 BM1930, 285 Himetite - S.W. Africa Olive-green crystals. 19 BM83397 Vanadinite - Hillsboro, New Mexico. Yellow crystals. 20 BMl929 , 193 Vanadinite - Abenab, Grootfontein, Orange-red crystals. S.W. Africa 21 BM1923, 822 Vanadinite - Jebel Mahser, Oudj de, Yellow-grey crystals. Morocco 22 BM1930, 417 Vanadinite - Old Yuma Mine, Nr. Tucson, Red prisms coated with calcite. Arizona TABLE 6

MINERAL SPECIMENS FROM CARROCK FELL AREA

I.D. Origin Description

DGB Drygill Pyromorphite (yellow barrel-shaped hexagonal crystais), psilomelane and quartz.

DGC Drygill Pyromorphite (orange-yellow hexagonal crystals) barytes (white tabular crystals), psilomelane and quartz (pseudomorphous after barytes).

DGD Drygill Pyromorphite (yellow hexagonal crystals), psilomelane and quartz (hexagonal crystals and pseudomorphous after barytes).

DGF Drygill Pyromorphite (yellow hexagonal crystals), psilomelane and quartz. Quartz contained manganese oxides in veins.

BMl964, R8l04 Rough ten Gill Pyromorphite (yellow hexagonal crystals), plumbogummite (blue crust), some Fe and }h1 oxides.

DV80.l Driggith (a) Pyromorphite (small , green, barrell-shaped crystals), quartz and limonite. (b) Barytes (white, massive).

CFS.2 Rough ten Gill Galena

CFlS.9 Driggith Sphalerite and galena.

SC Driggith Sphalerite and chalcopyrite.

GC Driggith Sphalerite and chal copyri te. 43

TABLE 7

ROCK SPECIMENS FROM CARROCK FELL AREA

Specimen Description Identification

DV80.1 Completely altered wall-rock.

DV80.2 Altered tuff.

DV80.3 Altered tuff.

DV80.4 Completely altered tuff near quartz vein.

DV80.5 Fresh porphyritic basalt.

See Fig. 3 for locations. 44

TABLE 2

CONDITIONS UNDER WHICH PRECIPITATES OF SYNTHETIC PYROMORPHITES WERE LEFT TO AGE

2- 2- Sample Digestion pH SO4 added CO3 added I.D. time (y moles) (y moles)

1 24 hours 2.8 - — 2 1 week 2.8 - - 3 12 weeks 2.8 — —

4 1 week 4.8 - - 5 1 week 6.6 - - 6 1 week 7.5 - - 7 1 week 8.7 — —

8 1 week 6.4 45 - 9 1 week 5.3 119 - 10 1 week 7.6 394 —

11 1 week 8.8 - 141 12 1 week 8.5 - 226 13 1 week 9.3 — 2179

*14a 1 week 2.8 - — b 3 weeks c 5 weeks d 9 weeks

*15 1 week 3.4 12 7 — *16 1 week 9.8 — 260

+17 1 week 2.8 — — +18 7 weeks 2.8

* Precipitate transferred into distilled water. f REE added before phosphate. 45

CHAPTER 4

RESULTS

4.1 SURVEY OF REE CONCENTRATIONS IN PYROMORPHITE-GROUP MINERALS

REE concentrations in ppm are given in Table 9 and chondrite- normalized values in Table 10. The values of REE concentrations in

chondrites which were used are those quoted by Haskin, et al. (1976),

except for the Dy value, which is taken from Table 39, 57-71 C2 in

the 'Handbook of Geochemistry* (Wedepohl, 1978). Figures 4-18 show

the chondrite-normalized concentrations plotted against atomic number.

REE were detected in all specimens except two mimetites,

BM1958,178 and BM1930,281. The highest concentrations were found in arsenious pyromorphite and phosphatian mimetite. Only La and Ce were detected in the vanadinite specimens. There is a general enrichment in light REE with the ratio of light to heavy REE being greater in mimetite and arsenious pyromorphite than in pyromorphite containing little arsenic.

In many specimens the chondrite-normalized abundances reach a maximum value at some point between the two ends of the series. The position of the maximum varies from Nd to Yb.

Five specimens had a negative cerium anomaly. Four of these,

BM27540, BM733, BM1964,R8102 and BM1964,R8103 are intermediate members of the pyromorphite-mimetite series (28-86% mimetite), the other, BM27542, is a near end-member pyromorphite. The non-detection of cerium in AG29 is thought to indicate an anomalously low Ce concentration in this speci- men also. It is not possible to say whether the cerium concentrations in the other mimetites and in the vanadinites are anomalously low. Small positive europium anomalies were obtained in six specimens, BM27540,

BM27542, BM733, BM42241 and BM1957,844. 46

4.2 STUDY OF PYROMORPHITE AND ACCOMPANYING MINERALS FROM THE

CARROCK FELL AREA

.(i) Primary minerals

The primary minerals analysed were barytes, BaSOi+, galena,

PbS, chaleopyrite, CuFeS, and sphalerite (Zn, Fe)S. No REE were detected in any of these minerals.

(ii) Pyromorphite and accompanying secondary minerals

Chondrite-normalized REE concentrations are given in Table 12 and are shown plotted against atomic number in Figs. 19 - 23. The

REE concentration profiles in the pyromorphite are similar to those found in specimens BM27540, BM27542 and BM733 (Figs. 13, 14 and 15) which are also from the Carrock Fell area and are of similar compo- sition. There is enrichment of light REE with a large negative Ce anomaly and, in some cases, a small positive Eu anomaly.

Traces of REE are found in the accompanying quartz but this is probably due to the presence of impurities.

High concentrations of REE were found in plumbogummite

PbAl 3(P01+) 2(0H) 5H20 and psilomelane (Ba,H20) 2Mn2010. The concentration profiles for these minerals show a maximum in the middle of the RE series, and the concentrations of heavy REE are much higher than in pyromorphite.

As in the pyromorphite, a large negative Ce anomaly was found in the plumbogummite specimen, but in psilomelane there were positive Ce anomalies.

Although three of the pyromorphite specimens had positive Eu anomalies, there were no Eu anomalies in psilomelane or plumbogummite. 47

Distribution coefficients for the partition of REE between

pyromorphite and psilomelane and between pyromorphite and plumbogummite

are given in Table 13 and are shown plotted against ionic radius in

Figs. 24 and 25.

REE in rocks from the Carrock Fell area

Chondrite-normalized REE concentrations for the five rock speci- mens are listed in Table 15, Figs. 26 - 30 show the normalized concen-

trations plotted against atomic number. The REE concentration profiles are similar for all five specimens, i.e. slight enrichments of light

REE with a small negative Eu anomaly. There is no difference between the altered tuffs DV80.2 - DV80.4 and the fresh basalt DV80.5. Concen- trations in the vein wall rock, DV80.1 are appoximately twice as high as in the others.

4.3 RESULTS OF DOPING EXPERIMENTS

The results of the doping experiments are given in Tables 16 - 18 and Figs. 31 - 35. The REE concentrations have been normalized by dividing the true concentration (ppm) by the concentration in the standard solution

(ppm) .

The distribution coefficients for REE between solid and solution were high, 10 - 10 , and in many cases REE were not detected in the solution.

Variation of REE concentration profile with time (Nos. 1,2,3,14,17,18)

With Nos. 1-3 and 17-18 the ratio light/heavy REE increases with time. This can be seen most clearly by comparing the distribution coefficients (Figs. 31 and 32). A higher proportion of REE enters the solid when the REE are present during the formation of pyromorphite than when they are added later, and comparison of Nos. 3 and 18 shows that this difference persists for several weeks at least. 48

In No. 14, where the precipitate was transferred into distilled

water, comparison of REE concentrations in successive extracts showed a

slight increase in REE concentrations with time but there was iio increase

in fractionation of light and heavy REE.

Effects of pH (Nos. 2 and 4-7)

With increasing pH the distribution coefficients become larger

and there is a decrease in fractionation of light and heavy REE. No S REE were detected in eolutions with pH > 5.

Effect of sulphate ions (Nos. 8-10 and 15)

In Nos. 8 and 9 the presence of sulphate ions had no noticeable

effect. In No. 10 higher concentration of sulphate, 394 ymole/100 ml

produced a lowering of the distribution coefficient, but no fractionation

of light and heavy REE. In No. 15, at a lower pH and with the precipitate

transferred into distilled water, lower concentrations of sulphate,

127 ymole/100 ml were able to produce a greater reduction in distribution

coefficient together with significant fractionation of light and heavy

REE (see Fig. 35).

Effect of carbonate ions (Nos. 11 - 13 and 16)

In Nos. 11 - 13 the addition of carbonate had no effect. In

No. 16 the distribution coefficients were lower than those for pyromorphite

in NaC£ solution of the same basicity. The apparent increase in REE con- cenlration with increasing atomic number in the precipitate is not statis- tically significant and neither the concentrations in solution, nor the distribution coefficients provide evidence for such fractionation. TABLE 9

RARE EARTH CONCENTRATIONS (ppm) IN PYROMORPHITE, MIMETITE AND VANADINITE

La Ca Nd Sm Eu Tb Dy Yb Lu Pyromorphite 1 BM42241 24.6 34 47 6.5 7.62 1.3 11.9 4.9 0.890 2 AG29 16.1 - 50 9.1 5.79 4.5 46.7 30.8 0.410 3 AG1 - -- 2.1 0.85 - 2.40 - 0.075 4 BM195 7, 844 8.4 - - 2.2 1.52 - 1.55 - 0.133 5 BM1964, R8100 34.0 212 - 10.2 2.90 2.1 8.34 7.0 0.860 6 AGIO 29.8 308 103 51.4 23.8 6.2 27.5 5.2 0.495 7 BM1956, 227 32.1 118 - 15.7 6.33 4.0 22.5 14.6 1.60 8 AG81 5.75 - - 2.41 0.72 - 1.55 - 0.203 9 BM1964, R8101 44.0 177 144 26.1 8.80 2.3 28.9 6.5 0.707 10 BM27542 59.6 15 139 38.6 23.8 9.8 48.5 13.5 0.130 Arsenious pyromorphite 11 BM27540 145 29 185 26.5 9.46 1.6 8.42 - 0.135 12 BM733 128 36 269 85 36.9 10.3 43.0 8.3 0.051 o . . I . Ph¢sphat1an m1me11te 13 BM1964, R8103 217 36 225 18 4.47 - 1.3 - 0.064 14 BM1964, R8102 468 67 206 18 3.36 0.3 0.7 - - Mimetite 15 BM1958, 178 ------16 BM1972, 385 35 -- - 1.52 - - - - 17 BM1930, 291 ------18 BM1930, 285 53 161 125 30.2 3.26 - 0.7 - - TABLE 9 (continued)

RARE EARTH CONCENTRATIONS (ppm) IN PYROMORPHITE, MIMETITE AND VANADINITE

La Ce Nd Sm Eu Tb Dy Yb Lu

Vanadinite 19 BM83397 - 27 ------20 BM1929, 193 27 11 ------21 BMl923, 822 10 9 ------22 BM1930, 417 31 ------.,.. -

For origins and descriptions of specimens see Table 5. TABLE 10

CHONDRITE-NORMALIZED REE ABUNDANCES IN PYROMORPHITE, MIMETITE AND VANADINITE

La Ce Nd Sm Eu Tb Dy Yb Lu Pyromorphite 1 BM42241 74.5 39 78 36 110 28 38 24 26 2 AG29 48.8 - 83 50 84 96 150 154 12 3 AG1 -- - 12 12 - 7.7 - 2.2 4 BMI957, 844 25.4 - - 12 22 - 5.0 - 3.9 5 BMl964, R8IOO 103 240 - 56 42 45 27 35 25 6 AGIO 90.3 350 172 284 345 132 89 26 14 7 BM1956, 227 97.3 134 - 87 92 85 73 73 47 8 AG81 17.2 - - 13 10 - 5.0 - 6.0 9 BMl964, R8101 133 201 240 144 l27 49 93 32 21 10 BM27542 180 17 231 213 345 208 156 67 3.8 Arseniolls pyromorphite 11 BM27540 439 33 308 146 137 34 27 - 4.0 12 BM733 388 41 448 470 534 219 138 41 1.5 Phesphatian mimetite 13 BMl964, R8103 657 41 375 99 65 - 4.2 - 1.9 14 BM1964, R8102 1420 76 343 99 49 6 2.2 - - Mimetite 15 BM1958, 178 ------16 BM1972 , 385 106 - - - 22 - - - - 17 BM1930, 291 ------18 BM1930, 285 160 183 208 167 47 12 2.2 - -

cont~nued TABLE 10 (continued)

CHONDRITE-NORMALIZED REE ABUNDANCES IN PYROMORPHITE, MIMETITE AND VANADINITE

La Ce Nd Sm Eu Tb Dy Yb Lu 0 Vanadinite 19 BM83397 - 31 ------20 BM1929, 193 82 12 ------21 BMI923, 822 30 10 ------22 BMl930, 417 94 ------

For origins and descriptions of specimens see Table 5. t§ ~ ~.., ~ 1 1 ~. ~ -5 .§ s

&. ~ E '} & ~ • -- --1 .~ 1 •< .~ ·f S .5 .! e f ~ a: !l: 100 \J1 • 100 \00 \,.oJ • • •

10 10 10 -

1~~~~~~~~~~~~~~r-T-4 I+-~~'--r~~~-T~~~~~--~ LA Ceo N~ 5rn Eu Tb l>j .rooioc. """'..... Yb LA. ~Ce. Net 8m ~11 Tb ~ at;;",", __ Yb '-'" 'l..Q. Ce. Ncl. 8m EI.\ -n ~ ~ ... .-,)'b !4 lOOi- _FICA- 4- . &M4-224-1 l1~rc)"'Ofph'ti. -.£is; 5" AGo 2'1 '100./0 ~ro ..... o .. ph\re. .£i~._6 ~ AG- I 100,," p~ro"'01>)\I~ ~ ~ .... ~ ~ ~ ~ ~ . ~ . S .~ (... • <>. , .- l :-.... - 1 I 1 •• f .S ( .: £ ..."- 1 \J1 100 10 10 .,..

••

10 (0

I 1 I I I I 1..0. Ceo Nd S", El.\. Tb ~ ".. ,.,'" n",..:!.b ~ Lo. Gt. NJ.. $", Eu Ib 1)3 ..10'",,,, "_b~b l...u.

-5~"8 eM (Q6lt, R8lOO 100;0 P'J1'01I'\O'pI-\\~ LI'S' 9.. Ali 10 100'1., P'6f'OfY\Of'pnite. F;~!a. 4. - \~

•

/0 10 10

I ;-~~~~~~~T-T-~~~~~~~ ,\ ,. I

LQ. Ce. Nt:t Sm Eu 1'b ~*';"""M •• ' yb Lv. LA Ceo Ncl Sm etA Tb l)~ "'om><- """,bo.< Yb Lu. FiCA- ·10 8M IQS6,221 '0070 ~ro"'Or,,"'ti.. -Ei~. II ~ A& g~ -- F15~ .Lt. - \ 8

1 ~ J -i 1 ~ /. ~ ~ -5 f • -~ t E ~ "- .' &: ---- "'i/--- -- ...... ~ 1 • 1 ~\ 'e .S -~ ~ 1 l... ~ V1 100 100 100- 0\

• • •

10 10 10

'~~~'-~r-~~'~~'~~I-r~-r~ I.A. Ce. Net &'" E:u. Th l)~ ~fO.,,~ _Vb Lt.\ .£ic~r __IS __ B=--M_t=-_3~_~~.,:!.:6;t.~ - ~ ~ ~ c ~ ~.;: 0 c -5 ~ I: .~ E ~ ..."- l .S -..... I>. E --- '"11- 1c --- 1 'f 1 .:' E r K. e .£ ... %: l... 100 V1 100 100 .....

10 10

Lo. ~ NO. 8m £I! Tb J)3 o.r;,,,,,e '\u!! l.t.\ .£I~. If, eMlqb4,R8tO~' ~~~: ~~~~;:hil€ TABLE 11

REE CONCENTRATIONS IN PYROMORPHITE AND ACCOMPANYING MINERALS FROM THE CARROCK FELL AREA REE CONCENTRATIONS (ppm)

Sample La Ce Nd Sm Eu Gd Tb Dy Ho Yb Lu 1. Pyromorphi te DGB 260 23 205 19.6 6.92 - 0.41 2.40 - - - DGC 194 - 204 41.9 21.0 21.6 0.63 1.43 - - - VI DGD 30 9.9 - 6.9 4.6 - 0.70 2.5 - - - (Xl DGF - - - 2.49 23.9 - - 30.6 - - - BMl964 , R8104 169 14.1 228 80.1 32.6 60.6 11.4 n.d. n.d. 12.4 1.42 DV80.1 142 37.6 286 77 .2 27.5 55.8 6.76 25.0 - 4.35 - 2. Psilome1ane DGB 221 2528 475 140 50.4 136 20.2 n.d. 9.89 22.6 1. 70 DGC 39.3 775 132 50.3 17 .8 51.3 6.99 n.d. 3.29 10.9 0.968 DGD 125 1507 225 76.0 26.1 72 .1 9.69 n.d. 4.04 7.03 - DGF - 161. 37.3 15.6 3.95 10.0 1. 76 n.d. 3.69 1.68 - 3. Plumbogunnnite BM1964, R8104 147 87.9 304 137 51.5 196 31.6 n.d. 45.7 81.6 11.1 4. Quartz DGB - 2.78 - - 0.35 - 0.13 - - - - DGC - - - 4.2 0.975 ------DGD - 3.24 - 1.80 0.330 ------DGF - 1. 94 - 2.80 0.297 - - - DV80.1 - - - 3.20 3.49 5.43 2.42 0.85 2.80 0.309 - - 0.428 0.044 n.d. not determined dash = not detected TABLE 12

CHONDRITE-NORMALIZED REE ABUNDANCES IN PYROMORPHITE AND ACCOMPANYING MINERALS FROM THE CARROCK FELL AREA (concentration in mineral/concentration in chondrites)

Sample Lu Ce Nd Sm Eu Gd Tb Dy Ho Yb Lu 1. Pyromorphite DGB 787 26.1 341 108 100 - 54.5 7.74 - - - DGC 588 - 340 271 304 86.7 13.4 4.61 - - - DGD 90.9 11.2 - 38.1 66.6 - - 8.06 - - - DGF - - - 13.8 34.7 - - 9.50 - - - BM1964, R8104 512 16.0 380 443 472 243 244 n.d. n.d. 61.9 41. 7 DV80.1 430 42.7 447 430 398 224 143 80.6 - 21. 7 - 2. Psi1ome1ane DGB 670 2872 792 772 730 548 430 n. d. 141 112 50 DGC 119 881 220 278 258 206 148 n.d. - 54.8 27.9 DGD 379 1713 426 420 378 289 206 n. d. 57.7 35.1 - DGF - 18.3 62.2 86.2 57.3 40.3 37.4 n.d. - 8.42 - 3. P1umbogummite BM1964, R8104 445 44.8 507 755 746 786 672 n.d. 652 408 327 4. Quartz DGB - 3.16 - - 5.07 - 2.76 - - - - DGC - - - 23.2 14.1 . ------DGD - 3.69 - 9.94 4.78 ------DGF - 2.21 - 15.5 4.3 ------DV80.1 9.70 3.96 9.05 13.2 11.5 13.0 6.57 n.d. - 2.14 1.29 n.d. = not determined dash = not detected 60

Figs- 25 Ifiondrife^normalizid REE conceflfraffons /'/i pgromorphilZ

,at\d accompanying minerals prom fe. tarrock fill area

3 -S

Q p«,\|orneJane- B piyomorphite.

ION

ION

10'-

NoI LOL Ci Sm Eu (xk Tb Dcj Ho atomic numb-er

Rcy fl Speam^n 3)G"E>: pyromor^ile U p&'i/omelane. 61

Ficjs. l°l - 25 fliondrife-normaliied REE concenlrarions in p^romorpK'ile and accompanjincj mmerals from the tarrock Ull area.

Specimer? I)£C : ptjromorphi/e he psilomelane 62

Figs. - 23 . CMnfc-normalized REE concentrations in puromorphifc and accomparw'mg minerals from fe Carrock fill area

21 - Specimen ])(TD: pijromorpKile & ps'ilomelane. 63

Figs. " Chofldnfo-normliied ftEE concgnlfafitins in p^romorphifeT

and accompanying minerals Prom ffe. tarrock fill area.

-cI E Jt -7V3

.E • _ p^romorpK»l«. £& IOM

|OlJ

/

fO J

' 1 r —r~ —r~ —r I r- La Ce Tu Sm Ea Gd Tb Ho atomic norr.ber Yb La

Fig. 22 Specimen l)GrF: pyomorpKife & psilomelanc. 64

Ffo. [°l~23 Chondrite-Morryialized REE concentrafens in

pyomorpkite and accompatmjirKj minerals from t\\t tarrock Hi! afea

lOH • plombo^ummit*. • p^romorpkife

u —i 1 1 r- ~1 1 r- La (U. Sm Eu. Ced Tb alormc number 1-U

11 .Specie BMl^RgW : pyromorphite. U plumbo^ammife. TABLE 13

DISTRIBUTION COEFFICIENTS FOR THE PARTITION OF REE BETWEEN PYROMORPHITE AND ASSOCIATED PSILOMELANE AND PLUMBOGUMMITE Concentration in psilomelane or plumbogummite/concentration in pyromorphite

La Ce Nd Sm Eu Gd Tb Yb Lu

DGB psilomelane 0.851 110 2.32 7.15 7.3 - 7.89 - - + pyromorphite

DGC psilomelane 0.202 - 0.647 1.02 0.849 2.38 11.0 - - + pyromorphite

DGD psilomelane 9.27 153 - 11.0 5.67 --- - + pyromorphite DGF psilomelane -- - 6.25 1.56 -- - - + pyromorphite

BM1964, R8104 0.86 2.80 1. 33 1. 70 1.58 3.23 2.75 6.59 7.84 pyromorphite + piumbogunnnite 66

Ih Partition of- belween plunftbogummile. arvci pcjromorphife', distribution coefficients plotted against" ionic radius 67

•i 1

< Vi

8

Q> t0 'I< c

1 c8 8 1.0—-

g/ Specimen ZD^C • speci^e-A 3>G-8 O.l n ji 11 i—i r ur yr ezmasx A*T tir 'ionic. rcvdi'ius

ParWOrx op" REE betwean

psvlomelane. and pyromorphile;

distribution coefficients plolled against"

lomc radius . TABLE 14

REE CONCENTRATIONS IN ROCKS FROM THE CARROCK FELL AREA

REE concentration (ppm)

Sample La Ceo Nd Sm Eu Gd Tb Ho- Yb Lu DV80.1 53.0 (vein wa11 rock) 91.6 39.6 8.40 1.99 - 2.10 2.46 7.06 1.07

DV80.2 30.3 (altered tuff) 53.3 25.8 5.74 1.44 - 1.13 1.51 3.21 .0.546

DV80.3 (altered tuff) 31.6 57.7 30.4 6.82 1.77 - 1.39 1.62 3.29 0.688

DV80.4 37.0 (altered tuff) 79.7 38.5 8.03 2.04 - 1.14 - 3.36 0.623

DV80.5 (fresh basalt) 32.8 72.2 42.2 7.71 1. 73 - 1.53 1.44 3.92 0.689

The positions from which the rock samples were taken are shown in Fig. 3. TABLE 15 Il REE CONDRITE-NORMALIZED ABUNDANCES IN ROCKS FROM THE CARROCK FELL AREA !. REE concentration in rock/REE concentration in chondrites

Sample La Ce Nd Sm Eu Tb Ho Yb Lu DV80.1 160 (vein wall rock) 104 66.0 46.4 28.8 44.7 35.1 35.3 31.5

DV80.2 (altered tuff) 91.8 60.6 43.0 31. 7 20.9 24.0 21.6 16.0 16.1

DVSO.3 94.8 (altered tuff) 65.6 48.1 37.7 25.6 29.6 23.1 16.4 15.8

DV80.4 112 (altered tuff) 90.6 - 44.4 29.6 24.2 - 16.8 15.3

DV80.5 (fresh basalt) 99.4 82.0 52.7 42.6 25.1 32.6 20.6 19.6 16.8 70

Fit*. 2.6-3*0 Ckonfete." rAprmal.iz.€.d conc^nlraHbris in rocks

from.rt \e..CarrocJ

%Z6 DV80-S. fresh basalt"

fy.27. 80- ^ altered faPF

Ra.28 3)V 80.3 altered faff

t 1—i r LA Cft Nd Sm Eu Tb Ho,. .BTortvc .. wwioe, Yrb LM 71

Figs. 2.6-50 Ckondirife.-r\ormalizedi REE'canckr\\rations,.!n rocks

U lfe. Cerrock Fell area TABLE 16

NORMALIZED REE CONCENTRATIONS IN SYNTHETIC PYROMORPHITE (ppm in mineral/ppm in standard solution)

Sample ID Time La Ce Nd Sm Eu Cd Tb Ho Yb Lu

1. Variation in digestion time 1 24 hrs 5.66 4.84 4.31 4.94 5.37 4.60 5.08 5.50 5.12 4.52 2 1 wk 4.46 5.27 5.08 4.58 4.96 4.18 4.75 4.30 4.15 3.83 3 12 wks 5.43 4.76 4.94 5.37 4.96 4.69 4.78 4.49 3.69 3.51 ...... *14a 1 wk 5.05 5.68 4.47 5.fo 5.09 4.27 4.70 3.22 3.76 3.50 N b 3 wks 5.88 5.73 5.47 5.14 5.15 4.86 4.63 3.71 4.00 3.59 c 5 wks 5.99 6.16 5.57 5.71 6.18 5.11 5.43 3.90 4.55 3.76 d 9 wks 6.03 5.58 5.02 5.33 5.80 4.58 5.42 3.94 4.66 3.90

Sample ID pH La Ce Nd Sm Eu Cd Tb Ho Yb Lu

2. Variation in EH 2 2.8 4.46 5.27 5.08 4.58 4.96 4.18 4.75 4.30 4.15 3.83 4 4.8 5.21 4.62 4.74 4.57 4.61 4.76 4.74 4.47 4.58 4.63 5 6.6 5.36 5.64 5.19 5.08 5.24 5.41 5.77 5.45 5.26 5.15 6 7.5 5.16 5.29 4.48 4.98 4.71 4.51 5.04 5.58 4.96 5.02 7 8.7 5.34 5.49 5.10 5.71 5.36 5.30 5.49 5.42 5.75 5.42

continued •••••••...••••• TABLE 16 (continued)

NORMALIZED REE CONCENTRATIONS IN SYNTHETIC PYROMORPHITE (ppm in mineral/ppm in standard solution)

2- S04 added Sample ID pH La Ce Nd Sm Eu Cd Tb Ho Yb Lu (lJ mole)

3. Effect of su1Ehate 8 45 6.4 5.17 5.68 5.52 5.36 5.17 5.74 5.32 5.35 5.31 5.34 9 119 5.3 5.03 5.50 5.15 5.33 5.48 5.44 5.45 4.94 5.28 5.34 10 394 7.6 5.51 5.49 4.97 5.36 4.97 5.02 5.35 5.54 5.44 5.17 *15 127 3.4 4.71 4.20 4.43 4.30 3.23 2.60 2.49 1.83 1.68 1.61 2- C03 added Sample ID pH La Ce Nd Sm Eu Cd Tb Ho Yb Lu (ll mole)

4. Effect of carbonate 11 141 8.8 5.28 5.87 5.73 5.21 5.80 5.87 5.40 5.35 5.22 5.37 12 226 8.5 5.76 5.57 4.77 5.50 5.33 4.96 5.46 5.36 5.21 5.60 13 2179 9.3 5.40 5.51 5.36 5.49 5.65 5.16 5.56 5.29 5.29 5.42 *16 260 9.8 3.70 3.31 4.21 4.25 4.55 3.76 4.05 4.74 5.34 4.90

Sample ID Digestion time La Ce Nd Sm Eu Cd Tb Ho Yb Lu

5. Addition of REE before formation of pyromorphite 17 1 wk 5.97 5.63 5.37 5.54 5.37 5.39 5.33 5.80 5.21 5.05 18 7 wks 4.77 5.39 4.90 4.33 5.20 4.70 5.32 4.00 4.29 4.05

*Precipitates transferred to distilled water before the addition of REE etc. TABLE 17

NORMALIZED CONCENTRATIONS OF REE REMAINING IN SOLUTION (ppm in solution x 104/ppm in standard solution)

Sample ID Time La Ce Nd Sm Eu Gd Tb Ho Yb Lu

1. Variation of di~estion time 2 1 wk 3.10 I 6.57 4.85 3.55 5.88 10.4 10.5 16.1 21. 3 23.5 3 12 wks < 3.23 0.82 2.33 < 0.46 1.89 2.82 3.39 8.20 19.4 24.6 Solutions 1I and 4 were not analysed

Sample ID pH La Ce Nd Sm Eu Gd Tb Ho Yb Lu

2. Variation of EH 2 2.8 3.10 6.57 4.85 3.57 5.88 10.4 10.5 16.1 21.3 23.5 4 4.8 < 6.12 0.73 < 3.12 0.98 < 0.45 < 0.54 0.33 0.27 0.63 0.30 5 6.6 < 5.34 < 1.02 < 1.36 < 0.78 < 0.36 < 0.35 < 0.12 < 0.16 < 0.31 < 0.11 6 7.5 < 4.94 < 0.63 <12.3 < 0.69 < 0.67 < 0.61 < 0.42 < 0.16 < 0.21 < 0.11 7 8.7 < 7.75 < 0.59 < 1.37 < 1.20 < 1.04 < 0.91 < 0.28 < 0.30 < 0.37 < 0.14

con tinued ....•.•. ~ ..•....••.••.• TABLE 17 (continued)

NORMALIZED CONCENTRATIONS OF REE REMAINING IN SOLUTION (ppm in solution x 104/ppm in standard solution)

2- S04 added Sample ID pH La Ce Nd Sm Eu Gd Tb Ho Yb Lu (].l mole) 3. variation in sol- concentration 8 45 6.4 < 5.05 0.73 < 3.17 < 1.80 < 0.45 < 0.54 0.33 0.15 0.64 < 0.42 9 119 5.3 < 6.75 < 0.56 < 1.09 < 0.87 < 0.48 < 0.75 < 0.23 0.21 < 0.30 < 0.08 10 394 7.6 < 9.94 3.19 2.87 2.00 2.02 2.84 3.01 2.96 2.62 2.65 *15 127 3.4 19.4 17.1 21.0 21.2 27.6 34.5 36.6 45.2 44.8 48.4 2- C03 added Sample ID pH La Ce Nd Sm Eu Cd Tb Ho Yb Lu (11 mole) 2- 4. Variation in COa concentration 11 141 8.8 < 7.01 < 0.48 < 1.84 < 0.78 < 0.60 < 0.54 < 0.19 < 0.16 < 0.44 < 0.16 12 226 8.5 < 5.39 < 0.52 < 0.99 < 0.75 < 0.82 < 0.71 < 0.23 < 0.17 < 0:23 0.15 13 2179 9.3 <15.3 < 0.64 < 6.83 1. 75 < 0.87 < 0.83 < 0.28 < 1.97 < 0.65 < 1. 73 *16 260 9.8 <10.2 11.8 3.94 6.80 5.94 5.52 6.20 6.36 3.92 7.00

Sample ID Digestion Time La Ce Nd Sm Eu Cd Tb Ho Yb Lu

5. Addition of REE before formation of J>Yromorp_hite 17 1 wk < 1.49 1.44 1.30 2.74 1.51 2.01 2.18 4.01 7.27 9.03 18 7 wks < 2.97 0.56 0.77 0.58 0.49 1.13 1.18 3.01 9.28 10.6

* Precipitate transferred to distilled water before addition of REE etc. TABLE 18 DISTRIBUTION COEFFICIENTS FOR THE PARTITION OF REE BETWEEN SYNTHETIC PYROMORPHITE AND AQUEOUS SOLUTION (distribution coefficient (solid/solution) x 10-3)

Sample ID La Ce Nd Sm Eu Gd Tb Ho Yb Lu , 2 14.4 . 8.02 10.4 12.8 8.43 4.03 4.43 2.68 1.95 1.63 3 - 57.9 21.2 - 26.2 16.6 14.1 5.47 1.91 1.43 4 - 63.6 - 46.6 -- 143 162 72.8 156 8 - 78.2 - - - - 161 352 84.4 - 10 - 17 .2 17.3 26.8 24.6 17.7 17.8 18.7 20.8 19.5 15 2.46 2.45 2.11 2.03 1.17 0.75 0.68 0.40 0.37 0.33 16 - 2.81 10.7 6.25 7.66 6.81 6.53 7.45 7.37 5.43 17 - 39.1 41.3 20.2 35.6 26.8 24.4 14.5 7.16 5.59 18 - 95.4 63.6 74.6 106 41.6 45.1 14.6 4.62 3.80

SamE1e ID Conditions 2 Digestion time 1 wk, pH 2.8 3 Digestion time 12 wks, pH 2.8 4 pH 4.8 2_ 8 pH 6.4 SO~_ cone = 45 ~ moles/lOO ml 10 pH 7.6 SO~_ cone = 394 ~ moles/lOO m1 15 pH 3.4 SO~_ cone = 127 ~ moles/100 m1 ppt. transferred to distilled water 16 pH 9.8 C03 conc 260 ~ moles/lOO ml ppt. transferred to distilled water 17 Digestion time 1 wk, pH 2.8) REE added before formation 18 Digestion time 7 wks pH 2.8) of ppt. 77

R£E distribution bdvveen sijntfefic pyomorphile ar\d a^eous solutfofl

10 I week 3 • offer 12 weeiu £

Jo

Vt fig. 31' l/ariatfon of- distribution |0T coefficient"w/ffi lime I: REE

added af/S* ifie formation of

pyromorphife.

—i—2—1— U Ce Nd Sm Eu Tb Hcolbm\» .c «\umbeVbr U

\/ar)£ifIbn of dklf/biifror)

co^fPi'aenlr witK tJrne 2: pyro-

morpKite precipitated in TRi

presence of REE.

T—i—i—r HL , yb U Sm Eu Gd Tt atomic number 78

f^s.Sl - REE dislributfon between s^nlfeKc

Fi^ ^^ . Variation of- diatnbvAnon coepfcciervl' WitK pH. 79

Fi^s. SI -35 REE distribution between

_syr\\Ket£ pyomorphile ancj acyeous solution.

lO

8

1 i .3

&

• -SO^ cent =

101 1 1 1 1—I 1 1 1 1— Co. La Nd Sn, B* fid Tb Haforav0f..c numbe"ftr U

Fig- 32+ Effect' of- sulphoE ions on

tfe distribution coefficient* I: in neutral

saline solution. 80

REE distribution beiwe&n "sijnfetic pyomorph'iK and aeneous solution

7TJ r- 1—i—i—r La Le Nd Sm Bx GA Tb Ho . Yb U atomic number

%2)5". Effetf of* sulpKale tons on tfe.

distribution coefficient" 2,: in aa4 2>olu(ibn . 81

CHAPTER 5

DISCUSSION

5.1 ANALYSIS OF MINERALS AND ROCKS FROM THE CARROCK FELL AREA

(i) The effects of crystal chemistry in determining REE concentrations

in contemporary minerals

The study of minerals and rocks from the Carrock Fell area was intended to provide information concerning the relative importance of various factors controlling REE concentrations in minerals precipitated from hydrothermal solutions, with particular reference to the pyromorphite group. Unfortunately, owing to the lack of suitable specimens, factors such as sulphate and carbonate complexation could not be investigated, but the analysis of the contemporary secondary minerals, plumbogummite and psilomelane, yielded information on the role of crystallochemical factors.

If geochemical factors were dominant in determining REE concen- trations in minerals, then minerals which are formed at the same time would be expected to exhibit similar REE concentration profiles. Analysis ol yy iromorphite and accompanying plumbogummite and psilomelane clearly show i"hat this is not the case. 3+ 2+ In both pyromorphite and plumbogummite, RE replaces Pb

The lead site in plumbogummite has 12-fold coordination compared with 8- and 9-fold in pyromorphite, and plumbogummite might therefore be expected to show a greater preference for larger RE ions than does pyromorphite.

However, according to Dana (1951), the lead site in plumbogummite is very tolerant to ionic size, and this is probably the reason for the increasing value for the ratio of concentration in plumbogummite/concen- tration in pyromorphite with decreasing ionic radius (Fig. 24). 82

The differences between pyromorphite and psilomelane are more striking because of the much greater difference between the concentrations of Yt-group elements in the two minerals, and the large cerium anomalies of opposite sign. It is not certain how RE ions are incorporated into 2+ . psilomelane, but it is most likely that they replace Ba in the tunnels which run through the Mn02 framework of the structure; however they could replace Mn2 + , or both' . Substitutio. n of barium would be expected to favour inclusion of large RE ions, although the barium site by its nature would be likely to tolerate a wide range of ionic sizes. On the other hand, substitution by small RE ions would be preferred if manganese were being replaced. The REE concentration profiles observed in psilomelane could result from (1) a preference for substitution by small RE ions and an enrichment of Ce-group elements in the solution, or (2) a preference for substitution by large RE ions but with the distribution coefficients for the partition of^REE between the precipitate and the solution being much higher in psilomelane than in pyromorphite. Whatever the explanation, the results show that geochemical factors are not the only ones determining REE concentrations in minerals precipitated from hydrotheraal solution. The crystal chemistry of the mineral also plays an important role. The importance of crystallochemical factors may mean that REE concentrations in a mineral can be greatly affected by the nature of co-precipitating minerals; for instance, the co-precipitation of pyromorphite and psilomelane may result in an increase in the Ce/Yt-group ratio in pyromorphite and a lowering of the concentration of Ce-group elements in psilomelane. 83

No explanation has been found as to why there are small emaopium anomalies in pyromorphite but not in the accompanying psilomelane.

(ii) Source of the REE in Carrock Fell pyromorphite Analysis of rocks from the region of the Driggith Vein shows that there is little difference in REE concentrations between the altered tuffs and the fresh basalt, but that REE concentrations are higher in the vein wall rock than in the altered tuffs. This suggests that at Driggith REE have not been leached from the surrounding country rocks but have been introduced into the vein from some other source. The apparent absence of REE in the primary sulphides and barytes make those unlikely to be the source, but it is possible that REE are present in very low concentrations below the detection limits of the method of analysis used, and are concentrated in the secondary minerals, as are the trace elements vanadium and chromium (Shazly et al. 1957). Another possible source is the apatite which is found in the tungsten veins further south at Carrock Mine and in the Carrock Fell gabbro, and which has been suggested as the source of phosphate, since there are no primary phosphate minerals in the Pb-Zn-Cu veins. (Thimmaiah 1956). Apatite from Carrock Mine and from the Carrock Fell gabbro is presently being analysed.

5.2 RESULTS OF DOPING EXPERIMENTS The partition of trace elements between a precipitate and a solution is discussed in standard textbooks dealing with the theory of precipitation such as those by McKay (1971) and Vogel (1966). A number of factors are involved. The partition will depend inter alia upon the 84

number of adsorption sites on the surface. This is determined by factors related to crystal growth and is independent of the trace element concentration. With the number of sites being fixed, if the concentration of the trace ions is relatively low a larger tKe. proportion will be adsorbed than if^concentration were higher. The distribution coefficient will therefore vary with trace element concentration as well as with the conditions under which the precipitate is formed. This problem is discussed in greater detail by Navrotsky

(1978). The partition will also be affected by the competition between different ions for the available sites. Electrostatic attraction be- tween the solid and the ion will favour adsorption of ions with high charge, but if surface exchange is allowed to occur before the foreign ion is buried in the bulk solid i.e. if the rate of crystal growth is slow, the relative stability of ions in the lattice also becomes im- portant. Surface exchange involving a lattice position requires an activation energy and therefore the partition would be expected also to depend on temperature. As the precipitate ages, large crystals grow at the expense of small crystals. Foreign ions on the surface of larger crystals are therefore incorporated into the crystallizing solid whilst those in small crystals are continually being released into solution. Equili- brium between trace elements in the bulk solid and trace elements in solution cannot be attained until complete recrystallization has taken place. 85

In the doping experiments which were carried out there was an increase in the total concentration of REE in pyromorphite and an in- crease in the Ce-group/Yt-group ratio with increasing digestion time. This was to be expected as recrystallization and surface exchange proceed and the competition for adsorption sites becomes keener as the surface area decreases. It is evident from these results that fraction- ation of REE can occur in the absence of complexing ions such as sulphate and carbonate. As a result of the reaction between lead chloride and sodium orthophosphate chloride ions were present in high concentrations in the solutions from which pyromorphite was precipitated. The experi- ment in which the precipitate was transferred into distilled water before the addition of the rare-earth elements (sample No. 14) shows that com- plexation by chloride ions is not responsible for the observed fraction- ation. Fractionation is therefore attributed to greater stability of the larger RE^+ ions in the pyromorphite lattice. Although in sample No. 14 the total concentration of REE in the precipitate increased with time, there was no increase in the Ce-group/ Y-group ratio in the precipitate. This is thought to be the result of selective removal of light REE from the system as successive extracts were taken. REE concentrations in pyromorphite were higher when the RE ions were present during the formation of the precipitate than when they were added after precipitation was complete. This is not surprising as an RE i-jii adsorbed on the surface of a growing crystal would rapidly become trapped as more precipitate was formed. Even after several weeks1 digestion REE concentrations remained higher in the precipitate which had been formed in the presence of RE ions, showing that equilibrium 86

between REE in the solid and in the solution was not achieved in this time. Since in other experiments only one week was allowed for the ageing of the precipitates, the results for the entire series of doping experiments were obtained under non-equilibrium conditions. With increasing pH there is less fractionation and the dis- tribution coefficients (solid/solution) become much higher (see Fig. 33). Pyromorphite is much less soluble in alkaline conditions than in acid, and the solubility may be depressed further by the presence of sodium ions in solution. (NaC£ is present as a by-product of the reaction to form pyromorphite, and NaOH was added to increase the pH.) The rate of recrystallization is therefore slower in alkaline solution. This might be expected to lower REE concentrations in the precipitate, since REE ions would be confined to surface layers, instead of becoming r distributed throughout the bulk of the solid, but the lack of rec^ystalli- zation also means that the surface area of the precipitate remains large, offering greater opportunity for surface adsorption. In addition, surface adsorption of RE ions may be promoted as a result of complex formation 2+ - . . 2+ . between Pb and OH , which would stabilize Pb ions in solution. The ability of sodium ions to enter the pyromorphite crystal structure, pro- viding a means for compensating for an extra positive charge on a sub- stituting ion elsewhere, raises the possibility that the higher sodium ion concentrations in alkaline solutions may promote substitution by RE ions. (Sodium concentrations in the precipitates are discussed in Appendix

2.) Similarly, substitution of phosphate b2y+ the comple3+ . x ion ((OH)^)^ may also facilitate the substitution of Pb by RE in alkaline solution. In situations where complex formation is involved the partition of RE ions between a precipitate and a solution will still depend on the number of adsorption sites available, but will also be influenced by factors which 87

affect the stability of the complex or which favour inclusion of the trace element in the precipitate. It is therefore not surprising that the ability of carbonate and sulphate complexes to hold back RE ions in solution and to produce fractionation was found to vary with the conditions under which the precipitate was left to age. In alkaline, saline solution the presence of carbonate ions had no noticeable effect on the partition of RE ions between pyromorphite and solution, but in the case where the precipitate was transferred into distilled water (No. 16) there was a significant lowering of the dis- tribution coefficients compared with those obtained under similarly alkaline conditions, although there was no fractionation of RE ions. These results may be due to the possible effects of the sodium ion concentration, mentioned above. In neutral or slightly acid solutions sulphate ions were effective in keeping RE ions in solution but did not produce fractionation. In sample No. 15, where the precipitate was transferred into distilled water and the pH was lower, the presence of sulphate ions had a greater effect and did produce fractionation of the Ce- and Yt-group RE ions. These + differences may again be partly due to differences in the Na concentration, but are probably due also to the greater degree of recrystallization which occurs in acid solution.

5.3 REE CONCENTRATIONS IN NATURAL MINERALS OF THE PYROMORPHITE-GROUP (i) Ce and Eu-anomalies co . Anomalous eud^pium and cerium concentrations are generally assumed to be the result of deviation from the 3+ oxidation state, which is characteristic of the rare-earth elements. Ce4 + .i s thermodynamically 88

unstable in aqueous solution with respect to Ce (E = 1.3-1.6, de- pending on the anion present), but it can be stabilized kinetically by complexing ions such as sulphate, fluoride and chloride. Since pyromorphite-group minerals are formed by the oxidation of sulphides, the hydrothermal solution will be oxidising and rich in sulphate at least initially in the oxidation zone, and it is therefore reasonable to assume that'the negative cerium anomalies in members of the mimetite- 4+ . pyromorphite series reflect the presence of Ce m solution. Eu2 + is also unstable in aqueous solution (E = -0.43) and is rapidly oxidise. . d by traces of dissolved. , elemental oxygen. Eu2 + is therefore likely to exist only in solutions where conditions are 4+ 2+ anaerobic and reducing. It is inconceivable that Ce and Eu could exist together in the same solution, and yet some specimens possess both negative cerium and positive europium anomalies. This could indicate a drastic change in the oxidation potential of the solution, but this is unlikely, and it is the author's opinion that europium is in the 3+ oxidation state, and that the small positive europium anomalies are due to anomalously high concentrations of europium in the solutions, (ii) Differences in total REE concentrations and Ce/Yt-group ratios The results of the analysis of natural minerals show a correlation between the type of REE concentration profile observed and the position of the mineral in the vanadinite-mimetite-pyromorphite series. Members of the vanadinite-mimetite series have low total REE concentrations combined with high Ce-group/Yt-group ratios, and arsenious members of the mimetite-pyro- morphite series have high total concentrations and high Ce-group/Yt-group ratios, whilst in end-member pyromorphites both the total concentrations and the Ce-group/Yt-group ratios tend to be lower. 89

Although crystallochemical factors are important in causing the general enrichment of Ce-group REE which is observed it is unlikely that the slight differences in the crystal structure of vanadinite, mimetite and pyromorphite could be responsible for the differences in REE concen- tration profiles between these minerals, and it is therefore assumed that the differences are due to these minerals being precipitated under different conditions. In the review of previous work on the behaviour of REE in hydrothermal solutions (Section 2.3) it was noted that, due to the formation of sulphate and carbonate complexes of RE ions, Ce-group elements are con- centrated in the earliest minerals to be precipitated and Yt-group elements in later minerals, and that the maximum REE concentrations are found in minerals which are precipitated during the period of precipitation of independent RE compounds. On this basis, the results lead to the conclusion that minerals belonging to the vanadinite-mimetite series are precipitated at an earlier stage than are minerals of the mimetite-pyromorphite series. The absence of vanadinite-type patterns in pyromorphite and vice versa suggests that there is little overlap between the periods of precipitation of these two minerals, whereas the similarity between some mimetites and vanadinites, and between some mimetites and end-member pyromorphites e.g. AGIO, BM1964, R8101 and BM1930,285 (Figs. 9, 12 and 18) suggests that the precipitation period of mimetite overlaps those of vanadinite and pyromorphite. The low REE concentrations in members of the vanadinite-mimetite series may indicate precipitation close to the oxidation zone, where acid conditions and high sulphate concentrations would tend to keep RE ions in solution. 90

However, the results of the analysis of minerals from the Carrock Fell area and of the doping experiments show that the formation of com- plexes of RE ions is not the dominant factor determining REE concentrations f , , in pyromorphite, and that the^e are other factors which should be considered. The doping experiments showed that the preference of the pyromorphite crystal structure for the larger RE ions could bring about fractionation of REE in the absence of complexing ions. The precipitates formed in the doping experiments were very fine with poorly-formed crystals: it is possible that in large, well-formed crystals, which have been grown slowly, fractionation to the extent observed in some natural minerals could be caused by crystal chemistry. However, the influence of complex formation, especially by sulphate ions cannot be ruled out. Comparison of Figs. 4* 32 and show that the type of fractionation produced by sulphate complexes, in which the distribution coefficient plotted against atomic number falls sharply in the middle of the RE series but levels out at either end, is distinct from that produced by crystal chemistry, in which there is a steady decrease in the value of the distribution coefficient with increasing atomic number among Yt-group elements. One of the mimetite specimens BM1964,R8103 Fig. 16, has a con- centration profile which may show the influence of sulphate complexes. The doping experiments suggest that complex formation may have a greater influence on REE concentrations in pyromorphite-group minerals precipitated under acid conditions than in those precipitated from alkaline solutions. The importance of crystal chemistry means that the rate of crystal growth will also be an important factor influencing REE concentrations in a mineral. In this context it is significant that all the vanadinite, mimetite and arsenian pyromorphite specimens in the survey were in the form of well-developed crystals and had high Ce-group/Yt-group ratios, whereas the majority of end-member pyromorphites were in the form of crusts and had low Ce-group/Yt-group ratios. The formation of well-developed crystals would be favoured by acid conditions in which pyromorphite-group minerals are more soluble. Alkaline solutions would be expected to give rise to less well-formed crystals. Since more acid conditions are likely to be found nearer to the oxidation zone the conclusion that, of the minerals used in the survey, the vanadinite and mimetites were formed nearer the oxidation zone than were the pyromorphites is probably correct, but the unequal distribution of crystal habits among the three minerals means that there may, in general, be greater similarity between REE concen- tration profiles in vanadinite, mimetite and pyromorphite than the results of this survey indicate. It would be interesting to see whether mimetite and vanadinite in which crystals were not well-developed exhibited REE concentration patterns similar to those found in pyromorphite. The comparison of REE concentrations in psilomelane and pyromor- phite and the doping experiments show that the nature of co-precipitating minerals and sodium ion concentrations may also play a part in determining REE concentrations in pyromorphite-group minerals. It is, however, imposs- ible to tell to what extent these factors may have influenced the REE con- centrations in the minerals used in the survey. Mineralogical evidence to support the conclusions which have been drawn concerning the relationship between vanadinite, mimetite and pyromor- phite is sparse and inconclusive. The limited substitution which is found between V and P in natural minerals, compared with the extensive substitution between V and As and As and P, supports the view that there is little overlap between the precipitation periods of vanadinite and pyromorphite, but that mimetite overlaps both pyromorphite and vanadinite. 92

Marshall (1973) and Nakomote et al. (1969) report the occurrence of mimetite coated with pyromorphite, proving that pyromorphite is precipitated after mimetite, but although Kautz and Gufcser (1969) too, in microprobe analyses across basal sections of pyromorphite- group minerals, found a mimetite specimen in which the outermost zone was enriched in phosphate, they also discovered an arsenian pyromorphite specimen in which arsenic was concentrated in the outermost zones, with higher phosphate concentrations in the centre of the grain.

5.4 SUMMARY Crystal structure and the rate of crystal growth are major factors influencing the partition of RE ions between aqueous solution and pyromorphite, although the formation of sulphate and carbonate complexes may also be important. Differences in crystal structure give rise to different REE concentration profiles in co-precipitated minerals. Substitution by sodium ions in pyromorphite provides a means of compensating for the extra positive charge on RE ions, and high concentrations of sodium ions in solution increase the proportion of RE ions entering the mineral. In the survey of natural minerals of the pyromorphite group there was a correlation between the type of REE concentration profile and the position of the mineral in the pyromorphite series, suggesting that minerals of different composition are formed under different con- ditions. Interpretation of the results on the basis of the effects of PXE complex formation and of the crystal structure of pyromorphite leads to the conclusion that vanadinite and mimetite are formed under 93

more acid conditions, nearer the oxidation zone than is pyromorphite. However, because of the importance of the number of adsorption sites in determining REE partition, and the non-random distribution of well- and poorly-developed crystals among specimens belonging to different parts of the pyromorphite series, it is thought that the differences in the REE concentration patterns in vanadinite, mimetite and pyro- morphite may not, in general, be as great as the results of this work seem to indicate. 94

CHAPTER 6 SUGGESTIONS FOR FURTHER RESEARCH

REE in natural pyromorphite-group minerals The survey of REE concentrations in pyromorphite-group minerals involved the analysis of a relatively small number of specimens. In addition, the non-random distribution of crystal habits among minerals belonging to different parts of the series has resulted in uncertainty as to whether differences in REE concentration profiles are the result of differences in the geochemical behaviour of RE ions under different conditions, or of differences in the rates of crystal growth in pyro- morphite-group minerals under different conditions. It is therefore recommended that additional specimens, including greater proportions of non-crystalline mimetite and vanadinite and of well-crystallized pyromorphite, should be analysed, in order to eliminate this uncertainty and to improve the significance of the results. In the analysis of REE in pyromorphite by instrumental neutron- activation analysis, the detection limits can be improved by placing a sheet of perspex between the sample and the detector. This absorbs beta radiation from 32P, without significantly attenuating the y-rays, thus reducing the amount of bremsstrahlung, which is a major component of the background radiation. The partition of REE between pyromorphite and aqueous solution was investigated here using synthetic minerals. It may be possible to measure the distribution coefficients for the partition of REE between natural minerals and aqueous solution by analysing liquid inclusions in pyromorphite-group minerals, possibly by means of inductively-coupled plasma techniques currently being developed. 95

A general problem encountered during this research was that relatively little work has been done on the thermodynamic aspects of the formation of pyromorphite-group minerals in the environment. Calculation of the stability fields of mimetite and vanadinite, as has been done for pyromorphite (Nriagu 1974), would provide more in- formation about the relationship between these minerals and possibly evidence to support the theory that they are formed under different conditions. This would, however, involve a considerable amount of work as many of the relevant thermodynamic data have yet to be determined.

Doping experiments The doping experiments were useful in assessing the role of various factors affecting the uptake of REE by pyromorphite, but the fineness of the precipitate may have allowed surface adsorption to play a significant role in determining the results. The length of time required to establish equilibrium between the solid and the solution was also a problem. It may be possible to overcome both these problems by carrying out the recrystallization of the pre- cipitate in a hydrothermal bomb. Posner and Perloff (1957) have used hydrothermal bomb techniques both to synthesize hydroxypyromorphite, Pbs(PO^) 3OH, and to increase the crystal size of pyromorphite prepared by other methods. 96

The problem of sodium activity in the analysis of the solutions can be avoided either by transferring the precipitate into distilled

water before the addition of RE ions, or by using (NHI+)2HP0I+ or

NHt+H2P0i+ instead of Na3P0i+ in the synthesis of pyromorphite.

Extension of the research to other mineral systems

The study of REE concentrations in pyromorphite-group minerals has shown that the REE distribution in minerals precipitated from hydro- thermal solutions is influenced by several factors, some of which depend upon the nature of the precipitated mineral, rather than upon chemical properties of REE. In order to discover more about the general be- haviour of REE in hydrothermal solutions it is suggested that REE dis- tributions in other minerals and in other types of ore deposits should be investigated. Because so many factors are involved, care should be taken to make sure that all possible influences on the behaviour of

REE are considered, and an effort should be made to obtain as much information as possible about the hydrothermal solution, possibly by means of stable isotope studies etc. Because of the difficulty in obtaining information about the exact conditions under which a mineral is formed, doping experiments using synthetic minerals may prove useful in elucidating this part of REE geochemistry. 97

APPENDIX 1

CHEMICAL ANALYSIS OF PYROMORPHITE-GROUP MINERALS

Analyses of the pyromorphite-group minerals used in the general survey of REE distributions are given in Table 19.

Lack of material was a problem in the chemical determination of lead and calcium, preventing the use of the usual gravimetric methods.

Lead and calcium were therefore determined by AAS. Vanadium in vanadinite was also determined by means of AAS.,

Phosphate was measured by colorimetry using the vanadomolybdenate method developed by Baghurst and Norman (1957). This method permits the determination of phosphate in the presence of arsenate.

Analysis of vanadium in pyromorphite and mimetite, and of chlorine and arsenic was carried out at the same time as the determination of the shorter-lived REE, using INAA. The Y~sPectra obtained in the course of

REE analysis also revealed the presence of other trace elements such as

Cr, Mn and Sr.

The minerals were also examined under the electron microprobe.

About half the specimens were analysed using a wavelength-dispersive

Cambridge Instruments Microscan Mark 9 microprobe, and the rest were analysed using an energy-dispersive Cambridge Instruments Geoscan micro- probe. The main problem in the electron microprobe analysis of these minerals is interference between the As K and the Pb L X-ray line. ai a The energy-dispersive instrument had a spectrum-stripping facility which enabled the contribution from Pb to the As peak to be removed. With the wavelength-dispersive instrument the As concentrations had to be corrected 98

by hand. Arsenic concentrations were calculated on the basis of stoichiometry using the Pb, Cu, P and V concentrations and from these the values for the arsenic concentrations before ZAF correction were estimated. These values were then used in the ZAF correction. The arsenic concentration was refined until the calculated value agreed v with the value obtained after ZAF correction.

The probe results agreed fairly well with the chemical analysis and therefore only the information concerning inhomogeneity, inclusions or substitution by elements not detected using the other techniques is given in Table 19.

From the results it can be seen that many specimens are apparently non-stoichiometric and/or give low analysis totals. It is thought that this is due to the presence of impurities, since inclusions of silica and unidentified lead phases were detected in many specimens in the course of microprobe analysis. TABLE 19

CHEMICAL ANALYSES OF MINERALS USED IN GENERAL SURVEY

3_ Pb Cu L 3_ CR, Total Stoichiometry Additional information from Specimen As0 4 V04 P0 4 (%) (%) (%) (%) L electron probe analysis and (ppm) (%) A: X0 4 : CR, * INAA P~romorEhite 1. BM42241 63.4 3.55 0.30n 85 22.4 2.87 92.5 4.98: 3.0: 1.02 Fe 0.23% (INAA) silicon in centre of probe grain 2. AG29 68.4 6.42 0.282% 900 22.7 2.38 100.2 6.10:3.0:0.83 Sr 20 ppm (INAA) Probe shows substitution by Si~- and also Si02 inclusions 3. AG1 65.4 2.74 25 ppm - 22.5 2.82 93.5 4.86:3.0:1.00 Ag 0.12%, Sb 200 ppm (INAA) Mn (probe) silica inclusions 4. BM195 7, 844 60.0 10.6 1.42% 1080 24.2 2.69 98.9 6.28:3.0:0.86 Some Mn (probe) As0 4 conc. varies 0-3% 5. BM1964, R8100 71.2 2.70 381 ppm 35 20.5 2.58 97.0 5.69:3.0:1.01 Cr 53 ppm SC 28 ppm (INAA) A second lead phase present in sample 6. AG10 70.2 2.70 85 ppm 20 20.5 2.52 95.9 5.66:3.0:0.99 Sb 200 ppm Sc 5.6 (INAA) Probe analysis revealed the presence of a second Pb phase 40% Pb 20% P04 no. CR, 7. BM1956, 227 69.3 0.89 0.086% 225 20.4 2.56 93.2 4.98: 3.0: 1. 00 Si04 0.5% (probe) 8. AG81 79.1 0.10 40 ppm 45 20.5 2.85 102.3 5.33:3.0:1.01 Cr 123 ppm Sc 1.2 ppm (INAA) Silica inclusions in probe specimens

continued •••••••••• TABLE 19 continued

CHEMICAL ANALYSES OF MINERALS USED IN GENERAL SURVEY

3_ L 3_ Specimen Pb Cu (%) (%) As04 V0 4 P04 CR- Total Stoichiometry Additional information from (ppm) (%) (%) (%) L electron probe analysis' and A: X0 4 :CR-* INAA

9. BM1964, R8101 70.0 1.58 0.572% 430 19.9 2.75 94.8 5.30: 3.0: 1.09 Fe 0.153% Si04 10.24% (probe)

1-3% ~ ;~ ~3;6:~~ ~~N~~4A~~!~ o in probe specimens· o 10. BM27542 69.2 0.50 0.570 13 20.1 2.58 93.0 4.85: 3.0: 1.01 Some Si02 (probe) Arsenious ElromorEhite 11. BM27540 69.6 0.10 8.33% 70 14.2 2.40 94.6 4.85:3.0:0.97 Some Mn (probe) 12. BM733 69.6 0.30 9.40% 63 11.5 2.68 93.5 5.48:3.0:1.20 As/P ratio varies. Probe spe1!mens ~~ntaines 11-15% As0 4 • Si04 0.12% PhosEhatian Mimetite 13. BM1964, R8103 68.7 0.28 20.5% 97 4.88 2.63 97.0 5.13:3.0:1.12 Two mimetite phases present with P0 4 conCR. of approx •. 5% and 2%. There is a 3rd phase containing Fe and Zn 14. BM1964, R8102 73.5 0.05 21. 7% - 2.42 2.53 100.2 5.87:3.0:1.18 -

continued •••••••••• TABLE 19 continued

CHEMICAL ANALYSES OF MINERALS USED IN GENERAL SURVEY

L 3_ L Specimen Pb Cu (%) (%) As0 4 V0 4 P04 C£ Total Stoichiometry Additional information from (%) (ppm) (%) (%) 3_ * A: X0 4 :Ct electron probe ana1y"sis and INAA Mimetite 15. BM195 8, 178+ - - 22.4 600 - 2.59 - - u. Sc (INAA) 2_ 16. BM1972 , 385 68.0 1.00 22.8 200 0.41 2.50 94.7 6.30:3.0:1.26 !::< 0.13% Si04 (probe) Traces of U (INAA) 17. BM1930, 291 62.1 0.18 23.6 112 0.78 2.45 89.1 5.12:3.0:1.16 - 18. BM1930, 285 63.2 0.16 22.5 63 0.75 2.45 89.1 5.45: 3.0: 1. 22 Variation in As/P ratio. There is a second phase containing Pb (30%), Ca (5.7%) and As (probe). Traces U, Zn, Sb (INAA) Vanadinite (%)

19. BM83397 64.6 0.12 5.90 22.3 0.30 2.34 95.6 3.95:3.0:0.83 As0 4 7.5-9% in probe sample 20. BMl927 , 193 60.8 0.04 3.65 19.7 0.38 2.16 86.7 4.37:3.0:0.90 Traces of Cr (INAA) 21. BM1923, 822 69.2 0.05 1.28 21.6 1.29 2.08 95.5 4.80:3.0:0.84 - 22. BM1930, 417 67.1 0.11 3.72 23.8 0.57 2.39 97.7 4.09:3.0:0.84 Variation of VIAs ratio As0 4 O.7-8%

* 3_ 3_ 3_ 3_ 2+ 2+ 2+ Where X04 = P04 , As04 ,V04 and A = Pb ,Ca + . . 3_ Insuff~c~ent material for determination of Pb, Ca and P04 NOTE: Values for trace elements (other than As and V) detected by INAA are approximate. 102

APPENDIX 2

N0N-ST0ICHI0METRY IN SYNTHETIC PYROMORPHITE

A full chemical analysis of the synthetic pyromorphites was not carried out, but chlorine and sodium concentrations in most samples was measured using INAA. The results are presented in Table 20. Sodium concentrations between 0.07% and 1.4% were found in all the samples analysed and chlorine concentrations were invariably higher than the expected 2.62%. It is possible, since the precipitates were finely divided, that these results are due in part to surface adsorption of

C£ and Na+. Surface adsorption would be expected to increase with increasing pH, since recrystallization takes place faster in acid solution, but the results show that both CI and Na+"concentrations tend to be higher in acid solution, indicating that substitution of

Pb and possibly PO^ is also occurring.

The chlorine concentrations are related to pH. Of the samples with chlorine concentrations greater than 3% all but one (No. 6) were aged in solutions with pH < 6.5, and of the eight samples where the chloride concentration is < 3% seven have pH > 6.5. This suggests that 3_ the high chlorine concentrations may be caused by substitution of PO^ 2- by HPO^ in acid solution, leading to the formation of compounds such 2 3 2 2 2 3 + + + as Pb9 (P04)lf~(HP0i+)2~C£2 (2.8% CL) , Pb^ Na (HP0lt) ~(P01+)2~C£~ (3.02% 2 2 3 + C£) and Pb4 (HP03)2"(P04) ~Cl~ (3.09% CI). 103

In Chapter 4, the concentration of sodium ions in solution was said to be one of the factors influencing REE concentrations in pre- cipitated pyromorphite, higher Na+ concentrations producing a greater 3+ degree of substitution by RE . However, the amount of sodium required 3+ to balance the extra positive charge on RE is small, corresponding to a sodium concentration of about 0.02% in the precipitate. On the other 3_ 2_ hand, in acid solution, extensive substitution of PO^ by HPO^ provides + a means by which much larger amounts of Na can be incorporated into the crystal. This is probably the reason why sodium concentrations tend to be higher in pyromorphite aged in acid solution than in those aged in alkaline solution, even though the Na+ concentration in solution is greater than in more alkaline conditions. 104

TABLE 20

SODIUM AND CHLORINE CONCENTRATIONS IN SYNTHETIC PYROMORPHITE

21 ZT— C £ Na Aging Cone. SOi+ CO3 Sample pH Cone. (%) (%) Time (ymole/100ml) (ymole/100ml)

Variation in aging time 1 3.65 1.40 2.8 24hrs 2 3.45 0.896 2.8 1 wk 3 not determined 14a 3.10 0.020 2.8 wk b 3.20 0.027 2.8 wks c 3.05 0.024 2.8 wks d 3.10 0.030 2.8 wks Variation in pH 3.45 I 0.896 2.8 1 wk not determined 1 wk 2.98 n.d. 6.6 1 wk 3.02 1.40 7.5 1 wk 7.73 0.116 8.7 1 wk Variation in sulphate concentrations 8 3.20 0.096 6.4 1 wk 45 9 3.15 0.086 5.3 1 wk 119 10 2.70 0.070 7.5 1 wk 394 15 3.10 n.d. 3.4 1 wk 127 Variation in carbonate concentration 11 3.00 0.086 8.8 1 wk 141 12 2.80 0.090 8.3 1 wk 226 13 2.80 n.d. 9.3 1 wk 2179 16 2.80 n.d. 9.8 1 wk 260 Formation of pyromorphite in the presence of REE 17 3.76 1.23 2.9 1 wk 18 2.80 1.33 2.8 7 wks

n.d. = not determined 105

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