sign in sign up
<<
Home , Cassiterite, Chalcocite

American Mineralogist, Volume 62, pages IN-106, 1977

Chemistryand mineralogy of wood-,Black Range,New Mexico

JoHN L. LurxlN

Department of Geological Sciences, The Uniuersity of Texas at Austin Austin. Texas 78712

Abstract

Wood-tin occurs as placer accumulations at several localities in the Black Range, south- western New Mexico. Individual nuggets, 3 cm or less in diameter, commonly enclose fragments of porphyritic rhyolite, and contain , , cristobalite, and/or chalcedony. Color zoning is well developed in wood-tin and appears to have chemical significance;the content in cassiterite-richlayers, as much as 7.8 percent Fero3 by electronprobe analysis,generally increases through the color sequence:tan, yellow, red, dark brown. The addition of iron is correlated with a decreasein both the c cell parameter and the cell volume of cassiterite,suggesting that Fes+has substituted for Sno+in the . Although wood-tin is rarely found as lode deposits,it probably was depositedoriginally as fillings in the Tertiary Taylor Creek rhyolite with which it is closely associated.A low temperature,low pressureenvironment of depositionis reasonablefor the wood-tin.

Introduction veinletsin the Taylor Creek rhyolite which is exposed Wood-tin, a term generally applied to sub- over an area of 177 kms'?.Almost half of the material microscopic to finely crystalline, concentrically-lay- studied has cores or inclusions of altered and re- ered cassiterite,occurs as placer accumulations in placed rhyolite, and the placer deposits are closely eluvial and alluvial gravels in the Black Range tin associatedspatially with outcrops of Taylor Creek district of southwesternNew Mexico. Rarely, it is rhyolite. At Squaw Creek, a small amount of wood- observed in situ as fracture fillings in the Taylor tin has been observedin place as fracture fillines in Creek rhyolite of Tertiary age (Fig. l) that forms this rhyolite. flow-domesexposed just west of the Continental Di- vide in Sierraand Catron Counties(Fries, 1940;Fries Mineralogy and Butler, 1943;Lufkin , 1972).Based on estimates Wood-tin nuggetsare rarely composedof pure cas- of the U.S. Bureau of Mines (Volin et al., 1947), siterite.Other phasespresent include, in or- production of tin concentratesfrom the district dur- der of decreasingabundance, hematite, cristobalite, ing World War II was about l0 long tons, averaging and chalcedony. Cristobalite and chalcedony are 50 per cent tin. Since 1970,small operations much less abundant, and commonly absent from have shipped approximately 40 short tons of wood- many nuggets.The typical colloform texturesuggests tin concentrateaveraging about 6l percent tin from sequentialencrustations of theseminerals on walls of the vicinity of H ardcastleCreek (Frank Gitman, oral fractures.Many of the nuggetcores consist of altered communication). The following account is basedon porphyritic rhyolite with a groundmassthat is divitri- petrographic, X-ray, and electron probe studies of fied to aggregatesof fine-grained and alkali more than twenty specimenscollected by the writer feldspar.These rhyolite inclusionsare typically elon- and ProfessorR. H. Jahns from eluvial gravelsand gate and as much as l8 mm long by 4 mm wide. The jig concentrates in Sawmill Canyon, Hardcastle cuspateor botryoidal depositionalpattern of wood- Creek, and Squaw Creek. tin adjacentto the rhyolite contact commonly reflects The Black Range wood-tin typically occurs in small irregularitiesin the planar to slightly convex Holocene gravel depositsas isolatednuggets, gener- rhyolite surface.In places,the rhyolite groundmassis ally less than 3 cm in diameter, although fist-sized replaced by fine-grained, disseminated to colloform specimenshave beenobserved. It seemshighly prob- cassiterite,with phenocrystsof quartz and sanidine able that the wood-tin was precipitatedoriginally as left intact. In one specimen, quartz, sanidine, and LUFKIN: WOOD-TIN FROM NEW MEXICO pm-*l f-r'rr-w* sonrcrc | i

OSocoro

/ l-l lTruth or Conlcqucncas

""*PJ- Lj "".tr-// r / osompl€ tocotions ,/

\/

/ L \ ( ""t I wmill Conyo Loco lity

c.--t ( ,-)'2 1 t' I T

After Fig. l. MapshowingoutcropdistributionoftheTaylorCreekrhyoliteandsamplelocationsofwood-tinexaminedinthisstudy Fries and Butler (1943). (?)were presentin the groundmassof the rhyo- hematite,cristobalite, and/ or chalcedony.Cassiterite to lite core, but not in the body of the surrounding varies in size from microcrystalline aggregates wood-tin nugget. elongate crystals, 2.2 mm long; much of the cassiterite Proceeding outward from the rhyolite core, the is presentas elongated,radial grains a few microns in body of the wood-tin nuggetsis composedprimarily length. There does not appear to be any systematic of microcrystallinecassiterite, with lesseramounts of variation in grain size,although cassiteritetends to be t02 LUFKIN'WOOD-TIN FROM NEW MEXICO coarsergrained in the rhyolite inclusionsand in pe- body of wood-tin it generally occurs in layers and ripheral shells of the nodules. Color variations in varies in shape from radial, needle-likecrystals to cassiteritehelp to define the colloform texture of more equant grains. Less commonly, it forms semi- wood-tin. In transmitted light, cassiteritedisplays circular massesof radiating crystals.The distribution shadesof tan, gray, yellow, orange, red, and brown, of hematite-rich layers is non-uniform; most of the or is opaque. Less commonly, it is almost colorless. hematite is depositedin areasadjacent to the nugget Where opaque, cassiteritegenerally contains abun- interiors, suggestiveof an early formation. However, dant inclusions of hematite. In reflectedlight, cas- some nuggetsalso featurehematite in their peripheral siterite appearsdull gray with low reflectivityin air. shellsas well. There is a generalcorrespondence between color and Where present, cristobalite and less commonly, crystal layering, but in coarser-grainedlayers, color chalcedony,form thin shells,0.4 mm or lessthick, on boundariesmay depart from crystal outlines, or ter- the wood-tin surfaces.Cristobalite occurs in equi- minations. Slight is noted, but only in granular mosaicsor is fibrous, and the X-ray diffrac- the larger crystals. tion pattern of this material matches that of low Hematite rangesfrom minute particlesto platesof cristobalite. In one specimen,cristobalite occurs as specularite,2.8 mm wide. In the altered rhyolite in- linear, mosaic aggregratesinterlayered with cassite- clusionshematite is disseminated,but throughout the rite. Portions of these mosaic patchesare optically continuous, possibly suggestiveof inversion to the Table I Electronprobe analysesofwood-tin, Black Range,New mofe stable alpha quartz. Chalcedony is the least abundant constituentof wood-tin. It typically forms a thin, light-colored coating on the nodules. Specimen No. Sn02 Fe203* Co]or of cassiteritei Chemistry Nine wood-tin nodules were chemically analyzed 94.3 0.3 ran by electron probe. Traverseswere made acrossindi- 92.6 0.7 col orl ess vidual nuggetsfrom rim to core to determine their 86.2 3.? yellol-green brovln 84.6 4.4 yelIov brown purity and to seeifthe variouscolors ofthe cassiterite 86.8 5.9 orange brovn layers had chemical significance.Analyses for SnO, 86.5 5.3 yellovl brown 86.5 6.9 red brolvn and FerOg were determined for several differently colored layers of cassiteritein each specimen,ex- 90.5 'I1.6 ran 9l.5 .6 yellov cluding grains of other .A beam diameterof 89.9 1.7 1ight brown 88.I 2.5 ye1I ol orange 2-10 microns was employed. Due to the generally 91.8 2.7 reo orange submicroscopictextures developed in wood-tin, grain 93.I 2.5 yellorv brown boundaries of cassiteritegenerally could not be re- 9t.6 3.7 ye11ow v l.o 4.2 orange red solved in polished sectionduring probe analysis. 90.3 4.5 yellow brovln Probe analysesof wood-tin are presentedin Table 89.5 5.3 dark brown I . With the exceptionof specimensl8 and 21, most of 96.1 0.8 f,an 91.6 2.5 ye1 1ow the analyzedcassiterite consists of aggregatesof tiny 9l .6 a.o brown 90.9 5.8 red crystals a few microns long. Specimens l8 and 21 86.8 5.8 dark brovn have largergrains, 2.2 mm or lessin length.The SnO, 90.7 5.0 brown orange and FerOsanalyses of the micron-sizecassiterite do 94.0 5.3 red orange 89.4 7.8 yel low orange not total 100 percent, whereas those of the coarser 'l0l grained material do. As an independentcheck, rela- .5 0.2 color]ess l0l.3 0.5 ye'l 1or orange tively pure wood-tin practicallyidentical to specimen 100.6 0.4 oran9e 2 was submitted to the Gulf Chemical and Metallur- 99.9 1.0 ye1 I our gical Company for wet chemicalanalysis. The partial 96.9 2.1 orange 96.8 2.8 red brovn analysisis: SnO2,97.17; FezOr,0.43; SiOr, 0.77. By comparison,the probe analysisfor SnO2in specimen xTotal Fe as Fe203. 2 is almost 3 percentless than the wet chemicalanal- ysis of similar material, while the iron content is tColor basedon observationsof wood-tin in thin section, transmitted light. about the same.The explanationfor the low totals of probe analyses,particular SnO2,probably is related LIJFKIN: WOOD-TIN FROM NEW MEXICO 103 to the fine-grainedtexture of wood-tinand micro-pore compositioninclude variationsin texture,porosity, spacesalong grain boundaries(which are observed in and mineralthickness from specimento specimen. thin-sectionof specimen2, but generallynot in other Much study has been devotedto color zoning in material).This suggestionis compatiblewith the re- coarse-grainedcassiterite of deep-seatedvein and sultsobtained on singlecrystals in specimensl8 and pegmatiteaffilitations, but very little on the finer 21, wherea completeprobe analysis is obtainedfor grainedwood-tin occurrences of volcanicorigin. The tin and iron alone.Therefore, it seemsunlikely that red color in someNew Zealandcassiterites has been other major elementsare presentin significantcon- attributedto (Hutton, 1950),although it centration in wood-tin. Where analyzed,SiOz and wasnot determinedwhether the tantalumwas pres- Al2O3are presentin amountsgenerally less than 0.5 ent in the crystalstructure or as inclusionsof tan- percentcombined. Trace elementspresent in wood- talum-bearingminerals. Some coloredcassiterites, tin, determinedby microprobewave length scans and typicallybrown to black,have been related to ferrous X-ray fluorescence,include As, Cl, Pb, Zn, Ag, In, iron content (Greaveset al., l97l; Leube and and S. Stumpfl,1963; Pecora et al.,1950).Color in cassiter- From the chemicaldata, it is apparentthat thereis ites from Aberfoyle,, zoned in shadesof a generalcorrelation between cassiterite color and its brown,red, and black, apparentlycannot be corre- iron content.With fewexceptions, the iron contentin latedwith iron content,but the deepred zonescon- cassiterite-richlayers increases progressively through tain highervalues of Mn andNb comparedwith pale the color sequence:colorless, tan or light brown, brownzones of the samecrystal (Edwards and Lyon, yellow,orange, red, and dark brown (Fig. 2). How- l9s7). ever,the iron contentcannot be accuratelypredicted Other investigationssuggest that there is no rela- from the color.Cassiterite of similarcolor may vary tion betweencolor and chemicalcomposition in cas- by severalpercent FezOs between different nuggets. siterite.Noll (1948,in Leubeand Stumpfl,1963), in For example,yellow cassiterite may contain from I to his studyof largelyEuropean tin deposits,considered 2.5percent FerOr, red cassiterite,4to 6 percent,and colorto berelated to submicroscopicexsolution bod- red brown varieties,3 to 7 percent Fe2O3.Factors ies. In their microprobestudy of zonedcassiterite that mav affectthe accuracvof color correlationwith crystalsfrom PelepahKanan, Johore,Grubb and

G o

o q

t

o o (o --.--)

COLORLESS TAN YELLOW ORANGE RED YELLOW ORANGE RED DARK BROWN EROWN BROWN BROWN

Fig. 2. Scatter diagram showing the relationship betweencolor of wood-tin and its iron content. Specimen nu mbers correspond to those presented in Table l. r04 LUFKIN: IYOOD-TINFROM NEW MEXICO

Hannaford(1966) observed that somelight-colored hematitewas the only iron oxideidentified by X-ray zonescontained more iron than dark-colored,mag- and polished-sectionstudy, it is reasonableto as- neticzones. Gotman (1938, in Singhand Bean, 1968) sume,without the aid of Mdssbauerspectra, that the concludedfrom researchof Russiantin occurrencesiron in cassiteriteis dominantlyFe3+. Several stand- that color and light absorptionof cassiteriteare in ard mineralogytexts statethat Fe3+may proxy for part due to changesin the stateof the crystallattice Sna+in cassiterite.Cations of comparablesize and underdiverse conditions of formationand also to the chargethat may substitutefor tin as well include presenceof variousimpurities. Ta andNb, common Tin+,Nbu+, Tau+, and Sc3+.This lattergroup of ele- in cassiteritefrom ,may replaceSn, caus- ments. however. was not detectedin this in- ing a disturbancein valencybalance and latticeof vestigation. cassiterite. To evaluatethe possibilityof Fe8+substitution for The first attempt to explaincolor variationsin Sn'+in thecassiterite lattice, six powdered specimens wood-tinwas probablyby Newhouseand Buerger of wood-tinof differentcolor and Fe contentwere (1928).In their petrographicstudy of severalspeci- mountedseparately on a polycrystallinesilicon disc, mens from Mexico and the westernUnited States, which providedan internalstandard. A PhillipsX- includingthe Black Range,they correlatedthe cas- ray diffractometerwas employed, using Cu radiation, siteritecolor to proximityof hematite,noting that a graphite reflectedbeam monochrometer,and a cassiteritetends to be a deeperred adjacentto hema- scanningspeed of 0.25o/minute. Lattice reflections titegrains. lf iron is theprincipal pigmenting agent in weremeasured between 60 and l16 degrees20, and the Black Rangewood-tin, as indicatedfrom this were used in a least-squaresrefinement of the cell microprobestudy, it couldbe presenteither as finely constants.Cell dimensionsof thesespecimens are dividediron oxideinclusions or incorporatedin the recordedin Table2 andcompared with dataof Swan- cassiteritestructure as Fe2+.Fe3+. or both. Becauseson and Tatge (1953).The Fe-bearingcassiterites

Table2. Unifcell dimensions of cassiteritein wood-tin, Black Range, New Mexico*

d V Fe20, Speci nen No. (A) (fr) ti3l (wt. percent)

2 4.7342(6) 3.r835(s) 7r.3s(2) 0.3 tt 4.733(r) 3.r833(e) 71.30(4) n.a. ?2 4.7341(7) 3.r828(e) 7r.33(2) n.a.

4.734(?) 3.r 77(3) 71.21(7) n.a. 9 4.736(2) 3.173(3) 7r. r6(7) 5.3-6 .9 t3 4.730(5) 3.177(4\ 7r.r(1) 5.0-7.8

2763 4.738 3.188 7.l.566 n.a. - not analyzed.

2. Tanto beigecassiterite, very fine grained. 17. Creamcolored cassiterite, very fine grained. 2?. Tanto beigecassiterite, similar to No.2. 5. 0range-redcassiterite, coarsegrained. 9. Yellow-brornto red-browncassiterite, very fine grained. 13. Red-orangeto orange-browncassiterite, fine grained. 2763. Sn02,National Bureauof Standards(Swanson and Tatge, 1953).

*The estimatedstandard deviation given in parenthesesrefers to the last decimal position. LUFKIN'WOOD-TIN FROM NEW MEXICO 105

(nos. 5, 9, and 13) consistentlyfeature a smallerc cell ulesof BlackRange wood-tin, however, feature both edge and a smaller cell volume, V; the a cell dimen- colloformand isolatedcrystals of cassiteritein the sion, however, does not seemto vary systematically.A samesection, suggesting that the colloformtexture is decreasein cell volume would be expectedwith Fes+ but onesize range, albeit very fine grained, in a con- substitution,since the Fe3+ion is smaller than Sna+ tinuum,as pointedout by Roedder(1968). Rapid (0.63 A and 0.67 A radii in 6-fold coordination, re- nucleationpromoted by highly saturatedsolutions spectively;Shannon and Prewitt, 1969,p. 943). (not necessarilycolloidal) may well accountfor these Diffraction patterns of Fe-bearing and Fe-poor fine-grainedtextures that are so commonin silica, cassiteritesfrom the wood-tin nuggets were com- carbonates,and iron andmanganese minerals. pared with that obtained from an unanalyzedsingle The chemicalstability of cassiteritein the zoneof cassiteritecrystal, Lost River, Alaska. The general weatheringis generallyrecognized, although it may appearanceof the peaks suggeststhat the wood-tin alter to the soft, hydrousoxide of tin, varlamoffite cassiteriteis not as well crystallizedand that the Fe- (Alexanderand Flinter, 1965; Singh and Bean, 1968). bearing cassiterite(no. 9) featuresbroader and less Secondarycassiterite may form in the oxidizedzone defined reflections-a situation that might be ex- of tin sulfidedeposits, apparently by alterationof pectedif Fe were presentin the cassiteritestructure. (e.g. Ramdohr, 1969). Considering the sta- bility of cassiterite,the absenceof ,and the Discussionand conclusions lackof supergeneenrichment in theexposed tin-bear- Wood-tin placers are widespread in the Black ing veinletsin the district,the precipitationof wood- Range, but are closely associatedwith the Taylor tin from hypogenerather than solutions is Creek rhyolite, which contains small amounts of favored. hematite and non-colloform cassiteritein veinletsat Acknowledgments severallocalities. This relationship,together with the This investigationwas funded in part by a grant from the Uni- presenceof rhyolite inclusionsin wood-tin common versity ResearchInstitute, The University of Texas at Austin nodules, indicates that the wood-tin was deposited (U.R.l. ProjectSRF-628). I thankProfessor R. H. Jahns(Stanford originally as veinlets in the Taylor Creek rhyolite. University)who kindly providedme with additionalwood-tin The general absenceof lode wood-tin suggeststhat material,C L. Sainsburyand G. A. Desborough(U S.Geological thatwas used as a tin most of the colloform cassiteritewas deposited in Survey)for theirgift of high-puritycassiterite standardin the microprobework, and Drs. J. Hogginsand H. stratigraphicallyhigher sectionsof the rhyolite that Steinfink,Department of ChemicalEngineering, for their valuable were subsequentlyeroded. assistancewith the cell refinementwork. This manuscripthas The initial environment of wood-tin deposition is benefitedfrom the reviewsof A. W. Laughlin,R. C. Ewing,and conjecture.Similarities of thesedeposits to the Mexi- D. S. Barker.The GeologyFoundation, The Universityof Texas publicationofthis article' can tin-bearingrhyolites bespeaka similar origin and at Austin,gave financial support for the environment. Heating and recrystallizationexperi- ments performed on Mexican wood-tin (Pan and References Ypma, 1973, 1974) appear to favor a low-temper- Alexander,J. B and B. H. Flinter(1965) A noteon varlamoffite ature ((150'C) genesis,although the substitutional and associatedminerals from the BatangPadang district, Perak, effectsof Fe for Sn on the stability of cassiteritehave Malaya,. Mineral. Mag., 35,622-627. Edwards,A. B. and R. J. P. Lyon(1957) Mineralization at Aber- not yet beeninvestigated. Concentration and precipi- foyle tin mine, Rossarden,Tasmania. Auslrulia Inst. Min Met' tation of tin oxide in volcanic rocks suggestslow all..l8 l .93-145. pressureas well (closeto I atm). The chemistryand Fries,C., Jr (1940)Tin depositsof the BlackRange' Catron and mineralogyof wood-tin nodulesimplies that the min- SierraCounties, New Mexico,a preliminaryreport. U.S Geol eralizing fluids contained appreciable iron and silica, SurueyBull . 922-M, 355-370. - and A. P. Butler(1943) Geologic map of the BlackRange in addition to tin. Zoning in wood-tin and cell refine- tin district,New Mexico.U.S. Geol. Surveyopen-file map. ments of cassiterite suggestfurther that most of the Greaves,G.. B. G. Stevensonand R. G. Taylor(1971) Magnetic iron was precipitated early as Fe3+in both the struc- cassiteritefrom Herberton,North Queensland,Australia. Econ tures of hematite and cassiterite.Most of the silica Ceol.66.480-487. (1966) and was deposited later in the parageneticsequence as Grubb, P. L. C. and P. Hannaford Ferromagnetism colour zoning in some Malayan cassiterite.Nature' 209' cristobaliteand chalcedony. 677-678. The colloform and botryoidal texture that charac- Hutton, C. O. (1950)Studies of heavydetrital minerals. Bull Geol terizes wood-tin has been widely attributed to col- Soc Am . 6l. 635-'116. loidal processessince the turn of the century. Nod- Leube,A. and E. F. Stumpfl(1963) The Rooibergand Leeuwport 106 LUFKIN: WOOD-TIN FROM NEW MEXICO

tin mines, Transvaal, South Africa. Part 2. Petrology, mlner- Ramdohr, P. (1969) The Minerals and their Intergrowths alogy and geochemistry. Econ. Geol, 58, 527-557. Pergamon Press,Oxford. I 174 p. Lufkin, J. L. (1972) Tin Mineralization within Rhyolite Flow-domes, Roeddcr, E. (1968) The non-colloidal origin of "colloform" tex- Black Range, New Mexico. Ph.D. Thesis, Stanford University, tures in . Econ Geol,63, 451-471 Stanford,California. Shannon, R. D., and C. T. Prewitt (1969) Effectiveionic radii in Newhouse, W. H. and M. J. Buerger (1928) Observationson and fluorides. Acta Crystallogr , 825,925-946. wood-tin nodules.Econ. Geol.,23, 185-192. Singh, D. S. and J. H. Bean (1968) Some generalaspects of tin Pan, Y and P J. M. Ypma (1973)The Mexican type tin deposit- minerafs in Malaysia. ln, A TechnicalConference on Tin, Yol.2. its occurrence,chemistry and physicalconditions of deposition Int. Tin Council,London. p.457-478. (abstr.).Ceol. Soc Am Abstr. Progr.,5,162. Swanson,H. E. and E. Tatge (1953) Standard X-ray diffraction - (1974) Heating and recrystallization of wood-tin-an in- powder patterns. Natl Bur Stand. Circ 539, Yol. l, 54-55. dication of low temperature genesisof cassiteriteand wood-tin Volin, M E., P. L. Russell,F. L. C. PriveandD. H. Mullen(1947) veins in volcanics (abstr.). Geol. Soc. Am. Abstr. progr., 6, Catron and Sierra Counties tin deposits,New Mexico. IJ.S Bur. 62-63. Mines Rept Inu. 4068, 60 p. Pecora,W. T , C. Switzer,A. L. Barbosaand A T. Myers(1950) Structure and mineralogy of the Golconda , Minas Mannscript receiued, January 14, 1976, accepted Gerais, Brazil. Am Mineral., JJ, 889-901. for publication, August 5, 1976 American Mineralogist, Volume 62, pages 107-l 14, 1977

Electron diffraction study of phasetransformations in coppersulfides

ANonswPurNIs Departmentof Mineralogyand Petrology,Uniuersity of Cambridge DowningPlace, Cambridge C82 3EW,

Abstract

The order-disordertransformation behavior of chalcociteand ,and digeniteand anilitehas beenstudied by direct observationsof the transformationsin a transmission electronmicroscope. and djurleiterepresent low-temperature superstructures of the high-temperaturedisordered chalcocite subcell, and there need be no compositional differencebetween them. The djurleite superstructureis the more stable form at room temperature.The intermediatestructure in digeniteforms through modulatedstructures ratherthan throughtwinning. A metastable6a digenitetype superstructure,isochemical with anilite, forms on heating.This 6a superstructureis metastablerelative to the formation of anilite.

Introduction cannotbe directlymeasured in this way,the courseof a transformationcan be followeddynamically, and For compositionsnear CurS a numberof phasesare subtle changeswhich would be very difficult to ob- known to existat room temperature.These may be serveby X-ray methodscan clearly be seen. dividedinto two broadcategories based on thenature of the close-packingin the structure:(1) I: The transformationbehavior of chalcociteand chalcociteand djurleitewith structuresbased on hex- djurleite agonal close-packingof sulfur atoms: (2) - like structuresand anilitewith sulfur atomsin ap- Chalcociteis generallyconsidered to have a com- proximatelycubic close-packing.The low-temper- positionvery closeto CuzS,whereas djurleite, origi- aturephase relations have been summarized by Mori- nallythought to beCur.rrS (Roseboom, 1966; Takeda moto and Gyobu (1971)and Barton (1973).The et at., 1967a)is now consideredto be a solidsolution transformationbehavior of someof thesephases and with a compositionrange from about Cut.*S to the relationshipsbetween them havebeen studied by Cu' nzS(Mathieu and Rickert,1972; Potter, 1974). directobservation of the transformationsin a trans- The phasesand cell dimensionsof chalcociteand missionelectron microscope. djurleite may be summarizedas follows. Between 104' and 435'C chalcociteis hexagonalwith an : Method 3.95,cn = 6.75A and spacegroup P6"/mmc. Above Naturally-occurringcrystals of the phaseswere 435"C it has the cubicclose-packed digenite struc- crushed,after chilling in liquid nitrogen to prevent ture.Below 104'C a pseudo-orthorhombiccell with a deformation.The finestgrains were collected from a : ll.92, b : 27.34, c : 13.44A, spacegroup Ab2m is suspensionin absolutealcohol and mountedon a formed (Buergerand Buerger, 1944).This can be standardcarbon-coated grid for observationsdescribedas a superstructureof the hexagonalform in an AEI EM6G transmissionelectron microscope with a : 3en,b : 4y3av,,c : 2cn.More recentlyit operatingat l00kV. Observationswere made firstly has been shown that low-chalcociteis monoclinic on untransformedgrains by usinga smallcondenser (Evans,l97l), but the pseudo-orthorhombiccell will apertureand a defocussedbeam to keepthe illumina- be usedhere for convenience. tion and the heatingeffect of the beam minimal. Djurleite is orthorhombic,diffraction aspect P*t?* : Transformationsin the grains,both on heatingand with celldimensions a :26.90, b 15'72,c: 13.57 cooling,can be observedby focussingand defocus- A. the diffractionaspect is alsocompatible with the singthe beamor by movingthe beamlaterally on and monoclinicspace group P2'ln. Takedaet al. (1967a) off the grain. Although the temperaturein the grain point out that thesecell dimensionsare closelyre- 107