THE RELATIONSHIP BETWEEN CRYSTAL STRUCTURE, BONDING AND CELL

Canadian Mineralogist Vol.23, pp. 6l-76 (1985)

THE RELATIONSHIP BETWEENCRYSTALSTRUCTURE,BONDINGAND CELLDIMENSIONSIN THE GOPPERSULFIDES RONALD J. GOBLE Deportment of Geology, University of Nebrasko-Lincoln, 433 Morrill Holl, Lincoln, Nebrosko 68588-0340,U.S.A,

ABSTRACT

de cui'ire de structureconnueindiquentque la structure de la yarrowiteet cellede la spionkopiteressembleraient Copper and copper-iron sulfidescan be classifiedinro surplusieurs pointsi celledela covelline,maison doit conthree generalgroups: (1) anilite, digenite,geerite,cubanite, siddrercommeprobableuneoccupation statistique dessites. chalcopyrite, haycockite, talnakhite, mooihoekite and bor- Lestentativesdeddtermination dela structuredela spionnite with structuresbasedupon approximatecubic close- kopiteont 6t€entravdes par I'imperfectiondescristauxet packing of the sulfur atoms; (2) djurleite and chalcocite le grandnombrede sitesde cuivrevacants.La stru4ure with structures basedupon approximate hexagonal close- de la geeriteestrhomboddrique (fi32?), a 15.77A, a packing oftle sulfur atoms; (3) covelline,yarrowite, spion- 13o56' i cellede la digdnite. , Z = l, et ressemblerait

kopite and idaite with a combination of hexagonalclosepacking and covalent bonding of the sulfur atoms. The average spacing D between layers in all groups can be expressed asD = 2.M3 + 0.654(Cu:S)+ l. 183@e:S).The ionic radius R of sulfur for group-l mineralsis R, = 2t7 Q'l2A), where D is from the previous expression;for group-2 minerals, Rz = 1.856+ 0.060 (Cu:S)+ 0.023 @e:S);for group-3minerals,R: = 1.857+ 0.039(Cu:S)0.020 @e:S). Consideration of bond lengths in coordination polyhedra of known copper sulfide structures indicatesthat major portions ofthe yarrowite and spionkopite structures will resemblethe covelline structure. vdth probable statistical site-occupancy.Attempts at the determination of the spionkopitestructurewere hamperedby the imperfection of the crystals and the partial occupancy of most structural positionsof copper. The geeritestructure is rhombohedral(R3n?) with a 15.77A, o 13.56,. Z = l, and will probably resemblethe digenitestructure. Keywords: yarrowite, spionkopite, geerite,copper sulfides, copper-iron sulfides, blaubleibender covelline, crystal structure, bonding, SoMMAIRE On peut regrouperles sulfuresde cuivre et de cuivre r fer en trois grandesfamilles: l) anilite, dig6nite,geerite, cubanite, chalcopyrite,haycockite,talnakhite, mooihoekite et bornite, dont la structure contient un empilement approximativement cubique compact des atomes de soufre; 2) djurl6ite et chalcocite,dont la structuremontre un empilementhexagonalcompact (6rossomodo) desatomes de soufre, et 3) covelline,yarrowite, spionkopiteet idaite, qui montrent une combinaison de I'agencementhexagonal compacfdesatomesde soufre avecliaisonscovalentes.Pour lestrois groupes,la s6parationmoyenneD descouchesest 6galed I'expression 2.063+ 0.654(Cu:S)+ 1.183(Fe:S). Le rayon ionique.R du soufre Cansles min6raux du premier groupe est 6gal d D/2 l2A. Pour les mindraux ou : 1.856+ 0.060(Cu:S)+ 0.023 deuxidmegroupe,on a .rR2 (Fe:S), et pour ceux du troisiime groupe, R: = l.8SZ + 0.039 (Cu:S) - 0.020 @e:S).Les longueurs de liaisons dans les polybdresde coordination des sulfures

61

(Traduit par la R6daction) Mots-clds:yarrowite,spionkopite,geerite,sulfurc decuivre, sulfuresdecuivreet fer, covellineblaubleibender, structurecristalline,liaisons. INTRODUCTIoN Eight copper-sulfideminerals havebeenidentified to date: covellineCul.6gs,yarrowite Cu1.12S, spionkopite Cu,.*S, geeriteCu1.6eS, anilite Cut.75S,digenite Cu,.*S, djurleite Cu1.eSand chalcociteCuz.mS. Partial or completestructural determinationshave been carried out for covelline (Oftedal 1932,Berry 1954,Kalbskopf et al. 1975,Evans& Konnert 1976), anilite (Koto & Morimoto 1970),digenite(Donnay et al. 1958,Morimoto & Kullerud 1963),djurleite (Takedaet al. 1967a,b,Evans 1979),and chalcocite (Sadanagaet al. 1965,Evans l97l). The structures of yarrowite, spionkopiteand geeritehavenot been determined.Yarrowite and spionkopite,two of the blaubleibenderor "blue-remaining" covellines,were for many yearsdescribedin terms of the hexagonal unit-cell of covelline,basedupon similaritiesof Xray powderpatterns(e.9., Frenzel1959,Moh 1971, Rickard 1972,Putnis et ql. 1977),However, Goble (1980)hasshownthat the unit cellsof yarrowiteand spionkopite are not the sameas that of covelline. Geeriteis a pseudocubiccopper sulfide that has only recently been reported (Goble & Robinson 1980). The known structurescan be divided into three generalgroupsbasedupon the natlue of packingof the sulfur atoms: (l) anilite and digenite,with structuresbasedupon approximatecubic close-packing, (2) djurleite and chalcocite,with structuresbased upon approximate hexagonalclose-packing,and (3) covelline, with a combination of hexagonalclosepacking and covalentbonding of the sulfur atoms.

62

THE CANADIAN MINERALOGIST

The resemblancebetweenthe well-developedyarrowite and spionkopite subcellsand the covelline unit-cell (Goble 1980)suggeststhat theseminerals belong to group 3; the pseudocubicnature and resemblanceof the geeriteunit-cell to a structureproducedby the leachingof anilite (Goble 1981)suggestthat geeritebelongsto group 1. Single-crystal X-ray studiesprovide a method of examiningthe structuresof yarrowite, spionkopiteand geeriteand of determiningthe true relationshipbetweenthese structure$and those alreadyknown for other copper sulfides. PRoCEDURES Cleavagefragmentsof yarrowite, spionkopite and

F

t.c

f ]t A. COVELUNE

B. COVELUNE 0120)ptone

I*I T IT -t_ C.'ANIUTE" (100) phne

l_Jl tY I

)-)

A

l..-/.........---.....n

D.SPHALERITE

geeritefrom the studiesof Goble (1980)and Goble & Robinson (1980)were used for all single-crystal patterns. Precession,Weissenbergand integrated Weissenbergphotographswere preparedfor selected orientations of the yarrowite and spionkopite reciprocallattices.Precessionphotographsof covelline in the equivalentorientationswere also prepared in order to checkthe supposedstructural similarity of the blaubleibendercovellinesto this mineral. For geerite,only precessionphotographswere prepared. Filtered Cu-radiationwas usedfor all Weissenbergfilms and Mo radiation for all precession films. The fragmentsof yarrowite, spionkopiteand geeriteusedmeasureapproximately0.14 x 0.08 x 0.03,0.25 x 0.08 x 0.08and 0.07 x 0.06 x 0.02 mm, respectively. For intensity determinations,0-, l- and 2level integratedWeissenbergphotographs(rotation axis c), weretaken of the yarrowite and spionkopite fragments.Exposuretimes for all levelswereon the order of 200hours using a Philips fine-focusCu X-ray tube (point-focusport) with a Philips PW 1008/85selfrectified high-voltagepower supply operatingat 40 kV and 12mA. Three-film packswereused,the films being separated by one sheet of black paper. Individual films of eachpack werescaledusing the "film factor" of Morimoto & Uyeda(1963).Different levelswerescaledby a comparisonof reflections present on two or more of the levels, with the assumptionthat the unit cellsare approximatelyhexagonal. Intensitieswere estimatedby visual comparison with the {110} reflection of covellineon standardUnobscalefilms preparedfrom multiple exposures. servedreflectionswere assignedmaximum intensities of one-half the minimum observableintensity at that point on the film. Raw intensity-datawere correctedfor Lorentz and polarization effectsand, in the caseof spionkopite,for the presenceof minor (= l59o) intergrownorientedyarrowite; absorption correctionswere not applied. Wilson plots (Wilsgn 1942)showôveralltemperature-factorsof 0.45 A2 and 0.75 A2 for yarrowite and spionkopite, respectively.

E. METASTABLE DI6EMTE

Frc. 1. The crystal structure of covelline(after Wuensch 1974),"anilite", sphaleriteand metastabledigenite.The "anilite" structureis an idealizedversion(seetext). In B t,Cu and tvCu indicate copper atoms in triangular and tetrahedralco-ordinationpolyhedra,respectively; S. indicates covalently bonded sulfur; lC and lI representthe spacingsof one covalentlybondedand one tetrahedral (ionically bonded)layer, respectively.Filled circlesin A, B and D indicate fully occupiedtriangular co-ordination polyhedra; filled and open triangles in C and E indicate fully and partly occupiedtetrahedral coordination polyhedra,respectively,with atomic displacement toward the four surroundingtriangular faces.

DATA STRUCTURAL

The crystal strucure of covelline, a group-3 mineral,is shownin Figure la (after Wuensch1974); equivalentsitesfor all atomslie on the (l l0) plane, and the structure can be representedby a section through the unit cell on this plane asshown in Figure lb. The crystal structure of anilite, a group-l mineral, can be representedin a similar fashion if the structural data of Koto & Morimoto (1970)are transformed into an idealized cubic close-packed unit-cell. Copperatomsin triangularly co-ordinated sitesare assignedto the tetrahedraof which these

63

CRYSTAL STRUCTURE OF THE COPPER SULFIDES a11.tlt3

Oooe

r663

a88e

Oli3

a-LZ1

.057

.T7.nn

4776

.n.n.i

)zza

o635 olo.fit.m

415.15.15

)zzt

a$r Ods-0

.t3.f3.tz

oiiS a66s-

an.ji.i3

i"

a7.t7

FIc. 2. The reciprocallattice of geeritewith rhombohedralindices.Both rhombohedral and cubic crystallographicaxesare indicated.

sites form a face, and displacementof the copper atoms from thesetetrahedrally co-ordinatedpositions areignored.A sectionthroughthis '(idealized" structure of anilite perpendicularto the close-packed layersof sulfur is shown in Figure lc; reduction of the cell dimensionsnecessitates averagingfull and empty tetrahedraand resultsin partial (%) occupancy by copper for some tetrahedra. The structure of metastabledigenite(Morimoto & Kullerud 1963)in the equivalentorientation is shown in Figure le. The crystal structuresof djurleite and chalcocite,group-2 minerals,involve extensivetriangular co-ordination of Cu and cannotreadily be representedon sections suchas lb, lc and le. Because ofthe strongresemblance betweenthe X-ray patterns of geerite and sphalerite,a sectionthrough the sphaleriteunit-cell in this orientation is representedin Figure ld. Goble (1980,Figs. l, 2) showedthat the reciprocal latticesof yarrowite and spionkopiteare similar to but distinct from that of covelline.Yarrowite Xray data were lndexed on a hexagonal cell with a 3.800,c 67.26A; spiopkopitewasshow4to be hexqsonal, with a 22.962A (i.e., 6 x 3.8n X), c 4t.429 A. Well-developedsubcellswith the approximate {imensionsof the covellineunit-c€ll(a 3.796,c 16.36 A) were noted. Tables I and 2, which list the observedstructure-factorsfor yarrowite and spionkopite, have been submitted to the Depository of UnpublishedData, CISTI, National ResearchCoun-

cil of Canada,Ottawa, Ontario KIA 0S2.For yarrowite, I 17nonequivalentreflectionswereobserved, 623 were unobserved(only lF1,e1lvaluesare listed in Table l, lF*:'l = lFnrrl). For spionkopite,147 nonequivalentreflectionswere observed,406 were unobserved;all supercellreflectionsof the tpe hkl wfih h or k + 6n were ignored (i.e., the a' : a/6 = 3.8?i/A subcellwasused).Examination of the structurefactors indicatesthat the spacegxoup for both yarrowite and spionkopiteis one of .P5ra1, P 3 m l , P 3 2 1 ,H l m , H l m o r B l 2 . Geeritewas describedby Goble & Robinson(1980) asbeingpseudocubic(possiblyorthorhombic),space groupF43m. Careful re-examinationof precession photographsshowsthat geeritecan be assignedconsistentindicesbasedupon a rhombohedralunit-cell similar to that proposedfor digeniteby Donnay e/ TABTE 3.

IATA ANDCUBICII{DICES FORI1IE GEERITEPOIIDER

r/r1 3. t28 2 ' I .. 7 91 12 8 1.870* '1.576 1.247 1.109 rrcflectlon

100 l0 50 l0 l0 30 l'10 0 20

Rhmbohedral Ind'l @s

Cubic lndices

221, 555 334 776 ll3,889 442,10.10.10 997,15.15.t5 il2, 13.t3.14

not obseryed on sJngle-crystal

patterns

3ll 222 422

64


o

o

o a

a O

o

Oor O

o

a

1u11u32u1.

orarOsvtovlev,

a

hlflrrr

a

o '

o

w36ut72n

o

o

aoautsznln

ota

o

o

a

a o

ln:rnifr.OO, oT cilTtVtTn

o o

o

hl01c

ouraraoz!'or

a

a

o

a

FIc. 3. The reciprocal lattice of anilite after leachingin ferric sulfate solution for twelvehours. Rhombohedralindicesconsistentwith the geeriteunit-cell are shown. Open circleslabeledT representreflectionsattributed to the secondindividual of a twin.

a/. (1958).The geeritereciprocallattice, indexedon this basis,is reproducedin Figure2; the X-ray powder pattern with both pseudocubicand rhombohedral indexingis givenin Table 3. Geeriteis rhombohedral Cu6S5(diffraction aspect R*1) with a 15.77A, a 13"56', Z = l, spacegroupR3m, R3m, or R32. These data are consistentwith the weak bireflectanceand moderateanisotropismobserved by Goble & Robinson (1980). The choice of the rhombohedral cell for geerite is substantiated by the leaching experiments conductedby Goble (1981),which produceda structure similar to that of geerite.The reciprocal lattice of this material is shown in Figure 3; indicesrefer to the geeriterhombohedralcell. Fractional indicesindicate that the leachedphasehas approximately tJree times the a dimension of geerite (a,1,47.L2 A, a 4"40'). Careful re-examinationof reflection intensitieson precessionfiLns of leachedanilite showsthat reflections indicated by open circles in Figure 3 are produced by twinning about [00]*]" or [110*]". Converting the rhombohedralcell tp a hexagonalcell producesa length cpu of l4\.2 A, approximately twice that in yarrowite (67.26A), thus explainingthe resemblanceof the two phaseson X-ray powder patterns, a$ noted by Goble (1981).

Relationship betweenstructure and composition The five known copper sulfide structures have either approximately cubic or hexagonal closepackedlayersof sulfur or, in the caseof covelline, a combination of hexagonalclose-packedwith covalently bonded layers of sulfur. In an ideal closepacked netwqlk the distance D between layers is equalto 2R l2A, whereR is the radius of the sulfur atoms. In copper sulfides such as covelline, with

OF DISTAI{CE BnIIEEIISULruRTAYERS(O) AI{ORADIUS TABLE4. AVERAGE (R) IN C'OPPER SULFIDES TAYERS SULFUR AIO.ISUTIFIN SULFIJR

D (8)

hkr

R (i)

@ell'lm vafMlte ;plonkopite geerite anlllte

2.735 2.7 2.964 3.128 3.20B

006 0.0.24 0.0. 14 555 [email protected]

dlgenlte

3 . 2 1 6 555 3.200 3 . 6 2 BO0 3.373 h4

1.8971 ll0 Poltet & Evans(1976) ll0 Goble (1980) 1.899 1.910 660 Coble (19B0) 776 Table 3 1.918 1.%3 040,400, Potter & Evans (1976) 224 1.969 10.10.0 ilorlrcto & Kullefud (1963) oonnayet al. (1958) 1.959 1.9556 046 potter & Evans (1976) (30 Potter & Evans (1976) 1.974n

nlreral

dJur'lelte chalcoclte

hkr

data source

For anilite D is taken on the averageof the four nonDarallel-il1!i pl'nes in the pseudocublc cell, that-ls, the orthoah@bic (0221 arld 12021 olaFs rith sDacinqsof 3.198 X and 3.2i8 A, respectively; R ls i.aken as the rciqhted averaqe of the six mn-parallel t440' pseu@cubic planes, th;t ls,-the odiorh@blc (40!), {040}, an! t224} plares xith spacingsof 1.977X' 1.953I, and 1.962A. respectlvelv.

65

CRYSTAL STRUCTURE OF THE COPPER SULFIDES

layersof covalently bondedsulfur, D will be an average of the covalent, C and close-packedor tetrahedral, I interlayer spacings,weightedasto the relative number of each (see,for saamplelFig. l). As noted by Goble (1981), planes with spacingscorrespondingto R and D are readily identifiable in the copper sulfides; theseare listed in Table 4 and shown as functions of compgqition in Figure 4. The ideal relationship,D = 2R.l2A,is also shown in Figure 4. For spionkopite and geerite, the compositions Cu1.a6S and Cu,.6oSbetter fit the data of Figure 4 than the compositions Cu1.r2Sand Cu1.53S determined from the microprobe data of Goble & Smith (1973),Goble (1980)and Goble& Robinson(1980). DigeniteD and djurleite R valuesare anomalouswith respectto the data of Figure 4. Figure 4 confirms that geerite has approximate close-packingof sulfur atoms,whereasyarrowite and spionkopite have structures with a combination of close-packing and covalent bonding of the sulfur atoms. In Table 5 the spacingbetweenclose-packed layers,D' = 2R .12A,has beencalculatedfrom the R values of Table 4 and combined with the spacing betweenlayqrs of covalently bonded sulfur in covelline, 2.071 A @vans& Konnert 1976), in order to determine the number of eachtype of layer parallel to the c axesof yarrowite and spionkopite. Comparable data are presentedfor covelline, geeriteand anilite. The geerite21d anilifs data are consistentwith pseudocubicunit-cells containing three and six layers of close-packedsulfur, respectively; the yarrowite and spionkopite data are consistent with a number of different combinations of covalently bonded and close-packedlayers correspondingto different unitcell contents. Becauseof the small sample-size,the presenceof impurities and the scarcity of yarrowite and spionkopite, no attempt was made to measuretheir density. Instead, the composition yersardensityrelationships for known copper sulfides shown in Table 5 and Figure 5 wereusedto determinethe most probable densityof yarrowite and spionkopite.Compositions of Cu1.,2Sand Cu132Swere used, as determined by Goble & Smith (1973)and confirmed by Goble (1980). A spionkopite composition of Cu1.asS, consistentwith the data of Figure 4, was also used.For spionkopite,the data of Table 5 and Figure 5 are consistentwith a unit-cell content of 14 Cu,.rr_,.*S;for yarrowite, cell contentsof either 24 Cus.12S or 25 Cu1.12S are acceptable(although 24 Cu1.12Sseemsmore likely), depending upon the choice of the density-compositionrelationship in Figure 5. The difficulty in the choice of a unit-cell content and density for yarrowite was resolved by examining the relationship betweenratios of the differently bonded types of layers (close-packedor covalently boqded) and composition. In those copper sulfides

D (A)

R6= 1.860.0.058(Cu:S),' --2-;lg-z-?

;iJ_ o

t

R (A)

,,

t

vfui -\, '--'l'

R,=1.850.0.036(Cu:s) I

1.0 1.2

ti,, '."1r"

1.8 2.0

Frc. 4. The relationship(A) betweenaveragedistanceD between sulfurlayersandcompositionand@)between radiusR of sulfur atomswithin sulfurlayersandcompositionfor thecoppersullides(afterGoblel98l). Open circlesfor spionkopiteand geeriteindicateanalyzed compositions(Cur.:zS,Cu1.53S); open circlesfor digenite(Cu1.6S) indicatetwo possible distances; open circlesfor djurleiteindicatetwo possiblecompositions (Cu1.e3S, Cur.c7).Correlationswere determinedby by opencirclc exceptthose omittingall datarepresented for digenite,whichwereaveraged. The dashedline in B represents the equationderivedin A if ideal closepackingof sulfur atomsis present.

with lessthan 1.75 copper atoms per sulfur atom, there is an increasingdegreeof developmentof covalent sulfur-sulfur bonding with loss of copper, as shownby the data of Table 5. Thesedata wereconvertedto aratio of thenumber of interlayersof covalently bondedsulfur to the number of close-packed or tetrahedrally co-ordinated interlayers, C/7, and plotted as a function of composition in Figure 6. For spionkopitea C/Tvalue of 0.167,correspondingto 2 covalent and 12 tetrahedral interlayers in the cell, is consistentwith the data; for yanowite, of the three possibleC/T values,only 0.412, correspondingto 7 covalent and 17 tetrahedral interlayers, is consis-

66

THE CANADIAN MINERALOGISI

TABLT 5. MSSIBLE NUI.IBERS OF COVALEIIY BONDED SULRJRLAYERSAI{D CLOSE-PACKED SULFUR LAYERSIlI ONEHEIAGOMLUNII CELLOF STIECTEDCOPPER SI'LFIDES

covell lle lamrlte

uoor{i)

D' (A)

16.341 67.26

3.098 3.101

splonkoplte 4t.429 geer'lte anll lte dlgenlt€ dJurlelte chalcrclte

9.37 19.24

3.119 3.132 3.189

nC + nI

zc+ 47 lC { 217 4Cr 197 7CI I7f 10cr l5T 2C1 l2'l 5C+ 107 8c+87 0c+ 3T 0c+67

o*r.(i) ' 67.L9 67.20 67.21 67.23 41.57 41.55 4t.52 9.40 19.14

22 23 24 25 14 15 16 3 6

0.

i;i*jj 4.68 4.48 4.69 4.89 5.09 5.49/5.74 5.8/6.12 3.42/5.61 5.63/5.71 5.76/5.82a 5.79

9ob;: obseryeda cell dlrensloni geerlte and anlllte cells ,ere converted b,thelr hexagonalequtyalents. R: observedrad.lus of sulfur atons wlthln sulfur layerslU': colculated dlstsnce betreen lryers of close-Dacked sulfur (" 2Rr'2l3i"R.frm Table 4). C: layers of covalenily bond;d sulfuri spacing ls 2.071 A (Evanst Konnert 1976). T: layers of cl;se-Dacked Sulfuri spoclng ls Dr A. nC + eT: n@ber of C and T lavers ln'one e 'lengthi other values of n lead to nonlntegral values oia. o.,t.: calculoted-d cell dimnslol. Z: nmber of suliur atom in lexagon'ii"unii-ceit; the a' - a/6. 3.827 A subcell of splonkoplte ls used. Deislty is calculated frcn unit-cell parareteE. Densltles of splonkopjte apfor Cu1.32S/Cu1.asS.Densitles of geerlte are for Cur_crSTCurrnS. Data sources are as in Table 4 ercept for dJurlelte (at;-iaken-lr6m rakeda at a/. (1967b1.

olss oss

pt 0./. 9le -ot t>o 0.3

:l: atr

EI;0.2

o23S

EE

014S

5+0.1 clo olq

Els >to ol ot

I

o22S

0.0

1.2 1.tl Cu:S rotio

1.6

Ftc. 6. Ratio between the number of layers of covalently bondedsulfur to the number of close-packedor tetrahedral sulfur layers (ionically bonded) as a function of composition. Different points shown for yarrowite, spi onkopite and geeriterepresentdifferent possiblestructures and compositions.

5.8 GPtss TABLE6. AVERAGE DISTNCE BfllEEN sU.FUR LAYERS,D, ANDMDIUS OF SIJI.FUR ATOMS HITHIN SOFUR LAYERS,R, IN COPPER-IRON SULFIDES

{/

56

oineral

E -u ;'|5'4

sllfur cubanlte chalcopyr'lte haycocklte talnakhlte @lh@klte ldalte 'idalte ? bonlte

6Pts

fic s.z (u

cu:s

Fe:s

0.0 0.0 0.33 0.667 0.5 0.5 0.5 0.625 0.55 0.55 0.5625 0.5625 0.833 0.167 0.845 0.155 I.25 0.25

D (i)

hkr

R d)

2.048 3.12 3.03 3.07 3.06 3.07 2,82 2,792 3.18

calc 002 lI2 226 222 221 006 0U 224

1.856 1.867 1.865 1.889 1.874 1.87',t 1.890 1.887 1.937

data $urce

calc 123,330 220 440 4N 4& uZ l',lo 440,408

1,2 3 3 4 5 4 6 7 3

T' The value of D for sulfur ls tlE average covalent S-S dlstane deternlred tun the strocture of o.thorhotrblc sulfur. The value of R for sulfur ls half the mlnlmm lntemolecular dlstance of Ab.ah@ (1955) scaled to allow for the revlsed cell-parileteB ol cooper et al.(196x). olta sources: (1) lbrahms (1955)i (2) Cooperet al. (1961)i (3) Berry & Tlmpson (1962); (a) Cabri & Hol1 (1972); (5) Hiller & Prcbsthaln (1956); (6) Frenzel (1959)i (7) Yund (1963).

5.0

oY%rs

1.0

1.2

1.t, 1.6 Cu:Srotio

1.8

2.0

Ftc. 5. Calculated density of the copper sulfides as a function of composition,Filled circlesrepresentvaluescalculatedfrom known structures;open circlesrepresent possibledensitiesfor yarrowite and spionkopite. Different points shown for a single mineral represent different possiblecompositionsfor this mineral. The two curvesrepresentthe range fo densitiesto be expected.

tent with the data. Once again, compositions of Cu1.a6Sand Cu1.66Sfor spionkopite and geerite better fit the data than do the analyzed compositions of Culj2s and Cu1.sS.

The crystal-structure - composition relationships shown in Figure 4 can be extendedto include sulfur and copper-iron sulfides,as shown in Table 6 and Figure 7. In Figure 7a the interlayer distance, for copper-iron sulfides,is plotted as a function of the ratio of metal (Cu + Fe) to sulfur. The linesplotted are for different ratios of copper to total metal and satisfy the equation: D = 2.O63+ 0.654 (Cu:S)+ 1.183@e:S),R2= 0.993.In Figure7b the ionic radius R of sulfur for the samecopper-iron sulfides is shown as a function of metal-to-sulfur ratio. Each copper-to-metalratio is representedby three straight-line segments.The central high-slope segment o!_each line represents the equation R: D/Q V%) plotted for the valuesof D in Figure 7a, and was used to determine whether a given sulfide belongs to the "low-metal" (eft) or "high-


67

3./,

32 30 J8

:s

o2.6 2.1

22 02

0.1

0.6

08 1.0 12 (Cu.Fe):S

1A

16

t8

1.98 1.96

29

Rl = 1.857.0.039(Cu:5)- 0.020(Fe:S)

1.0(r)

Rh=1.856. 0060(Cu:S) " 0.023(Fe:5) R =Dr(2lm)

0a3(4

1.91

0.5(.) 0.33(+)

_1.92

:g

Et.9o

0.0

1.88 1.86 1.81t 0.0

{-------q2

0.1

06

0.8 1.0 12 (Cu'Fe):S

1!

16

18

2p

Ftc. 7. The relationship (A) betweenaveragedistanceD betweensulfur layers and compositionand (B) betweenradius R of sulfur atoms within sulfur layersand compositionfor the copperand copper-iron sulfides.Valuesof the Cu,z(Cu+ Fe) ratio represented are 1.0(-.-),0.S3 (-4. .),0.5 (-+),0.33 (..+..) and 0 (-). The open diamond in B representsa Cu,/(Cu+Fe) value of 0.44,buta separateset of curvesfor this value is not included; this point is obscuredby a closeddiamond in A. In B, each group of minerals with a singlevalue of the Cu,/(Cu + Fe) ratio is representedby tlree straight-line segmpqts;the middle, highslopesegmentrepresentsthe ideal relationshiF,Rc: D/2,JrA, with D taken from A; the high-Cu (right-hand) segment represenrsthe calculated relationship Ra: 1.856+ 0.060 (Cu:S)+ 0.023 (Fe:S); rhe low-Cu (efr-hand) segment representsthe calculatedrelationshipRr : 1.857+ 0.039 (Cu:S) - 0,020 (Fe:S).

metal" (right) eroup of sulfides. Approximate equations usedfor the low-metaland high-metalcopperiron sulfidesare: R1= 1.857+ 0.039(Cu:S) - 0.020 ( F e : S ) , R 2 : 0 . 9 9 2 , a n d R a : 1 . 8 5 6+ 0 . 0 6 0 (Cu:S)+ 0.0234 (Fe:S), R2:0.999, respectively.

The positions of the lines shown are more tenuous than in Figure 7a becauseof the possibility of a sulfide having a behavior similar to that of high-Cu sui. fides (anilite to chalcocite)or low-Cu sulfides(coveiline to geerite).

68

THE CANADIAN MINERALOGIST A,COVELUNE

"--.-....----.L-'F

\ -\ O

,J

aq

=.4

€>

/'

flc \

/a

$"/ e

/fi

F^$ -ua

ca

( /c \*6

FIc. 8. /20/Pattersonprojectionsfor covelline,yarrowiteand spionkopite.Peak positions are indicated by filled circles.

In Figure 7, an increasein iron contentresultsin an increasein D but a decreasein R. This is due td the presenceof iron in the structuresinhibiting the formation of covalentS-S layers(1.e.,covellineand chalcopyriteboth havea metal-to-sulfur ratio of one, but only covelline has layers of covalent sulfur). BecauseD is an averagespacingand doesnot include any of the small covalenti^nterlayerdistancesin the copper-ironsulfides(- 2 A for a covalentinterlayer as opposedto = 3 A for a tetrahedralinterlayer), u an increasein iron content increasesthe averagespaY cing of interlayers.Also, the lack of covalentlayers U resultsin fewer metal atoms, on average,per tetraoU hedral interlayerin the copper-iron sulfidesthan in the correspondingcopper sulfides, and a decrease in R. As an example,covellineand chalcopyriteboth u E have a metal-to-sulfur ratio of one. D for chalinterlayers copyrite is the averageof six tetraJnedral ..ANtLIIE" of approximately3 A each,or 3 A; D for covelline is the average oj four tetrahedral interlayers of approximately 3 A eqchand two covalentinterlayers of approximately2 A each,or 2.7 A. Similarly,in chalcopyrite there is one metal atom for eachsulfur or one per tetrahedralinterlayer, whereasin covel0 2 1 6 8,(A)10 12 14 16 l8 20 line, there are six metal atoms spreadthrough four tetrahedral interlayers or one and one-half per tetra= Ftc. 9. Pattersonsectionsalong x: 0,./ 0 for coveline, hedral interlayer. This increasesthe S-S distanceand, yarrowite, spionkopite,sphaleriteand "anilite". Lines therefore,the radipsof sulfur R from 1.80A in chalindicate the interatomic distancesof Table 7. copyrite to 1.90 A covelline. F

/lL


69

It is interestingto note that the minerals shown in Figure 7b tend to clusteron or near points at which the high-slopeR*. Iines or extensionsof the R"d" lines cut the observedR1and R7,lines for the various metal-to-sulfur ratios, evenif the various lines havedifferent valuesof the ratio Cu:(Cu + Fe) [i.e., B.YARROWITE idaite lies at the point where the R.4. line for a Cul(Cu + Fe) value of 0.33 cuts the R, line for a Cu/(Cu + Fe) value of 0.331.These presumably representparticularly stable structural arrangements in the copper-iron sulfides.It is also interestingto note that the chalcopyrite,mooihoekite, haycock- UJ C.SPIONKOPITE ite, talnakhite group of mineralsrepresentsa series I extending along a single Rcal" line at Y Cul(Cu + Fe) :0.5 (haycockite is actually on a uL slightly different line). A secondseriesextendsfrom u : anilite and digenite to geerite.L. Whiteside (pers. comm. 1983)has observedfive intermediatestruc- trl E tures in a leachingstudy of membersof this series. In Figures4 andT, the spacingsR and D represent the radius of sulfur atoms and the averagespacing betweensulfur layersin the copperand copper-iron sulfides,respectively.For the coppersulfidesin the composition ranges Cul.e_,.6S and Cur.rr_2.6S, R would appear to vary continuously. However, although not representedin this way in the figures, D must vary in a stepwisefashion sinceit depends partly upon the number of times a constant quanI -10 12 11 16 l8 20 0 2 4 6 tity, the spaqingof layersof covalentlybondedsulz (A) fur (= 2.07A) is averagedwith a variablequantity, sectionsalongx = a/3, y = 2a/3 for 2R l2A. A similar relationship would apply to the FIc. 10. Patterson yarrowite,spionkopite, sphalerite covelline, and"anilvarious compositionrangesshown for copper-iron ite'' . Linesindicatetheinteratomicdistances of Table7. sulfidesin Figure 7. Theiefore, for eachmineral in Figures4 and7, D will be nearly constant,particularly becauseof the limited solid-solutionshownby of sphalerite,which are also presentedin Figures9 most copper and copper-iron sulfides. and 10. Representativeinteratomic distancesfrom Iron sulfide data are not shown in Table 6 and the covelline,sphaleriteand "anilite" structuresof Figure 7. Both pyrite and pyrrhotite have octahedral Figure I are listed in Table 7 and indicated in Figures Fe co-ordination and do not fit the data presented. 9 and 10.The sphaleriteand "anilitd" distanceshave TheoreticalFeSin the sphaleritestructureis consis- beenscaledto the covellinecell and are therefore distent with the data of Figure 7a but not 7b. placedslightly from the positions of the Patterson peaks. Pqtterson synthesesand space-group determination Severalfeaturesare immediatelyapparenton the yarrowite, spionkopite, covelline, sphalerite and Pattersonprojectionson (010)for yarrowite and "anilite" one-dimensionalsyntheses: spionkopite are shown in Figures 8b and c (space a) The Pattersonsynthesesof yarrowite and spigroupBml); filled circlesrepresentPattersonpeaks. onkopite are, in general,intermediatein form with A similar projection for covellinepreparedfrom the respectto the covellineand "anilite" syntheses. This data of Berry (1954)is shown in Figure 8a. One- is to be expectedbecausetheseminerals are interdimensionalPattersonsynthesespreparedfor lines mediate in the alteration sequenceanilite - spionthroughx: 0, a/3 and?-a/3for theyarrowite,spionkopite - yarrowite* covelline and presumably kopite, covellineldata from Berry (1954)]and "ideal- representstructuresintermediateto those of anilite ized" anilite unit-cells are shown in Figures9 (x = 0), and covelline.However,the Pattersonsyntheses for l0 (x : a/3) and I I (x = 2a/3), Becauseof a lack of yarrowite and spionkopite more closely resemble reliable data, Pattersonsynthesescould not be pre- those of covellinethan those of "anilite". Patterpared for geerite.However,the strong resemblance sonslnthesesof yarrowite closelyresemblethoseof of the geeriteand sphaleriteX-ray patternssuggests covelline,whereasspionkopitesynthesesmore closely that the Pattersonsyntheseswould resemblethose resemblethose of sphalerite.

70


A.COVELUNE

B.YARROWTE

C.SPIONKOPITE F

(, U Y tl

D.SPHALERIIE (:seite)

u u E

E:ANIUTE"

0

2

1

6

8 .10 z (A)

12

11

16

t8

20

FIo. 11. Patterson sections along r = 2a/3, y = a/3 for covelline, yarrowite, spionkopite, sphalerite and "anilite".

TABLE7. OISTAI{CES BFmEEN ATOl'lS IN THEcOvELtINE,SPHALER]TE A{D'NILI'TE' S1RUCTURES

nJneral

(E) distance befreen atons along z-*t. atms Lx'Af - ol3'2a/3 Ar,Ay, " 0,0

atons

2 . 0 7( : l C )

!c-sc

:.'lsr.. Ai'ss. i:ii LzrCi-.

5c-5c !c:sc

Cu

Cu-S

sphale|ite

"anilite"

s-d'cu

5.d'c,t ""Cu-LuCu

5.88 6.10 8.t7 8.17

2.2g

2.2g 4.58

s_tinc,

0.00

s-ducu

0.76

s-<

l-u5

fuiu-ducu "cu-s ircu-s 5-S Lttu-Luu

3.05 5.34

s--atcu Lrcu-i.i.nc! Sc-z,Cu s--s irci-iucu S-:S s:-1ucu !i ",Cu s.-zocu aocu_arc! 5-s

S-",cq Lrc!-Lrcu

0.76 2.29 2.83 3 . 0 5( = l T ) 3.60 5.12 5.34 5,8a 7.41 g. 13 8.17

0.76 3.05 3.05 s.34 7.62

iiicu ls @pper ln trlangular coordinat'lon. ircu ls coppe|in tetrahedral coordlnation. Sc ls covalently bondedsulfur. S i s l o n l c a l l y b o n d e ds u l f u r . itcu-s may refer The ordJr of ltms l4-.aton palrs -1g arblt.ary (i.e., to both distances ""Cu-S and S-""Cu). are based upon the TtE dlstances listed for sphalerlte and "allllte" ,tcu-s S-s and dlstances of covelline (1.e., are mt to the sue scale as the unlt cells for these nlnerals). Thls result! in negatlve dlsplacments of these dJstances frm the Patterson peaks 'ln Flgures 5 and 6.

b) Sets of pseudomirror^planes ile present at approxir4ately16.8and 8.4 A for yarrowiteand 17.6 and 8.8 A for spionkppite.Thesereflectthe presence of the I 6.8 and I 7.6 A subcellsnotedby Goble(1980) and puggestpossibleadditignal subcellsat 8.4 and 8.8 A. The 16.8and 17.6A subcellsshow a closp in peak positionswith the 16.36A correspondence unit-cell of covellineand explain the application of the covelline unit-cell to the bloubleibender covellines by earlier workers. A similar set of pseudomirror planesexistson the Pattersoq synthesesof "anilite", representingthe basic 5.5 A cubic closepacked subcell. c) The strongpeakat a z of 8.210 8.8 A Q = c/2 for covelline)is shifted somewhatfrom x = a/3 in the covellinesynthesisto x: 0 in the yarrowite and The shift is more pronounced spionkopitesyntheses. in the spionkopite than in the yarrowite syntheses and is further developedin the "anilite" syntheses. d) The Patterson synthesesof yarrowite and spionkopite show small peaks at x:a/3, z:0, correspondingto triangularly bondedcopperatoms. The small sizeof thesepeaksand the lack of suchpeaks on P6/mmm Patterson maps indicate that space groups having equivalentpositionswith x differing by a/3 ard zequal(Hlm, P312,P3lm) arenot possiblefor theseminerals.Therefore,the spacegroup for yarrowiteand spionkopitemust be oneof Hml, Hml or P321. e) The peak at x = 0; z = 4.6 A on the Patterson synthesescorrespondsto the distancebetweentwo copper atoms occupying apex-sharingcopper-sulfur tetrahedra(i.e.,iucu-iucu in covelline).It is moderately strong in covelline,very strong in "anilite", but weak in both yarrowite and spionkopite. This suggeststhat in yarrowite and spionkopite the occupiedcopper-sulfur tetrahedra,rather than facing alternatelyup and down along c as in covelline and "anilite" (seeFig. l), face either up or down along c in a given region of the cell. f) Thereis a generalbroadeningof peaksin the Patof both yarrowite and spionkopite, tersonsyntheses suggestingthat there may be considerabledisplacement of copper atoms from the ideal tetrahedral and triangular co-ordination polyhedra found in covelline and in the idealized"anilite" structure of Figure l. This will probably be reflected in statistical occupancyof the four triangularly co-ordinatedsites surroundinga given tetrahedrallyco-ordinatedsite in the structure. Relationship between structure qnd bonding Structureshavebeenproposedfor covelline,anilite, digenite,djurleite and chalcocite.In the covelline structure (Fig. l), Evans& Konnert (197Qdetermined copper to be in tetrahedral (4 sites) and triangular (2 sites) co-ordination, with Cu-S dis-

7l


tancesof 2.31 and2,l9 A, respectively.In the anilite structure (approximated in Fig. l), Koto & Morimoto (1970)found copper in distorted tetrahedral (8 sites)and triangular (20sites)co-ordinatiory with Cu-S distancesvaryinC from 2.28 to 2.52 $ (weighted average 2.37^L) and 2.2.4 to 2.35 A (weightedaverage2.30A), respectively. Of the 112 possiblebondsin the tetrahedralco-ordinationpolyhedrq 84 clustercloselyaround a Cu-S distanceof 2.30A, with therem*inderhavingdistances of 2.52 A (8 bonds), 2.94 A (8 bonds), arLd3.22 L 62 bonds). In the structure proposed for metastable digenite (Frg. l) by Morimoto & Kullerud (1963), copper atoms are in distorted tetrahedral coordination, with copper atoms statistically displaced toward the triangular facesof the tetrahedra.Cu-S distancesare not given by the authors but, by analogy with the relatedstructureof metastablebornire (Morimoto 1964),the averagedistancefrom the copper atom to the thrye closestatoms of sulfur can be estimatedas 2.29 A, whereasthe averagedirlance from the fourth atom of sulfur would be 2.74A,.In the djurleite and chalcocitestructures,Evans(1981) determinedthat the copperatomsweremainly in distorted triangularco-ordinatjon,with Cu-S {istances varyingfrom 2.18to 2.90A (averageZ.3l A). Two copper atoms in chalcociteand one in djurleite have or approachlinear two-fold co-ordigation,with CuS distancesof approximately2.2 A. Calculatedand observedCu-S bond lengthsfor the copper sulfide minerals are presentedin Table 8. Observe{ bondJengthsclustef at approximately 2.2 and2.3 A. Comparisonwith the calculatedbondlengthsin Table 8 showsthat yarrowite, spionkopite and geerite will all readily accommodatecopper atoms in both triangular and tetrahedral coordination without sienificantly distorting thesesites. Assuming that the differencein size of the copper atoms in thesesitesreflects a difference in charge (l'.e., Cu+ being the larger and Cu2+ the smaller ion), it may be possibleto predict the relativedistribution of copper atoms betweenthe two types of sites.Whereasstudiessuchasthoseof Tossell(1978) and Vaughan & Tossell (1980) show that such a bonding model is grosslyoversimplified,it may be used as a first approximation in predicting coordination. If sulfur is regardedas having a chargeof -2, the covalently bonded 52 pair in covellinea chargeof -2, the tetrahedrally bonded Cu in covellinea charge of + l, and the triangularly bondedCu in covelline a chargeof +2, one can make the following empirical observationsusing an ionic model (whetheror not the reducedCu-S distahcein triangularly bonded Cu is due to Cu* bondedto S- rather than to Cu2+ bonded to S2-,as suggestedby Folmer & Jellinek (1980), is immaterial; the result is the same).The

IABLE 8.

nlneral

avelllne yarMlte splonkop ite ger'lte mll.lte dlgenlte djurlelte chalc@lte

CALCULATDANDOBSERVED BOND.LA{6IHSIN COPPER SULFIDES

iil7,,_e (olcul-aic-a, 8)

2.'191 2.193 2.205 2,215 2.267* 2.262- 2.274* 2.255- 2.3.9,* 2.280- 2.352*

iv",,_. ^ averaqe Cu-s (catiirlaiea, X) (olseivea, X)

2.323 2,326 2.339 z.Ug Z,cBA, 2.399- 2.412* 2.462 2.477

data source

2 . 1 9 ,2 . 3 1

2.{,2,31 2.29 2.2' 2.31 2,2,2.31

2 3,4 5 5

Bond lengths are calculated for ideal closFpacked structures. *For anlllte the 'ld@llzed" cublc close-packed cell is used; varlatlons in the dlgenlte data reflect dlfferent data sources. warlations and chalcoclte data reflect differences ln the djurleite ln locatlon and shape of tie oordlnation polyhedra (parallel or obllque to the close-packed layers. Data sources: (l) Evans & Konnert (1976); (2) Koto & t{orlrcto (1970); (3) l'lorlmto & Kullerud (1963)i (a) Horlmto (1964); (5) Evans (1981).

superscriptson the copper ions refer to the coordination number: a) The formula of covelline can be written as (tcu+)4(',Cu2*)r(Sr),r(S,-)r; t'cu-s -2.32 A and 'tu-S - 2.19 A, Overall charge-balanceis maintained; chargebalancebetweenthe close-packedsulfur layersis also maintained, as shown in Figure l2a. b) The formula of chalcocite.panbe written as iiiCu-S - 231 L. overall charge(tiCu+)re2(S2-)n;' balance and charge balance betweenthe layers is maintainedas shown in Figure l2b (modified after Evansl98l). c) The formula of djurleite can be written for cul.eoS or as as (it,Cu+)601iigrz+)r(s2-)32 * * Evans (l 979) 1,tu lur1ttcu2)z(52)n for Cu1.e7S. reported only 62 copper atoms in the structure, of which one, Cu{62),is in linear two-fold co-ordination (tu-S - 2.2 A); a second,Cu(13),would appearto approachlinear co-ordination. The remainder of the copper ions is approximately in trianeular three-fold co-ordination (ttcu-S - 2.3 A). As shown in Figure l2c (modified after Evans l98l), whether or not overall charge-balanceand a balance of charge betweenthe layersare maintainedwould dependon the location of a secondcupric ion or on the location of the "missing" cuprous ion in Cu6S32. d) Distortions in tetrahedrally co-ordinated sitesin anilite and displacements from tetrahedrally coordinated sitesin metastabledigenitereflect the fact that neither tetrahedral nor triangular co-ordination polyhedrain thesemineralsfit our model. Instead, both Cu+ and Cu2* tend to be in intermediatepositions. e) Figure 4 showsthat there is a basic difference betweencoppersulfideswith Cu:S > 1.75and those with Cu:S < 1.60. This is reflected in the coordination number of cuprous and cupric ions. Structurally, yarrowite, spionkopite, and geeritewill approximate the behavior of covelline and contain cuprousions in tetrahedrallyco-ordinatedsitesand cupric ions in triangularly co-ordinatedsites.Also,

72


r

tu-2

rrl

i"-r

l^

cr1

)

i"-z

91

si

Cs2 5 Cu{ c!-1 s &-2 Cu-2 5

c( &{ 92 Cu-3 Cu? tu-l

"64$:5 - -6-.;:;u*,, 3Yi

A.COVELLINE

C)

ovr

3it-) o&o3#

-

Cu-l

C.DJURLEIIE

l-llPil1n .uL l-rt Nr4ti .3L t^ .%

llt lni4|( .78 l^ .7tg _1

Fl

''

1

t0

!1il.] 11 .1 l0 -11

.36 ffi) om-

q2

AJ

D.YARROWITE D.YARROWITE

E.SPIONKOFNE

F.OEERITE F. OEERITE

Frc. 12. Ionic bonding in covelline, low chalcocite(Evans 1981),djurleite @vans l98l), yarrowite, spionkopiteand geerite.Numberswithin the covellinestructure in A representionic charges;in other structuresthey representsite occupancies other than one. Numbersto the right of eachstructurerepresentcharges;anows indicatethe amount of chargeabove(f) and below (l) the ion.

atoms in the structuresof yarrowite and spionkopite are, like thosein covelline,constrainedto atomic g4rlte ylrelts spJonkopite @velllne positionswith x: 0, o/3,2o/3 by peakpositionson Pattersonsyntheses. cu1.32s cur.s3s Cul.l2S analyzedconposltion: tot.ooS structuralcmposltlon'-The formula of yarrowite can be writtpn as f) cur.60-r.6rs (fm FJgs. 6'9): c'l.36-r.4os Cut.]lS Crt-ooS (,cu*)r(,iCu'*)r(sttr(stl10; 'vcu-s - 2.33A and rhmbohedral huagonal iExagonal heFgonsl crystdl syst4r rttcu-S=2.19 A. To maintain charge balance RSr? PSnt, P32l P6y'@c space sroupr or P301 Tl,rHi betweenthe sulfur layers, a distribution of copper I @ t l d i m n s l o n s : a . 3 . 7 $ 8 8 a . 3 . e O 0 E a - 22.s628 aa :"1 15.77 3056' cE4l.429I c"16.34lX c"67,264 atomssuchas in Figure l2d is required.Within this c!8sg c'rg.s\q hnSzq Cus fomla: generalmodel, sulfur atomscan be shifted to many z r 2"36 7'1 2"6 unlicell @ntent: possiblepositionsas long as the symmetryrequireg/q3 g/m3 5.61 q/cti3 5.33 4.91 +.oe E/o3 catdlated d€nsliy: nunber of sulfut mentsof the spacegroup are maintainedand seven '14 5 24 6 layeB in unft celt: layersof covalentlybondedsulfur are retained.Cu nwber of covalently'layers bondedsulfur atomsin triangularly co-ordinatedsitescanbe shifted 7 20 2 ln unlt cell: perpendicularto c, and Cu atoms in tetrahedrally avefage n@bet of @pper a&ns pea co-ordinatedsitescan be shiftedwithin the two sites close-Dacked 1.59-1.63 1.53-1.60 1.58 1.50 sllfur laFr: in any layer. The formula of spionkopite can be written Data are fw Pottet & Evans (1976), coble a smith (1973)' 0ob1e (1980)' and e) Goble& Roblnson(1980). The denslty of splonkoplte is ronglv repoded iucu-s as 36^('ucu*)rr(t'icu2*)c.s(FJ2-r(S2-)'oi as 5.13 g/@3 by coble (1980); thls ls th value for the analyzed@mosltlon, Cll.32S. 234 A and 'tu-S - 2,20 A. To maintain charge

TABLE 9.

SPIONrcPII! AilD CEERITE DATAFORCOVEIIIIIS.YARROIITTE, 'IRTJCTUML


73

balancebetweenthe sulfur layers,a distribution of copperatomsfor the a' subcellsuchasin Figure 12e is required.Shifts in the position ofsulfur and copper atoms are possibleas long as the requirements of symmetry are maintained and two layersof covalently bonded sulfur are retained in the structure. h) The formula of geerite can be^ written as (tcu * )6(,:cu2* )r(sr-),;,-,cu-s = 2 3 s x and ttr.S - 2.22A. To maintain chargebalancebetweenthe sulfur layers,a distribution of copperatomssuchas is shown in Figure l2f is suggested.This structure is basedupon that presentedfor metastabledigenite by Morimoto & Kullerud (1963),with the restrictions that copperatomsbe locatedonly in undistoftedtriangular and tetrahedralco-ordination polyhedra and that rather than one out of everyfive Cu-S-Cu sandwicheshaving one empty Cu position, two of five must be empty. It is to be expectedthat in eachof thesestructures (yarrowite, spionkopite,geerite),therewill be some distortion of the ideal Cu-S triangular and tetrahedral co-ordination polyhedra, as has beennoted in previously determinedcopper sulfide structures. The basicstructuraldata for covelline,yarrowite, spionkopiteand geeriteare summarizedin Table 9. For each structure, the averagenumber of copper atomsper tetrahedral layer is calculatedbasedon the Frc. 13. A, B, C, D: Possiblearrangementof layers of ionically bonded and covalentlybonded sulfur in spiideal compositionas determinedfrom Figures4 and onkopite. E. Attempted crystal-structuredetermination 6 and structural data. For yarrowile, spionkopite and of spionkopite;fractionsindicatesiteoccupancy;scatgeerite,this numberapproximates1.60,whereasfor tering factors for copper were used for all positions. covellineit is 1.50. This suggeststhat in the tetraF. Possibleinterpretation of the partial crystal-structue hedral layersbetweenthe layersof covalentlybonded of E. sulfur, the structuresofyarrowite, spionkopiteand geeriteprobablyhavea similar co-ordinationof copper atoms, and that this is somewhatdifferent than in covelline. of the 41.4A cellwith atomicpositionsindicatedfor PARTIAL DETERMINATION that structure giving the lowest R-value. R for this OF THE CRYSTAL STNUCTUNE OF SPIONKOPITE structureis 0.280for observeddata but increasesto A.404 if unobserved data are included. This was The numerousmethods of arranging 2 covalent found to be the generalcasefor spionkopite;R could and 12tetrahedrallayersparallel to the c axis of the readily be reducedto below 0.20 for observeddata, spionkopite cell may be reducedto four types as but inclusion of unobservedreflections causeda shown in Figure l3a,b,c,d (assumedspacegxoup sharpincreasein R. For this reason,an attemptwas IF,ml). All other possiblearrangementsmay be der- made to find the relationship governingthe unobived by shifting the sulfur atomsparallel to the c axis servedreflections.It should be noted that for most (i.e., by changingthe origin). All possiblepositions of the attemptsto determinethe structure, scatterfor copperin tetrahedralco-ordination are indicated; ing factors for copper were used; sulfur would be triangularly co-ordinatedsites have been omitted. expectedto show up as Cu with approximately Vz Numegousattemptsweremadeâtsolving the 8.8 and occupancy.Attemptsto changeselectedpositionsof 1?.6A subcellsand the 41,4A cell of spionkopite, copper to sulfur did not result in an improvement starting with the basicstrucfural arrangementsshown in the R factor. A possibleinterpretation of the strucin Figure 13and assumingthat large areasof the sub- ture in Figure l3e is shownin Figure l3f. The intercellswould resembleeither the covellineor 'Jdeal- pretedstructureis inconsistentwith a centricspaceized anilite" structures(thea' = q/6 = 3.827 Asub- group. It is also missing severalcopper atoms. cell was used in all attempts at determining the An examinationof the structurefactors (Table2) structure). In everycase,splitting of atomic positions showsthat for the spionkopite cell (a' = a/6), the becamea problem.Figure l3e showsthe bottom half / Miller index for reflectionsof the type ft minus fr

74

"l;"-l"l--.:l


045 o

o20

[_] 1__j

8i: o 4 5 9ro o20 o

AB

hrin oxis f

FIc. 14.A. Atomic positionsin the 8.9 A subcellof spionkopitenecessary to account for the systematicabsences;fractional occuppnciesare indicatedas percentages. B. Positjon of Pattersonpeaks for the 8.9 A subcell. C. Structure derived for the 8.9 A subcell:positions onx = a/3 and:c = 2a/3havethe sameoccupancy. D. A twiqned-sphaleritesubcellfor comparisonwith C. E, Structurederivedfor the 35.5A subcell;positionson x: a/3 and x: 2a/3 havethe sameoccupancy. F. A possibleinterpretationof the structurein E. G. A tqinned-sphaleritesubcell for comparisonwith E. H. A twinned-sphalerite41.4 A cell for spionkopite.

e q u a l s3 n i s 4 , 7 , 1 1 , 1 4 , 1 8 ,2 1 , 2 5 , 2 9 , 3 2 ^ 3 6 I. f the cell is re-indexedon the basisof a 35.5 A pseudocell, this index approximates3, 6, 9, 12, 15, 18, 21, 24, 27, 30. The data setre-indexedon this basis has two setsof extinctions:if I minus k is equal to 3n, then / is not equalto 3n, and if Z minus k is not equal to 3r, then / is equal to 3n (but / is not equal to 0). Theseextinctionsrequirethat atomsbe placed in the cell in the positions0, 0, zi V3,2A,z + tA; 2/t,rA,z, * 2A;r/t,2A,z + 2/z;2/t, rA,z + rA(frml spacegroup). The atom at 0, 0, z must havedouble the weight of the remaining atoms. Theseatomic positions (with z = 0) are indicated on Figure l4a, togetherwith peak positionsfor a Pattersonsynthesis on an 8.9A subcellbasedupon the abovere-indexed data (Figure l4b). The peak positions on this Patterson map are identical with those of a sphaleritetype $tructureusing the body diagonal as a c axis.

In Figure 14, lessthan full occupancyof the sites is indicated as a percentage. o Various attemptswere made at solving the 8.9 A re-indexedsubcell of spionkopite using least-squares refinement and the minimum residual method o.f Ito (1973).The final structurederivedfor the 8.9 A reindexed subcell is shown in Figure 14c. R for all reflectionsis 0.292(againscatteringfactors wereused for copper only). The correspondencewith the atomic positions derived from consideration of extinction data is obvious, as is the similarity to a 'jtwinned-sphalerite"structure(Fig. lad). This 8.9 A re-indexedsubcell*u, e"p*d"Jinto ihe 35.5 A re-indexedcell as shown in Figure l4e. A possible interpretation is s.lrownin Figure l4f. A "twinnedsphalerite" 35.5A cell is again shown for comparison (Fig. l4g). The interpretedcell shown in Figure l4f is againinconsistentwith a centricunit-cell and,

75


onceagain, severalatoms of copperare missing.The prediction that there would be fewer apex-sharing tetrahedrathan in the covellinestructure is shown by the distributign of copperatoms.All attemptsat solving the 41.4A cell of spionkopiteusingthesedata failed. No atomic arrangementcould be derivedthat would account for the observedsystematicextinctions, afthoughexpansio! would appearto take place by the insertion of a 6Â segmentbetweenthe two halves of the 35.5 A re-indexed subcell, such as is slown in Figure l4h for a "twinned-sphalerite" 41.4A structure. DISCUSSIoN In attempting to solve the structure of spionkopite, we are trying to derive a minimum of seventeen independentatomic positions using just 147 observedreflections.If, as is suggestedby the partial structures shown in Figures 13f and l4f, the structure is acentricrather than centric (Azl rather than frml), this number would increaseto thirtyfour. The full cell is in fact thirty-six times this large. In addition, featuressuch as the diffuse nature of the X-ray patterns,broadeningof peakson Patterson synthesesand splitting of atomic positionsduring structure determinationsall suggestthat Cu is statistically distributed between the two possible tetrahedrally co-ordinatedsites of copper between sulfur layersand among the four possibletriangularly co-ordinatedsites of copper situated around eachtetrahedrallyco-ordinatedsite. Any attemptat removing constraintson the x andy atomic positions during determinationsinvariably lead to splitting of peak positions in these directions as well as along z. Given theseobservations,it is extremelydoubtful that the structure of spionkopite can be fully solved with the currently availabledata. Most of the problemsthat exist in attemptingto solvethe spionkopitestructurecan be applied equally well to the yarrowite structure.The only factor that might make it more amenableto structuredetermination would be the previousobservationthat large areasof the cell should strongly resemblethe covelline structure(e.9., Fig. l2d). However, attemptsat solvingthe spionkopitestructureusingthe structure of covellineas a model were unsuccessful. The structure of geeritemay be solvable.Peaks observedon X-ray patternsare relativelysharp.The major hindrancein determiningthe structure may lie in the scarcityand small sizeof natural material. Experiments such as those conducted by Globe (1981)may provide a methodfor synthesizinglarger fragments of geeritefor single-crystalstudies. ACKNOWLEDGEMENTS

The author thanks Dr. G.W. Robinson for provid-

ing the specimenof geeriteusedin the single-crystal studies.Discussionswith Dr. J.D. Scott were particularly helpful during the attemptsto solvethe crystal structureof spionkopite.Finally, I thank Mr. L. Whitesidefor helpful discussionsduring the writing of this paper and Helen Betenbaughfor tlping the manuscript. REFERENCES ASRAHAMS, S.C. (1955): The crystal and molecular structure of orthorhombic sulfur. Acta Cryst. E, 66t-671. Bsnnv, L.G. (1954): The crystal structure of covellite, CuS, and klockmannite, CuSe.Amer. Mineral.39,

s04-509. R.M. (1962): X-ray powder data & THor"rpsoN,

for ore minerals:the PeacockAtlas, Geol. Soc. Amer. Mem. E5. Casm, L.J. & Har.r-,S.R. (1972):Mooihoekiteand haycockite,two newcopper-ironsulfides,andtheir relationshipto chalcopyriteand talnakhite.Amer. Mineral. 57, 689-708. S.C. (1961): Coopsn,A.S., Bor.ro, W.L. & ASRAHAMS, The latticeand molecularconstantsin ofihorhombic sulfur.Acta Cryst.14, 1008. DoNNev, G., DoNNav, J.D.H. & Kur.lsnuo,G. (1958): Crystal and twin structure of digenite, CueSr. Amer. Mineral. 43, 228-242. EvaNs,H.T., Jn. (1971):Crystalstructureof low chal' cocite.Noture 232. 69-70. (1979):Djurleite (Cur.e4s)and low chalcocite (Cu2S):new crystalstructurestudies.Science203, 356-358. (1981):Coppercoordinationin low chalcocite and djurleite and other copper-rich sllfides. Amer. Mineral. 66. 807-818.

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Fottr,mn,J.C.W. & JBrrrurr, F. (1980): The valence of copper in sulphides and selenides:an X-ray photoelectron spectroscopystudy. J. Ler;sCommon Metals 76. 153-162. FnENzBr,G. (1959): Idait und "blaubleibender Covellin". Neues Jahrb. Mineral. Abh.93,87-132. Gogre, R.J. (1980): Copper sulfides from Alberta: yarrowite CueSr and spionkopite Cu3eS2s, Can. Minerol. lE. 5ll-518.

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(1981):The leaching of copper from anilite and the production of a metastablecopper sulfide structnre. Can. Mineral. 19. 583-591. & Rosrxsox, G. (1980): Geerite, Cu1.56S,a new New York. York. rer sulfide sulfide from from deKalb deKalb Township, Township. New copper Can. Mineral. lE, 519-523.

PurNrs, A., Gnacr, J. & CavenoN, W.E. (1977): Blaubleibender covellite and its relationship to normal covellite. Contr. Mineral. PetrologJt 60,2@-217. Rrcrenp, D.T. (1972): Covellite formation in low temperature aqueous solutions. Minerql. Deposita 7, 180-188.

& Svrrr, D.G.W. (1973):Electronmicroprobe investigationof coppersulphidesin the Precambrian SeoeNeca,R., OHuasa, M. & Monluoro, N. (1965): On the statistical distribution of copper ions in the Lewisseriesof S.W. Alberta. Canada.Can.Mineral. structure of 6-chalcocite, Mineral. J. 4' 275-290. 12,95-103, K. (1956):Thermische Tarepa, H., DowNav, J.D.H. & Apprnvar, D.E. Hnrsn, J.-E. & PnossruerN, Untersuchungen an Kupund rdntgenographische (1967a):Djurleite twinning. Z. Krist. 125, 414-422. ferkies.Z. Krist. l0E, 108-129. Iro, T. (1973): On the application of a minimum residualmethodto the structuredeterminationof Z, Krist. 137, 399-411. superstructures, Kalssropr, R., PEnrrm,F. & ZSBMAN, J. (1975):Verfeinerungder KristallstrukturdesCovellins,CuS, mit Einkristall daten. Tscher maks Mineral. Petrog. Mitt. 22. U2-249.

Rosssool\a,E.H. & ApprsueN, D.E. (1967b): The crystallography of djurleite, Cu1.e7S. Z. Krist. 125, 404-413. Tossrlr, J.A. (1978): Theoretical studies of the electronic structure of copper in tetrahedral and triangular coordination with sllfur. Phys. Chem. Minerals 2,225-236.

Koro, K. & Monnaoro,N. (1970):The crystalstructure VaucneN, D.J. & Tossplr, J.A. (1980):The chemical of anilite. Acta Cryst. 826,915-924. bond and the properties of sulfide minerals. l. Zn, Mon, G.H. (1971):Blue-remainingcovelliteand its relationsto phasesin the sulfur rich portion of the copper-sulfursystemat low temperatures.Mineral. Soc.JapanSpec.Pap.1,226-232.

Fe and Cu in tetrahedral and triangular coordinations with sulfur. Can. Mineral. lE, 157-163. Wrrsor, A.J.C. (1942): Determination of absolute from relative X-ray intensity data. Nature 150, 15t-t52.

Monnaoro,H. & Uvene,R. (1963):A secondcomparison of various commerciallyavailableX-ray films,Acta Cryst.16, 1107-1119. WurNscu, B.J. (1974): Sulfide crystal chemistry. /n Monurroro,N. (1964):Structuresof two polymorphic Acta Cryst. 17, 351-360. forms of Cu5FeSa. & Kur-renup,G. (1963): Polymorphismin digenite.Amer. Mineral. 4E, 1.10-123.

Sulfide Mineralocy (P.H. Ribbe, ed.). Mineral. Soc. Amer., Rev. Mineral. 7, W2l-M. YuNo, R.A. (1963): Crystal data for synthetic Cu5.5ferS6.5,(idaite). Amer. Mineral. 48, 672-676.

OrrspAr, I. (1932):Die KristallstrukturdesCovellins (CuS).Z. Krist.83,9-25. Porrnn,R.W. II & EvaNs,H.T., Jn.(1976):Definitive X-ray powderdata for covellite,anilite, djurleite, ReceivedFebruary 7, 1984, revised manuscript accepted andchalcocite.J. Res.U.S.Geol.Sun. 4.205-212. July 31, 1984.

THE RELATIONSHIP BETWEEN CRYSTAL STRUCTURE, BONDING AND CELL

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