American Mineralogist, Volume61, pages996-1N0, 1976

Crystalstructure refinement of covellite

HowlRn T. EveNs,Jr. nNoJunrrH A. KonNenr U.S. GeologicalSuruey, Reston, Virginia 22092

Abstract

: The crystal structure of covellite CuS [space Erotp P6"/mmc, a : 3.7938(5)4,c 16.341(l)Al hasbeen refined using 237 counter-measured (MoKa) intensitydata. The classic structureis confirmed,and z parametersdetermined as 0.10733(9) for Cu(2)and 0.06337(15) for S(2).The S-S bond lengthin the S, group is 2.071(4)A,and the Cu-S bond lengthsare 2.1905(2)Ain the CuSgtriangle and 2.302(1)(3 bonds),2.331(2)A in the CuSrtetrahedron.

Introduction ville, Colorado,was of good quality as shownby precessionphotographs. The crystalstructure of covellite,CuS, was solved heavily exposed Buerger photographs 44 yearsago by Oftedal(1932). This structuredeter- These showed no trace of any Bragg glideplane mination is one of the classicsof the early era of reflectionsthat would violatethe c charac- group (or structureanalysis, and the atomic arrangement,al- teristicof the space P6r/mmc P6"mc),origi- (1932). though somewhatunusual from a crystal chemical nally determinedby Oftedal The cell dimen- point of view, has beenillustrated without further sions have been determinedby Potter and Evans powder questionin countlessreviews and textbookssince (1976)by least-squaresanalysis of datato be a : 3.7938(5)A, : 16.341(l)4.The crystal used for that time. The first attemptto verify the structureby " more modernmethods was madeby Berry (1954). intensitymeasurement was a plate0.021 by 0.045cm profiles His study,based on trial anderror adjustments using in.sizeand 0.00029cm thick. The scanning two-dimensionalphotographic data, served mainly to wereirregular and of varyingwidth, indicatingsome peaks confirm Oftedal's proposed structure and also, to texturaldistortions in the crystal,but no were (plus some degree,to refine it. Bernardini and Catani more than lo in width. With 0-?i scansof 3o (1968)again reaffirmedthe structure,but usedin- the a;a2 dispersion),the full integratedintensity tensitiesmeasured from powderpatterns, so that our was effectivelyregistered for all reflections. knowledgeof the structuraldetails was not much All data within one half of the Ewald spherede- furtheradvanced. Recently, Kalbskopf et al. (1975) finedapproximately by (sin 0)/X= 1.0,including re- havereported a newdetermination based on a least- flectionsforbidden by the spacegroup, were meas- squaresanalysis of 59 photographic single-crystal ured using Nb-filtered MoKa radiation.In fact, data, giving for the first time reliable interatomic becausein this structureall atomsare confined to x, y distancesin the structure.Unaware of the work of parametersof 0, 0; l/3, 2/3; and2/3, l/3, the glide Kalbskopfe t al. we havealso redetermined the struc- plane extinguishesnot only the reflectionshhl with I , : ture of covellite, using 367 independentcounter- odd, but all hkl reflectionswith h-k 3n and / odd. measureddata. Our resultsare in substantialagree- None of thesereflections showed significant intensity ment,but becauseof theimproved data we havebeen in the measurement,and they have beenomitted able to attain a considerablyhigher degreeof accu- from further consideration.The total of 1958reflec- racy. tions (276 forbidden)were reduced by averaging,af: ter corrections were applied, to 367 independent structureamplitudes. Of these,237 had valuesgreater Experimentalprocedure than 4o on the basis of counting statistics(that is, Covellitegrows in soft, thin, hexagonalplates, so wereabove a thresholdof lfl :6.2) andwere used for that it is difficult to find a crystalsufficiently free of the structureanalysis. distortionfor reliableintensity measurements. The Absorptioncorrections were applied to theindivid- crystalwe used,taken from a speclmentrom Summit- ually measuredreflections using a Gaussianquad-

996 CRYSTAL STRUCTURE REFINEMENT OF COVELLITE rature method basedon 32 samplingpoints within (2) Least-squaresrefinement in P6g/mnc of aniso- the crystal. The calculated linear absorption tropic atoms, based on 2 structure and 8 thermal coefficient,determined from the atomicmass absorp- parameters,led to R : 0.054. tion coefficientsof Cromerand Liberman(1970), is 1i (3) Least-squaresrefinem ent in P6smcof isotropic : 165.7cm-l. The transmissionfactors A varied atoms,based on 5 structureand 6 thermal parame- from 0.35 to 0.95;75 percentof the measuredreflec- ters.led to R = 0.058. tionshad ,{ between0.85 and 0.95.Of the 367inde- (4) Leas-squaresrefinement in P6gmcof aniso- pendent reflections,six equivalentreflections were tropic atoms, basedon 5 structureand 12 thermal measuredfor 162.and threewere measured for 190. parameters,led to R = 0.045. Thus, a good internal estimateof the standarderror The convergenceof runs I and 2 was smooth and of lFl couldbe obtained.The overall standard devia- secure.lt is surprisingto find a considerableimprove- tion is 2.4.Thedeviations were uniformly distributed, ment in R in proceedingfrom 2 to 4, but in runs3 and showedno systematicvariation with lFl, andno con- 4 the convergenceis unstable.In both the last two nectionwith low I values,or any other parameter. cases,correlation factors greater than 0.95 wereen- On this basis.a minimum R factor of about 0.05 countered,especially between the z parametersof the could be expected.Comparison of the final F(obs) atom pairsoriginally related by the mirror plane.The and F(calc) values showed no evidenceof the in- noncentrosymmetricanalysis therefore cannot be fluenceof secondaryextinction, and no correction consideredreliable. has beenmade for it. Kalbskopf et al. (1975)suspected that the prolate The scattering functions for neutral atoms of anisotropyof thermalmotion for Cu(l) whichthey Doyle and Turner(196E) and the dispersioncorrec- found. and which hasalso been observed by us (Fig. tionsof Cromerand Liberman ( 1970)were used in all l), might actuallyrepresent a configurationin which structurefactor calculations.All computationswere this atom is at the vertex of a low pyramid with executedon an IBM370/155 computer using the respectto the coordinatedtriangle of S(2)atoms. We XRAY72 systemedited by J. M. Stewartand co- alsotested this hypothesis,as they did, by placingl/2 workersat the Universityof Maryland. Cu(l) at 0.24, and continuingleast-squares analysis. Keeping this atom isotropic, 2 cyclesled to U = Structureanalysis 0.029(1)A'and z = 0.2435(5)(0.106(8)A from the The structureanalysis began with Oftedal'sstruc- mirror plane)for Cu(l), with a correlationratio of ture and proceededthrough four stages: 0.64for these2 parameters.In 2 cyclesof anisotropic (l) Least-squaresrefinement in P6s/mmcof iso- refinement,this atom attained Ugs = 0.031(13)A'and tropic atoms, basedon 2 variable structureand 4 z : 0.2439(34),with a correlationratio of 0.991.In thermal parameters,led to a conventionalreliability both refinementsthe reliability factor remainedat R factor R = 0.072. : 0.054.

Ftc. I . Stereoscopicview of thecrystal structure of covellite.The thermalellipsoids repres€ntthe 50 percentprobability surfaces. H. T EVANS, JR AND J, A KONNERT

Thus, no valid evidencecan be found for either a The S, group has a bond strengthof 2.071(4)A, noncentrosymmetricor a split-atommodel for cov- whichis considerablyshorter than those found in the ellite. The resultsof the anisotropiccentrosymmetric polymorphsof FeS, by Brostigenand Kjekshus refinement(stage 2 above)must therefore be accepted (1969),2.177(4) in pyrite and2.223(3)Ain marcasite; as the best.These results are shownin TableI com- but longer than2.042(3)Aand 2.032(3)Aas found in paredwith theparameters obtained in previousstruc- patroniteV(Sr)z by Allmann et al. (1964).The aver- ture studies.The structurefactors are listed in Table2 agebond lengthin the Cu(2)tetrahedron is 2.3124, (in thistable, lF(obs)l is left blankwhen its measured slightlylarger than the bestrecent determinations of valuefalls below4o). tetrahedralCu-S bond lengths:2.302(l)A in chal- The possibilityof the existenceof a homometric copyrite (Hall and Stewart, 1973)and 2.305Ain structure(that is, a whollydifferent arrangement that cubanite(average; Szymanski, 1974). The Cu(l)-S(2) would yield the samestructure amplitudes as the clas- bond lengthsof 2.1905(2)Ain the triangulargroup sic structure)was explored. Many differentarrange- areconsiderably less than the averagevalue of2.33A mentsboth in P6r/mmcand P6rmc, especially those found for 24 independentCuS, triangles in low- basedon close-packedsulfur atoms in ABCACB chalcociteby Evans(1971). stacking,were tested without finding any that would The thermalmotion is markedlyvarying and ani- giveR(0.3. Sucha possibilitynow seemsextremely sotropicfor the differentatoms in covellite(Fig. l), unlikely. but the anisotropyis largelywhat would be expected, A AF Fourier synthesiscontaining 85 terms for consideringthe surroundings of eachatom. Thus, the which(sin 0)/\<0.4 showedmaxima of 0.4 electron triangularCu(l) has a largemotion along c [rz : (e)/A' at I/3,2/3,0.145and 0,3 (e)/As at 2/3, l/2, 0.203(4)Al,where no other atomsoffer any serious l/4; alsominima of -0.6 (e)/A' at2/3,l/3,O.fi 6 and restraint[the nearestCu(2) atom is at a distanceof -0.4 (e)/As at 0, 0, l/4. Thereappears to beno rea- 3.199(1)Al.Along the Cu-S bond rz : 0.170(3)A, sonableexplanation for thesefeatures, and we regard similar to the correspondingmotion of S(l), for them as spurious. whichu = 0.160(2)4.The S(l) atom,held betweer, The crystalstructure two Cu(2) atoms, has r.t : 0.100(6)Aalong these bonds(c axis).The tetrahedral practically Covellitehas an unusualstructure for suchsimple Cu(2) is isotropic,with rz: 0.118(2)4.In the group, stoichiometryas CuS.Of the 6 formulaunits in the S, S(l) hasr.r :0.109(2)A normalto the S-S bond, unit cell,4 of the Cu atomshave tetrahedral and 2 and u : 0.083(5)Aalong the bond. triangularcoordination; 4 of the S atomsform di- While sulfideS, groups,and 2 aresingle sulfide ions. The tetrahedralcoordination for copperin sul- fidesin common, structureis illustratedin a stereoscopicdrawing triangularcoordination has been foundin few (1974) showingthe thermal ellipsoidsin Figure L Bond only a species.Lewis and Kupcik listed (1ow-chalcocite, lengthsand anglesare shown in Figure2. 5 wittichenite, anilite, hod- rushite,and syntheticBi2CusSaCl) to which mustbe Tlnlr l. Structuraland thermalparameters for covelUte addedcovellite, stromeyerite, AgCuS (Frueh, 1955),

Kalb skop f tetrahedrite,Cu12SboS,, (Wuensch, 1964), bornite, Atom Parameters This work et al. (1975) Berry(1954) CuuFeS,(Koto and Morimoto, 1975),and synthetic Struclural paraneters : CunBin$,(Takeuchi and Ozawa,1975). The lastcom- cu(1) xt!tz 2/3,I/3,I/4 2/3,L/ 3,r/ 4 2/3,L/ 3,1/4 pound(CurBioSr) contains S, groupsand linkagesof Cu(2) x,y 7/3,2/3 r/ 3,2/3 r/3,2/3 CuSntetrahedra and CuS,triangles resembling those z 0.10733(9) 0.1073(3) 0.107 seenin portionsof the covellitestructure. Covellite s(1) x,!,2 r13,2/3,t/ 4 r/3,2/3,r/ 4 r/3 ,2/3,r /4 containsa continuouslayer of S atomsin triangular S(2) x,y 0,0 0,0 0,0 z 0.06337(1s) 0.0624(8) 0.064 array with every other triangleoccupied by a Cu atom.An present Thernal paraneters (in A2): exactlysimilar layer is in stromeyer- (1 ite, and similarbut interruptedlayers also appear in cu ) llr r rrrr 0.0289(10) 0.015 Ug:'-' 0.0414(I7) 0.045 low-chalcocite(Evans, 1971). Evidently this arrange- Cu(2) Ulr (rr) O,0142(4) mentis characteristicof metalsulfides rich in u:s.-. 0,0r35(5) 0.0147(6) copper and may be expected s(1) urI 0.0255(15) to appearin other complex r,,, (14) u::r-, 0.0100(13) 0.0180 coppersulfide phases. S(2) Ull r,,r 0.0II7(6) A rigorousdiscussion of the electronicand bond- U::'-' 0. 0108(10 ) 0.0069(8) ing charactersof thecovellite structure is beyondthe CRYSTAL STRUCTUREREFINEMENT OF COVELLITE 999

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Acknowledgments We aregrateful for the helpof R. W. Potterof theU.S. Geologi- cal Surveywho suppliedour study specimen,and to ProfessorJ. Zemann of the University of Vienna for sending information about the work on covelliteby him and his colleaguesprior to publication.

References 2,33t(2) ALLMANN,R., l. BlurrlnrN, A. Koroclu, H. RdscH ero E. Hslr-- xrn (1964) Die Kristallstruktur des Patronits V(Sr)r. l{arnru'ss. 51,263-2U. BenNenorrr,G. P. rr.ro A. CereNr (196E)Raffinamento della strutteradella covellina mediante fotogrammi dipolvere. Lincei- Rend.Sci. fn. e nat. 44,290-298. 2,305 Bsnnv, L. G, (1954)The crystalstructure of covellite,CuS, and klockmannite,CuSe.,4rn. M ineral.39, 504-509. s(2) Bnosrtcrn, G. erp A. Krerssus (1969) Redeterminedcrystal structureof pyrite.Acta Chem.Scand. 23, 2186-2lEE. Cnouen,D. T. lNo D. Lrsenruex(1970) Relativistic calculation of anomalousscattering factors for X-rays. "/. Chem.Phys. 53, l89l-1898. Dovle, P. A. nNo P. S. TunNsR(1968) Relativistic Hartree-Fock Ftc. 2. Portion of the covellitestructure showing bond lengths X-ray and electron scattering factors. Acta Crystallogr. A24, (in A) and angles.Numbers in parenthesesrepresent standard 390-39't. deviationsin termsof last significantfigures. Evnus,H. T., Jn. (1971)Crystal structure oflow chalcocite.ffa- ture ( Phys Sci.), 232, 69-70. Fnueu, A. J., Jn. (1955)The crystalstructure of stromeyerite, scopeof this paper,but oneor two empiricalobserva- AgCuS:a possibledefect structure. Z. Kristallogr.106,299-307. Hnll, S. R. lr.ro J. M. Srewenr (1973) tionsmay bemade. Clearly, the strongbond in the S, The crystal structure group requires refinement of chalcopyrite, CuFeS2.Acta Crystallogr. B,29, that the averagevalence of Cu be 579-585. Klr-nsxorn, R., F. Penrux nNo J. ZrervreNN(1975) Verfeinerung der Kristallstrukturdes Covellins,CuS, mit Einkristalldaten. Tschermak' s M ineral Pet rog r. M i t t. 22, 242-249. Koro, K. ero N. Monnroro (1975)Superstructure investigation of bornite,CuuFeS., by the modifiedpartial Pattersonfunction. A cta Crystallogr. B3l, 2268-2273. If S(l) is assumedto have a valenceof 2, then it Lrwrs, J., Jn. rno V. Kupcfx (1974)The crystalstructure of followsthat Cu(l) also will havea valenceof 4/3. BirCu'S.Cl. Acta Crystallogr.830, 84E-E52. According to Pearson'sextension of the g-i{ rule Orrrn,rr, I. (1932)The crystalstructure of covellite.Z. Kristallogr. E3,9-25. (Pearson,1964), the number of valenceelectrons ne andthe number Pennson,W. B. (1964)The crystalstructures of semiconductors of electronsinvolved in anion-anion and a generalvalence rule. Acta Crystallogr.17, l-15. bonding6" shouldsatisfy the relation (n.lbo)/no = g, PorrER, R. W. nNo H. T. EvrNs, Jn. (1976)Definitive X-ray whereno is the numberof cations.Applying this rule powderdata for covellite,anilite, djurleite and low chalcocite. to covellite,CusS(S2), we find that it is satisfied(when U.S.c.S.J. of Res.4,205-212. (1974) bo =2) only if the 3 Cu atomscontribute 4 electrons. SzvrrreNsxr,J. T. A refinementof the structureof cubanite. Z. Kristallogr. 140, 2lE-239. This is consistentwith the interpretation of the bond Texrucur, Y. nNo T. Ozewn (1975)The structureof Cu.Bi.S" lengthsgiven above. and its relationto the structuresof covellite,CuS and bismuthi- Evidentlythe perfect(001) cleavageof covellite nite, Birss.Z. Kristallogr.l4l,2l7-232. resultsfrom the fact that thereis onel/3 bond in the WueNscu, B. T. (1964) The crystal structure of tetrahedrite CutrSb.S,r.Z. Krtstallogr. ll9, 437-453. CuS. tetrahedronparallel to the c axis, which is the only bond in the unit cell holding the structureto- Manuscriptreceiued, January 23, 1976;accepted getherin this direction. for publicatton,May 14, 1976.