Phase Relations in the Cu-Co-S System and Mineral Associations Of

Home , Carrollite, Cattierite, Djurleite, Gersdorffite, Siegenite

Economic Geology Vol. 74, 1979, pp. 657-671

PhaseBelations in the Cu-Co-SSystem and Mineral Associationsof the Carrollitc ( CuCoS4)- Linnaeite (Co3S4)Series

JAMESR. CRAIG,D^vm J. VAUGItAN, ANDJoHs B. HmG•ss

Abstract Phase relationsin the systemCu-Co-S have been studiedusing sealedsilica capsule methodsbetween 400 ø and 900øC. The high-temperatureportion of the systemis dominatedby the coexistenceof the chalcocite-digenitesolid solution•vith cobaltsulfides (COS2, COl_xS, Co4S:•) . AS temperature is decreasedto 880ø___5øC, the dominant ternary phase, the carrollite-linnaeite solid solution. appears with a compositionof •Cu0.aCo2.aS4. The copper-richcompositional limit is extendedon cooling,reaching a maximum (CuCo2S4) at •,500øC. Below 507øC covellite is stable and coexistsin the sulfur-richportion of the systemwith copper-bearingcattlerite. Below500øC the major changesinvolve a changein stableassemblages (at •,450øC), in whichthe high-temperature pair Co + Cu2S is replacedby the pair Cu +Co0Ss. the disappearanceof Co•_xS (<_460øC)and digenite (_<70øC), and the appearanceof low chalcocite(103øC), djurleite (93øC), and anilite (70øC). Optical and electronmicroprobe studies of natural as- semblagespermit speculation on phaserelations at 25øCand suggestthe topologyof the systemis similar to that at 400øC exceptfor the appearanceof the low-temperaturecopper sulfides.Similar studiesalso permit speculationon the low-temperaturephase relationsin the relevantportion of the Cu-Co-Fe-Ssystem in whichcarrollite frequently occurs in closeassociation with chalcopyriteor borniteand, in somecases, cobalt-pyrite. More cobalt-rich"carrollites" are foundto occur in assemblageswith more cobalt-rich "pyrites"and this correspondsto decreasingas2. Assemblages such as carrollite-chalcopyrite-bornite-cobaltianpyrite and carrollite-cobaltianpyrite-bornite-digenite correspond to a furtherdecrease in as2. The actualas.o values during formation of assemblagescon- taining thesephases are also consideredusing thermochemicaldata for sulfidationequi- libria. In the carrollite-linnaeiteseries, the systematicdecrease in unit cell parameterand increasein reflectivity(at 589 nm) as a functionof composition,as well as the actual compositionalvariations in the natural phases,are consistentwith crystalchemical models.

Introduction was later consideredto be merelya slightly iron- and T• Cu-Co-Ssystem is representedin numerousore nickel-bearing linnaeite with intimately intergrown depositsby minor amountsof membersof the car- copper sulfides (Dana and Ford, 1932). In the rollite (CuCoeS•)-linnaeite (CoaSt) series.In the interim, Laspeyres (1891), unable to obtain "car- Zambian Copperbelt,however, this mineral seriesis rollite" for comparison,assigned the name "sychno- a major ore constituentand is the principalsource of dymite" and the composition(Cu,Co)iS, to similar the cobalt extracted (Whyte, 1971; Notebaart and material from Siegen. The true character of car- Vink, 1972). The presentstudy reports on the phase rollite as a thiospinelbecame apparent with a series equilibriaof the Cu-Co-Ssystem and mineralogicof goodchemical analyses (Shannon, 1926) and with occurrencesof coexistingcopper- and cobalt-bearingX-ray crystallographicstudies (Menzer, 1926) sulfides with emphasison the carrollite-linnaeite demonstratingthe essentiallyidentical nature of series. carrolliteand linnaeite.X-ray data also demonstrated Carrollite and linnaeite are members of the thio- the completeidentity of carrollite and sychnodymite spinel mineral group, a group long consideredto (de Jong and Hoog, 1928). Althoughthe 4th edi- possessthe commonalityof the spinelcrystal struc- tion of Dana's Textbook of Mineralogy (Dana and ture. Linnaeite, CoaSt, named for the famed Lin- Ford, 1932) essentiallydiscredited the name carrol- naeus,was the first thiospinelto be recognizedas a lite, the name was restored by the 7th edition of mineralspecies (Brault, 1746) and its identityhas Dana's Manual of Mineralogy (Palache et al., 1944) beenreaffirmed many timessince. Carrollite,origin- at the expenseof sychnodymite.Some texts suchas ally describedas Cu=Co=S•from specimensfrom Hey's (1962) ChemicalIndex of Minerals still re- Finksburg,Carroll County,Maryland (Faber, 1852), tain sychnodymitefor nickel-rich varieties but the 657 658 CRAIG, VAUGHAN, AND HIGGINS

trend has beento replacethat name (as doesFleis- plicationsin the studyof sulfidephase equilibria, cher, 1975) with carrollite. particularlyat temperaturesbelow •400øC where reaction mechanismsand kinetic factors becomein- Previous Work in the Co-Cu, Cu-8, Co-S, creasinglyimportant. and Cu-Co-8 Systems Co-Cu system Co-S system The phaseequilibria in the Co-Cu systemas deter- The cobalt-sulfursystem, reviewed by Hansenand mined by numerousauthors have been reviewedby Anderko(1958), Elliott (1965), and Shunk (1969), Hansen and Anderko (1958) and Elliott (1965). containsfive intermediatephases, three of whichcor- Although there are no intermediatebinary com- respondto mineralspecies: Co9S8 (cobalt pentlandite, poundsin this system,cobalt acceptsapproximately co-pent,stable below 835øC); CoaS4(linnaeite, ln, 10 weight percent copper in solid solution above stablebelow about 680øC); and CoS2 (cattierite, ct, 340øC and lessthan 2 percentat 300øC, althoughthe stablebelow about 950øC). Co1_xS melts congruently latter amountis not known with certainty. The solid at 1,182øC;its lowerstability limit hascommonly solubilityof cobaltin copperis about1 weightpercent been reportedas •460øC. Co4Sais stablebetween at 700øC but lessthan 0.1 weight percentat 500øC 930ø and approximately785øC. and below. Cu-Co-S system Cu-S system The generalconfiguration of the Cu-Co-Ssystem The copper-sulfursystem contains six phases between400 ø and600øC has been outlined by Craig known as mineral species. These phases (their and Higgins(1973) and Wang (in Moh, 1976). standard abbreviations and their maximum thermal stabilitiesgiven in parentheses)are chalcocite,Cues Experimental Methods (cc, 103.5øC,although there is an intermediateform Experimentalcharges in the presentstudy were stable up to •430øC and a high-temperatureform preparedby conventionalsilica tube techniques (Kul- which melts at 1,129øC): djurleite, Cu•.90S (dj, lerud, 1971) and by meansof the multiplereaction 93ø); anilite,Cu7S4 (al, 75øC); digenite,Cu0S5 (dg, techniquespreviously used for violarite synthesis stableonly between70 ø and •80øC in the Cu-S sys- (Craig, 1971). Reagentswere 99.999+ percentpure tem and forming completesolid solution with Cues cobalt,copper, and sulfuras indicatedby supplier's above 430øC; Morimoto and Koto (1970) suggest (ASARCO) analyses.Copper was freshlyfiled. that iron-containingdigenite is stablein the Cu-Fe-S Cobaltsponge was reduced in a streamof hydrogen systemas low as roomtemperature); "blaubleibender at 700ø to 800øCfor 6 hoursprior to usein experi- covelline"CUl+xS (bcv, 157øC); and covellite, CuS ment preparation. Experiments were carried out in (cv, 507øC). The phaseequilibria among the copper Nichrome wound resistance furnaces controlled to sulfides have been reviewed by Roseboom (1966), ---3øC. At the termination of each experiment, Morimoto and Koto (1970), and Craig and Scott chargeswere rapidly chilled in cold water and the (1974) and recentlyrefined by Potter (1977). reactionproducts were examinedby X-ray powder Additional copper sulfideswhich are known only diffraction, electron microprobe,and reflectedlight as synthetic phasesinclude: two high-temperature microscopictechniques. Cell dimensionswere deter- polymorphsof Cues (a hexagonalform stable between 102 ø and 430øC and a cubic form stable above m/ned by X-ray powder diffraction employing National Bureau of Standardssilicon (a0 = 5.43088 430øC); a tetragonalpolymorph of Cues stable at A) as an internal standard. Electron microprobe high pressurebut at 1 atmosphereonly below 10øC; data were obtainedusing either an ARL-EMX or an a cubic form of Cu0S5 stable above 73øC; and a CuSe ARL-SEM•O instrument operating at 15 kV ac- phasestable at high temperatureand pressure(Mun- celerating voltage and 0.15 microampssample cur- son, 1966; Taylor and Kullerud, 1972). rent. Synthetic CoaSi, CoSe, Cu•FeS4, NiS, FeS, Relationshipsinvolving the low-temperaturecopper and CuS were used as standards. sulfideshave also been clarified by the recent electron microscopestudies of Puthis (1976, 1977). These Experimental Data on the Cu-Co-S System show that, in addition to the ideal transformation from the stablehigh-temperature form to the stable Phaseequilibria in the Cu-Co-S systemhave been low-temperature form, alternative metastable pro- examined experimentally between 400ø and 900øC cessesmay operate when the formation of the low- and are summarized in isothermal diagrams in temperature state is impeded. Such observationson Figure 1. Selectedrepresentative experimental data these and other sulfide systemshave important im- are given in Table 1. Cu-Co-S PHASE RELATIONS lIND CuCo2S•-Co.S•MINERIlL lISSOCIlITIONS 659

s s

Cu Co Cu Co

500øC

CoS•

E Cu Co

O0 ø C

/ s,

F Cu Co FIG. 1. Isothermal sectionsof the condensedphase relations in the Cu-Co-S system at' A, 900ø; B, 800ø; C, 700ø; D, 600ø; E, 500ø; F, 400øC. At 900øC (Fig. 1A) two sulfide-richliquids co- tends from the Cu-S join approximately 18 weight exist in the interior of the system. Liquid I contains percent Co into the system;Liquid 11 containsbe- betweenabout 25 and 30 weight percentS and ex- tweenabout 26 and 30 weight percentS and extends 660 CRAIG, VAUGHAN, AND HIGGINS

TABLE 1. Experimental Data in the Cu-Co-S System

Cu Co S days Product (wt %)

A. 400øC 15 28 57 21+ ct(4.4% Cu) + cv + S-liq 15 75 10 84 Cu(0.5% Co) + cc(1.2% Co) + Co0Ss(1.1%Cu) 25 55 20 84 Co(1% Cu) + cc(3.8% Co) + Co0Ss(2.3%Cu) 27 33 4O 84 cv(0.6% Co) 3. ct(1.8% Cu)l•cr(13.4% Cu) 3. Co0S•(1.3% Cu)3 35 35 3O 84 cc(1.9% Co) + cr(8.7% Cu) 3- Cm_xS(3.4% Cu) 59 9 32 84 *cc(0.5% Co) + cv + ct(4.9% Cu) [-3- cr(11.3% Cu)3 70 20 10 27 Cu(0.4% Co) 3- cc(1.3% Co) 3- Co(0.5% Cu) B. 500øC 20.4 21.0 58.6 13 ct(14.3% Cu) 3- cv(1.0% Co) 3- S-liq) 20.5 38.1 42.4 33 cr(20.5% Cu) 25 55 2O 35O cc(3.7% Co) + Co + Co0S•(0.4% Cu) 25 25 5O 13 ct(18.4% Cu) 3- cv(0.5% Co) 3- S-liq 28 32 40 350 cc(0.5% Co) 3- cr(20.0% Cu) 3- ct(0.9% Cu) 32 31 37 13 cr(20.2% Cu) + ct(0.5% Cu) + cc(2.5% Co) 32 37 31 350 cc(3.3% Co) + Co•_xS(0.6% Cu) 59 9 32 350 ct(16.1% Cu) 3- cc(0.5% Co) 3- cv 70 15 15 13 Co + cc(5.3% Co) + Co0S•(2.5% Cu)

C. 600øC 15 35 52 212 ct(12.1% Cu) + cc(2.8% Co) + S-liq 2O 45 35 72O cr(7.9% Cu) + Cm_xS(0.7% Cu) + cc(4.4% Co) 20.5 38.1 41.4 7 cr(17.4% Cu) + cc + ct 30 40 30 720 cc(7.0% Co) + Co,_xS(0.5% Cu) 3- Co0Ss(0.4% Cu) 32 31 37 2 cr(19.3% Cu) 3- ct(0.6% Cu) 3- cc(2.5% Co) 40 40 20 720 cc(3.4% Co) + Co9S•(1.2% Cu) + Co 55 10 35 720 ct(14% Cu) + cc(33% Co) + S-liq 70 20 10 720 cc(2.6% Co) + Co + Cu(0.5% Co)

D. 700øC 15 35 5O 52 ct(7.0% Cu) + cc + S-liq 2O 45 35 28O cc(4.9% Co) + cr(10.2% Cu) + Cm_xS(1.0% Cu) 20 60 20 34 cc(3.3% Co) 3- Co0Ss(0.3% Cu) 3- Co(1.%Cu) 20.5 38.1 41.4 280 ct(1.7% Cu) + cr(16.5% Cu) + cc(3.2% Co) 40 20 40 280 ct(7.1% Cu) 3- cc(1.0% Co) 3- S-liq 65 25 10 34 cc(1.5% Co) + Co(1% Cu) + Cu(1% Co)

E. 800øC 4 54 42 285 ct(0.4% Cu) 3- cr(8.6% Cu) 3- Co•_xS(1.0% Cu) 2O 4O 4O 285 ct(0.6% Cu) 3- cr(12.2% Cu) 3- cc(7.9% Co) 10.3 48.0 41.7 5 ct + cr(10.4% Cu) 25 25 5O 38 ct(4.4% Cu) + cc + S-liq 30 40 30 285 Col_xS(0.8% Cu) 3- cc(11.5% Co) 3- CooSa 35 45 20 285 Co + CotSa + cc(10.6% Co)

F. 900øC 10 59 31 3 Cm_xS(0.9% Cu) 3- Liq-II(Cu-12.3; Co-57.2; S-30.5) 15 7O 15 3 Co(1.2% Cu) + cc(10.5% Co) + Liq-II (Cu-7.3; Co-64.6; S-28.1) 20 40 40 3 ct(0.6% Cu) + Co•_xS(2.4% Cu) + Liq-I (Cu-54.2; Co-17.7; S-28.1) 20 40 40 7 ct(0.7% Cu) + Cm_xS(2.5% Cu) + Liq-I (Cu-56.3; Co-17.0; S-26.7) 35 37 28 7 Liq-I(Cu-58.5; Co-17.2; S-24.3) 3- Liq-II (Cu-20.6; Co-49.2; S-30.2) 3- Cm_xS(0.8% Cu) 45 15 4O 3 ct (2.0% Cu) + Liq-I (Cu-61.7; Co-10.7; S-27.6) + S-Liq 65 25 10 7 Cu(3.8% Co) + Co(5.9% Cu) + cc(2.0% Co)

All data in weight percent. Abbreviations: ct = cattierite, cr = carrollite, cc = chalcocite-digenitesolid solution, cv = covellite; liq = liquid; *[- 3 indicated disequilibrium phase(s) present. from the Co-S join approximately20 weight percent percent Cu, coexistwith Liquid II. CoSe which co- Cu into the system. In the sulfur-deficientportion exists with Liquid I containsup to 2 weight percent of the system,Co and Cues solid solutionscontaining Co (Fig. 2). The mutual solid solutionlimits of Cu up to 11.5 weightpercent Co exist with Liquid II. In and Co are taken from Hansen and Anderko (1958). the more sulfur-rich portion of the system,Co4Sa, Relationshipsin the 500ø to 800øC range are char- Co•_xS,containing a maximum of about 2.5 weight acterizedby coexistenceof the high-temperaturechal- percentCu, and CoSe,containing less than 0.7 weight cocite-digenitesolid solution with cobalt sulfides. Cu-Co-S PHASE RELATIONS AND CuCo•,e•-Co•,c, MINERAL ASSOCIATIONS 661

At 800øC (Fig. 1B) the high-temperaturechalcocite- s digenitesolid solution, which acceptsup to 11 weight • LOW percent Co into solid solution,coexists with cattierite, PERATURE carrollite-linnaeite solid solution, Co•_xS, CooSs, Co4Sa,and cobalt. The copper contentsof Co9Ss and Co•_xSare limited to < 1 weight percent,whereas that of Co4Sa reachesa maximum of •4 percent. Cattierite takesonly 1.5 weight percentCu into solid solutionat 800øCwhen coexistingwith the carrollite- linnaeite solid solution and the chalcocite-digenite / solid solutionbut will acceptup to 4.4 weight percent Cu when coexisting with S-liquid. The carrollite- ' -7 - .... co-,. linnaeite solid solutionis restrictedto a composition between8 and 12.5weight percent Cu and is bounded on the Cu-rich and Cu-poor limits by chalcocite- digenite solid solution-I- cattlerite and cattierite -I- Co•_.•S-bearingassemblages, respectively. Cu Co The dominantternary phase,the carrollite-linnaeite solidsolution, is stableup to 880ø -- 5øC abovewhich Fro. 3. Inferred low-temperaturephase relations in the Cu-Co-S'xsystemon the basisof the experimentalwork in it breaksdown to Liquid I, CoSu,and Co•__xS.The this studyand the observednatural mineral assemblages. compositionof maximum stability is •10 weight percentCu correspondingclosely to Cuo.sCou.sSo.•. the carrollite-linnaeite seriesto stoichiometriccarrol- The general topologyof the systemchanges little lite,CuCo•S;, composition. The maximumsolubility as the temperatureis decreasedto 700øC (Fig. 1C). of Cu in CoS• which coexistswith the Cu•S ss and The CogSaphase is not stable below about 780øC S-liqincreases to • 12weight percent. Below507øC, (Elliott, 1965); no evidencewas observedto suggest CuS is a stablephase and coexistswith Cu•S and that the presenceof Cu might stabilize this phase. cattierite;CuS contains a maximum of about1 weight The carrollite-linnaeitesolid solutionphase increases percent Co. its width by extending the Cu-rich and Cu-poor Below500øC (Fig. 1E) the majorchanges which limits to 16.5 and •2 weight percent,respectively. occurinvolve the shrinkageof the Cu•S solid solu- The maximum solubilityof Co in chalcocite-digenitetion fieldand the appearanceof the low-temperature solidsolution at 700øCis •5 weightpercent and the copper sulfidessuch as digenite,djurleite, anilite, maximumsolubility of Cu in CoSuis 7 weight per- and low chalcocite.Relationships here are not un- cent. equivocallyestablished even in the binary system. Further decreaseto 600øC (Fig. 1D) resultsin the The calculatedsulfidation curves for cobaltand cop- extention of the carrollite-linnaeite series to CoaS• at per (seeFig. 11) crossat 404øCthus indicating that 680øC and to a maximum Cu contentof 18 weight the stable high-temperaturetie line between cobalt percent when coexisting with CuuS and cattierite. andchalcocite is replacedby a tie linebetween copper Continued decrease to 500øC results in extension of and cobalt-pentlandite.Experiments at 400øC.con- firm thistie line change(Fig. 1F). Anotherproblem concernsthe low-temperaturestability of the mono- sulfideCo•_•S, which may occurnaturally as the mineral jaipurite (Clark, 1974),' althoughsome studies on the Co-S system show that it is unstablebelow 460øC (Rosenqvist,1954). Our experiments,dis- cussedbelow, suggest that Co•_•Sis not stablebelow 460øC but that it may persistmetastably at lower temperature for long periods of time. The low- temperaturestability relations in the Cu-Co-S system have been estimatedfrom the study of natural as- semblagestogether with extrapolationof the trends observedin the high-temperaturedata (Fig. 3). Co- existingcarrollite and Cu•S phasesare commonin-' natural assemblages,but the exact tie-line arrange- FIG. 2. Rounded grains of CoS• in a matrix of Liquid I. Sample rapidly cooled from 900øC; the width of the figure ment in the Cu•S region is uncertainas many is approximately 0.75 mm. workersare not explicit in the identificationof these 662 CRAIG, VAUGHAN, AND HIGGINS

TABLE 2. Thermal Stability Data for the Carrollite-Linnaeite Series

Time, Carr-l.inn Composition Temp, øC days Products a0A(4-0.002)

CoaSt 671 « CoaSt 9.404 CoaSt 676 1 CoaSt -- CoaS4 684 1 CoSs q- Co•_,,S -- CoaS• 687 1 CoSs q- Co•_,,S --

Cu0.5Cos.5S4 800 1 Cu0. •Cos.•S4 9.439 Cu0. •Co2.•S4 850 1 Cu0.5Co2.•S4 9.442 Cu 0.• Co`,.•S• 857 1 Cu 0.•Cos. •S• 9.439 Cu0.scoa. •S• 872 I Cu0.•Cos. •S4 -- Cu0.•Co.,.•S• 882 1 CoS•, q- Cu•S q- Co•_xS --

CuCosS• 500 40 CuCosS4 9.472 CuCo=S• 600 7 Cu•_,,Cos+xS4q- CoSsq- Cu,,S 9.464 CuCosS, 700 7 Cu•_,,Cos+,,S4q- COS,,q- Cuss 9.459 CuCo=S• 800 300 Cu•_•Cos+.•+ CoSs+ Cuss 9.444 CuCo=S• 875 « Cm-,,Cos,,,S• + CoSs + Cuss -- CuCosS, 890 2 CoSs + Cuss + Co•_xS -- phases. As suggestedby the high-temperaturerela- Carrollite and linnaeiteare both reportedto have tions, covellite does not occur in associationwith car- the spinel crystal structure normally consideredas rollire. There are conflictingreports of jaipuriteand spacegroup Fd3m. However, additional reflections of cobalt pentlandirecoexisting with carrollire. Co- incompatible;vith this spacegroup have been ob- balt pentlandireand "carrollire" (the composition servedin both carrollireand linnaeiteby Higgins along the carrollite-linnaeitejoin is unknown) have et al. (1975). Suchadditional reflections may indi- been reported from the Rhokana South erebody catethe correctspace group to be F43mand result (Notebaart and Vink, 1972) whereasClark (1974) from displacementsof the octahedral site ions--a has described coexisting carrollite, djurleite, and model suggestedto explain similar data for certain jaipurite from Carrizal Alto in Chile. Since neither oxide spinels (Grimes, 1972). An alternative ex- the validity nor the stability of jaipurite has been planation,cation orderingon the tetrahedralsites as clearly establishedas a •nineral species,the former found by Hill et al. (in press) for indite, Feln2S4, associationis favoredby us as representingthe stable couldalso be responsiblefor the forbiddenreflections. assemblageat low temperatures.

Properties of the Cu-Co-S Phases TABLE 3. Unit Cell Parameter Data Measured at Room Temperature for Carrollite-Linnaeite Series and Copper- bearing Cattlerites Carrollite-linnaeite series

A. Carrollite-linnaeite series The maximumthermal stabilityof linnaeite,CoaS4, Unit cell has previouslybeen given as from •625 ø to 680øC Synthesis Time, dimension, (Hansen and Anderko, 1958-Kullerud, 1968; Shunk, Composition temp, øC days A4-O.001 1969). In the present study this value was deter- CoaSt q- Co•_•S 600 180 9.405 minedto be 680ø -- 4øC (Table 2). At temperatures CoaSt q- CoS= 600 180 9.404 below approximately500øC, completesolid solution CoaSt 500 60 9.404 exists betweenlinnaeite and carrollitc, CuCo2S4.The Cu 0.25Cos.,.•S, 500 600 9.425 Cu 0.• 0Cos.•0S, 500 300 9.442 thermalmaximum for this series(Table 3) is 880ø ---+ Cu0.7•Cos. ssSl 500 600 9.459 5øC and lies at a compositionof approximately CuL 00Cos.00Si 500 600 9.472 Cu0.•Co2.sS4.This compositionbreaks down to a B. Copper-bearing cattlerites copper-containingcattieritc plus a melt with a com- Unit cell positionestimated to containabout 65 weightpercent mole % Synthesis Time, dimension, Cu, 10 weightpercent Co, and 25 weightpercent S Wt % Cu CuSs temp, øC days A 4-0.001 plus Co•_xS. Below 800øC the compositionalrange 0 0 700 13 5.535 of the solid solution series is extended toward both 0.6 1.2 800 250 5.538 more Cu-rich and Co-rich compositions.At 800øC 1.9 3.7 900 3 5.543 4.4 8.6 800 37 5.553 the solidsolution extends from 12.5 to 8 weightper- 6.8 13.2 700 22 5.566 centCu, at 700øCfrom 16.5to 2 weightpercent Cu, 12.1 23.6 600 210 5.590 and at 600øC from 18 to 0 weightpercent Cu. Cu-Co-S PHASE RELATIONS AND CuCo2S•-CoaS•MINERWE ASSOCIATIONS 663

CARROLLITE- LINNAEITE SERIES The unit cell dimensions of the carrollite-linnaeite i i • i i series have been determinedby X-ray powder diffraction (Table 3).. Samplesfor this study were preparedusing a three-stagemethod of synthesis E (initial homogenizationat 500øC; sulfurizationat •40 300øC; annealingat 500øC). X-ray data were col- lectedusing monochromatized CuK•l radiationat an oscillationscanning rate of 1/2ø0 per minute, and National Bureau of Standardssilicon (ao = 5.43088 A) asan internalstandard. The unit celldimensions 2C I I I I I were calculatedusing the program of Evans et al. A cuco2s4 cu0 75co2 25s4 Cuo5oCO2 5oS4 Cuo.25co275s4 co5s4 (1963). As shownin Figure4, the variationin unit REFLECTIVITY PROFILES: carrollire- linnoeite series cell parametersbetween carrollite and linnaeiteis i i i [ i i i i i i i i

_.....- linear and the relationshipbetween unit cell and .....- • compositionis expressedby the followingequations: wt % Cu(-+0.3) = -2798.88 + 297.58ao(A) a0(A) (-+0.001): 9.4057+ 0.0033wt % Cu 40 Natural phasesin the carrollite-linnaeiteseries with minor impurityconcentrations fit this line in Figure4. The unit cell dimension of CoaS4 samples syn- thesizedat 500øC in the presenceof excessCoS2 or .... #136 Co3S4 Co•_xS was identical to that of stoichiometricsyn- --+ •$7 Cuo•CozsS4 •,o #33 CuCozS4 thetic CoaS4(i.e., 9.404--+-0.001A) suggestingno metal:sulfur nonstoichiometry,at least in pure syn- S 20420 I I 460I I 500I I 540I I 580I I 620I I thetic samples. Availabledata on the Vickersmicro- FIC. 5. A. Reflectivity variation at 589 nm for the carrol- hardness of the carrollite-linnaeite phases are con- lite-linnaeite series. sistentwith the predictionthat microhardnessshould B. Reflectivity spectral profiles for the carrollite-linnaeite decreasefrom CoaS• to CuCoeS• becausethe number series. of electrons in antibonding orbitals increases (Vaughan et al., 1971). to a hexagonal NiAs structure. He reported that Variation in reflectivity as a function of composi- CoaS4undergoes an inversion at about 2.5 kb and tion has also been examined in this work. At 589 640øC. We have heated stoichiometric CuCo2S4 at 6 nm, reflectivity shows a systematicdecrease from kb and 444øC and at 40 kb and 500øC each for 18 linnaeite to carrollite as illustrated in Figure 5A. hours. In both experimentsthe principal phasere- This trend is seenover most of the visibleregion of sulting was carrollite (a0 = 9.471 ----+-0.002A from 6 the spectrumas illustratedin the spectralprofiles of kb and 9.451-+ 0.002 A from 40 kb), but small Figure 5B and is alsoconsistent with the predictions amounts of cattierite and chalcocite-digenitesolid of the thiospinelbonding models (Vaughan et al. solution were also present. There was no evidence 1971). of a hexagonalphase being present or having existed Kullerud (1968) has suggestedthat carrolliteand under the conditionsof the experiments. many other mineralthiospinels invert underpressure

i i i

9 48O 5.610 Wt % Cu = -2798 88 + 297 58( (3o} I I I I I Wt. % Cu: -1214.19 + 219.59(Q o)

9 460 5.590

o 9440 "S 5.570 -

9,420 5.550

9 400 WI % Cu Wt. % Cu '? zo5• i•0 ? i 5.530 I '• I'? •i • • I f I I00 715 •0 25 0 25 20 15 I10 5 0 CuCozS4 Co3S4 CoS2 Cerrollite Lmnaeite Cottierite Mal % CuCo•S4 Mole % CuS3 FIC. 4. Unit cell dimension of the carrollite (CuCo.oS,) FIC. 6. Unit cell dimensions of synthetic copper-bearing - linnaeite (CoaSt) series, measured at room temperature. cattierites, (Co, Cu)S•, measured at room temperature. 664 CRAIG, VAUGHAN, AND HIGGINS

Cattlerite lessthan 1 mm away but in contactwith the covellite and sulfur,contained 14.4 weightpercent Cu. Cattlerite, COS2,was encounteredin the present studyas a promineptphase in sulfur-richassemblages The presenceof copper in the cattlerite structure results in an increase in unit cell dimension from and as a breakdownproduct of the carrollite-linnaeite 5.535 ñ 0.001 A for CoS2 to 5.590 ñ 0.002 A for a series. Althoughcattlerite coexisting with carrollite and the chalcocite-digenitesolid solution between cattleritecontaining 12.1 weight percentCu (Table 400ø and 900øC containsless than 1 wt percentCu, 3; Fig. 6). The variation of unit cell parameter cattlerite coexistingwith the chalcocite-digenitesolid with coppercontent is linear and is expressedby the solutionand liquid sulfur containsup to 18 weight following equationswhich are based on the data in percent Cu. Cattierire, like other disulfides,is rather Table 3: refractory and grains frequentlypersist metastably wt % Cu(ñ0.3) = -1214.19 + 219.39ao(A) in disequilibriumassemblages, especially at 400ø or ao(A) (ñ0.001): 5.535 + 0.00455wt % Cu 500øC. In oneinstance, a chargeprepared by direct reaction of Cu, Co, and S and annealed at 500øC for The changein the unit cell parameterof cattierire 13 days containedstoichiometric carrollite which was parallels that in the carrollite-linnaeite series when rimmed by cattlerite,which in turn was rimmed by copper substitutesfor cobalt. Although cattierire covelliteand liquid sulfur. The insideof the cattier- samplescontaining up to 18 weight percent copper ite rim, where in contact with the carrollire, con- were encounteredduring microprobe analysis, ac- tained 3.3 weight percentCu; the outsideof the rim, curate unit cell dimensions were not obtained from

S .•o-py C•o_pybn•COrr Co-py carrpyrite cp finn er.ite.

Fro. 7. Inferred low-temperaturephase relations in the central portion of the Cu-Co-Fe-S system. The lower portion of the figure is a wedge boundedon the left by the Cu2S-CuS portion of the Cu-S join, at the top by the join FeS2--CoS•, and to the right by the CoS•-- Co2S portion of the Co-S system. The front face is thus the CusS--CuS-CoS.o-Co•S portion of the Cu-Co-S system. The three upper parts of the figure have been drawn out of the main figure for clarification of some important mineral assemblages. Cu-Co-SPHASE RELATIONS AND CuCo•S•-Co•S•MINERAL ASSOCIATIONS 665

TABLE 4. Natural Cu-Co(-Fe)-S Mineral Assemblages

Assemblage' Locality Reference

CF -'{- CC Kamoto, Zaire Bartholomd (1962) cr + Cu-sulfide Borras, Norway Vokes (1967) cr(20% Cu) + dg Luanshya, Zambia This study cr(20.S% Cu) + dj + Co1_xS Carrizal Alto, Chile Clark (1974) cr + bn + dg Kamoto, Zaire Bartholomd (1962) Kolwezi, Zaire This study cr + bn -4- cc Kamoto, Zaire Bartholomd (1962) cr(20.4% Cu) -4-bn q- cc Kohlenbach, Siegen This study cr(18% Cu) + bn q- dg gorras, Norway This study cr + bn + cpy Nkana, Zaire Jordann (1961) Chibuluma West, Zambia Whyte (1971) crq- ,p,y (Co bearing?) Kamoto, Zaire Bartholomd et al. (1973) Copperbelt Grimmer (1962) cr(15.8% Cu) q- py q- cpy Shirataki, Japan Itoh et al. (1972) Carrizal Alto, Chile Clark (1974) cr(14.,2,%Cu) q- py q- cpy q- bn Kolwezi, Zaire This study cr q- py q- cpy q- bn q- sph Sazare, Japan Tatsumi et al. (1975) cr q- py q- po q- cpy q- bn Ruby Creek, Alaska RunnelIs (1969) cr q- ct q- py q- cpy Nkana, Zaire Grimmer and O'Meara (1959) cr q- co-pn + po + cpy q- mk Rokana South, Zambia Notebaart + Vink (1972) cr q- Co-pn (10% Cu) q- cpy + sph Vauze, Quebec Stumpfl and Clark (1964) ct q- py q- sieg Shinkolobwe, Zaire Derriks and Vaes (1956) ct q- "polydymitedinnaeite" Shinkolobwe, Zaire Kerr (1945) ct q- Co-py(15-20% Co) q- py q- cr(12% Cu, 12% Ni, 34% Co, 0.4% Fe) Shinkolobwe, Zaire Craig and Vaughan (1979) Co-py(7% Co) q- cpy Luanshya, Zambia This study Co-py(10% Co) q- Co-py(20% Co) q- cpy q- mk Luanshya, Zambia This study Co-py(4.1% Co) q- Co-vs (8-11% Co; 11-2% Fe) q- cpy q- Meg (24.3% Co; 6.5% Cu; 0.5% Fe; 29% Ni) Katanga, Zaire This study Co-pn q- po q- cpy Varislahti, Finland Kouvo et al. (1959) Outokumpu, Finland This study co-pn q- po q- cpy q- Meg Kamaishi, Japan Imai et al. (1973) Co-pn q- py q- mc q- Meg Langis Mine, Ontario Petruk et al. (1969) lang q- bv

Abbreviations:cr = carrollite;ct = cattlerite; py = pyrite; bn = bornite;cpy = chalcopyrite;dg = digenite;dj = djurle- ire; po = pyrrhotite;cc = chalcocite;mk = mackinawire;Co-pn cobaltpentlandire; mc = marcasite;sieg = siegenite;bv = bravoire; lang = langisite; sph = sphalerite; vs = vaesite. t Percent Cu in carrollireand % Co in pyrite indicatedif data available. suchsamples because of highly variablecompositional is not stablebelow •450øC but that this phasewill zoning. persistmetastably for long periodsof time.

C o1 -x• Cobalt pentlandite, The Cox_xSphase was reported by Rosenqvist Cobalt pentlanditewas encounteredin this study (1954) to decomposebelow 460øC. However, this at 700øC and below. The maximum copper content value is not consistentwith other reports listed by of Co•Ss in the presenceof coppersulfides is 2.3 Hansen and Anderko (1958). In the presentstudy, weightpercent at 400øC. The unit celldimension of we have found that compositionof Co0.s7S(prepared pure Co•Ss preparedat 700øC, as determinedby initially as a homogeneousphase at 600øC) did not measuringthe (044) reflection,is 9.928- 0.002A; decomposewhen annealedat 400øC for 10 days but the unit cell dimensionof CogSaprepared in the did shift to a slightly more S-rich compositionby presenceof the chalcocite-digenitesolid solutionat exsolving CosSs. Additional annealing of such 700øC is 9.930 ñ 0.002 A. samplesat 400ø and 440øC for periodsof two months resultedin no recognizablechange. A seriesof syn- Copper sulfides thesisexperiments in which sulfur was reactedwith The only coppersulfides encountered in the ex- CosSs(to a bulk compositionof Co0.•7S)resulted in perimentswere covelliteand the chalcocite-digenite formation of Co•_xS at 460 ø and 500øC but in mix- solid solution. Covellite is restricted to sulfur-rich tures of CosSsand CoaS4at 400 ø and 440øC. These portionsof the Cu-Co-Ssystem and is prohibited results,though not unequivocal,suggest that Co•_xS from stable occurrencein carrollite-bearing assem- 666 CRAIG, VAUGHAN, AND HIGGINS

Natural Assemblagesin the Copper-Cobalt- Iron-Sulfur System Since copper-cobaltsulfides most commonlyoccu in associationwith copper-ironsulfides, phase rela tions in the Cu-Co-Fe-S quaternary system are o more direct interestthan thosein the ternary system Using the data available for the relevant ternar' systems,together with examinationof natural assem blages,speculative (and rather simplified) phaserela tionsfor the relevantportion of the Cu-Co-Fe-S sys ternare proposedin Figure 7. In the main diagran (center) the Cu-Co-S systembecomes the front fac of a tetrahedron(bottom omitted) with sulfur at th apex and iron extendingback into the plane of th diagram. Some of the more important carrollire containingassemblages are illustrated by the pyra mids shownabove the main diagram. Carrolliregenerally occurs in closeassociation wit] chalcopyriteand althoughthe carrollireis usuallyen• member CuCo•S4,it may show a range of Cu/C, ratios. When end member carrollire coexists witl chalcopyriteand cobaltiferous-pyrite,the cobaltcon tent of the pyrite is •12 wt percentas shownin th top right of Figure 7. However,the thiospinelcorn positioncan vary toward CoaS4when coexisting witl a morecobalt-rich disulfide or (if the geometryof th. phasediagram is examined) if the aa2is lower. Th. other assemblagesshown (carrollite-chalcopyrite bornire-cobalt pyrite and carrollire-cobalt pyrite FiG. 8. A. Euhedral carrollite crystal (white) with digenite (light gray) and bornite (dark gray). This sample,typical bornite-digenite)correspond to decreasingaa2. of vein deposit occurrencesof carrollite is from Borras, Nor- The degreeto which low-temperatureadjustment way. The field of view is 0.75 mm. B. Anhedral mass of carrollite (white) serving as host of phasecompositions alter (stablyor metastably)o for an intergrowth of chalcopyrite and bornite •vhich ap- obscureprimary mineral assemblagesis difficult t, parently formed as a single phase. This sample is typical of assess.For example,the assemblagedigenite-bornite the copper-cobalt ores of the Zambian Copperbelt in which carrollite serves as a major source of the cobalt and chalco- carrollite-chalcopyritelisted in Table 4, probablydoe pyrite and bornite are the major sources of the copper. not representequilibrium if our and other workers USNMNH #102608. The field of view is 0.2 mm. interpretationof a stable low-temperaturepyrite bornite-chalcopyriteassemblage (as shownin Fig. 71 is correct. It is likely that what is presentlyobserve{ as a digenite-borniteintergrowth formed as a singl, blages by tie lines between the chalcocite-digenitephase of-intermediate composition. Similarly, it i solid solution and cattierite. The covellite observed likely that some presentlyobserved chalcopyrite was normal in all respects;the maximumcobalt con- bornireassemblages formed as singlephases (Fig tent observedwas 1 wt percentat 500øC. In con- 8B). trast, the chalcocite-digenitesolid solution dissolves Examplesof theseassemblages reported from th, up to 11 weight percentCo when in equilibriumwith studyof naturalores are shownin Table 4. As thi Co•_xSand Co0S8at 800øC and up to 8 weight per- table indicates,the carrollite-linnaeiteminerals occu: cent Co when in equilibrium with carrollite and in a varietyof geologicalsettings ranging from "epi cattieriteat 800øC. During the rapid coolingof the thermal" vein depositssuch as thoseof Borras,Nor chargesat the end of the experiment,the cobalt is way, shownin Figure 8A and describedby Voke.. ejected from the chalcocite-digenitesolid solution, (1967) to the possibly"syngenetic" copper ores o ultimately appearingas small anhedralgrains of co- Zaire and Zambia. The Zambian Copperbelt, a: balt sulfidesdispersed in a chalcocite-digenitesolid exampleof whichis shownin Figure 8B, containsth• solution matrix. most economicallyimportant copper-cobalt ores it Cu-Co-S PHASE RELATIONS' AND CuCo.oS4-Co•S'•MINERAL ASSOCIATIONS' 667

T^m• 5. New Analysesof CarrolIite-Linnaeite SeriesMinerals

Weight percent Mole percent Cu Ni Co Fe S Total MxS4 Cu3S4 Ni3St Co3S4 Fe3S• Locality

16.96 2.16 39.66 0.08 41.82 100.69 3.00 27.29 3.76 68.80 0.15 Finksburg, Maryland 16.98 4.40 36.95 0.18 41.68 100.19 2.99 27.48 7.71 64.68 0.33 Kohlenbach,Ge•'manv 17.06 0.52 42.52 0.82 40.45 101.37 3.21 26.49 0.87 71.19 1.45 Baluba,ZarffBia • 17.59 0.43 41.30 0.02 41.73 101.07 3.03 28.10 0.74 71.12 0.04 P,hokana, Za mbia 17.83 3.55 37.62 0.24 41.57 100.81 3.04 28.53 6.15 64.89 0.44 Kohlenbach, Germany 17.96 2.65 38.53 0.00 40.86 100.00 3.08 28.80 4.60 66.61 0.00 Borras, Norway 18.17 2.82 37.45 0.06 41.51 100.01 3.00 29.46 4.95 65.48 0.11 Borras, Norway 18.57 0.93 39.31 0.43 40.95 100.19 3.08 29.74 1.61 67.87 0.78 Kambove, Zaire 19.23 0.57 38.62 0.71 40.11 99.24 3.13 30.87 0.99 66.84 1.30 Chibuluma, Zambia 19.37 2.53 37.98 0.58 39.61 100.07 3.25 30.40 4.30 64.27 1.04 Chibuluma, Zambia 19.41 0.57 39.70 0.65 40.13 100.46 3.20 30.53 0.97 67.33 1.16 Chibuluma, Zambia 19.57 0.34 39.24 0.31 41.82 101.28 3.02 31.26 0.59 67.59 0.56 Kolwezi, Zaire 19.62 0.41 39.93 0.65 40.54 101.15 3.18 30.73 0.69 67.42 1.16 Chibuluma, Zambia 19.95 2.84 37.00 0.64 40.04 100.47 3.21 31.35 4.83 62.68 1.14 Kohlenbach, Germany 19.95 0.59 39.04 0.69 39.43 99.70 3.25 31.43 1.01 66.32 1.24 Chibuluma, Zambia 20.01 1.06 38.02 0.68 40.63 100.40 3.13 31.80 1.82 65.15 1.23 Kolwezi, Zaire 20.08 2.50 38.23 0.70 39.87 101.38 3.28 30.99 4.18 63.61 1.23 ChJbuluma, Zambia 20.20 0.45 38.88 0.82 40.02 100.37 3.20 31.79 0.77 65.97 1.47 Chibuluma, Zambia 20.21 0.18 37.24 0.31 41.51 99.45 2.96 33.18 0.32 65.92 0.58 Kambove, Zaire 20.34 2.91 35.75 0.59 40.86 100.45 3.10 32.44 5.02 61.47 1.07 Kohlenbach, Germany 20.62 0.07 37.85 0.57 41.37 100.48 3.03 33.18 0.12 65.66 1.04 Kolwezi, Zaire 20.63 0.07 37.35 0.57 41.74 100.36 2.98 33.48 0.12 65.35 1.05 Kolwezi, Zaire 20.68 0.38 37.96 0.86 39.53 99.41 3.22 32.83 0.65 64.97 1.55 Kolwezi, Zaire 20.70 0.35 39.38 0.00 40.00 100.43 3.21 32.58 0.60 66.82 0.00 Kamoto, Zaire 20.73 0.10 38.87 0.42 39.88 100.00 3.20 32.79 0.17 66.29 0.76 Kamoto, Zaire 20.82 0.86 37.57 0.51 39.80 99.56 3.19 33.13 1.48 64.46 0.92 Chibuluma, Zambia 20.89 0.35 39.11 0.10 38.48 98.93 3.33 32.87 0.60 66.35 0.18 Kamoto, Zaire 20.96 0.03 36.85 0.60 40.77 99.48 3.05 33.97 0.53 64.40 1.11 Kolwezi, Zaire 21.06 0.48 37.25 0.82 40.99 100.60 3.09 33.60 0.83 64.08 1.49 Kolwezi, Zaire 21.12 1.04 36.86 0.49 39.35 98.86 3.21 33.77 1.80 63.54 0.89 Chibuluxna, Zambia 21.16 0.44 38.76 0.18 39.68 100.22 3.24 33.25 0.75 65.68 0.32 Kamoto, Zaire 21.17 0.45 37.92 0.63 41.73 101.90 3.06 33.47 0.77 64.63 1.13 Kolwezi, Zaire 21.17 0.56 38.82 0.00 40.09 100.64 3.20 33.27 0.95 65.78 0.00 Kamoto, Zaire 21.18 0.47 36.98 1.01 39.84 99.48 3.18 33.78 0.81 63.58 1.83 Kolwezi, Zaire 21.37 0.32 37.83 0.52 39.70 99.74 3.21 33.87 0.55 64.64 0.94 Kolwezi, Zaire 21.40 0.25 38.10 0.63 41.39 101.77 3.10 33.72 0.43 64.73 1.13 Kolwezi, Zaire 21.52 0.44 38.28 0.45 39.39 100.08 3.27 33.74 0.75 64.71 0.80 Kolwezi, Zaire 23.39 0.34 36.99 0.91 39.44 101.70 3.31 36.17 0.57 61.66 1.60 Kamoto, Zaire

the world. X,Ve have analyzedthe samplesshown as compositionsthat CuCo2S4are from samplescontain- well as a large numberof other carrollite-linnaeite- ing inclusionsof copper-sulfides(Powell, 1967). containingores by electronmicroprobe. The results In Figure 10, the carrollite-linnaeiteanalyses are are presentedin Table 5 and in Figures 9 and 10. plottedon a Cu-Co-Scomposition diagram. The re- In Figure 9 microprobeanalysis of Cu-Co-Ni-Fe sultsshow a spreadin metal: sulfur ratiosabout the thiospinelsfrom this work and from the literature idealcomposition (shown by the line) whichcor- are plotted to illustrate the range of variation in respondsto ñ weightpercent sulfur. The scatterof metal ratios. This figure showsthat, although some analysesabout the CoaS4-CuCo2S4join couldbe just minerals of the carrollite-linnaeite series are pure analytical error, consistentwith the experimental Cu-Co sulfides,many also contain nickel. There is findingsthat there is no deviationfrom MaS4 stoi- no evidencethat the coppercontent of the carrollite- chiometry,or may representsome real variationsin linnaeite seriesextends beyond one-third of the total stoichiometryof natural samples. Further investiga- metal. Since more copper-richcompositions would tions are requiredto clarifv this point. imply the existenceof the relativelyunstable Cu +a oxi- dation state and the CoaS4 - CuCo2S4 series involves Discussion a regular substitutionof Cu+-• for Co+-• in the tetra- The study of the stable sulfide assemblagesin- hedral sites, the CuCo2S• compositionlimit is also volving copper-cobaltsulfides is an important aspect predicted by models of thiospinel crystal chemistry of determiningthe temperatureof formationof ores (Vaughan et al., 1971). The few analysestaken containingthese phases. However, an additionalcriti- from the literature which plot at more copper-rich calparameter is the activityof sulfur(Barton, 1970). 668 CRAIG, VAUGHAN, AND HIGGINS

½%$4 Corrollite.•

Fig. 9. A plot of thiospinels,in terms of CthS•-Co•S•-NiaS•--FeaS• components,which showsthe compositionalrange of the carrollite-linnaeiteseries relative to other thiospinels. Data are from: Smith and Brush (1853), Genth (1857), Cleve (1872-74), Laspeyres(1891), Stahl (1902), Johansson(1924), Shannon(1926), de Jong and Hoog (1928), Tarr (1935), Hiller (1935), Kazitsyn (1959), Permingeatand Weinryb (1960), Darnley and Killingworth (1962), Richards(1965), Vokes (1967), Powell (1967), Itoh et al. (1972), Clark (1974), Filimonovaand Slyasarev(1974), Pavlovaand Polyakova(1974), Tatsumiet al. (1975), Minceva-Stafanova(1975), Karup-M½ller(1977), and Craig and Vaughan(1979).

We have determined the activity of sulfur over a Acknowledgments range of temperaturesfor the sulfidationreaction in We gratefullyacknowledge the supportof NATO which stoichiometric carrollite breaks down to form Grant No. 966 and NSF Grants Nos. DMR75- digeniteand cattlerite. The data were obtainedusing 03879 and DMR78-09202 in this work. We are also the solid electrolyteelectrochemical cell Ag/AgI/ indebted to the U.S. National Museum, the British Agu+xS,S2(g) describedby Schneeberg(1973). Museum(Natural History), and Drs. A. Brown,A. This sulfidationreaction is plottedon a log cts2-- 1/T Annels, A. Criddle, and F. M. Vokes for natural diagram in Figure 11, together with other relevant carrollite-linnaeite-bearingsamples. The critical sulfidation reactionstaken from the compilationsof comments of G. Kullerud who reviewed an early Barton and Skinner (in press) and Vaughan and versionof this manuscriptand of StevenD. Scott Craig (1978). Diagramsof this type enablean esti- have been most helpful. mate of sulfur activity during ore formation to be made through an examinationof the coexistingsul- J. R. C. fide assemblages.This approachwill be considered DEPARTMENT OF GEOLOGICAL SCIENCES in more detail in its application to certain of the VIRGINIA POLYTECHNICINSTITUTE AND STATE Zambian ores in a separate publication. By com- UNIVERSITY bining estimatesof sulfur activity during ore forma- BLACKSBURG,VIRGINIA 24061 tion with data on tie lines from the phasediagrams, it D. J. V. DEPARTMENT OF GEOLOGICALSCIENCES is also possibleto define the mineralogicalchanges UNIVERSITY OF z•STON IN BIRMINGHAM which will occur with an increase or decreasein as.o. BIRMINGHAM B4 7ET, ENGLAND For example,a decreasein cts2can result in a more j. B. It. Co3S4-richthiospinel when the assemblageis chal- DEPARTMENT OF GEOLOGICAL SCIENCES copyrite-cobaltpyrite-thiospinel or an increasein as2 UNIVERSITY OF TENNESSEE can producemore cobalt-poorpyrite and CuCouS4 KNOXVILLE,TENNESSEE 37916 thiospinelin this assemblage. February17, September5, 1978 Cu-Co-SPHASE RELATIONS AND CuCo2S4-Co•S,MINERAL ASSOCIATIONS 669

Wt. Percent Cu $0J 20 I0 ^ ^ 50

• A A A ß o-I..• o - e' ß ß ß

v v v v 40 50 60 Wt. Percent Co

Fro. 10. A portionof the Cu-Co-S systemonto whichcarrollite-linnaeite analyses have been plottedto illustratethe coppercontent and the metal'sulfur stoichiometry.The centralline representsthe ideal CuCo•S4- C•S, join.

-5

-15 cc 300 40Jg c.,• 500 600 700eC

1.8 1.4 1.0 I000 / T, '•K Fro. 11. A plot of the log as_•--1,000/Trelationships in the Cu-Co-Ssystem. The pyrite+ pyrrhotiteand bornire+ pyrite+ chalcopyritecurves have also beenincluded. The abbrevia- tionsare: py--- pyrite; po= pyrrhotite;bn = bornire;cpy = chalcopyrite;cv = corellite;dg= digenite;cart -- cattlerite;cart -- carrollire;linn ---- linnaeite; cc = chalcocite;Co-pn = cobalt pentlandite.Based on datafrom Bartonand Skinner(in press)and this work. 670 CRAIG, VAUGHAN, AND HIGGINS

REFERENCES Hansen, M., and Anderko, K., 1958, Constitution of binary alloys: New York, McGraw-Hill, 1305 p. Bartholom•, P., 1962, Les minerais cupro-cobaltiferesde Hey, M. H., 1962, An index of mineral speciesand varieties Kamoto (Katanga-Ouest)I. Petrographie:Studia Univ. arrayed chemically: London, British Museum, 728 p. Lovanium (Republicdu Congo), Fac. des Sci. 14, 40 p. Higgins, J. B., Speer, J. A., and Craig, J. R., 1975, A note Bartholom•,P., Evrard, P., Katekesha,F., Lopex-Ruiz,J., on thiospinel spacegroup assignment: Philos. Mag., v. 32, and Ngongo,M., 1973,Diagenetic ore-forming processes at p. 683-685. Kamoto,Katanga, Republicof the Congo,in Amstutz, G. Hill, R. J., Craig, J. R., and Gibbs, G. V., 1978, Cation order- C., and Bernard, A. J., Ores in sediments: Berlin, ing in the tetrahedral sites of the thiospinel Feln•S• Springer-Verlag, p. 21-41. (indite): Jour. Phys. Chem. Solids, v. 39, p. 1105-1111. Barton,P. B., Jr., 1970,Sulfide petrology: Mineralog. Soc. 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