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

Phase Belations in the Cu-Co-S System and Mineral Associations of the Carrollitc ( CuCoS4)-

Linnaeite (Co3S4) Series

JAMES R. CRAIG, D^vm J. VAUGItAN, AND JoHs B. HmG•ss

Abstract

Phase relations in the system Cu-Co-S have been studied using sealed silica capsule methods between 400 ø and 900øC. The high-temperature portion of the system is dominated by the coexistence of the chalcocite-digenite solid solution •vith cobalt sulfides (COS2, COl_xS, Co4S:• ) . AS temperature is decreased to 880ø___ 5øC, the dominant ternary phase, the carrollite-linnaeite solid solution. appears with a composition of •Cu0.aCo2.aS 4. The copper-rich compositional limit is extended on cooling, reaching a maximum (CuCo2S4) at •,500øC. Below 507øC covellite is stable and coexists in the sulfur-rich portion of the system with copper-bearing cattlerite. Below 500øC the major changes involve a change in stable assemblages (at •,450øC), in which the high-tempera- ture pair Co + Cu2S is replaced by the pair Cu +Co0S s. the disappearance of Co•_xS (<_460øC) and digenite (_<70øC), and the appearance of low chalcocite (103øC), djurle- ite (93øC), and anilite (70øC). Optical and electron microprobe studies of natural as- semblages permit speculation on phase relations at 25øC and suggest the topology of the system is similar to that at 400øC except for the appearance of the low-temperature cop- per sulfides. Similar studies also permit speculation on the low-temperature phase rela- tions in the relevant portion of the Cu-Co-Fe-S system in which carrollite frequently oc- curs in close association with chalcopyrite or bornite and, in some cases, cobalt-pyrite. More cobalt-rich "carrollites" are found to occur in assemblages with more cobalt-rich "pyrites" and this corresponds to decreasing as2. Assemblages such as carrollite-chalco- pyrite-bornite-cobaltian pyrite and carrollite-cobaltian pyrite-bornite-digenite correspond to a further decrease in as2. The actual as.o values during formation of assemblages con- taining these phases are also considered using thermochemical data for sulfidation equi- libria. In the carrollite-linnaeite series, the systematic decrease in unit cell parameter and increase in reflectivity (at 589 nm) as a function of composition, as well as the actual compositional variations in the natural phases, are consistent with crystal chemical models.

Introduction

T• Cu-Co-S system is represented in numerous ore deposits by minor amounts of members of the car- rollite (CuCoeS•) -linnaeite (CoaSt) series. In the Zambian Copperbelt, however, this mineral series is a major ore constituent and is the principal source of the cobalt extracted (Whyte, 1971; Notebaart and Vink, 1972). The present study reports on the phase equilibria of the Cu-Co-S system and mineralogic occurrences of coexisting copper- and cobalt-bearing sulfides with emphasis on the carrollite-linnaeite series.

Carrollite and linnaeite are members of the thio-

spinel mineral group, a group long considered to possess the commonality of the spinel crystal struc- ture. Linnaeite, CoaSt, named for the famed Lin- naeus, was the first thiospinel to be recognized as a mineral species (Brault, 1746) and its identity has been reaffirmed many times since. Carrollite, origin- ally described as Cu=Co=S• from specimens from Finksburg, Carroll County, Maryland (Faber, 1852),

was later considered to be merely a slightly iron- and nickel-bearing linnaeite with intimately intergrown copper sulfides (Dana and Ford, 1932). In the interim, Laspeyres (1891), unable to obtain "car- rollite" for comparison, assigned the name "sychno- dymite" and the composition (Cu,Co)iS, to similar material from Siegen. The true character of car- rollite as a thiospinel became apparent with a series of good chemical analyses (Shannon, 1926) and with X-ray crystallographic studies (Menzer, 1926) demonstrating the essentially identical nature of carrollite and linnaeite. X-ray data also demonstrated the complete identity of carrollite and sychnodymite (de Jong and Hoog, 1928). Although the 4th edi- tion of Dana's Textbook of Mineralogy (Dana and Ford, 1932) essentially discredited the name carrol- lite, the name was restored by the 7th edition of Dana's Manual of Mineralogy (Palache et al., 1944) at the expense of sychnodymite. Some texts such as Hey's (1962) Chemical Index of Minerals still re- tain sychnodymite for nickel-rich varieties but the

657

658 CRAIG, VAUGHAN, AND HIGGINS

trend has been to replace that name (as does Fleis- cher, 1975) with carrollite.

Previous Work in the Co-Cu, Cu-8, Co-S, and Cu-Co-8 Systems

Co-Cu system

The phase equilibria in the Co-Cu system as deter- mined by numerous authors have been reviewed by Hansen and Anderko (1958) and Elliott (1965). Although there are no intermediate binary com- pounds in this system, cobalt accepts approximately 10 weight percent copper in solid solution above 340øC and less than 2 percent at 300øC, although the latter amount is not known with certainty. The solid solubility of cobalt in copper is about 1 weight percent at 700øC but less than 0.1 weight percent at 500øC and below.

Cu-S system

The copper-sulfur system contains six phases known as mineral species. These phases (their standard abbreviations and their maximum thermal

stabilities given in parentheses) are chalcocite, Cues (cc, 103.5øC, although there is an intermediate form stable up to •430øC and a high-temperature form which melts at 1,129øC): djurleite, Cu•.90S (dj, 93 ø); anilite, Cu7S4 (al, 75øC); digenite, Cu0S5 (dg, stable only between 70 ø and •80øC in the Cu-S sys- tem and forming complete solid solution with Cues above 430øC; Morimoto and Koto (1970) suggest that iron-containing digenite is stable in the Cu-Fe-S system as low as room temperature); "blaubleibender covelline" CUl+xS (bcv, 157øC); and covellite, CuS (cv, 507øC). The phase equilibria among the copper sulfides have been reviewed by Roseboom (1966), Morimoto and Koto (1970), and Craig and Scott (1974) and recently refined by Potter (1977).

Additional copper sulfides which are known only as synthetic phases include: two high-temperature polymorphs of Cues (a hexagonal form stable be- tween 102 ø and 430øC and a cubic form stable above

430øC); a tetragonal polymorph of Cues stable at high pressure but at 1 atmosphere only below 10øC; a cubic form of Cu0S5 stable above 73øC; and a CuSe phase stable at high temperature and pressure (Mun- son, 1966; Taylor and Kullerud, 1972).

Relationships involving the low-temperature copper sulfides have also been clarified by the recent electron microscope studies of Puthis (1976, 1977). These show that, in addition to the ideal transformation from the stable high-temperature form to the stable low-temperature form, alternative metastable pro- cesses may operate when the formation of the low- temperature state is impeded. Such observations on these and other sulfide systems have important im-

plications in the study of sulfide phase equilibria, particularly at temperatures below •400øC where reaction mechanisms and kinetic factors become in- creasingly important.

Co-S system

The cobalt-sulfur system, reviewed by Hansen and Anderko (1958), Elliott (1965), and Shunk (1969), contains five intermediate phases, three of which cor- respond to mineral species: Co9S8 (cobalt pentlandite, co-pent, stable below 835øC); CoaS4 (linnaeite, ln, stable below about 680øC); and CoS2 (cattierite, ct, stable below about 950øC). Co1_xS melts congruently at 1,182øC; its lower stability limit has commonly been reported as •460øC. Co4Sa is stable between 930 ø and approximately 785øC.

Cu-Co-S system

The general configuration of the Cu-Co-S system between 400 ø and 600øC has been outlined by Craig and Higgins (1973) and Wang (in Moh, 1976).

Experimental Methods

Experimental charges in the present study were prepared by conventional silica tube techniques (Kul- lerud, 1971) and by means of the multiple reaction techniques previously used for violarite synthesis (Craig, 1971). Reagents were 99.999+ percent pure cobalt, copper, and sulfur as indicated by supplier's (ASARCO) analyses. Copper was freshly filed. Cobalt sponge was reduced in a stream of hydrogen at 700 ø to 800øC for 6 hours prior to use in experi- ment preparation. Experiments were carried out in Nichrome wound resistance furnaces controlled to

---3øC. At the termination of each experiment, charges were rapidly chilled in cold water and the reaction products were examined by X-ray powder diffraction, electron microprobe, and reflected light microscopic techniques. Cell dimensions were deter- m/ned by X-ray powder diffraction employing National Bureau of Standards silicon (a0 = 5.43088 A) as an internal standard. Electron microprobe data were obtained using either an ARL-EMX or an ARL-SEM•O instrument operating at 15 kV ac- celerating voltage and 0.15 microamps sample cur- rent. Synthetic CoaSi, CoSe, Cu•FeS4, NiS, FeS, and CuS were used as standards.

Experimental Data on the Cu-Co-S System

Phase equilibria in the Cu-Co-S system have been examined experimentally between 400 ø and 900øC and are summarized in isothermal diagrams in Figure 1. Selected representative experimental data are given in Table 1.

Cu-Co-S PHASE RELATIONS lIND CuCo2S•-Co.S• MINERIlL lISSOCIlITIONS 659

s s

Cu Co Cu Co

E

s

500øC

CoS•

F

Cu Co

S

O0 ø C

/

s ,

Cu Co

FIG. 1. Isothermal sections of the condensed phase relations in the Cu-Co-S system at' A, 900 ø; B, 800 ø; C, 700 ø; D, 600 ø; E, 500 ø; F, 400øC.

At 900øC (Fig. 1A) two sulfide-rich liquids co- exist in the interior of the system. Liquid I contains between about 25 and 30 weight percent S and ex-

tends from the Cu-S join approximately 18 weight percent Co into the system; Liquid 11 contains be- tween about 26 and 30 weight percent S 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

B. 500øC

C. 600øC

D. 700øC

E. 800øC

15 28 57 21+ 15 75 10 84 25 55 20 84 27 33 4O 84

35 35 3O 84 59 9 32 84

70 20 10 27

20.4 21.0 58.6 13 20.5 38.1 42.4 33 25 55 2O 35O 25 25 5O 13 28 32 40 350 32 31 37 13 32 37 31 350 59 9 32 350 70 15 15 13

15 35 52 212 2O 45 35 72O 20.5 38.1 41.4 7 30 40 30 720

32 31 37 2 40 40 20 720 55 10 35 720 70 20 10 720

15 35 5O 52 2O 45 35 28O 20 60 20 34 20.5 38.1 41.4 280 40 20 40 280 65 25 10 34

4 54 42 285 2O 4O 4O 285 10.3 48.0 41.7 5 25 25 5O 38 30 40 30 285 35 45 20 285

F. 900øC 10 59 31 3

15 7O 15 3

20 40 40 3

20 40 40 7

35 37 28 7

45 15 4O 3

65 25 10 7

ct(4.4% Cu) + cv + S-liq Cu(0.5% Co) + cc(1.2% Co) + Co0Ss(1.1% Cu) Co(1% Cu) + cc(3.8% Co) + Co0Ss(2.3% Cu) cv(0.6% Co) 3. ct(1.8% Cu)l•cr(13.4% Cu)

3. Co0S•(1.3% Cu)3 cc(1.9% Co) + cr(8.7% Cu) 3- Cm_xS(3.4% Cu)

*cc(0.5% Co) + cv + ct(4.9% Cu) [-3- cr(11.3% Cu)3

Cu(0.4% Co) 3- cc(1.3% Co) 3- Co(0.5% Cu)

ct(14.3% Cu) 3- cv(1.0% Co) 3- S-liq) cr(20.5% Cu) cc(3.7% Co) + Co + Co0S•(0.4% Cu) ct(18.4% Cu) 3- cv(0.5% Co) 3- S-liq cc(0.5% Co) 3- cr(20.0% Cu) 3- ct(0.9% Cu) cr(20.2% Cu) + ct(0.5% Cu) + cc(2.5% Co) cc(3.3% Co) + Co•_xS(0.6% Cu) ct(16.1% Cu) 3- cc(0.5% Co) 3- cv Co + cc(5.3% Co) + Co0S•(2.5% Cu)

ct(12.1% Cu) + cc(2.8% Co) + S-liq cr(7.9% Cu) + Cm_xS(0.7% Cu) + cc(4.4% Co) cr(17.4% Cu) + cc + ct cc(7.0% Co) + Co,_xS(0.5% Cu)

3- Co0Ss(0.4% Cu) cr(19.3% Cu) 3- ct(0.6% Cu) 3- cc(2.5% Co) cc(3.4% Co) + Co9S•(1.2% Cu) + Co ct(14% Cu) + cc(33% Co) + S-liq cc(2.6% Co) + Co + Cu(0.5% Co)

ct(7.0% Cu) + cc + S-liq cc(4.9% Co) + cr(10.2% Cu) + Cm_xS(1.0% Cu) cc(3.3% Co) 3- Co0Ss(0.3% Cu) 3- Co(1.%Cu) ct(1.7% Cu) + cr(16.5% Cu) + cc(3.2% Co) ct(7.1% Cu) 3- cc(1.0% Co) 3- S-liq cc(1.5% Co) + Co(1% Cu) + Cu(1% Co)

ct(0.4% Cu) 3- cr(8.6% Cu) 3- Co•_xS(1.0% Cu) ct(0.6% Cu) 3- cr(12.2% Cu) 3- cc(7.9% Co) ct + cr(10.4% Cu) ct(4.4% Cu) + cc + S-liq Col_xS(0.8% Cu) 3- cc(11.5% Co) 3- CooSa Co + CotSa + cc(10.6% Co)

Cm_xS(0.9% Cu) 3- Liq-II(Cu-12.3; Co-57.2; S-30.5)

Co(1.2% Cu) + cc(10.5% Co) + Liq-II (Cu-7.3; Co-64.6; S-28.1)

ct(0.6% Cu) + Co•_xS(2.4% Cu) + Liq-I (Cu-54.2; Co-17.7; S-28.1)

ct(0.7% Cu) + Cm_xS(2.5% Cu) + Liq-I (Cu-56.3; Co-17.0; S-26.7)

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)

ct (2.0% Cu) + Liq-I (Cu-61.7; Co-10.7; S-27.6) + S-Liq

Cu(3.8% Co) + Co(5.9% Cu) + cc(2.0% Co)

All data in weight percent. Abbreviations: ct = cattierite, cr = carrollite, cc = chalcocite-digenite solid solution, cv = covellite; liq = liquid; *[- 3

indicated disequilibrium phase(s) present.

from the Co-S join approximately 20 weight percent Cu into the system. In the sulfur-deficient portion of the system, Co and Cues solid solutions containing up to 11.5 weight percent Co exist with Liquid II. In the more sulfur-rich portion of the system, Co4Sa, Co•_xS, containing a maximum of about 2.5 weight percent Cu, and CoSe, containing less than 0.7 weight

percent Cu, coexist with Liquid II. CoSe which co- exists with Liquid I contains up to 2 weight percent Co (Fig. 2). The mutual solid solution limits of Cu and Co are taken from Hansen and Anderko (1958).

Relationships in the 500 ø to 800øC range are char- acterized by coexistence of the high-temperature chal- cocite-digenite solid solution with cobalt sulfides.

Cu-Co-S PHASE RELATIONS AND CuCo•,e•-Co•, c, MINERAL ASSOCIATIONS 661

At 800ø C (Fig. 1 B) the high-temperature chalcocite- digenite solid solution, which accepts up to 11 weight percent Co into solid solution, coexists with cattierite, carrollite-linnaeite solid solution, Co•_xS, CooSs, Co4Sa, and cobalt. The copper contents of Co9Ss and Co•_xS are limited to < 1 weight percent, whereas that of Co4Sa reaches a maximum of •4 percent. Cattierite takes only 1.5 weight percent Cu into solid solution at 800øC when coexisting with the carrollite- linnaeite solid solution and the chalcocite-digenite solid solution but will accept up to 4.4 weight percent Cu when coexisting with S-liquid. The carrollite- linnaeite solid solution is restricted to a composition between 8 and 12.5 weight 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-bearing assemblages, respectively.

The dominant ternary phase, the carrollite-linnaeite solid solution, is stable up to 880 ø -- 5øC above which it breaks down to Liquid I, CoSu, and Co•__xS. The composition of maximum stability is •10 weight percent Cu corresponding closely to Cuo.sCou.sSo.•.

The general topology of the system changes little as the temperature is decreased to 700øC (Fig. 1C). The CogSa phase is not stable below about 780øC (Elliott, 1965); no evidence was observed to suggest that the presence of Cu might stabilize this phase. The carrollite-linnaeite solid solution phase increases its width by extending the Cu-rich and Cu-poor limits to 16.5 and •2 weight percent, respectively. The maximum solubility of Co in chalcocite-digenite solid solution at 700øC is •5 weight percent and the maximum solubility of Cu in CoSu is 7 weight per- cent.

Further decrease to 600øC (Fig. 1D) results in the extention of the carrollite-linnaeite series to CoaS• at 680øC and to a maximum Cu content of 18 weight percent when coexisting with CuuS and cattierite. Continued decrease to 500øC results in extension of

FIG. 2. Rounded grains of CoS• in a matrix of Liquid I. Sample rapidly cooled from 900øC; the width of the figure is approximately 0.75 mm.

s

• LOW PERATURE

/ ' -7 - .... co-,.

Cu Co

Fro. 3. Inferred low-temperature phase relations in the Cu-Co-S'xsystem on the basis of the experimental work in this study and the observed natural mineral assemblages.

the carrollite-linnaeite series to stoichiometric carrol- lite, CuCo•S;, composition. The maximum solubility of Cu in CoS• which coexists with the Cu•S ss and S-liq increases to • 12 weight percent. Below 507øC, CuS is a stable phase and coexists with Cu•S and cattierite; CuS contains a maximum of about 1 weight percent Co.

Below 500øC (Fig. 1E) the major changes which occur involve the shrinkage of the Cu•S solid solu- tion field and the appearance of the low-temperature copper sulfides such as digenite, djurleite, anilite, and low chalcocite. Relationships here are not un- equivocally established even in the binary system. The calculated sulfidation curves for cobalt and cop- per (see Fig. 11) cross at 404øC thus indicating that the stable high-temperature tie line between cobalt and chalcocite is replaced by a tie line between copper and cobalt-pentlandite. Experiments at 400øC. con- firm this tie line change (Fig. 1F). Another problem concerns the low-temperature stability of the mono- sulfide Co•_•S, which may occur naturally as the min- eral jaipurite (Clark, 1974),' although some studies on the Co-S system show that it is unstable below 460øC (Rosenqvist, 1954). Our experiments, dis- cussed below, suggest that Co•_•S is not stable below 460øC but that it may persist metastably at lower temperature for long periods of time. The low- temperature stability relations in the Cu-Co-S system have been estimated from the study of natural as- semblages together with extrapolation of the trends observed in the high-temperature data (Fig. 3). Co- existing carrollite and Cu•S phases are common in-' natural assemblages, but the exact tie-line arrange- ment in the Cu•S region is uncertain as many workers are not explicit in the identification of 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 CuCo=S• 600 7 CuCosS, 700 7 CuCo=S• 800 300 CuCo=S• 875 « CuCosS, 890 2

CuCosS4 9.472 Cu•_,,Cos+xS4 q- CoSs q- Cu,,S 9.464 Cu•_,,Cos+,,S4 q- COS,, q- Cuss 9.459 Cu•_•Cos+.• + CoSs + Cuss 9.444 Cm-,,Cos,,,S• + CoSs + Cuss -- CoSs + Cuss + Co•_xS --

phases. As suggested by the high-temperature rela- tions, covellite does not occur in association with car- rollire. There are conflicting reports of jaipurite and of cobalt pentlandire coexisting with carrollire. Co- balt pentlandire and "carrollire" (the composition along the carrollite-linnaeite join is unknown) have been reported from the Rhokana South erebody (Notebaart and Vink, 1972) whereas Clark (1974) has described coexisting carrollite, djurleite, and jaipurite from Carrizal Alto in Chile. Since neither the validity nor the stability of jaipurite has been clearly established as a •nineral species, the former association is favored by us as representing the stable assemblage at low temperatures.

Properties of the Cu-Co-S Phases

Carrollite-linnaeite series

The maximum thermal stability of linnaeite, CoaS4, has previously been given as from •625 ø to 680øC (Hansen and Anderko, 1958-Kullerud, 1968; Shunk, 1969). In the present study this value was deter- mined to be 680 ø -- 4øC (Table 2). At temperatures below approximately 500øC, complete solid solution exists between linnaeite and carrollitc, CuCo2S4. The thermal maximum for this series (Table 3) is 880 ø ---+ 5øC and lies at a composition of approximately Cu0.•Co2.sS4. This composition breaks down to a copper-containing cattieritc plus a melt with a com- position estimated to contain about 65 weight percent Cu, 10 weight percent Co, and 25 weight percent S plus Co•_xS. Below 800øC the compositional range of the solid solution series is extended toward both

more Cu-rich and Co-rich compositions. At 800øC the solid solution extends from 12.5 to 8 weight per- cent Cu, at 700øC from 16.5 to 2 weight percent Cu, and at 600øC from 18 to 0 weight percent Cu.

Carrollite and linnaeite are both reported to have the spinel crystal structure normally considered as space group Fd3m. However, additional reflections incompatible ;vith this space group have been ob- served in both carrollire and linnaeite by Higgins et al. (1975). Such additional reflections may indi- cate the correct space group to be F43m and result from displacements of the octahedral site ions--a model suggested to explain similar data for certain oxide spinels (Grimes, 1972). An alternative ex- planation, cation ordering on the tetrahedral sites as found by Hill et al. (in press) for indite, Feln2S4, could also be responsible for the forbidden reflections.

TABLE 3. Unit Cell Parameter Data Measured at Room Temperature for Carrollite-Linnaeite Series and Copper- bearing Cattlerites

A. Carrollite-linnaeite series

Unit cell Synthesis Time, dimension,

Composition temp, øC days A4-O.001

CoaSt q- Co•_•S 600 180 9.405 CoaSt q- CoS= 600 180 9.404 CoaSt 500 60 9.404 Cu 0.25Cos. ,.•S, 500 600 9.425 Cu 0. • 0Cos. •0S, 500 300 9.442 Cu0.7 •Cos. ssSl 500 600 9.459 CuL 00Cos. 00Si 500 600 9.472

B. Copper-bearing cattlerites

Wt % Cu

Unit cell mole % Synthesis Time, dimension,

CuSs temp, øC days A 4-0.001

0 0 700 13 5.535 0.6 1.2 800 250 5.538 1.9 3.7 900 3 5.543 4.4 8.6 800 37 5.553 6.8 13.2 700 22 5.566

12.1 23.6 600 210 5.590

Cu-Co-S PHASE RELATIONS AND CuCo2S•-CoaS• MINERWE ASSOCIATIONS 663

The unit cell dimensions of the carrollite-linnaeite

series have been determined by X-ray powder dif- fraction (Table 3).. Samples for this study were prepared using a three-stage method of synthesis (initial homogenization at 500øC; sulfurization at 300øC; annealing at 500øC). X-ray data were col- lected using monochromatized CuK•l radiation at an oscillation scanning rate of 1/2ø0 per minute, and National Bureau of Standards silicon (ao = 5.43088 A) as an internal standard. The unit cell dimensions were calculated using the program of Evans et al. (1963). As shown in Figure 4, the variation in unit cell parameters between carrollite and linnaeite is linear and the relationship between unit cell and composition is expressed by the following equations:

wt % Cu(-+0.3) = -2798.88 + 297.58ao (A) a0(A) (-+0.001): 9.4057 + 0.0033 wt % Cu

Natural phases in the carrollite-linnaeite series with minor impurity concentrations fit this line in Figure 4.

The unit cell dimension of CoaS4 samples syn- thesized at 500øC in the presence of excess CoS2 or Co•_xS was identical to that of stoichiometric syn- thetic CoaS4 (i.e., 9.404--+-0.001 A) suggesting no metal:sulfur nonstoichiometry, at least in pure syn- thetic samples. Available data on the Vickers micro- hardness of the carrollite-linnaeite phases are con- sistent with the prediction that microhardness should decrease from CoaS• to CuCoeS• because the number of electrons in antibonding orbitals increases (Vaughan et al., 1971 ).

Variation in reflectivity as a function of composi- tion has also been examined in this work. At 589

nm, reflectivity shows a systematic decrease from linnaeite to carrollite as illustrated in Figure 5A. This trend is seen over most of the visible region of the spectrum as illustrated in the spectral profiles of Figure 5B and is also consistent with the predictions of the thiospinel bonding models (Vaughan et al. 1971).

Kullerud (1968) has suggested that carrollite and many other mineral thiospinels invert under pressure

E

•40

CARROLLITE- LINNAEITE SERIES i i • i i

2C I I I I I A cuco2s 4 cu 0 75co2 25s4 Cuo 5oCO2 5oS4 Cuo.25co2 75s4 co5s4

40

REFLECTIVITY PROFILES: carrollire- linnoeite series i i i [ i i i i i i i i

_.....-

.....- •

.... #136 Co3S4 --+ •$7 Cuo•CozsS 4 •,o #33 CuCozS 4

S 20 I I I I I I I I I I I I 420 460 500 540 580 620

FIC. 5. A. Reflectivity variation at 589 nm for the carrol- lite-linnaeite series.

B. Reflectivity spectral profiles for the carrollite-linnaeite series.

to a hexagonal NiAs structure. He reported that CoaS4 undergoes an inversion at about 2.5 kb and 640øC. We have heated stoichiometric CuCo2S 4 at 6 kb and 444øC and at 40 kb and 500øC each for 18

hours. In both experiments the principal phase re- sulting was carrollite (a0 = 9.471 ----+-0.002 A from 6 kb and 9.451-+ 0.002 A from 40 kb), but small amounts of cattierite and chalcocite-digenite solid solution were also present. There was no evidence of a hexagonal phase being present or having existed under the conditions of the experiments.

9 48O

9 460

o 9440

9,420

9 400

zo 5• i•0 ? I00 715 0

CuCozS4 Co3S4 Cerrollite Lmnaeite

i i i

Wt % Cu = -2798 88 + 297 58( (3o}

WI % Cu

'? i

•0 25

Mal % CuCo•S 4

FIC. 4. Unit cell dimension of the carrollite (CuCo.oS,) - linnaeite (CoaSt) series, measured at room temperature.

5.610 I I I I I

Wt. % Cu: -1214.19 + 219.59(Q o)

5.590

"S 5.570 -

5.550

Wt. % Cu

'• '? • • • f I 5.530 I I i I 25 20 15 I10 5 0 CoS 2

Cottierite Mole % CuS3

FIC. 6. Unit cell dimensions of synthetic copper-bearing cattierites, (Co, Cu)S•, measured at room temperature.

664 CRAIG, VAUGHAN, AND HIGGINS

Cattlerite

Cattlerite, COS2, was encountered in the present study as a prominept phase in sulfur-rich assemblages and as a breakdown product of the carrollite-linnaeite series. Although cattlerite coexisting with carrollite and the chalcocite-digenite solid solution between 400 ø and 900øC contains less than 1 wt percent Cu, cattlerite coexisting with the chalcocite-digenite solid solution and liquid sulfur contains up to 18 weight percent Cu. Cattierire, like other disulfides, is rather refractory and grains frequently persist metastably in disequilibrium assemblages, especially at 400 ø or 500øC. In one instance, a charge prepared by direct reaction of Cu, Co, and S and annealed at 500øC for 13 days contained stoichiometric carrollite which was rimmed by cattlerite, which in turn was rimmed by covellite and liquid sulfur. The inside of the cattier- ite rim, where in contact with the carrollire, con- tained 3.3 weight percent Cu; the outside of the rim,

less than 1 mm away but in contact with the covellite and sulfur, contained 14.4 weight percent Cu.

The presence of copper in the cattlerite structure results in an increase in unit cell dimension from

5.535 ñ 0.001 A for CoS2 to 5.590 ñ 0.002 A for a cattlerite containing 12.1 weight percent Cu (Table 3; Fig. 6). The variation of unit cell parameter with copper content is linear and is expressed by the following equations which are based on the data in Table 3:

wt % Cu(ñ0.3) = -1214.19 + 219.39 ao(A) ao(A) (ñ0.001): 5.535 + 0.00455 wt % Cu

The change in the unit cell parameter of cattierire parallels that in the carrollite-linnaeite series when copper substitutes for cobalt. Although cattierire samples containing up to 18 weight percent copper were encountered during microprobe analysis, ac- curate unit cell dimensions were not obtained from

S

.•o-py

C•o_py bn•COrr Co-py

carr pyrite cp finn er. ite .

Fro. 7. Inferred low-temperature phase relations in the central portion of the Cu-Co-Fe-S system. The lower portion of the figure is a wedge bounded on 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-S PHASE RELATIONS AND CuCo•S•-Co•S• MINERAL ASSOCIATIONS 665

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

Assemblage' Locality Reference

CF -'{- CC cr + Cu-sulfide cr(20% Cu) + dg cr(20.S% Cu) + dj + Co1_xS cr + bn + dg

cr + bn -4- cc cr(20.4% Cu) -4- bn q- cc cr(18% Cu) + bn q- dg cr + bn + cpy cr q- ,p,y (Co bearing?)

cr(15.8% Cu) q- py q- cpy cr(14.,2,% Cu) q- py q- cpy q- bn cr q- py q- cpy q- bn q- sph cr q- py q- po q- cpy q- bn cr q- ct q- py q- cpy cr q- co-pn + po + cpy q- mk cr q- Co-pn (10% Cu) q- cpy + sph ct q- py q- sieg ct q- "polydymitedinnaeite" ct q- Co-py(15-20% Co) q- py

q- cr(12% Cu, 12% Ni, 34% Co, 0.4% Fe)

Co-py(7% Co) q- cpy Co-py(10% Co) q- Co-py(20% Co)

q- cpy q- mk 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)

Co-pn q- po q- cpy

co-pn q- po q- cpy q- Meg Co-pn q- py q- mc q- Meg

lang q- bv

Kamoto, Zaire Borras, Norway Luanshya, Zambia Carrizal Alto, Chile Kamoto, Zaire Kolwezi, Zaire Kamoto, Zaire Kohlenbach, Siegen gorras, Norway Nkana, Zaire Chibuluma West, Zambia Kamoto, Zaire Copperbelt Shirataki, Japan Carrizal Alto, Chile Kolwezi, Zaire Sazare, Japan Ruby Creek, Alaska Nkana, Zaire Rokana South, Zambia Vauze, Quebec Shinkolobwe, Zaire Shinkolobwe, Zaire

Shinkolobwe, Zaire Luanshya, Zambia

Luanshya, Zambia

Katanga, Zaire Varislahti, Finland Outokumpu, Finland Kamaishi, Japan Langis Mine, Ontario

Bartholomd (1962) Vokes (1967) This study Clark (1974) Bartholomd (1962) This study Bartholomd (1962) This study This study Jordann (1961) Whyte (1971) Bartholomd et al. (1973) Grimmer (1962) Itoh et al. (1972) Clark (1974) This study Tatsumi et al. (1975) RunnelIs (1969) Grimmer and O'Meara (1959) Notebaart + Vink (1972) Stumpfl and Clark (1964) Derriks and Vaes (1956) Kerr (1945)

Craig and Vaughan (1979) This study

This study

This study Kouvo et al. (1959) This study Imai et al. (1973) Petruk et al. (1969)

Abbreviations: cr = carrollite; ct = cattlerite; py = pyrite; bn = bornite; cpy = chalcopyrite; dg = digenite; dj = djurle- ire; po = pyrrhotite; cc = chalcocite; mk = mackinawire; Co-pn cobalt pentlandire; mc = marcasite; sieg = siegenite; bv = bravoire; lang = langisite; sph = sphalerite; vs = vaesite.

t Percent Cu in carrollire and % Co in pyrite indicated if data available.

such samples because of highly variable compositional zoning.

C o 1 -x•

The Cox_xS phase was reported by Rosenqvist (1954) to decompose below 460øC. However, this value is not consistent with other reports listed by Hansen and Anderko (1958). In the present study, we have found that composition of Co0.s7S (prepared initially as a homogeneous phase at 600øC) did not decompose when annealed at 400øC for 10 days but did shift to a slightly more S-rich composition by exsolving CosSs. Additional annealing of such samples at 400 ø and 440øC for periods of two months resulted in no recognizable change. A series of syn- thesis experiments in which sulfur was reacted with CosSs (to a bulk composition of Co0.•7S) resulted in formation of Co•_xS at 460 ø and 500øC but in mix- tures of CosSs and CoaS4 at 400 ø and 440øC. These results, though not unequivocal, suggest that Co•_xS

is not stable below •450øC but that this phase will persist metastably for long periods of time.

Cobalt pentlandite,

Cobalt pentlandite was encountered in this study at 700øC and below. The maximum copper content of Co•Ss in the presence of copper sulfides is 2.3 weight percent at 400øC. The unit cell dimension of pure Co•Ss prepared at 700øC, as determined by measuring the (044) reflection, is 9.928- 0.002 A; the unit cell dimension of CogSa prepared in the presence of the chalcocite-digenite solid solution at 700øC is 9.930 ñ 0.002 A.

Copper sulfides

The only copper sulfides encountered in the ex- periments were covellite and the chalcocite-digenite solid solution. Covellite is restricted to sulfur-rich portions of the Cu-Co-S system and is prohibited from stable occurrence in carrollite-bearing assem-

666 CRAIG, VAUGHAN, AND HIGGINS

FiG. 8. A. Euhedral carrollite crystal (white) with digenite (light gray) and bornite (dark gray). This sample, typical of vein deposit occurrences of carrollite is from Borras, Nor- way. The field of view is 0.75 mm.

B. Anhedral mass of carrollite (white) serving as host for an intergrowth of chalcopyrite and bornite •vhich ap- parently formed as a single phase. This sample is typical of the copper-cobalt ores of the Zambian Copperbelt in which carrollite serves as a major source of the cobalt and chalco- pyrite and bornite are the major sources of the copper. USNMNH #102608. The field of view is 0.2 mm.

blages by tie lines between the chalcocite-digenite solid solution and cattierite. The covellite observed

was normal in all respects; the maximum cobalt con- tent observed was 1 wt percent at 500øC. In con- trast, the chalcocite-digenite solid solution dissolves up to 11 weight percent Co when in equilibrium with Co•_xS and Co0S8 at 800øC and up to 8 weight per- cent Co when in equilibrium with carrollite and cattierite at 800øC. During the rapid cooling of the charges at the end of the experiment, the cobalt is ejected from the chalcocite-digenite solid solution, ultimately appearing as small anhedral grains of co- balt sulfides dispersed in a chalcocite-digenite solid solution matrix.

Natural Assemblages in the Copper-Cobalt- Iron-Sulfur System

Since copper-cobalt sulfides most commonly occu in association with copper-iron sulfides, phase rela tions in the Cu-Co-Fe-S quaternary system are o more direct interest than those in the ternary system Using the data available for the relevant ternar' systems, together with examination of natural assem blages, speculative (and rather simplified) phase rela tions for the relevant portion of the Cu-Co-Fe-S sys tern are proposed in Figure 7. In the main diagran (center) the Cu-Co-S system becomes the front fac of a tetrahedron (bottom omitted) with sulfur at th apex and iron extending back into the plane of th diagram. Some of the more important carrollire containing assemblages are illustrated by the pyra mids shown above the main diagram.

Carrollire generally occurs in close association wit] chalcopyrite and although the carrollire is usually en• member CuCo•S4, it may show a range of Cu/C, ratios. When end member carrollire coexists witl

chalcopyrite and cobaltiferous-pyrite, the cobalt con tent of the pyrite is •12 wt percent as shown in th top right of Figure 7. However, the thiospinel corn position can vary toward CoaS4 when coexisting witl a more cobalt-rich disulfide or (if the geometry of th. phase diagram is examined) if the aa2 is lower. Th. other assemblages shown (carrollite-chalcopyrite bornire-cobalt pyrite and carrollire-cobalt pyrite bornite-digenite) correspond to decreasing aa2.

The degree to which low-temperature adjustment of phase compositions alter (stably or metastably) o obscure primary mineral assemblages is difficult t, assess. For example, the assemblage digenite-bornite carrollite-chalcopyrite listed in Table 4, probably doe not represent equilibrium if our and other workers interpretation of a stable low-temperature pyrite bornite-chalcopyrite assemblage (as shown in Fig. 71 is correct. It is likely that what is presently observe{ as a digenite-bornite intergrowth formed as a singl, phase of-intermediate composition. Similarly, it i likely that some presently observed chalcopyrite bornire assemblages formed as single phases (Fig 8B).

Examples of these assemblages reported from th, study of natural ores are shown in Table 4. As thi table indicates, the carrollite-linnaeite minerals occu: in a variety of geological settings ranging from "epi thermal" vein deposits such as those of Borras, Nor way, shown in Figure 8A and described by Voke.. (1967) to the possibly "syngenetic" copper ores o Zaire and Zambia. The Zambian Copperbelt, a: example of which is shown in Figure 8B, contains th• most economically important copper-cobalt ores it

Cu-Co-S PHASE RELATIONS' AND CuCo.oS4-Co•S'• MINERAL ASSOCIATIONS'

T^m• 5. New Analyses of CarrolIite-Linnaeite Series Minerals

667

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

Finksburg, Maryland Kohlenbach, Ge•'manv Baluba, ZarffBia • P, hokana, Za mbia Kohlenbach, Germany Borras, Norway Borras, Norway Kambove, Zaire Chibuluma, Zambia Chibuluma, Zambia Chibuluma, Zambia Kolwezi, Zaire Chibuluma, Zambia Kohlenbach, Germany Chibuluma, Zambia Kolwezi, Zaire ChJbuluma, Zambia Chibuluma, Zambia Kambove, Zaire Kohlenbach, Germany Kolwezi, Zaire Kolwezi, Zaire Kolwezi, Zaire Kamoto, Zaire Kamoto, Zaire Chibuluma, Zambia Kamoto, Zaire Kolwezi, Zaire Kolwezi, Zaire Chibuluxna, Zambia Kamoto, Zaire Kolwezi, Zaire Kamoto, Zaire Kolwezi, Zaire Kolwezi, Zaire Kolwezi, Zaire Kolwezi, Zaire Kamoto, Zaire

the world. X, Ve have analyzed the samples shown as well as a large number of other carrollite-linnaeite- containing ores by electron microprobe. The results are presented in Table 5 and in Figures 9 and 10. In Figure 9 microprobe analysis of Cu-Co-Ni-Fe thiospinels from this work and from the literature are plotted to illustrate the range of variation in metal ratios. This figure shows that, although some minerals of the carrollite-linnaeite series are pure Cu-Co sulfides, many also contain nickel. There is no evidence that the copper content of the carrollite- linnaeite series extends beyond one-third of the total metal. Since more copper-rich compositions would imply the existence of the relatively unstable Cu +a oxi- dation state and the CoaS4 - CuCo2S4 series involves

a regular substitution of Cu +-• for Co +-• in the tetra- hedral sites, the CuCo2S• composition limit is also predicted by models of thiospinel crystal chemistry (Vaughan et al., 1971). The few analyses taken from the literature which plot at more copper-rich

compositions that CuCo2S4 are from samples contain- ing inclusions of copper-sulfides (Powell, 1967).

In Figure 10, the carrollite-linnaeite analyses are plotted on a Cu-Co-S composition diagram. The re- sults show a spread in metal: sulfur ratios about the ideal composition (shown by the line) which cor- responds to ñ weight percent sulfur. The scatter of analyses about the CoaS4-CuCo2S4 join could be just analytical error, consistent with the experimental findings that there is no deviation from MaS4 stoi- chiometry, or may represent some real variations in stoichiometry of natural samples. Further investiga- tions are required to clarifv this point.

Discussion

The study of the stable sulfide assemblages in- volving copper-cobalt sulfides is an important aspect of determining the temperature of formation of ores containing these phases. However, an additional criti- cal parameter is the activity of 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 shows the compositional range of the carrollite-linnaeite series 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), Permingeat and Weinryb (1960), Darnley and Killingworth (1962), Richards (1965), Vokes (1967), Powell (1967), Itoh et al. (1972), Clark (1974), Filimonova and Slyasarev (1974), Pavlova and Polyakova (1974), Tatsumi et al. (1975), Minceva-Stafanova (1975), Karup-M½ller (1977), and Craig and Vaughan (1979).

We have determined the activity of sulfur over a range of temperatures for the sulfidation reaction in which stoichiometric carrollite breaks down to form

digenite and cattlerite. The data were obtained using the solid electrolyte electrochemical cell Ag/AgI/ Agu+xS, S2(g) described by Schneeberg (1973). This sulfidation reaction is plotted on a log cts2 -- 1/T diagram in Figure 11, together with other relevant sulfidation reactions taken from the compilations of Barton and Skinner (in press) and Vaughan and Craig (1978). Diagrams of this type enable an esti- mate of sulfur activity during ore formation to be made through an examination of the coexisting sul- fide assemblages. This approach will be considered in more detail in its application to certain of the Zambian ores in a separate publication. By com- bining estimates of sulfur activity during ore forma- tion with data on tie lines from the phase diagrams, it is also possible to define the mineralogical changes which will occur with an increase or decrease in as.o. For example, a decrease in cts2 can result in a more Co3S4-rich thiospinel when the assemblage is chal- copyrite-cobalt pyrite-thiospinel or an increase in as2 can produce more cobalt-poor pyrite and CuCouS4 thiospinel in this assemblage.

Acknowledgments

We gratefully acknowledge the support of NATO Grant No. 966 and NSF Grants Nos. DMR75- 03879 and DMR78-09202 in this work. We are also indebted to the U.S. National Museum, the British Museum (Natural History), and Drs. A. Brown, A. Annels, A. Criddle, and F. M. Vokes for natural carrollite-linnaeite-bearing samples. The critical comments of G. Kullerud who reviewed an early version of this manuscript and of Steven D. Scott have been most helpful.

J. R. C. DEPARTMENT OF GEOLOGICAL SCIENCES

VIRGINIA POLYTECHNIC INSTITUTE AND STATE UNIVERSITY

BLACKSBURG, VIRGINIA 24061 D. J. V.

DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF z•STON IN BIRMINGHAM

BIRMINGHAM B4 7ET, ENGLAND j. B. It.

DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF TENNESSEE

KNOXVILLE, TENNESSEE 37916 February 17, September 5, 1978

Cu-Co-S PHASE RELATIONS AND CuCo2S4-Co•S, MINERAL ASSOCIATIONS 669

s

Wt. Percent Cu $0 J 20

• A A A ß

40

I0 ^ ^ 50

ß ß

o- I..• o - e' ß

ß ß

v v v v 50 60

Wt. Percent Co

Fro. 10. A portion of the Cu-Co-S system onto which carrollite-linnaeite analyses have been plotted to illustrate the copper content and the metal' sulfur stoichiometry. The central line represents the ideal CuCo•S4 - C•S, join.

-5

o

-15 c c

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/T relationships in the Cu-Co-S system. The pyrite +

pyrrhotite and bornire + pyrite + chalcopyrite curves have also been included. The abbrevia- tions are: 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 data from Barton and Skinner (in press) and this work.

670 CRAIG, VAUGHAN, AND HIGGINS

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