VIOLARITE STABILITY RELATIONS Jalms R. Cnarc, Department of Geological, Sciences, Virginia Polytechnic Institute and State Uniae

TtI]' AMERICAN MTNIRALOGIST, \/OL 56, JULY AUGUST, 19i1

VIOLARITE STABILITY RELATIONS

Jalms R. Cnarc, Departmentof Geological,Sciences, Virginia PolytechnicInstitute and State Uniaersity, Blacksburg, V ir ginio 24061.

Aesrnlcr

Violarite, FeNizSr, a member of the spinel-sulfrde groups, has a maximum thermal' stability of 461 * 3oC in the Fe-Ni-S system Violarite solid solution extends toward more 'fhe Ni-rich compositions at lower temperatures becoming complete with NiaSr at 356oC. unit cell varies Irom 9.465+ 0.003A f or FeNLST to 9.489+ 0.003A for NiaSr.It is likely that much hypogene violarite is formed by exsolution from initially deposited (l'e, Ni)r-'S monosulfide solid solution.

IurnooucrroN Violarite, FeNizS+,one of the less common iron-nickel sulfides, is a member of the spinelsulfide mineral series,MaS+ (where M:Fe, Ni, Co, Cu, Cr). Although rarely constituting a major component of nickel- bearing ores, it is commonly present in small amounts, especially where weathering of pentlandite by meteoric waters has occurred.Typically such occurrencesare in pyrrhotite-rich sulfide ores associatedwith ultra- mafic intrusives such as those at Sudbury, Ontario and Marbridge' Que- bec.

Pnevrous Wonx Violarite was originally describedby Lindgren and Davy (1924)' who suggested from analyses of contaminated samples' a composition of NiSz. Short and Shannon(1930), on the basisof new analyses,proposed the now acceptedformula of FeNizN+.Tarr (1935) compiled all available analysesof the spinel-sulfideminerals and concluded that in spite of the frequent association of violarite with polydymite (NiaSa)and the simi- larity of their compositions,the two minerals were distinct. The first synthesisof violarite was reportedby Lundqvist (1947),who reacted S with Fe-Ni filings in evacuated glass tubes. He observed violarite at 200oC,but not at 480oCand above,noting " . . it is not obvious whether this phase is a ternary phase separated from Ni:S+ through a two-phase area, or if it is linked with this phase . . . there are somefacts which suggestthat the phase with iron content has properties different from NiaSn."Kullerud (1963)in his extensivestudy of the Fe-Ni-S system did not observea violarite phase at 400"C or above. Craig (1968) reported preiiminary findings on the phase relations involving violarite; that work has been expandedin the present paper. 1303 1304 rAMES R. CRAIG

Exprnnanlrra.r, Rnacrl.Nrs aNn Pnocnluxrs

Experiments were prepared using iron, nickel, and sulfur of 99.999+ percent purity as indicated by suppliers analyses. Oxide films on the metal powders were removed by reduc- tion in a stream of hydrogen at 700oC for eight hours prior to use. Experimental charges were prepared in evaluated fused quartz tubes which were heated in mufle furances in which the temperature was maintained + 2oC. At the termination of each experiment, the charges were rapidly cooled by immersion in ice water. The reaction products u'ere anzlyzed by X-ra1' diffraction and reflection microscopic techniques.

ExrrnrunNrAl Rrsur,rs AND DrscussroN or SvNrrlBsrs

Violarite was first encountered by accident during experiments at 3000 and 400"C de- signed to define the composition limits of the monosulfide solid solution (Mss) in the central porition of the Fe-Ni-S system. Starting with Mss compositions near the sulfur-rich limit at 600oC, it was expected that annealing at 300"C or 400oC, would result in exsolution of pyrite (FeS) and/or vaesite (NiSt In addition, however, several percent of fine lamellar laths of a light violet-gray phase, subsequently identified as violarite, also appeared. At- tempts to synthesize violarite of FeNizSr composition by direct reaction of iron, nickel, and sulfur at 3000 and 400oC were futile and resulted in formation of variable but large per- centages of pyrite, vaesite, and Mss with but traces of violarite. Repeated regrinding of these charges over periods of several months increased the violarite content to only a {ew percent. The immediacy with which the metastable assemblage of pyrite, vaesite, and Mss formed and the very slow rate at which these phases then reacted to form violarite possibly explains why Kulierud (1963) did not obtain violarite in his experirnents at 400oC. To circumvent the rapid formation and metastable preservation of py'rite and vaesite, a two-step process was employed in violarite synthesis. (1) Preparation of an Mss containing Fe and Ni in 1 : 2 atomic proportions, by direct reaction of the elements at 5000 to 70C"C; one grinding Iollowed by heating for two to three additional days was usually sufficient for hornogenization; (2) Reaction at 2000 to 300oC of this finely ground Mss, u,'ith the ap- propriate amount of sulfur to achieve the FeNizSr composition for five to ten days. Best results, producing 95 to 100 percent violarite, were achieved when the Mss and the sulfur were physically separated by silica wool, thus limiting the reaction to sulfur transported in the vapor state. At 400oC this second step resulted in formation of considerable amounts of pyrite because of the extension of pyrite-Mss tie lines (as noted in Figure lc) between Mss of Fe:Ni:1:2 atomic proportions and l'eNi:Sr. As with attempts of direct one step slmtheses, pyrite once formed broke down oniy slowly. Emplovment of the two-step slmthesis permitted preparation of homogeneous M3Sa compositions between FeNizSr and Feo zsNir 75Sa.Attempts to synthesize homogeneous M3Sa compositions more nickel-rich than Fe0.2b-Ni275Sa resulted in formation of 10 to 20 percent NiSz and residual Mss in addition to the spinel phase; this probably resulted from sluggishness of formation of the spinel phase and buildup of the sulfur vapor pressure to a degree sumcient to nucleate vaesite.

Vroranrrn Rpr,arroNs AND SrABrLrry Once a satisfactory method for violarite synthesis was found, it was possible to examine the relations surrounding this phase and its thermal stability. The results of these investigations between 500oand 300oCare noted in Figure 1; selectedexperiments defi.ning the assemblagesshown in Figure 1 are given in Table 1. The compositionsof the pyrite, (Fe, 40 30 Weighl per cent Fe Weighf centNi 450"C ( Fe,N o)

aos

q"t ,

40 30 Weight pel cenl Fe cenl Ni ,oweighl 400"c

40 Weighl per cenl

Wcight pei cent Fc

Frc. 1. Phase relations involving the appearance of violarite and development of the violarite-polydymite solid solution at: A-500"C, B-450'C, C-400oC, D-300"C. 1306 7AMES R. CITAIG

TAer,n 1. ExpunrnnNrs DnlrNrNc Pnasr RererroNs SunnouxorNc rHE FeN izSr-NisS + Sorrn SolurroN

Composition, wt. /a Erp. Temp. Time Reactantsa Products No Days

496 29.8 307 395 Mss+vs(tr.) 500 26 Msslpyfvs 497 300 31 0 390 Mss 500 26 Mss*py 498 303 31.2 38.5 Mss 500 26 Mss 569 250 3s.5 395 Mss+vs(tr.) 500 26 Msslvs 605 200 41 0 390 Mss+vs(tr.) 450 30 Msslpyfvs(tr ) 596 160 45.0 390 Mss+vs(tr.) +viol(tr.) 450 37 Msslviolfpy 120 50.0 380 Mss+vs(tr ) 450 43 Mssfvs 309 20.0 41.0 390 Mss+vs(tr ) 400 t70 Mss*py 572 16.0 450 390 Mss+vs(tr.) 400 38 Msslviolfvs(tr.) 611 11 4 49.6 39.0 Mss{S 400 7? Mssfviol 494 6.0 55.5 38.5 Mss+vs(tr ) 400 29 Mssfvs 474 29.8 307 39.5 Mssfvs(tr ) 300 34 Mss{py 571 25.O JJ. J 395 Mss+vs(tr.) 300 38 Mssfviol 480 200 41 0 390 Mss+vs(tr ) 300 34 MssSviol 548 12.O 50.0 38.0 Mss 300 60 Mss{viol 487 60 J5.5 385 Mss+vs(tr.) 300 .3.3 Mss*viol

a tr. indicates trace

Ni)Sz, and vaesite,(Ni, Fe)Sz,have beentaken from Clark and Ktillerud (1963) who extrapolated data from higher temperature studies. It is noteworthy that violarite bearingassemblages may contain as little as 5 weight percent Ni. The maximum thermal stability of violarite in the presence of an equilibrium vapor is 461+3oC, at which temperatureit decomposesto form (Fe, Ni)52, (Ni, Fe)Szand Mss containing Fe and Ni in the atomic ratio 1:2.4. As noted in Figure 2a, a sectionalong the FeNizSE-NiaSnjoin, the compositonwith maximum thermal stability is Feo.gzNizosS+. Below 461'C the violarite solid solution expandsto reach FeNizSaat 356+3oC, the upper stability of polydymite (Kullerud and yund, 1962). Homo- geneousFeNirS+-NisSi+ solid solutions were synthesized at 200ocand it was found that previously prepared compositions did not break down when heatedat 200oCfor periodsup to six months. Combination of S2vapor pressuredata for the py*vs*Mss assem- 8 blage at 600'C (10-1 atm) from Naldrett and Craig (1968),and for the Py*violf Mss assemblageat 4500 (10-n.tatm) and 400' (10-6.0atm) from craig and Naldrett (r97l) with an estimatedentropy for FeNizsaof Szse:46.01permit formulation of the free energyequation for formation of violarite as: I Szss:46.0 is the average value obtained from Latimer,s (1951) and Grlnvald and Westrum's (1962) approximations and the reactions FeSz*2NiS:FeNirSr and FeS*NiS *NiS::FeNirSl assuming AS for these reactions is 0. The free energy equations given in the text assume that AS and AH for the formation of FeNi:Sr are temperature independent. VIOLARITE STABI LITY RELATIONS 1307

356t 3"

(J o ?nn

o (l, o E 2OO o) F

Mol 7o FeNi.So

B. r-Experimenlol dolo points

o E o it, 9.48 or g

.E o o 947

Weight "/" Fe 15 to 9.46 loo 75 50 25 0 Mol oh FeNirSo

Fro. 2. A. Section along the FeNirSrNraSr join. B. Variation of unit ceII parameter with variation in composition along the FeNizSrNiaSr join' I3O8 JAMES R. CRAIG

I/2Fe* Ni+ S,:1/2FeNizSa AG: -69,575 +41.97 2(FeS'2NiS)+ Sz:2FeNizSa2 AG: -57,900+59.47 (T is degreesKelvin).

X-ney D,qre Violarite and polvdymite belong to the spinel series (MrS4) of sulfide minerals which crystallize in the space grottp Fd3m. The variation in the unit cell from the FeNizSavah.re of 9.465+0.003A to the NirS+value of 9.489+0.003A is shown in Figure 2b. Values were measuredwith a diffractometer using nickel filtered Cu radiation, and silicon as an internal standard. Naturallv occurring violarites commonly contain 0.1 to 7.0 weight percentcobalt substitutingfor iron and nickel. CoeSnis isostructuralwrth violarite and polvdymite and has an a of.9.43A iR".rv and Thompson, 1962),thus the substitution of Co into the violarite Iattice will tend to reducethe unit cell dimension.

SpBcurerroNs oN PHASERnrarroNs ar Low TBlrpnnarunos The low temperaturephase relations involving violarite are uncertain. Kullerud et al., (1969) suggestedthat the common occurrenceof secondary violarite on weathering pentlandite may indicate a low temperature tie line between the phases.Buchans and Blorves(1963) and Naldrett (personalcomm. 1970) on the other hand report hypogenepyrite-millerite intergrowths in ores from Marbridge, Quebec. A pyrite-millerite join would invalidate a stable join between violarite and pentlandite. The analysesof Buchansand Blowes (1963)indicate Fe-deficiencyof the violarites and 1 to 2 percent Fe in solid solution in the millerite; these variations from FeNirS+and NiS might be sufficient to permit stable pyrite-millerite tie hnes. The violarite and polydymite anall'seslisted by Tarr (1935) and by Buchans and Blowes (1968) reveal a bimodal distribution with Fe con- tents Iessthan 3 percentand greaterthan 9 percent.However, co-existing violarite and polydymite, indicative of a solvus,have not been reported. Low temperaturesilica tube and precipitationexperiments prepared with FeSOr,NiSOa, and NarS solutionshave thus far failed to vield data ro support or refute the existenceof a solvus.

Gror,ocrcar.CoNsrpBnauoxs Violarite, where encountered,has generallybeen considereda super- , The AS values in this equation takes into account the inversion of FeS and NiS to high temperature forms; thus it is onll' applicable in the temperature range 379" to 467oC. VIOLARITE STABI LITY RELATIONS

caO\n€O \o\oNdo\ $<1<'4{'<'r I

X- + 1.9; E>> ''TT c al -taa o o o o LU?.u? > > > > a,aa

trh rrrrrN I \o\o\o\o\o: -H z O oocT)o€dr a F z N a o -:- s a z >>>> r> ^/. 1111(r)1, I rtttltl aaaa I a I +++*@** z a@@6la - a.a.66.ia s g z g> O' O\ O\ 01 O\ dr oO o E dl::dO -H (€

X (-) E 8888888 o -ri hnqn4\o

F a a a a -ri o z 6AAAA@ o a >>=E"zEE o N r i I r ii t a aaaaEaa 6 ++++4++ ^/.o zzzz6zz.i | .i.d F +++++++ o o q oir Q o aO F !rFrtrAZtuH E.- :

xll = NQ a 6= 'a h: = o

o F O

-No

'fler,r 3. Staerr,rtv ExprnnanNts Ar,oNc FeNizSr-Ni3SaJorN

Composition Time (in order of) Exp. No. T', oC Reactants mol 0/6 !'eNi2Sa Days (abundance)

579 100 300 JI MssiS viol 632 100 400 viol violfMssfpy 593 100 450 T4 viol violf Nlssfpy 616 100 458 + NIssf S Msslpyf vsf viol 628 100 458 4 viol violf Mssf py 623 100 46+ 4 viol Mss-f.vsfpy 622 100 481 I viol N{ssf vsf py 724 50 400 7 viol viol 725-r 410 + viol viol 725-2 420 A viol viol*NIss*r's 777 50 430 6 viol violf Mss*vs 724-l 50 440 4 viol Mss*vs 24-l 25 400 10 viol viol*Mss*vs gene mineral formed as the result of surficial alteration of iron-nickel sulfides, expecially pentlandite. The present study demonstrates that violarite, stablebelow 461+3"C in the presenceof an equilibrium vapor, may form as a hypogenephase by primary direct crystallizationor by secondary exsolution from an initialll' homogeneous Mss phase. Vio- Iarite formation is not necessarilyconfined to Ni-rich environments, but may form as a hvpogenemineral in orescontaining as little as 5 percent nickel provided that a nickeliferous pyrite is present. Violarite formation through alteration of pentlandite may not represent an equilibrium assemblage,but only failure of the alternative pyrite-millerite assemblage to nucleate.

AcrNowl,tocunNts The author is grateful for the opportunity of conducting a portion of this study in G. Kullerud's laboratory at the Geophysical Laboratory and to G. Kullerud and A. J. Naldrett for critically reviewing the manuscript.

BretroGnepHv

BrntoN, P. B., aNo B. J. SxrnNrn (1967) Sulflde mineral stabilities. In H. L. Barnes, eil. Geochemistry oJ Hyd.rothermd Ore Deposits. Holt, Rinehart and Winston Inc., N. Y., 23G333. Brnny, L. G., aro R. M. Tnonpsox (1962) X-ray powder data for ore minerals; The Peacock fll,las. Geol..Soc. Amer. Mem.85rp.287. Bucnmvs, R., .a.NoJ. H. Rlowns (1968) Geology and mineralogy of a millerite nickel ore deposit, Marbridge No. 2 Mine, Malartic, Quebec. Can. Inst. Mineral,. Bultr. Aptr 1968, 599-534. Cr-.tm, L. A., exo G. Kullrnuo (1963) The sulfur-rich portion of the Fe-Ni-S system. Econ. Geol.58, 853-885. VIOLA RITE STA BI LITY RELATION S 1311

Cnerc, J. I{. (1968) The Fe-Ni-S system: Violarite stability relations CarnegieInst. Wash. l'eor Book 66, 434-436. AND A. J. N.llrnnrr (1971) Phase relations and PSrT variations in the I,'e-Ni-S system Geocltemical Stud.ies Perl,inent to Sud.bu.ry, Mineral Assoc. Can. Symp., (in press). Gnlwvor-o, F., eNo E F. Wrsrnuu (1962) Heat capacities and thermodyrramic functions of iron disulfide (pyrite), iron diselenide, and nickel diselenide from 5 to 350oK. The estimation of standard entropies of transition metal chalcogenides. Inorg. Chem. l, 36-48. Kurtnnuo, G. (1963) The Ire-Ni-S system. Carnegie Inst. Wosh. Year Book 62, 175-189. Kulmnuo, G., R. A. Yumo, nNo G. H. Mou (1969) Phase relations in the Cu-Fe-S, Cu- Ni-S, and Fe-Ni-S systems. In H. D. B. Wilson (ed.) Magmatic Ore Deposit:s Econ. Geol,. M onogr. 4, 323-343 Kur,r.nnur, G., e.NoR. A. Yuno (1962) The Ni-S system and related minerals. J. Pel.rology 3, t26-t75. Lrxucnnt, W., aNo W. M. Dew (1924) Nickel ores from Key West Mine, Nevada. -Ecoz. Geol., 19. 309-319. Lumoqvrsr, D. (1947) X-ray studies on the ternary system Fe-Ni-S. Arkio Kemi MineraL Geol.24A. 72. Nalonnrr, A. J., eNt J. R. Cnr,ro (1968) Partial pressure of sulfur in the vapor coexisting with the Fer_"S-Nir_*S solid solution at 600" and 40OoC.Carnegie Inst. Wash. year Book 66.43M. Snonr, M N., axn E. V. SnenroN (1930) Violarite and other rare nickel sulfid.es.Amer. MineraL. 15. 1-22. Tatn, W. A. (1935) The linnaeite group of cobalt-nickel-iron-copper sulfides. Amer. Min- eral.2O. 69-80.

Manuscript recei.t;ed.,October19, 1970; acceptedJor publication, February 26, 1971.