(1,97La) and Potruk Et Al. (1971). in Thi Present Work We Have

Catudian Minerologtst Vol. 13, pp. 8-14 (1975)

TEXTURAL A,ND COMPOSITIONAL VARIATIONS IN A Ni-Co-As ASSEMBIAGE

K. C. MISRA Departmento! Geophysics M. B. FLEET Departmentof Geology University of Western Ontario, London, Ontario

AISTRACT semblage from the Cobalt area, hoping to clarify tle mode of formation and chemical A Ni-Co-As assemblage from Cobalt, history of Ontario, phases, consistlng of plumose rosettes of rammelsbergite its constituent The material studied was enclosing large prismatic crystals of pararammels- obtained from Ward's Natural Science Estab- bergite and rimmed by cobaltite and safflorite has lishmenf Rochester, New York, and was used been examined by electron microprobe analysis originally as the source of pararammelsbergite for Fe, Ni, Co, S, As and Sb. Approximate aver- for the crystal structure investigation of Fleet age metal compositions for these phases are: para- (1,972). A preliminary discussion of the texture raormelsbergite, axial blades, Ni.su Co.g3 Fe.or, in- of the assemblage and of the chemistry of tle terlocking crystals at the cores of rosettes, Ni.", pararammelsbergite was included in his paper. Co.67 Fe.s1; rammelsbergite, Ni.67 Co.ss Fe.6s; cobaltite, Ni.uo Co.ae Fe.16 to Ni.24 Co.76 Fe.sa; and safflorite, inner zone, Ni.26 Co.6s Fe.2o, outer zone, Onr MrNsRALocv Ni32 Co.3s Fe.rs. The rammelsbergite is relatively homogeneous but there are significant composition- The ore mineral assemblage in the specimen al variations within the cobaltite zone. Ni de- consis8 of pararammelsbergite,rammelsbergite, creases and Co increases outward, and Fe and cobaltite, safflorite and skutterudite in a cherty S distributions across the cobaltite zone have fairly rnatrix. Petruk (l97La) classified the arsenide pronounced V-shaped profiles, both tending to be assemblagesof the Cobalt-Gowganda area into relatively enriched toward the inner and outer fivo gradational classes: (1) Ni-As, (2) Ni-Co- margins. As, (3) Co-As, (4) Co-Fe-As, and (5) Fe-As. Al- Although pararammelsbergite appears to have though the assemblageunder study does not formed by inversion of rammelsbergite, tle Co re- fit stdctly leased in this process has migrated away from into any of the above classes, it most pararammelsbergite and must have been redis- nearly correqponds to the Ni-Co-As assernblage tributed within tle remaining rammelsbergite and, of Petruk, which is transitional between the Ni- probably, within the cobaltite. Also, the V-shaped As and Co-As assemblages: the NiAs assem- Fe and S profiles across the cobaltite zone, idio- blage is characterized by nickeline as tle main morphic cobaltite grains cutting across the grain Ni-arsenide and the presence of rammelsbergite, bouodaries of rammelsbergite and of safflorite, and the Co-As assemblagesby pararammelsber- and cobaltite islands enclosed in rammelsbergite gite as the main Ni-arsenide rather than nickel- aod safflorite are inconsistent with sequential, ine. The phases notably absent in present primary deposition from the ore-forming fluid. the Thus, whilst the gross chemical 2ening may be assemblage are nickeline, breithauptite, "Ag-Sb primary, other features in these rosettes seem to minerals", gersdorffite and sulphides. be the result of later chemical readjustments through solid state diffusion. Dendrites and zoned rosettes

INrnopucrroN The most striking textural features of the specimen are the over-all dendritic, almost fern- Tho mineralogy of the Ni-Co-Fe arsenide and like, habit of the arsenide assemblage(Fig. 1), sulpharsenide assemblages associated with the and the consistent zonal arrangement of the ar- silver-bearing veins in the Cobalt-Gowganda re- senide minerals in individual rosettes. A typical gion of Ontario has been described by Petruk dendrite is about 5 cm long and 5 mm across, (1,97La)and Potruk et al. (1971).In thi present the main and subsidiary branches of which are work we have investigatedin detail the textural characterized by narrow (less than L mm across) and compositionalvariations in a Ni-Co-As as- axial zones of bladed parargmmelsbergite(and Frc. 1. A portion of the specimen showing a typical dendrite, with axial pararammelsbergite (black) and plumose rosettes, occurring in a cherty matrix. Frc. 2. Photomicrograph of a portion of a typical zoned rosette showing roughly concentric zones of pararammelsbergite (white, interlocking prismatic crystals in the core, prm), rammelsbergite (greyish white, rm), cobaltite Oight grey, cob) and safflorite (greyish white, conical prismatic crystals at ttre rim, saf). The safflorite zone consists of two subzones WA and IVB); the sharp boundary between them is marked with minute inclusions of gangue. Frc. 3. Photomicrograph showing nature of tle boundary between rammelsbergite (liebt grey) and cobaltite (medium grey) zones. The idiomorphic grain boundaries of relatively large cobaltite crystals cut acres many smaUer grains of recrystallized, polygonal rammelsbergite. Frc. 4. Photomicrograph showing a typical g;radational boundary between the cobaltite (medium grey) and safflorite (greyish white) zones. Frc. 5. Photomicrograph showing the junction between outward growing safflorite (light gey) zones of two rosettes. Many of the islands of euhedral cobaltite (medium grey) in the safflorite zones cut across the grain boundaries of prismatic safflorite. Fro. 6. Photomicrograph showing idiomorphic skutterudite (ight grey, sk) transecting zone boundaries in a rosette, and containing islands of pararammelsbsrgite (white, prm), rammelsbergite (greyish white, rm) and cobaltite (medium grey, cob). Note the discontinuity of tle safflorite zone (greyish white, conical prismatic crystals, saf) as a result of the growth of the skutterudite. l0 TIIE CANADIAN MINERALOGIST

clert) and longi.tudinal and lateral envelopes of no consistent compositional difference between plumose rosettes (Fig. 1). the two types, .and the turbid appearanc€ ap- - The rosettes, which apparently are lateral sec- pears to be due to a contrast in polishirig char. tions of dendrite branches, range from less than acteristics of the two minerals, or to minu& in- I mm to about 3 mm in diameter. and are clusions of gangue phases in rammelsbergite, or chglagtgrized by arsenide minerals arranged in to a combination of both. welldefined, roughly concentric zones. The sequence of phase zoning outward from the core Cobaltite rq a typical rosette (Frg. 2) is: pararammelsber- qtte (zone I), rammelsbergite (zone II), cobaltiG Cobaltite is easily identified in tle present as- (zone III), and safflorite (zone fV). ihis zonal semblage because of its high relief and its pink- pattern is consistent in all the rosettes associated ish tinge, under reflected light, against the white cf,ith dendrites. The same zonal pattern may be of the other arsenide minerals. The wavy na- observed in isolated rosettes emfeAaea in- the ture of the boundary between rammelsbergite cheffy matrix, but many of them lack either the and cobaltite zones (see Fig. 2) is due to idio- ;nrarammelsbergite zone or the -Thepararammels- morphic outlines of relatively large cobaltite bergite and rammelsbergite zones. rosettes crystals that terminate against several smaller have preferred orientation within a dendrite. so grains of rammelsbergite (Fig. 3). On careful [hat their appearance varies with the orientation examination one can discern suggestions of out- of a section. No case of oscillatory or reverse lines of relict rammelsbergite grains within the zoning was observed among the hundreds of ro- cobaltite areas, and, along cobaltite-rammels- settes studied in polished sections of different bergite grain boundaries, these relict outlines orientation. Aa outer cobaltite zone following appear to pass into the grain boundaries of the the safflorite zone, as reported by petruk (L97La, rammelsbergite. The boundary between the co- !ig. 57) was not found in the present assem- baltite and safflorite zones may be sharp locally, hlage. but typically it is gradational, with the contact zone marked by innumerable islands of cobal- Patarammelsbergite tite, often with idiomorphic outlines, in safflorite and a few tiny islands of safflorite in cobal- TWo textural varieties of pararammelsbergite tite (Fig. 4). The resulting texture resembles the can be recognized in the specimen: (1) axial pseudo-eutectictexture of Lindgren (1930). Pods blades, some of which are up to 2 cm long, that of polygonal rammelsbergite in the cobaltite rnark the .axial zones of the main and subiidiary zone and islands of cobaltite in the rammlesber- branc,hes of dendrites, and (2) interlocking ag- gite zone are common, and in all such casesco- gregates of prismatic crystals, with minor baltite shows its characteristic idiomorphic out- amounts of interstitial rammelsbergite, in cores line against rammelsbergite. Islands of cobaltite of rosettes (Fig. 2) which represenithe terminal are also common in the safflorite zone and many and lateral limbs of the axial blades. Three com- of them are euhedral grains cutting across the positiolatly distinct textural varieties of para- boundaries of safflorite prisms (Fig. 5). They rammelsbergite were recognized by Fleet (1972) also ocsur in the pararammelsbergite zone at in a crushed concentrate of the present assem- the cote of rosettes, but invariably enclosed in blage. The first two o( these coriespond to the patches of rammelsbergite that are interstitial to axial blades and interlocking aggregates de- the pararammelsbergite. scribed. above; the third variety was possibly rammelsbergite. Isolated prismatic crystats of Safllorite pararammelsbergite occur in tle rammelsbergite zone, but never beyond this zone. Safflorite in the present assemblageshows its typical habit of interfingering, conical, prismatic Rammelsbergite crystals, with their apices pointing away from the core of the rosette. The texture is higbly - The pararammelsbergite zone is invariably suggestive of outward growth of the safflorite followed by a rammelsbergite zone, the width oi crystals rn an open-space environmsntr and is which tends to be inversely proportional to the documented best where two adjacent rosettes width of the central pararammelsbergite zone. coalesce through the outward gfowth of their Jhe loundary between the two zonc is grada- respective safflorite zones (Fig. 5). The safflorite tional. The rammelsbergite interstitial and adla- zone consists of two subzones (IVA and IVB) cent to pararammelsbergite has a distinctive which often can be recognized under reflected turbid appearance whereas furtlrer awav it is Iight by slight differences in colour and reflec- clear and granular (Fig. 2). ,However, tirere is tivity. The boundary between the two subzones TExTuRAL AND coMposrrroNAl vARIATIoNS rN e Ni-Co-As AsT;EMBLAGn 11 is usually welldefined, marked by inclusions of The standards used were: synthetic maucherite, gangue (Fig. 2), and is easily correlated \rith NilrAse (Fleet 1973) for Ni and As, synthetic the marked compositional discontinuity across Feae.arSro.rg(Misra & Fleet 1973) fot Fe and S, a typical safflorite zone (discussed later). and pure metals for Co and Sb. The data were reduced using program EMPADR-VII (Ruck- Skutterudite lidge & Gasparrini 1969). Some typical compositions of the phases are given in Table 1. Skutterudite is not a constituent of the zonal Compositional data obtained through spot pattern of the anenide assemblage. It occurs analyses of parar.ammelsbergite, rammelsbergite, either as clusters of small euhedral crystals at cobaltite and safflorite across randomly-chosen gan- cores of rosettes, embedded in the cherty rosettes, recomputed on the basis of assumed gue, or as isolated, stout euhedral to subhedral stoichiometric M:X ratio of 1:2 (M = FefCo preference crystals in the rosette, without any X = S*As*Sb) are presented in an particular *Ni; for a mne and often transecting one atomic per cent Fe-Ni-Co triangular plot (Fig. or more of the zone boundaries. The former 7). The plot shows a restricted compositional type is free of inclusions whereas the latter type field for rammelsbergite, but a wide variation in pararam- uoually contains corroded islands of the Ni:Co ratios for cobaltite, the compositions cobaltite (Fig. melsbergite, rammelsbergite and with high Ni:Co ratio being characteristic of the 6) which may be interpreted as relicts of the cobaltite zone ne.ar its contact with rammelsber- phases the skutterudite has replaced. Islands gite and those with low Ni:Co ratios, in gen- of safflorite have not been observed in skutter- eral, being characteristic of the cobaltite zone grows be- udite, but skutterudite often into and near its contact with the safflorite zone. Para- yond the safflorite zone, abruptly terminating rammelsbergite occurring as axial blades has a (Fig. - the safflorite layer on either side 6) a slightly higher Ni:Co ratio than that occurring of the texturs suggestive of later development as interlocking prismatic crystals in cores of ro. skutterudite. settes Cfable 1), though each of these two groups is fairly homogeneous in composition. For sim- Corvrposrrror* oF PHASES plicity of representation no distinction between plot. The safflo- All the compositional data on the anenide the two has been made in this two distinct gloups, phases of the assemblage were obtained with a rite compositions fall into tbree-channel MAC-Z$00 electron microprobe corresponding to the two optically distinguish- - Co-rich analyzer under the following operating condi- able subzones discussed earlier the subzone (fVA) tions: 20 kv, 0.12pamp on willemite, counting group corresponding to the inner time 20 to 60 secs, and spot diameter of about adjacent to the cobaltile zone, and the relative' group subzone 1#,m. A1l the phases were analyzed for Ni, Fe, ly Co.impoverished to the outer (tVB). subzones Co, Sb, As, S. The count data were obtained for Althoueh both of the safflorite latios, the Kc lines (Za line for Sb) using LiF (for Fe, Ni, show a significant spread in metal seems to Cq As) and PET (for S, Sb) analyzing crystals. compositional variation between them be abrupt rather than one of continuous grada- IIBLE l. TYPICALCllEl'llCAL coUroSITtot{S 0F PAlAMti!,tELSBERcIn, RA!0,|ELSBERGIIE, (also Fig. 8). The over-all trends of the @BALTITE, ArD SAFfLoRITE (ELECTR0IilittCROpnoEE SpoT AMLySES) tion see chemical gradients across the ros,ettesconsist of Pamn@ls' Nl Co S As Sb Total F€ ltl Co S As Sb a continuous decreasein the Ni:Co ratio (with berglte 't.2 &T 0.'l 26.9 t.8 70.60.3 [email protected] 0.1 30.9 1.4 3.8 63.60.2 much smaller, increase b14le3 0.14.0 1.2 2.t 70.60.3 101.3 0.130.8 1.4 4.463.1 0.2 a concomitant, although 0.327.0 0.5 t.9 69.60.2 99.5 0.331.4 0.7 4.163.4 0.1 in the Fe:(Ni+Co) ratio) outward from the pa- s.2 26.0 2.5 2.4 68.50.4 100.050.2 A$ 2.9 5.'t 61.70.2 0.2 6.6 2.4 2.6 67.30.5 99.6 0.2 30.6 2.8 5.5 60.60.3 tarammelsbergite core, an increase in the Co: 0.226.2 2,4 1.768.90.3 99.4 0.230.5 2.7 3.662.80.2 0.226.5 2.3 2.269.00.4100.6 0.230.4 2.6 4.662.0 0.2 (Ni*Fe) ratio inward from the inner safflorite 0.2 26.7 2.1 2.3 6.2 0.4 99.9 0.2 30.8 2.4 4,9 61.50.2 (IVA), a conv€rgence of the two Ra@ls- 0.8 t9.1 9.7 0.768.9 0.3 99.5 1.022.5 11.3 t.5 68.50.2 subzons and bergt t€ 0.9 1B.58.7 0.669.9 0.3 98.9 1.122.0 10.3 t.3 65.10.2 gradients 0.8 18.88.6 0.969.6 0.2 98.9 1.022.3 l0.l 2.064.5 0.1 in the cobalfite zane. 0.8 19.0 8.6 1.270.4 0.3 100.3 1.02..1 10,0 2.5&.2 0.2 0.919.5 8.6 1.569.7 0.2 100.4 l.t 22.5 9.9 3.26tr.20.1 A plot of tle compositional data obtained 0.919.4 8.1 0.870.4 0.2 99.8 l.l 22.8 9.5 1.764.8 0.1 through analysis of spots at measuled intervals Cobll tl t€ 2.0 5,9 25.618.1 46.9 0.0 98.5 2.1 5.7 24.732.0 35.5 0.0 2.4 9.9 23.017.3 48.5 0.0 r0l.l 2.4 9.4 2t.A30.2 36.2 0.0 across a typical zoned rosetle (about 1.1 mm in 4.0ll.4 20.2t8.0 46.1 0.0 99.7 4.010.9 t9.2 3t.4 34.5 0.0 2.9lr.8 t9.8 16.348.9 0.0 99.7 3.011.5 l9.l A.t 37.30.0 diameter) illustrates the nature of the composi- 3.2 14.216.5 17.5 48.3 0.0 99.7 3.2 13.715.9 30;8 36.4 0.0 3.2 18.313.4 15.1 49.9 0.04 100.9 3.3 17.612.9 A.3 37.70.02 tional variation in each of the mineral zones Ssfflorlte l@ 5.1 3.2 20.1 2.8 67.50.3 99.0 6,2 3.7A.l 5.86t.0 0.2 (Frg. 8). The following points about the dis- atb@ 6.1 4.3 18.0 1.8 67.4 0.4 98.0 7.5 5.12l.l 3.8 62.30.2 6.0 5.4 18.0 2.8 6.2 0.4 t00.8 7.1 6.1 20.3 5.8 60.50.2 tribution may be noted: 6.2 5.4 16.6 3.0 66.9 0.1 98.2 7.5 6.2l9.l 6.4&.7 0.1 (i) ct& 7.3 8.7 l2.l 0.271,3O.2 99.8 9.1 10.314.2 0.3 66.00.1 Relative to cobaltite and safflorite, ram- a/hffi 6.6 9.3 ll,7 0.2 70.8 0.2 98.8 8.3 ll.l 13.9 0.4 66.20.1 7.9 8.0 11.2 0,271.60.2 99.1 9.9 9.6 13.3 0.4 66.70.1 melsbergite and pararammelsbergite ars 9.0 7.9 11.0 0.2 71.40.t 99.6 lt.2 9.4 12.9 0.5 66.00.0, fairly homogeneous in composition. The t2 TgE CANADIAN MINERALOGIST

Fe solid solution is uniformly low in both, melsbergite(also seeTable 1). but there is a preferential concentration of (ii) As discussedearlier, the safflorite zone S in lnrarammelsbergite and of Co in ram- consists of firo compositionally distinct 507"Fe

oof sof qa Aa "dffi T dfrg'-"' q;g Co *] Ni Fr

.--.Af

*:3.1 o

Frro+

*r1 soF4A&

disfqnce("rlo-rt Frc. 8. Compositional variations across a typical zoned rosette about 1.1 mm in diameter. Olrn squares - :-. Fe; open triangles Co; open inverted triangles - S; open circles - Ni; and squires witn a bar - As. TEXTURAL AND COMPOSITIONAL VARIATIONS IN A NICO-AS ASSEMBLAGE 13

subzonesoa low Co- low S outer subzone work out a paragenetic sequence based on stabi- and a high Co- high S inner subzone, sepa- lity of coexisting phases. Based on textural fea- rated by a sharp compositional disconti- tures and compositional variations within and nuity. between the phases, we suggest the following in- (iii) Element distribution patterns across the terpretation. cobaltite zone are marked by a significant It is hypothesized that the initial precipitation discontinuity in the middle portion of the was in the form of a high temperature (below zone. Judging from continuous microprobe 800oC, as the experimental data of Roseboom scans across several rosettes, the type of (1963) woutd suggest)Ni, Co and As-rich metal distribution $€ems to be a consistent and diarsenidedisulphide solid solution. This pre- characteristic feature of the cobaltite zone. cipitate was chemically zoned, with a relatively (iv) gradients The over-all of Ni and Co distri- Ni-rich core [Ni(Co,Fe)(As,S)z]- high-temper- butions across the ros€tte are similar to ature lammelsbergite solid solution - grading those in Figure 7 compiled for many ran- into a relatively Co-rich margln [Co(Ni,Fe)(As, domly chosen rosettes. The convergence of S),1 - high temperature safflorite solid solution. the gradients in the cobaltite m\e is marked The well-defined inclusiondotted boundary be- by cobaltite compositionswith the lowest S tween the two safflorite subzones and the cor- contents. responding sharp compositional discontinuity suggest that the outer subzone (IVB) represents a later, separate phase of precipitation. Re- Dtscusstox equilibration of the solid solution at lorner tem- The phaseand compositional zoning in similar peratures led to recrystallization and redistribu- anenide assemblagesfrom the Cobalt-Gowgan- tion of the elements, rezulting in segregation of da area have been interpreted to represent se- the phases in roughly concentric zones. In the quential crystallization of the phases, outward Ni-rich core, pararammelsbergite developed by from the cores, in an open-space environment, inversion of the high-temperature rammelsber- reflecting the progressivechange in the composi- gite solid solution, and the Co-rich rim recrystal- tion of the precipitating solutions (Petruk l97Ia, lizqd to low-temperature safflorite (zubzone 1,97lb). Pararammelsbergite has been interpreted IVA). The formation of cobaltite was a "sink" to have been formed variously: as a late mineral for the excessCo, Fe, and S that could not be in the Ni-As assemblage,cutting across other accommodated in the recrystallized pararam- arsenide phases and formed through inversion melsbergite, rammelsbergite and safflorite. The of earlier rammelsbergite; as intergrowths, elements migrated within the assembl.age by formed contemporaneously with the other arse- solid state diffusion. The relative homogeneity nide phasesin the NiCo-As assemblage,at tem- of the rammelsbergitesuggests that this \ryasan peratures below that of rammelsbergrte-pararam- effective processwithin the time-scale of the re- melsbergite inversion. crystallization process; for example, there can A number of textural and compositional fea- be little doubt that the pararammelsbergitede- tures of the zoned arsenide assemblageunder veloped by recrystalltzation of a hig!-temper- study do not fit a model of sequential precipi- ature rammelsbergite solid solution, yet there tation of the phases.These are: are no concentration gradients within the rammelsbergite,toward the grain with pararammels- (i) the idiomorphic outlines of cobaltite cut- bergite. It is expectedthat the element redistribu- ting across grain boundaries of rammels- tion took place in the virtual absenceof a fluid bergite (and of safflorite), suggestinga la- phase within the mass of metal diarsenidedisul- ter development of cobaltite in the para- phide solid solution, so that marked discontinui- genetic sequence; ties in the assemblage,for example, the gangue- (ii) outlines of relict rammelsbergite within filled boundary between the IVA and fVB saf- the cobaltite; florite subzones,or the cherty gangue between (iii) the presenceof cobaltite islands in all the the rosettes, would form effective barriers to other three zones; such redistribution. The cobaltite developedpre- (iv) the compositional varintions in th,e cobal- ferentially between the rammelsbergite and saf- tite zone; and florite zones becauseof the diffusion of excess (v) the relatively homogeneouscomposition of S and Co from these zones and its probable early the rammelsbergite. nucleation along the margins of the Ni-rich areas In the absence of data on the synthetic Fe- which were inverting to pararammelsbergite. Ni-Co-As-S system and on the chemistry and With regard to the stability of the proposed nature of the precipitating solution, one cannot high-temperature solid solution, extensive solid t4 THE CANADIAN IvTINERALOGIST solution between the Fe-Ni-Co diarsenides is some typical cobaltite compositions in the pre- known from experimental studies (Roseboom sent assemblage), and the Ni-rich and Co.rich 1963; Radcliffe 7966). The S*olubiliry of the subzones we have reported in the cobaltite zone FeAsg-NiAsz-CoAsz solid solution has not beeu (see Fig. 8) may be a reflwtion on the supposed determined experimentally, but it may be esti- instability of intermediate compositions at the mated approximately from the available data temperature of formation of the cobaltite. on the condensed Fe-As-S .and Ni-As.S svsterns. Finally, although pararammelsbergite ap- At 700'C the S-solubility of ramrnelsboigite is pears to be replacing rammelsbergite in the only about 1.1 wt. % (aboat 2.2 at. Vo) CYvnd present assemblage, the two phases may coexist 1962) and that of loellingite about 3.4 wt. Vo over a range of temperature, since neither is (about 7.0 at. Vo) (Clark 1960). However, the S strictly stoichiometric and rammelsbergitg in contents of the pararammelsbergite and ram- particular, has about 19 at, Vo Co. melsbergite are of the order of 4-5 ar. Vo Q-3 wt. %) and 1-3 at. Vo Qes than 1 to 2 vtt. Vo\, respectively, and the data for these two phases RpnsnsNcEs reported by Petruk et aI. (I97L) also show in CLanK, L. A. (1960): The Fe-As-S system: phase many cases S contents higher than the L vft. % relations and applications. Econ, Geol. 55, limit suggestedby Yund (1962) for natural para- t345-1381 ard 1.631-1652. rammelsbergite and rammelsbergite. The higher Fr-nrr, M. E, (1972): The crystal structure of pa- S contents may well be due to appreciable solid rarammelsbergite (NiAsr). Amer. Mineral. 67, solution of Co in the natural phases. Further, it t-9. seems likely that the S solubility in diarsenides (L973): The crystal structure of mau- (NitAsg). may increase in the sequenceNiAsa(CoAss( cherite Amer. fuIineral. 58, 203-210. KLEUM, D. D. (1965): Synthesen und Analysen in FeAs*, and this may be related to the order of den Dreieckdiagrammen FeAsSCoAs,S-NiAsS insrease of the metal-sulphur bond strength in und Fe$-CoSr-NiSr. Neaes lahrb. Iltlineral. Abh. tho disulphides NiSz