AmericanMineralogist, Volume61, pages913-920, 1976

Orientationof exsolvedpentlandite in naturaland synthetic nickeliferous

Cenr A. FRANCISI,MtcHesl E. FLestz, KuL,q MlsnA.8,AND JenrEs R. Cnntcl rDepartment of Geological Sciences, Virginia Polytechnic Institute and State Uniuersity, Blacksburg, Virginia 24061 2Department of , The Uniuersity of Western , London, Ontario N6A3K7 sDepartment of Geological Sciences, The Uniuersity of Tennessee,Knoxuille, Tennessee37916


Pentlandite(pn), (Fe, Ni)rSB,exsolves from nickeliferouspyrrhotite (po), (Fe, Ni)r-,S, in the form of flamesexhibiting weak reflectionanisotropism and preferredorientation. X-ray precessionstudies on both natural and syntheticcrystals have establishedthe orientation relation:(111)pnll (00.1)po; (0Tl )pn ll (l 1.0)po;(TT2)pn ll (10.0)po.In synthesisexperiments, rapidlyquenched samples of (Fe,Ni)r-,Ssolid solutions contain randomly oriented blebs of ,whereas slowly cooled charges contain coherent lamellae of pentlanditeparallel to (00.1)of pyrrhotite.

Introduction typical Sudbury-typeFe-Ni sulfideassemblages lie within the monosulfidesolid-solution at higher Threetextural varieties of pentlandite,(Fe,Ni)156, temperatures(Naldrett and Kullerud, 1967;Craig are common in the pyrrhotite-bearing(Sudbury- and Kullerud, 1969) further supportsthis inter- type) naturalnickel assemblages: (l) massive pretation.Thus the subsolidusexsolution origin of (blocky) pentlandite,often containingislands of pentlanditein -sulfideassemblages is well es- pyrrhotite;(2) partialto completerims of pentlandite tablished. aroundpyrrhotite grains; and (3) stringersand blebs The presentcontribution is thecombined report of of pentlanditein pyrrhotiteoften showingpreferred two independentinvestigations, one on a natural orientation."Flame" or "brush" texture,consisting sampleby MEF andKCM andthe other on synthetic of fine subparallelplates or needleswith sharp to materialby CAF andJRC. The crystallographic rela- raggedborders, is characteristicof the latter variety tionshipbetween host pyrrhotiteand exsolvedpent- of pentlandite(Fig. l), but it alsooccurs at the mar- landite is established,correcting previous reports, ginsof rim-typeand (morerarely) blocky pentland- and structuralcontrols on the exsolutionprocess are ite, projectinginto the adjacentpyrrhotite. All these discussed. textural varietiesare often found within the limits of Orientedintergrowths of severalore- pairs a singlepolished section; however, detailed electron- wereexamined by Gruner (1929),who recognizeda microprobeanalyses have failed to establishany sys- commonstructural feature in them.The arrangement tematiccompositional differences among them. of atomsin one or more atomiclayers of the first is Crystallizationof pentlanditeduring cooling nearlyidentical with that in oneor morelayers of the of synthetichigh-temperature (Fe,Ni)r-,S (mon- second.Further, a singleelement, usually oxygen or osulfide)solid solutions(Newhouse, 1927; Hewitt, , populatesthose layers. For example,there is a 1938;Hawley and Haw, 1957;Kullerud, 1956,1962), closedimensional fit betweenthe oxygenpositions in often with texturestypical of naturalpyrrhotite-pent- the close-packed(lll) layersof magnetiteand those landite assemblages,strongly suggests that the pent- in the close-packed(00.1) layers of .Such an landitein naturalassemblages has exsolved from a arrangement,like that of coherenttwin boundaries, high-temperaturenickeliferous pyrrhotite. The with- tendsto minimizeinterfacial energy because the first drawalof themonosulfide solid-solution (mss) solvus coordinationspheres of the atomson the common in the Fe-Ni-S systemto progressivelymore S-rich planeare satisfied by both structures(Buerger, 1943; compositionsat decreasingtemperature (Naldrett, e/ 194s). al.,1967;Shewman and Clark, 1970; Misra and Fleet, Gruner (1929)concluded on the basisof crystal- 1973a),and the fact that the bulk compositionsof the structuralarguments that true "orientedintergrowths 913 914 FRANCIS.FLEET, MISRA AND CRAIG

pyrrhotite exsolvedfrom pentlandite.In the former example,"cut parallelto the base"[presumably the (00.1)of pyrrhotitel,pentlandite occurs in threedis- tinct orientationsresembling a snowflake.In thelater example,apparently hexagonal-shaped pyrrhotite crystalslie in the (l I l) planeof pentlandite.Ehren- bergalso described the only knownexample of pent- landiteepitaxially overgrown on pyrrhotite(Fig. 2). Pentlanditeoccurs as tri-radiateovergrowths in two orientationson the (00.1)face of pyrrhotitewith the rays extendingperpendicular to the {10.0}form of pyrrhotite.These "stars" are composed of smallcrys- tals that havefaces normal to the ray directionsand henceparallel to i10.0)of the pyrrhotitesubstrate. Assumingthat the three-foldsymmetry of the stars was not merelyinherited from the substratebut ac- tually representsthat of [1I l] of pentlandite,Ehren- bergassigned the smallvertical faces to the dodecahe- dron,{l l0}.He thusdeduced that the (ll l) and(l l0) planesof pentlanditeparallel the (00.1)and (10.0) planes of pyrrhotite respectivelyli.e. ( 1I 1) pn ll(00.1)poand (110)pn ll (10.0po1. Orientedintergrowths of pentlanditeand pyrrho-

FIG I Photomicrographs illustrating some typical pentlandite (pn)-pyrrhotite (po) textural relationships. The bar in each photo- micrograph represents0.04 mm. Oil immersion, reflected light. (a) Stringer of flame-textured pentlandite (light gray) in pyrrhotite (darker gray). Note the preferred orientation of the flames parallel to the pyrrhotite .Specimen 52, Falconbridge mine, Sudbury, Ontario, . (b) Oriented blebs of flame pentlandite (light gray) in pyrrhotite (darker gray). Specimen9868, , Sudbury, On- tario, Canada.

of pentlandite and pyrrhotite . . . are improbable if not impossible," despiteprevious reports of such in- tergrowths by Newhouse (1927) and others. Sub- sequently Lindqvist et al. (1936) showed that the pentlandite structure determined by Als6n (1925), which formed the basis of Gruner's argument, was incorrect;that structureassumes a unit-cetlcomposi- tion of 8(FeNirSn)and has the S and positions reversed relative to the acceptedstructure. Responding to Gruner's paper, Ehrenberg (1932) pentlandite documentedoriented intergrowths of and Frc. 2. Pentlandite epitaxially overgrown on pyrrhotite (vertical pyrrhotite. His polishedsections from Sudbury show dimension2.5 mm) from Miggiadone bei Pallanza,Piemont, Italy both pentlandite exsolved from pyrrhotite and (from Ehrenberg, 1932, with permission). ORIENTATION OF EXSOLVED PENTLAN DITE 915

off-setvery slightlyfrom the crossedposition. A reex- aminationof otherpolished sections from the earlier studiesconfirms that weak anisotropismis a charac- teristic feature of the flame pentlandite,and to a lesserextent, of the smallergrains of rim-typepent- landitein theseassemblages. When muchof the en- closingpyrrhotite is removed,the anisotropismis markedly reduced,suggesting that it results from elasticstrain. Such pentlandite is likely to havepre- servedits originalorientation relationship with the host pyrrhotiteand wastherefore considered ideal for the presentstudy. A small fragment(about 0.01 mm in longestdi- Frc. 3. Photomicrographof flame-texturedpentlandite (light mension)of an intergrowthof pyrrhotiteand flame grey) in pyrrhotiteshowing distinct anisotropism. Analyzer 2o off pentlanditewas removed from a polishedsection of crossedposition, oil immersion,reflected light. Bar is 0.04 mm. specimen9868 and examinedby the precession Specimen9868, Creighton mine, Sudbury, Ontario, Canada. method.A representativeX-ray diffractionpattern is shownwith an interpretationof the cerltralarea of tite arecommonly encountered in experimentalprod- this photographin Figure 4. The verticalreciprocal ucts as well as in natural samples.They werefirst latticerow in Figure4a includesboth pyrrhotite(po) producedexperimentally by Hawleyand Haw (1957) 00./ and pentlandite(pn) hhh diffractions,thus con- andhave subsequently been reproduced by a number of workers(e.9. Naldrett et al. 1967;Misra, 1972: Craig,1973). The crystal structuresof both pentlanditeand high-temperaturepyrrhotite are now well-known. High-temperaturepyrrhotite, which is the end- memberof the mss,has the NiAs structure(Harald- sen,1941). Lundqvist (1947) confirmed that the mss alsohas the NiAs structure.Pentlandite was shown to be isostructuralwith CoeSe,by Lindqvistet al. (1936)on the basisof powdermethods. The structure was confirmedby Pearsonand Buerger(1956) and Knop and lbrahim(1961), and has since been refined by Rajamaniand Prewitt(1973).

Experimentalmethods and results Natural samples The specimensused in this portion of the in- vestigationwere polishedsections from earlier elec- tron-microprobestudies of naturaf pentlanditeas- semblages(Misra and Fleet, 1973a,1974). More detailed examination of the assemblagefrom the Creightonmine, Sudbury(Specimen 9868) reveals that the flame pentlanditehas a weak but distinct reflectionanisotropism (Fig. 3) with parallel ex- tinction. As far as we are aware, anisotropic pentlanditehas not beenreported in the literature. precessionphotograph This anisotropismis often maskedby the Ftc. 4n. Zero level showingthe crystal- much lographicorientation of pyrrhotite and exsolvedpentlandite in strongeranisotropism ofthe enclosingpyrrhotite, but specimen9868. [01.0]po, [10]pn precessionaxis; p = 20o:Zr- is readilyrecognized when the analyzer is filteredMoKa radiation,35 kV, 20 mA, l4 daysexposure. 916 FRANCIS,FLEET, MISRA AND CRAIG t [oo.r]no [rr t]pn I I to.e / to]pn ,/ [t ,a / +

o/ +

Fro. 4s. Interpretation of the central part of the precessionphotograph of Fig. 4a. Large solid circles, pyrrhotite (po) subcell reflections;small solid circles, pyrrhotite 4C supercell reflections;large plus signs, reflections of pentlandite (pn) in the first orientation; small plus signs, reflections of pentlandite in the second orientation; weak reflections unlikely to survive reproduction of the original film have been omitted. firmingthat [111]pnll[00.1]po and consequently that (<0.01), S (53.37);pentlandite, Fe (24.72),Ni (l I l)pn ll (00.1)po.The direction normalto I l1]*pn (27.68),Co (0.40),Cu (0.00),S (47.20)(Misra and in the plane of the photographis I l2]*pn; thus Fleet,1973a, Table 5). The latticeparameters deter- (TT2)pnll (10.0)po.In confirmationof this,zero-level mined from the uncalibratedprecession films are: precessionfilms taken with the crystal rotated 30o pyrrhotite(subcell), a : 3.$44, c : 7.054; pent- about [00.1]poshow parallelismof the 0kkpn and landite,a : 10.050A.The valuefor the penttandite hh,}po reciprocal lattice rows. giving (0Tl)pnll latticeparameter is in good agreementwith that of (11.0)po.The orientationrelationship between pen- Knop e/ al. (1965)for Creightonmine pentlandite tlanditeand pyrrhotiteis establishedas (1ll)pnll (lo.042A). (00.1)po,(011)pnll (l 1.0)po,(If2)pnll (10.0)po and is In the earlierstudy (Misra and Fleet,1974) only summarizedstereographically in Figure5. monoclinicpyrrhotite (4C) was recognized. However The compositions(atomic percent) of the coexist- the presentstudy indicatesthat the pyrrhotite in the ing pyrrhotite and pentlanditein specimen9868 are: vicinity of the flamepentlandite is apparentlyhex- pyrrhotite, Fe (46.01),Ni (0.62), Co (0,00), Cu agonal. Careful examinationof precessionfilms ORIENTATION OF EXSOLVE D PENTLA N DITE 9t'7

limationin a thermalgradient of 5'lcm. The result- ing singlecrystals and crystalclusters were euhedral for the most part and averagedabout 0.I mm in diameter.The presenceof well-developedpinacoids (00.1)and first order dipyramids(ft0.1) facilitated orientingthe crystalson the precessioncamera. Polishedsections of crystalsthat had subsequently beenannealed at 400oCfor one weekclearly show exsolvedpentlandite. The pentlandite(- 8 modalper- cent) occurspredominantly along cracks,edges, and grain boundariesas small oriented flame-like la- mellaewhich coalesce to form irregulargrains, and in someinstances completely rim a crystal.Larget la- mellae often occur within grains. Generally, the largerthe pentlanditegrain or lamellathe more ir- regularits shape. The unit-celldimensions of the monosulfidesolid- solutionwere refined using the least-squares program of Appleman and Evans (1973). Averaged-values weremeasured from diffractometertracings recorded FIG. 5. Stereogramshowing the orientationrelationship of ex- solvedpentlandite (pn) in pyrrhotite(po), (lll)pn (00.1)po, with monochromatizedCur(a radiation from 24" to (0Tl)pn ll(l 1.0)po,and (f-12)pnll(10.0)po. Pentlandite indices are 75" U, scanningat a rate of 0.5o 2A per minute. on the left-handside of the poles,pyrrhotite indices on the right. Bariumfluoride (a : 6.197lA)was used as an inter- nal standard. The crystalsquenched from above900'C havea = 'equivalent' shows that, in each of the three settings 3.449(l)Aand c = 5.758(\A, wherethe numberin aboutthe pyrrhotitec-axis, the distribution ofthe 4C parenthesesis the estimated standard deviation. superstructurereflection intensities has mm symme- Thesevalues are statisticallyidentical to the cell di- try, and the pentlanditeand pyrrhotite retain the mensionsof the crystalsthat had beenannealed at parallelismof their orientationrelationship. Thus, 400'C. Although distinctin polishedsections and this 4C superstructureis the hexagonal2A,4C one precessionphotographs, there is not sufficientpent- reportedby Fleet(1968). landitein the annealedcrystals to be recordedon the diffractometertracings. Thus, it is not surprisingthat Syntheticsamples no expressionof nickelloss is apparentin the mono- Crystalswere synthesizedby evacuatedsilica tube sulfidesolid-solution cell dimensions.Furthermore, techniques(Kullerud, l97l) in Kanthal-woundfur- in this compositionalregion of the solid-solution, naces.A monosulfidesolid-solution composition unit-celldimensions are not a particularlysensitive (Fe:Ni:S= 53:10:37weight percent) was prepared by measureof nickel loss.since the increasein c due to directreaction of high-purity(99.999 percent) sulfur the decreasein Ni is compensatedby the decreasein c (Asenco),iron (UnitedMineral), and nickel(John- due to the decreasein total metal content.The cell son, Matthey and Company,Ltd.); the iron and dimensionof pentlandite[a : 10.15(2)A]was com- nickelhad previouslybeen reduced at )700oC in a puted from measurementsof precessionphoto- streamof .Initial reactionin evacuatedsi- graphs.This valuecorresponds to an iron-richcom- lica tubesat 600'C for twenty-fourhours resulted in position(34 atomic percentFe) (Misra and Fleet, discretebeads of Fer-,S and Ni,-,S. Thesewere 1973b)as would be expectedfrom the known phase ground under acetoneand then annealedin evac- relations(Misra and Fleet, 1973a). uated tubes for an additional twenty-four hours to Precessionphotographs of crystalsthat had been homogenizethe mixture.Single crystals suitable for quenchedin air from above 900'C were recorded X-ray precessionstudies were obtained by suspend- about the [00.1]and [0.0] axesusing MoKa radi- ing a 3 cm long silicatube containingthe homoge- ation. Neither thesephotographs nor thoseof the nizedpowder in a verticalfurnace at temperaturesin annealedcrystals show pyrrhotite superstructurere- excessof 900'C for one week.Crystals grew by sub- flections. Precessionphotographs of the annealed 918 FRANCIS,FLEET, MISRA AND CRAIG

and Francis et al. (1974).The orientation relation- [,'o]. ship hereestablished for intergrowthsis not the same orientationrelationship that Ehrenberg(1932) deter- mined for epitaxial overgrowthsof pentlanditeon pyrrhotite. Since intergrowths are constrainedin three dimensionsrather than two and sincemore than one mutualorientation may be possible,inter- growthsand overgrowths need not in generalbe gov- ernedby the sameorientation relationship. In this case,however, if the crystalscomprising the pentland- ite stars in Figure 2 are interpretedas octahedra ratherthan dodecahedra,then the orientationrela- tion of the overgrowthsis identicalto that of the intergrowths.We preferthis interpretationbecause it is consistentwith the octahedralmorphology of syn- thetic pentlanditecrystals (C.T. Prewitt, personal communication,1974). Sincepentlandite and pyrrhotiteform orientedin- tergrowths,it is reasonableto inferclose dimensional and structuralsimilarities. The crystalstructure of FIc. 6. Zero-levelprecession photograph showing crystallogra- pyrrhotiteis a metal-deficientderivative of the simple phic orientationof pyrrhotiteand exsolvedpentlandite in a syn- hexagonalNiAs-type structure (spacegroup thetic crystal.[0.0]po precessionaxis; p : 25o; unfilteredMoK radiation,35 kV, l6 mA, 100hrs. exposure. Small "powder" arcs P6"/mmc,Fig. 7). The sulfur atomsare arrangedin representpentlandite lamellae slightly misaligned with respectto hexagonalclose-packed arrays parallel to (00.1),with the pyrrhotitehost. metal atoms occupyingthe octahedralinterstices. Thereare two formulaunits of (Fe,Ni)t-,Sper unit cell.The idealcrystal structure of pentlandite(space crystals exhibit short arc-shaped pentlandite diffrac- groupFm3m) is basedon cubicclose-packed arrays tionssuperimposed on the pyrrhotitereciprocal lat- tice. The pentlanditediffractions describe arcs of about I Vz"on eitherside of [ 1.0]*in the a-b plane and 8V2" about the same direction in the I l0]*-c*plane.The 5.88Apentlandite d-value mea- suredalong c* of pyrrhotiteis in closeagreement with the publishedvalue of 5.86A for dnt of iron-rich pentlandite(Misra and Fleet, 1973b).Likewise, the 3.58Apentlandite d-value measured along [ 1.0]*of ooo pyrrhotiteis in good agreementwith the value of 3.59Afor dor"of pentlandite.Indexing of thesepent- landited-values is unequivocal,as no otherpermit- ted diffractionsof pentlanditeor pyrrhotite corre- /* "/* "/ spondto thesed-values. The colinearityof [111]*pn ooo with [00.1]*poand [001]*pnwith [ 1.0]*pofixes the l* \ mutual crystallographicorientation of theseinter- [to.o]po Irt.o]po grown structures.In terms of the direct lattice (l I l)pnll(00.1)po,and (01 l)pnll (l 1.0)po. [ri-z]pn [oTr]pn Discussion l\t\ The sameorientation relationship has beenestab- FIc. 7. Idealized structure of the pyrrhotite subcell projected lishedfor both naturaland synthetic exsolution inter- parallel to c axis. Large stippled circles, S at z : 3/4; large open growthsof pentlanditeand pyrrhotite.This corrects circles. S at z = l/4: small solid circles, Fe at z : 0, l/2. The the orientationpreviously reported by Francis(1974) orientation relationship is indicated by vectors. ORIENTATION OF EXSOLVED PENTLAN DITE 919 of sulfuratoms parallel to ( I I I ), with metalatoms in and chemicalcomposition. For constantsulfur con- both tetrahedraland octahedralinterstices (a useful tentsthe volumechange is about 9 percent,based on diagrammaticrepresentation is given in Misra and the lattice parametersof the natural crystal. Also, Fleet, 1974,Fig. 5). In eachunit cell there are four transformationof the sulfur arrays from hexagonal formula units of (Fe,Ni)rSr;thirty-two of the metal to cubic close-packing,assuming no cooperative atomsare in tetrahedralcoordination with S and four mechanism,requires redistribution of shuffiingof 4 are in octahedral coordination. The averagesul- out of every6 sulfur layers.Thus the departurefrom fur-sulfur distancein the close-packedlayers of pen- parallelismshown by the arcson the precessionpho- tlanditeis 3.46A,identical to that in pyrrhotite,and tographof thesynthetic crystal (Fig. 6) is not surpris- the interplanarspacings normal to the close-packed ing. It is surprising,however, that no suchdeparture layersof both structuresare within two percentof can be detectedin the natural sample.The radial one another. displacementof reflectionsin Figure4a is duemerely Orientationof the phaseboundaries between pent- to mechanicaldisruption on removalof the crystal landiteand pyrrhotiteis not fixedby establishingthe fragmentfrom the polishedsection, since the pentland- mutual orientationof the respectivelattices, although ite and pyrrhotite reflectionsare displacedby corre- certainplanes may becomeenergetically favored. The spondingamounts. preciseoptical determination of the phaseboundary In subsequentexperiments (Francis, 1974) mono- orientationis difficult becausethese do not sulfide solid-solutioncrystals with a wide range of form the long, sharpcontacts characteristic of other nickel-to-ironratios have beensynthesized. Crystals exsolvedpairs (e.9. -),but the quenchedfrom 600'C into ice water as well as those weightof texturalevidence (Ramdohr, 1969), albeit quenchedin air contain0.5-5 micronblebs of pent- not rigorouslydetermined, supports the view that landite.The presenceof completepowder rings of pentlanditelamellae exsolved parallel to (00.1) of pentlanditeon precessionphotographs establishes pyrrhotite.However, the snowflake-likeillustration their randomorientation. During one experimenta in Ehrenberg(1932) suggeststhat in at least some fuseblew out and the furnacecooled from 600oCto instancespentlandite exsolves along pyramidal pyr- room temperaturein about ten hours. The crystals rhotite planes. that experiencedthis slow cooling contain pentland- It is not fortuitousthat the pentlandite-pyrrhotite ite in orientedlamellae rather than randomly orien- phaseboundary commonly parallels the close-packedted blebs.Thus pentlanditeexsolution textures are sulfur layers.The closedimensional fit and the stack- dependenton coolingrate. An investigationof reac- ing continuityacross the phaseboundary produce a tion kinetics,similar to that by Yund and Hall (1970) coherentinterface parallel to the close-packedlayers. on exsolutionof pyrrhotite, would contribute Thus pentlanditeforms as coherentlamellae along significantly to understandingthe exsolution of (00.1)planes of pyrrhotitein conformitywith the pentlanditefrom pyrrhotite. criteriafor orientedintergrowths recognized by Gru- ner(1929) and rationalizedbyBuerger (1934,1945). Acknowledgments Pentlandite exsolvesfrom pyrrhotite with one Specimensof natural pentlanditeassemblages, including the Ill]pn axisparallel to [00.I]po.The [111]pnis a 3- Creightonmine specimen9868, were obtainedmostly from the fold axiswhereas is a 6-foldaxis. Thus two Suffel Collection of the University of WesternOntario. Other [00.1]po Mr. J. Holland pentlandite, specimenswere obtained from Dr. R. Ruchanand orientationsof relatedeither by reflec- of theFalconbridge Nickel MinesLtd. Originalphotographs of the tion on (ll2)pn or by 2-foldrotation about Ill]pn Miggiandonespecimen were kindly providedby ProfessorP. Ran- are predictedand two are observedon precession dohr. Discussionwith L. A. Taylor and reviewsby G. V. Gibbs photographs(Fig. a). The reflectionsof the second and D. A. Hewitt werehelpful. The authorsare particularlyin- orientationare much weakerthan thoseof the first, debtedto associateeditor C. W. Burnhamfor his suggestionsand was a WalterS. Varr fellowship within volume diplomacy.C. A. F. supportedby suggestingthat the of the sampleana- administeredby the HoraceSmith Fund (Springfield,Massachu- lyzedthe two orientationsdid not nucleatewith equal setts).Experimental facilities were made possible by National Sci- probability,The two orientationsare not resolvedon enceFoundation Grant DES74-22684to J. R. C; M. E. F. and K. zero-levelprecession photographs precessed about C. M. were supportedby an NRC operatinggrant. M. E. F.'s and sincethese axes are contributionto the preparationof the manuscriptwas made whilst [01.O]po,[0.0]po, [1.0]po leaveat the Departmentof Mineralogyand Petrol- normalto the twin planes the on sabbatical or twin axis. ogy, Universityof Cambridge,using facilities kindly madeavail- The transformation of pyrrhotite to pentlandite able by ProfessorW. A. Deer. We acknowledgegratefully these involvessubstantial changes in both crystalstructure contributionsto this study, 920 FRANCIS,FLEET, MISRA AND CRAIG

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