The Stereochemistry of Iron Sulfides-A Structural Rationale for The

American Mineralogist, Volume 65, pages 1026-1030, 1980

The stereochemistryof iron sulfides-a structural rationale for the-crystallization of somemetastable phases from aqueoussolutionr

Pnrnn Tnvron Research Chemistry Branch, Atomic Energy of Canada Limited Whiteshell Nuclear Research Establishment Pinawa, Manitoba, ROE lLO

Abstract

Stereochemicaland topological argumentsare used to rationalize the metastableoccur- rence of mackinawite and marcasitein laboratory experiments.The dimeric species,(FeS), and (FeSr)r,are postulatedas intermediatesin the crystallizationof mackinawiteand marcasite, respectively.

Introduction of oxidants (Berner, 1964, 1967;Rickard, 1969).It widely in nature (e.g.Evans et al., 1964;' The only stable binary solids in the Fe-S system also occurs al., 1972)and as a corrosionproduct on iron above 200"C and near ambient pressureare the pyr- Schotet steel exposed to aqueous HrS (Berner, rhotites, Fe,-,S (including troilite, FeS) and pyrite, or carbon 1962,1964;Takeno et al.,1970;Wikjord et al.,1980; FeSr.Several other phases,at leastsome of which are Shoesmithet al., 1980).Pyrrhotites are not readily metastable,occur widely at lower temperaturesin prepared from aqueous solution below l00oC, al- both natural and artificial environments (Ward, though troilite does occur as a low-temperaturecor- 1970; Power and Fine, 1976). Many workers have rosion product (Takeno et al., 1970;Wikjord et al., studied the synthesisand interconversionof these 1980;Shoesmith et al., 1980).Cubic FeS is known phases(e.9. Berner, 1962,1964, 1967; Rickard, 1969; corrosion product (M€dicis, l97oa,b; Takeno et al., 1970; Rising, 1973; Taylor et al., only as a al., l97O;Wikjord et al.,1980; Shoesmith l979a,b; Shoesmithet al.,l98O; Wikjord et al., 1980). Takeno et In this paper, stereochemicalarguments are used to et al.,1980). nature of most phasetransformations postulate crystallization mechanismsfor some iron The sluggish low temperatureshinders the deter- sulfides.Intermediates proposed in thesemechanisms in this system at of equilibrium phase relations. One ex- may exist in solution, or they may occur only at sur- mination ception is the transformation of cubic FeS to mack- faces.Similar conceptswere first proposedby Bloom inawite, which occurs spontaneously at room (1939).This work was part of an investigationof cor- temperature,demonstrating that cubic FeS is not a rosion and depositionphenomena in CanadianGird- phase (M6dicis, l97oa,b; Takeno et al., 1970; ler-Sulfide heavy-waterproduction plants. stable Shoesmithet al., l98O). Discussion The upper limit of thermal stability of mackinawite is about l30oC; small amountsof Ni and Co in- I ron monosulfides-mackinawite, troilite, and cabic corporatedin the structuretend to en-hanceits stabil- FeS ity (Takeno, 19651'Clark, 1966;Takeno and Clark, A striking feature of the low-temperatureforma- 1967).lt is not known whether this limit reflects a tion of iron sulfidesfrom aqueoussolution is the fre- true thermodynamic stability field, or kinetic limits quent occurre4ce of mackinawite. It is typically the on the transformationof a metastablephase. sole crystalline product of precipitation of ferrous Solubility studies (Berner, 1967; Tewari and ions by HrS or its saltsbelow l00oC, in the absence Campbell, 1976; Tewai et al., 1978) demonstrate that solutionswhich are saturatedwith respectto HrS rlssued as Atomic Energy of Canada Limited Report No. and synthetic mackinawite at ambient temperature AECL 6645. and pressureare supersaturatedwith respectto troil- 0003-004x/80/09I o- I 026$02.00 1026 TAYLOR: STEREOCHEMISTRY OF IRON SULFIDES to2'l ite. However, since mackinawite usually has a metal:sulfur ratio greaterthan 1.00,the possibleex- istence of a low-temperature stability field is not ruled out. Nonetheless,mackinawite is frequently formed under conditions where it is metastablewith respectto troilite and/or more sulfur-rich sulfides. Figure la showsthe crystal structure of mackinawite (Berner, 1962;Taylor and Finger, l97l). Iron atomsoccupy every site in alternatelayers of tetrahe- dral intersticesin a cubic close-packed(c.c.p.) sulfur sub-lattice.The Fe-S bonding network consistsof in- terconnectedFerS, rings, and Fe-Fe bonding also oc- Fig. 2. Part of the Qrystal structure of troilite, showing the curs within the metal-atom layers. The FerS, rings, environment ofone Fe3 cluster. Interactions with neighboring Fe3 and similar structural units discussedbelow, are not clusters are omitted. The solid bonds delineate an Fe3S, ring discretemoieties, but they are recognizableelements which may be a precursor to troilite crystallization. Dashed lines of the structural network. show Fe-Fe interactions. Cubic FeS (Fig. lb) is isostructuralwith sphalerite (M6dicis,1970; Takeno et a1.,1970),and differsfrom and subsequent polymerization of FeSH* with elimi- mackinawitein the distribution of iron atoms,which nation of protons and water of solvation. A similar one haHofeach layer oftetrahedralholes in occupy mechanism has been proposed for the growth of thio- c.c.p.sulfur sub-lattice.The closestructural rela- the ferrite salts from aqueous solution (Taylor and Shoe- betweenthese phasesaccounts for the ease tionship smith, 1978). Few SH- complexes of transition met- conversion of cubic FeS to mackinawite, cited of als are known, but CrSH2* has been reported FerS.but not FerS, rings, above.Cubic FeS contains (Ramasami and Sykes, 1976). If the initial polymeri- no Fe-Fe bonds. and there are zation step is a dimerization (2), then further associa- The structure of troilite is derived from the NiAs tion of the dimer is readily envisaged to lead to the Both Fe and S are six-coordinate,and iron type. layer structure of mackinawite by a simple tessella- are drawn together into triangular clusters. atoms tion, with formation of additional bonds. The topology of the structureis complex;both FerS, and FerS. rings can be discerned (Fig. 2) (Evans, Fer* + SH------+FeSH* (l) 1970). (

Fig. 3. (a) Part of the crystal structure of pyrite, viewed in projection down the a axis, showing the system of fused FerS3 rings. Bonds nearly vertical to the projection are omitted for clarity. The dashed lines delineate the unit cell. (b) Part of the crystal structure of marcasite, viewed in projection down the a axis, showing the system of fused Fe2S, and Fe2Sa rings. Some bonds are omitted for clarity, as in Fig. 3a. The dashed lines delineate the unit cell; b- and c-axis projections closely resemble Fig. 3a. TAYLOR: STEREOCHEMISTRY OF IRON SULFIDES to29

S bility relations of mackinawite. Neues Jahrb. Mineral. Monatsh., s-s I s-s 300-3M. /\S J, A. Caron and E. Goldish (1961). The crystal and \/\ Donohue, Fe Fe -' structure of 56 (sulfur-6). J. Am. Chem. Soc.' 83, re Fe Fe Fe molecular 's /\/ 3'148-f'l5l s s-s Evans, H. T., Jr. (1970) Lunar troilite: crystallography. Science, lg 167. 621-623. Sg - C. Milton, E. C. T. Chao, I. Adler, C. Mead, B. Ingram and R. A. Berner (1964) Valleriite and the new iron sulfrde, I ll lll mackinawite. II.S. Geol. Surv. Prof' Pap 475-D,64-69. Giggenbach, W. (1972) Optical spectra and equilibrium distribu- Fig. 4. Possible structures for an (FeS2)2 dimer tion of polysulfide ions in aqueous solution aI 2O". Inorg. Chem.. 1 l. 120l-1207 of the three-dimensional tessellation of such units leads di- Jones, P. E. and L. Katz (1967) The structure tris(pentasulphido) platinum (IV) anion. Chem' Commun., 842- marcasite structure (Fig. 3b). This in- rectly to the 843. volves the formation of additional Fe-S bonds, but Koepf, H., B. Block and M. Schmidt (1968) Di-z-cy- no further rearrangement of the structural net. clopentadienyltitanium (IV) pentaselenide and pentasulfide, High concentrations of S3- would favor the reac- two heterocyclohexachalcogens in fixed conformation. Cftenr. tion sequence (4) to (6), although not necessarily to Ber., 101, 272-216. Kullerud, G., and H. S. Yoder (1959). Pyrite stability relations in of pyrite precipitation. This is consis- the exclusion the Fe-S system Econ. GeoI.,54,533-5'12. tent with the observations that oxidizing conditions, M€dicis, R. de (1970a) Cubic FeS, a metastable iron sulfide. Sci- which favor polysulfide generation, also favor marca- ence,170, I l9l-l 192 site formation. It is noted that the equilibrium con- - (1970b) A new form ofiron sulfide: cubic FeS. Rev. Chim. centration of S3- in the system H,S-S-HrO is ex- Miner , 7,72f-728. Okamoto, S. (1968) Polymorphic transformation of sodium ortho- pH : 4 (Giggenbach, 1972; Teder, tremely low at fetites. Z. anorg. allg. Chem.,363,222-224. l97l). However, the same structural arguments apply Power, L. F. and H. A. Fine (1976). The iron-sulfur system. Part if the S, moiety is generated only as a surface species l. The structures and physical properties of the compounds of at the crystallization site, or if FeS, monomer is pro- the low-temperature phase fields. Minerals Sci. Eng., & 106-128. T and A. G. Sykes (1976) Further characterization duced by an alternative route. Ramasami, and aquation of the thiolopentaaquochromium (II! complex, not Unfortunately, iron sulfide formation is ame- CrSH2*, and its equilibration with thiocyanate' Inorg. Chem., nable to most probes which might test the hypotheses /J, l0l0-1014. put forward here. Some relevant information might R. S. Taylor and A. G Sykes (1976) Synthesis and charac- be obtained from a matrix isolation spectroscopic terization of the p-disulphido complexes CrS2Cro* and Chem. Commun, 383-384. study of the reactions of Fe atoms with S atoms and CrS2HFea*. Rickard, D. T. (1969) The chemistry of iron sulphide formation at S, molecules, or from the chemistry of other systems low temperatures. Stockholm Contributions in Geology, 20,67- in which similar polymorphism occurs. 95. Rising, B. A. (1973) Phase relations among Pyrite' marcasile and Acknowledgments pyrrhotite below 300"C. Ph.D. Thesis, Pennsylvania State Uni- I am gratefulto Drs. T. E. Rummery,D W. Shoesmith,and versity, University Park, Pennsylvania' A. G. Wikjordfor stimulatingdiscussions of thiswork. Rochow, E. G. (1973) Germanium. In J. C. Bailar, lt., et 4l', Eds', Comprehensive Inorganic Chemistry, Vol. 2' p l-41' Pergamon' References Oxford. (1973) Sulphur. In J. C. Bailar, Jr., et Berner,R. A. (1962)Tetragonal iron sulfide. Science,137,669. Schmidt, M. and W. Siebert Y ol. 2' p. 79 5-933. - (1964)Iron sulfidesformed from aqueoussolution at low al., Eds, Comprehensive I nor ganic Chemistry, temperaturesand atmosphericpressure. J. Geol.,72,293-306. Pergamon, Oxford. - (1967)Thermodynamic stability of sedimentaryiron sul- Schot, E. H., J Ottemann and P. Omenetto (1972) Some new ob- fides.Am. J. Sci.,265,773-785. servations on mackinawite and valleriite ' Rend. Soc. Ital. Min- Bloom, M. C. (1939)The mechanismof the genesisof polymorph- eral. Petrol., 28, 241-295 ous forms. Am. Mineral., 24,281-292 Shoesmith, D. W., T. E. Rummery, M. G. Bailey and D. G. Owen Brostigen,G. and A. Kjekshus(1969) Redetermined crystal struc- (1979) Electrocrystallization ofpyrite and marcasite on stainless ture of FeS2(pyrite). Acta Chem.Scand., 23,2186-2188. steel surfaces.J. Electochem' Soc', I26,9Il-919.

marcasitetype crystal structure.VIII. Redeterminationof the mation of ferrous monosulfide polymorphs during the corrosion prototype.Acta Chem.Scand., 27,2791-2796. of iron by aqueous hydrogen sulfide at 22"C. J' Eleclrochem. Buerger,M. J. (1934)The pyrite-marcasiterelation. Am. Mineral., Soc.,127, 1007-1015. /,9.3'l-61. Takeno, S. (1965) Thermal studies on mackinawite. J. Sci Hir- Clark, A. H. (1966)Some comments on the compositionand sta- oshima Univ., Ser. C,4,455478' 1030 TAYLOR: STEREOCHEMISTRY OF IRON SULFIDES

- and A. H. Clark (1967) Tetragonal (Fe,Ni,Co)1*.S, mack- aqueous polysulfide solutions. Acta Chem. Scand., 25, 1722- inawite. J. Sci. Hiroshima Univ., Ser. C, 5,28'7-293. 1728.

fide-with special reference to mackinawite . Am. Mineral., 55, fide (troilite) in aqueous sulfuric acid. J. Phys. Chem., 80, 1844- 1639-t649. 1848. Taylor,L.A.andL.W.Finger(l97l)Structurerefinementand composition of mackinawite. Carnegie Inst. Wash. Year Book, iron sulfides and their role in mass transport in Girdler-Sulfide 69,318-323. heavy water plants- Atomic Energy of Canada Limiled Report Taylor, P. and D. W. Shoesmith (1978) The nature of green alka- No. AECL-5960. Iine iron sulfide solutions and the preparation of sodium iron Ward,J. C. (1970)Thestructureandpropertiesof someiron sul- (III) sulfide, NaFeS2. Can. J. Chem., 56,279'l-2802. phides. Rev. Pure Appl. Chem., 20, l'15-2M. T. E. Rummery and D. G. Owen (1979a) On the con- Wikjord, A. G., T. E. Rummery, F. E. Doern and D. G. Owen version of mackinawite to greigite- J. Inorg. Nucl. Chem., 41, (1980) Corrosion and deposition during the exposure of carbon 595-596. steel to H2O-H2S solutions at elevated pressures. Corrosion Sci- '-- and - (1979b) Reactions ofiron monosulfide ence, rn press. solids with aqueous hydrogen sulfide up to 160'C. J. Inorg. Nucl. Chem., 41, 1683-168'1. tVanuscript received, October 24, 1979; Teder, A. (1971) The equilibrium between elementary sulfur and acceptedfor publication, May 5, 1980.