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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 marca- site, 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 mackina- wite 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 mackina- wite (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). (<l pm) The easeof precipitation of fine-grained 2FeSH*-------+FerS, + 2H* (2) mackinawite demonstratesthat homogeneousnucle- ation of this phaseis facile. This nucleation presum- (l), ably proceedsby complexationof Fe2* and SH- nFerS2-------+ mackinawite (3) Although FerS, rings are alsopresent in the troilite structure, the greater complexity of their inter- connectionmay account for the evident difficulty of troilite nucleation. Analogous situations, in which phaseswith high coordinationnumbers and complex structural networks are more difficult to crystallize than metastablepolymorphs with sirnpler structural networks,are common, e.g.,GeOt (Rochow, 1973,p. 26), NaFeO, (Okamoto,1968). When troilite occursas a corrosionproduct on iron Fig. l. (a) Projection of four unit cells of the crystal structure of or carbon steel in aqueousHrS at low temperatures, mackinawite down the c axis. Large open and small closed circles it appearsto arisefrom high local iron concentrations represent S and Fe atoms, respectively; this convention is used in at the corroding surface (Shoesmithet al., 1980).A (b) all figures. Dashed lines delineate Fe-Fe interactions. trimeric precursor,FerSr, may be involved in its nu- Projection of the crystal structure of cubic FeS down the a axis. Broken bonds proceed to Fe atoms above and S atoms b€low cleation. those shown. The absenceof closeFe-Fe contactsin cubic FeS TAYLOK STEREOCHEMISTRY OF IRON SI]LFIDES probably resultsin a low activationenergy for its nu- three-dimensionalnetworks of interJinked Fe and S, cleation, but its low stability confinesits occurrence moieties(Brostigen and Kjekshus, 1969;Brostigen e/ to conditions ofhigh supersaturationand short reac- al., 1973 and referencestherein). In each structure, tion times. Like troilite. its formation as a corrosion iron has approximateoctahedral coordination by sul- product is associatedwith high local supersaturation. fur, and sulfur has a distorted tetrahedralcoordina- There is evidencefor competition betweenthese two tion by three iron atoms and the secondsulfur atom phasesas sinks for dissolvediron at corroding sur- of the S, unit. The two structuresdiffer in their net- faces(Shoesmith et al., 1980). work geometries,and can be describedin terms of fused Fe-S ring systems(Rising, 1973). I ron disulfides--ayrite and marcasite The pyrite structureconsists entirely of FerS, rings, Both pyrite and marcasitehave compositionsvery I (Figs. 3a and 4), whereasmarcasite contains FerS, closeto FeSr*. Buerger(1934) suggested that an ap- and Fe,Sorings, II and III (Figs. 3b and 4) as well. parent slight sulfur-deficiency of marcasite (ca. There is no significantFe-Fe bonding in either struc- FeS,,rr) might be related to its mechanismof forma- ture. Clearly, a crystallizationmechanism which fa- tion. However, Kullerud and Yoder (1959) con- vors the formation of rings II or III will favor forma- cluded that the two phases are true dimorphs of tion of marcasite rather than pyrite, as outlined by FeSr-. Although the possible occurrenceof a low- equations(a) to (6). temperature stability field for marcasite cannot be Fe't + S3--------- *Fe-S-S- (4) completelyruled out, there is evidencethat it is often formed under conditions where it is metastablewith 2 *Fe-S-S- ----+ II or III (5) respectto pyrite. It may well be metastableunder all ------+ (6) conditions (Rising, 1973). n(II or III) marcasite Recent work by Shoesmithet al. (1979),Taylor et Although (FeSr), could have any of the structures al. (1979b),and Wikjord et al. (1980)showed that py- I to III, III is
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