Mineral Spectroscopy; A Tribute to Roger G. Bums © The Geochemical Society, Special Publication No.5, 1996 Editors: M. D. Dyar, C. McCammon and M. W. Schaefer Spectroscopic studies of iron sulfide formation and phase relations at low temperatures ALISTAIRR. LENNIE and DAVID J. VAUGHAN Department of Earth Sciences, The University of Manchester, Oxford Road, Manchester M13 9PL, England Ahstract- The iron mono sulfides mackinawite (Fe1+xS) and "amorphous FeS," the first formed iron sulfides in aqueous systems at low temperatures, have been studied using X-ray photoelectron and X-ray absorption spectroscopies. The divalent oxidation state of Fe and the local structure environment of Fe and S in mackinawite have been confirmed. Comparisons of the Fe K-edge X-ray absorption spectra of synthetic mackinawite with those of "amorphous FeS" precipitates subjected to ageing for periods ranging from 1 to 1800 seconds, and precipitated under pH conditions between pH 3.9 and pH 7.4, show the precipitates to have local structure consistent with that found in crystalline mackinawite. Combining these data with results in the literature leads to the proposal of a model which accounts for the formation from aqueous solution of Cubic FeS, troilite, and macki- nawite. We use this model, which takes account of the effect of electron withdrawal by H+ at the crystal surface, to propose that these three phases can all be formed from Fe in tetrahedral coordina- tion with S. This model facilitates understanding of the phase relationships in the Fe-S system at low temperatures, whereby initial formation is of iron "monosulfides" with either cubic close- packed (ccp) or hexagonal close-packed (hcp) S sublattices. Subsequent sulfidation of either sub- lattice type will form pyrite (or marcasite). However, the predominant sulfidation sequence at low temperatures is: [Cubic FeSj/amorphous FeS ---> mackinawite ---> greigite ---> marcasite/pyrite. Both the nucleation of hcp S sub-lattice phases in low temperature aqueous iron sulfide systems, and the formation of hcp phases by transformation from the cubic monosulfides, are inhibited by kinetic factors. INTRODUCTION and, perhaps, in catalysing reactions leading to the build-up of complex organic molecules. THE MINERALS of the iron-sulfur system are im- At temperatures in excess of ~ 350°C, the phase portant Earth and planetary materials for a number relations in the Fe-S system are relatively straightfor- of reasons. Troilite (FeS) is an important phase in ward, but at lower temperatures the Fe-S system be- iron meteorites, suggesting that sulfur may coexist comes much more complex and less well understood. with iron in the core of the Earth. Iron sulfides, as In part, this arises from ordering of vacancies upon well as occurring as accessory minerals in many cooling of the pyrrhotite (Fel_xS) solid solution, form- common rocks, are major phases in the sulfide ore ing complex superstructures. A further complexity deposits that are still the main suppliers of many concerns phases found only in relatively low tempera- base (Cu, Pb, Zn), ferroalloy (Ni, Co, Mn), and ture environments in nature, which can be synthe- rare or precious (Ga, Ge, Ag, Pt) metals. Studies sized only in solution. For this reason, these lower of iron sulfide formation in Recent sediments via temperature phases may not even appear in the (equi- sulfate-reducing-bacteria, and in hydrothermal sys- librium) phase diagrams that have generally been tems presently active at or near mid-ocean ridges, constructed on the basis of experiments involving have stimulated new ideas regarding the role of reaction between elemental iron and sulfur in sealed sulfides in the geochemical cycling of the elements. evacuated silica capsules. Several important phases These studies have not only furthered understand- (notably marcasite, FeS2, and mackinawite Fel+xS) ing of the processes of formation of metal sulfide are of this type, along with a number of rarer minerals ore deposits, but have also suggested that metal (greigite Fe3S4 and, possibly, smythite ~ F~S 11) and sulfides may entrap heavy metal pollutants in sedi- phases so far found only in synthesis experiments ments. Furthermore, the discovery of life forms (e.g., Cubic FeS). It seems very likely that all of these adapted to the warm but anoxic environments of phases are metastable; indeed calculations of the free' sea-floor hydrothermal systems, and the known energies of formation of these phases confirm this sulfur-metabolizing ancient bacteria, has fueled a view (see Fig. I). Although some of these minerals debate that anoxic sulfide environments could have appear to be rare in their geological occurrence, they been the sites for the emergence of life on Earth. are of considerable importance as transient species This debate has centred around iron sulfides which produced when iron and sulfur react in aqueous solu- may have played a central role in supplying energy tions, subsequently transforming to the more stable 117 A. R. Lennie and D. J. Vaughan 118 -60-.---------------, formation of mackinawite, Cubic FeS and troilite. This work forms part of a series of contributions !::, -SO ;" to studies of the crystal chemistry and phase rela- S -100 tions of the iron sulfides (LENNIE,1994; LENNIEet :;;i at. 1995a, 1995b). Q' -120 ~ ~ -140 MACKINAWITE 2!, Mackinawite possesses a layer structure similar ~-160-ISO L~--,--__:_r:-__;~-?:~~ to that found in tetragonal lead oxide (see Table 1.0 1.2 1.4 1.6 I.S 2.0 1) with Fe and S occupying the sites of 0 and Pb, FeS FeS 2 respectively, in PbO. Here we investigate further Composition (FeS,) the oxidation state and local structure of Fe and S FIG. 1. Schematic free energy-composition diagram for in synthetic mackinawite. the iron sulfide minerals. The gain in free energy, ~fGO(298.l5 K), in kl.mol " (of Fe), is that obtained from forming the various sulfides from the elements. Open cir- Synthesis cles represent Amorphous FeS (1), mackinawite (2), grei- gite (3) and marcasite (4). Filled circles represent the Several different methods for synthesis of mack- pyrrhotites: Troilite (5), FellS 12 (6), FelOS11(7), Fe9SlO (8), inawite have been described in the literature. Fe7SB (9), and pyrite (l0). Data for 1 - 3 are calculated BERNER(1964) found that reaction of metallic Fe from solubility data of DAVISON (1991); for 4 and 10 with a saturated solution of H2S gave well crys- from CHASE et al. (1985) and for 5 - 9 from GRPNVOLD and STPLEN (1992). The dashed line is the join between tallised tetragonal FeS. Alternatively, mackinawite monoclinic pyrrhotite and pyrite; the solid line is the join has been prepared by reacting aqueous solutions between troilite and monoclinic pyrrhotite. Free energy of Fe(U) ions and HS- ions forming an X-ray amor- values for phases 1, 2, 3, and 4 lie above these lines phous precipitate which, even on ageing, gives a suggesting metastability for these phases at this poorly crystalline mackinawite. temperature. In the present work, sulfidation of a commercial iron wire (Roncraft, Sheffield) was used to produce crystalline mackinawite. Here, a solution of Na2S O.S phases (BONEVet al., 1989). The keys to understand- was added slowly to an molar acetic acid/ace- ing these phases are concerned with their mechanisms tate buffer (pH = 4.6) in which approximately 4- of formation in solution, and with relationships to Sg of finely divided iron wire had been previously other iron sulfides, as determined by the kinetics and immersed. Dissolution of the iron wire by reaction mechanisms of transformations. The known iron sul- with the acetate buffer for 30 minutes or so evolved fide minerals, their structures and stabilities are sum- H2, providing a reducing solution environment. marized in Table 1. Detailed accounts of the mineral The Na2S solution was then added slowly, and ex- chemistry of these sulfides are provided by WArm ceeded the capacity of the acetate buffer, increasing (1970), POWERand FINE (1976), VAUGHANand the pH to a value of approximately pH = 6.S. This CRAIG(1978), VAUGHANand LENNIE(1991). solution, containing the iron wire, was allowed to Formation of iron-sulfur phases is affected by stand for 24 hours. The generated H2 trapped in the d-electron configuration of Fe, resulting in a the wool matte floated the iron to the top of the more complex range of structures than would be solution: Mackinawite forming on the iron surface otherwise expected. As an example of this, the for- spalled off and dropped to the bottom of the flask. mation of (metastable) marcasite (FeS2) from After reaction, the remaining iron wool was re- acidic aqueous solution has been explained by the moved, the supernatent solution poured off, and effect of bonding of H+ at the crystal surface on the mackinawite carefully rinsed then dried under the Fe 3d electronic structure. We were interested vacuum. Samples were sealed under vacuum in in examining the formation of the reduced phase(s) glass tubes until required for further analysis. to determine if electron withdrawing effects might also be responsible for formation of Cubic FeS and Crystal structure troilite, both of which, like marcasite, show pH- dependant formation. The mackinawite synthesised as described above In the present article, we examine the monosul- was examined using high resolution X-ray powder fide mackinawite and explore the properties of diffraction from which a Rietveld structure re- "amorphous FeS" as a key to understanding the finement was obtained. The structural parameters Iron sulfide formation 119 Table 1. The iron sulfides, their structures, stabilities and natural occurrence' Composition Crystal structure type Stability:j: Natural occurrence mineral structural data t FeS2 pyrite-type (cubic) The most abundant sulfide. Found as an accessory pyrite Pa3; a= 5.42 mineral in many igneous, metamorphic and sedi- mentary rocks including coals.