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. The major sulfide in many massive, bedded or vein type ores. FeS2 marcasite-type (orthorhombic) metastable marcasite Pnnm; a = 4.44; b = 5.41; Occurs as a primary mineral in certain sediments c = 3.38 and lower temperature hydrothermal deposits. Also formed through weathering of other sulfide minerals.

spinel-type (cubic) metastable ?Fd3m; a = 9.88 A very rare mineral reported from Recent sediments and low temperature hydrothermal deposits. -Fe9SIl(?) ?pseudorhombohedral A very rare (?) mineral reported from magmatic smythite a = 3.47; c = 34.4 and hydrothermal sulfide ore deposits. -Fe7SB NiAs-type; superstructure (mono.) ?stable As an important mineral in sulfide ores of monoclinic F2/d; a = 11.90; b = 6.86; -254°C "magmatic' origin and an accessory mineral in pyrrhotite c = 22.79; f3 = 90°26' ultramafic and mafic rocks. Also in metamorphosed massive sulfides and some hydrothermal ores. Fe9SlO NiAs-type; superstructure ?stable intermediate (hexagonal) (? - 100°C) pyrrhotite (5C) a = 6.88; c = 28.70

FewS II NiAs-type; superstructure ?stable intermediate (orthorhombic) Cmca or C2Ca; (?- 100°C) pyrrhotite (llC) a = 6.89; b = 11.95; c = 63.18

FeIIS12 NiAs-type; superstructure ?stable intermediate (hexagonal) (?-I00°C) pyrrhotite (6C) a = 6.89; c = 34.48

-Fe9Sw-FeIlS12 NiAs-type; superstructure ?stable non-integral (orthorhombic or monoclinic); (?-2200C) pyrrhotite (nC) a = 6.89; b = 11.95; c = variable

FeS NiAs-type (distorted super- stable Found in iron meteorites and lunar rocks. Rare on troilite structure) (hexagonal) (<138°C) Earth but found in some serpentinized ultramafic P62c; a = 5.97; c = 11.75 rocks. .

Fel+xS PbO-type (tetragonal) metastable (x = 0.01-0.08) P4lnmm; a = 3.60; c = 5.03 As the first-formed very fine grained black sulfide mackinawite precipitated in Recent sediments. Also as small grains and lamellae within other sulfides (pyrrhotite, chalcopyrite etc.) from a variety of environments. Fel+xS Sphalerite-type (cubic) metastable Known only from synthetic studies at present and (x = 0.03-0.10) F43m; a = 5.42 cubic FeS (synthetic) formed as a layer on sulfidized iron surfaces.

* Data from Vaughan and Craig (1978), Kissin and Scott (1982) t Information given is on structure type (crystal class); space group; unit cell parameters (in A) :j: Upper temperature of stability in parentheses

obtained from this refinement, reported elsewhere erating at a base pressure of less than 1 x 10-9 T. (LENNIE et al., 1995b), are given in Table 2. Removing mackinawite from the glass tube and subsequent transfer into the spectrometer vacuum X-ray photoelectron spectra chamber was carried out under Oj-free N2 condi- tions. Narrow region XPS spectra were recorded X-ray photoelectron spectra were obtained on a at an analyser pass energy of 20 eV. The spectrom- Kratos XSAM 800 surface analysis system op- eter was calibrated using the peaks: Au (4f7l2) 84.00 120 A. R. Lennie and D. J. Vaughan

Table 2. Refined structure parameters for synthetic mack- with the presence of iron hydroxide on the surface inawite. Errors (a) are in parentheses (after LENNIEet al., of mackinawite (McINTYRE and ZETARUK, 1977). 1995b) The pyrite XPS Fe (2p3/2) spectrum binding en- Space group P41nmm (No. 129) origin at 4m2 ergy is 707.2 eV, and the S(2p) spectra were fitted with symmetric Gaussian 50%, Lorentzian 50% Cell parameters (Rietveld) a (A) 3.6735(4) b(A) 3.6735(4) curves at 162.5 eV and 163.7 eV respectively. Fol- c(A) 5.0328(7) lowing Ar" etching, the Fe (2p3/2) peak showed V(A3) 67.914(24) considerable broadening and increased asymmetry towards the higher binding energy side of this peak. Atomic positional parameters x y z B(iso) The S(2p) spectrum showed formation of addi- 0.41(2) Fe in 2(a): 0 0 0 tional peaks at lower binding energies which were S in 2(c): 0 0.5 0.2602(3) 0.20(2) fitted with symmetric Gaussian 50%, Lorentzian Bond distances (A) 50% curves at 161.3 eV and 162.5 eV respectively. 2.2558(9) Fe-S: These changes in Fe and S photoelectron peaks are Fe-Fe: 2.5976(3) consistent with the partial reduction of S~- to S2- species, and partial oxidation of Fe(U) which would occur upon reduction to pyrrhotite. The S(2p3/2) peak of pyrite (S-) has a higher binding energy (162.5 eV) than that observed for S (S2-) in

eV, Cu (2p3/2) 932.66 eV, Ag (3ds/2) 368.27 eV mackinawite (161.85 eV), consistent with observed with Mg Ka X-rays (1253.6 eV) as the exciting trends which show increasing binding energy with radiation, and the static charge effect was corrected increase in formal oxidation state. using adventitious C with the Is electron binding The binding energy may also reflect differences energy of C taken as 284.6 eV. For comparison in the lattice site environment between two differ- with mackinawite, XPS measurements were also ent phases of the same oxidation state. In the case made on a natural sample of pyrite cleaved under of mackinawite, S (S2-) is coordinated to four Fe Oz-free N2 conditions prior to installation in the atoms in a pyramidal arrangement, with a lone pair spectrometer. A succession of spectra of the of electrons at the apex of this pyramid. In contrast, cleaved pyrite were taken before, and after, a series S in pyrrhotite (S2-) is coordinated to 6 Fe atoms of Ar" etches of the surface. Fig. 2 shows the Fe, in a trigonal prism arrangement. The binding en- Sand 0 XPS spectra obtained for synthetic macki- ergy of S in mackinawite (161.85 eV) is greater nawite. These spectra are fitted using a Shirley than the binding energies reported for pyrrhotite background (SHIRLEY, 1972). phases (161.1 - 161.25). This suggests a slight de- The Fe (2p3/2) peak of mackinawite has a binding crease in the electronic population of the S valence energy of 707.25 eV, similar to that of pyrite (707.2 band(s) for mackinawite compared with that of the eV), which provides confirmation of the Fe(I!) oxi- S valence band(s) for the pyrrhotites. dation state predicted for mackinawite. The macki- A compilation of S (2p312)and Fe (2p3/2) binding nawite Fe(2p3/2) peak shows considerable asym- energies reported in the literature for Fe-S miner- metric broadening at the higher binding energy side als, together with those obtained for pyrite and of this peak, which may arise from the high con- mackinawite from our experiments, is presented in ductivity (WERTHEIMand BUCHANAN,1977) in the Table 3. (001) plane of mackinawite due to Fe-Fe interac- tions inferred by KJEKSHUSet al. (1972). X-ray absorption spectra Following background subtraction, the S (2p) photoelectron spectrum of mackinawite has been X-ray absorption spectroscopy (XAS) and the fitted with a doublet in a 2:1 ratio to match the 2:1 analysis of the fine structure in spectra at and near intensity ratio of the spin-orbit splitting of S the absorption edge (so called 'XANES'), and in 2(P3/2) and 2(P1l2) peaks. These have been fitted the energy region extending well above the edge with asymmetric curves made up of symmetric (so-called 'EXAFS'), have been very comprehen- 80% Gaussian, 20% Lorentzian components. The sively described in the literature (see CALASet al. binding energies of the fitted doublet are 161.82 1984, 1990; BROWNet al., 1988; WAYCHUNASand eVand 163.02 eV for the 2(P3/2) and 2(pl/2) peaks BROWN, 1984, for excellent reviews). Here only respectively. The 0 (1s) spectrum in Fig. 2 shows specific details of our experiments and their inter- the small fraction of 0 in the sample of macki- pretation will be discussed. nawite. The peak position (532 eV) is consistent Mackinawite was examined by X-ray Absorp- Iron sulfide formation 121

(a) (c)

164 162 160 Binding Energy (eV)

720 710 700 Binding Energy (eV) (d)

164 162 160 540 535 530 Binding Energy (e V) Binding Energy (e V)

FIG. 2. XPS spectra of synthetic mackinawite: (a) Fe (2p) spectrum and (b) S (2p) spectrum. (c) The S (2p) spectrum following background subtraction and fitted with a doublet of asymmetric curves at 161.82 eV(2p3/Z) and 163.02 eV (2pl/z). (d) 0 (Is) spectrum: the small fraction of oxygen in the synthetic material, due to surface hydroxide species, is indicated by the low signal to noise ratio.

Table 3. XPS binding energies of S (2P3n) and Fe (2P3/Z) found in iron-sulfur minerals, including results from this study

Species Mineral Binding energy (eV) Reference FeS2 (pyrite) 707 BUCKLEY and WOODS (1987) 707.2 this study Fe,+xS (mackinawite) 707.25 this study FeO.89S (pyrrhotite) 708 BUCKLEY and WOODS (1985) Fe7S8 (pyrrhotite) 707.45 PRATT et al. (1994) Ar" reduced FeS2 surface 707.8 this study

S(O) 163.7 HYLAND and BANCROFT (1989) FeS2 (pyrite) 162.3 BUCKLEY and WOODS (1987) 162.4 HYLAND and BANCROFT (1989) 162.5 this study Fel+,S (mackinawite) 161.82 this study FeO.89S (pyrrhotite) 161.1 BUCKLEY and WOODS (1985) Fe7S8 (pyrrhotite) 161.25 PRATT et al. (1994) Ar" reduced FeS2 surface 161.24 this study 122 A. R. Lennie and D. J. Vaughan tion Spectroscopy using synchrotron radiation to by R(min), the value of the fit index at predicted establish the local enviroment around Fe and S, minimum (JOYNERet al.,1987). 3 and to provide model XAS data for amorphous iron Fig. 3 shows k weighted spectra from EX- sulfide precipitates. To obtain Fe K-edge absorp- CURV92 for S and Fe of mackinawite plotted tion data, a solid synthetic mackinawite sample was against the inverse wave-vector, k, and Fourier pressed into Al samples holders with adhesive-tape transforms of these spectra giving the radial dis- windows and cooled under vacuum using a liquid tance from the central scattering atoms. Data ob- N2 cryostat. The spectrum for Fe was collected in tained from the iterative fitting of these spectra are transmission mode on Station 7.1 at the Daresbury given in Table 4. Both S K-edge and Fe K-edge Synchrotron Radiation source, operating at 2 Ge V spectra have been fitted with three shells. The with an average current of 150 rnA. A Si(ll1) model set up for the S K-edge data uses an approxi- double-crystal monochromator was used with the mation for the second and third shells-the second crystal faces offset to give 50% rejection of the shell was modelled by 12 S atoms at 3.62A. (instead beam in order to reduce harmonic contamination. of 4 S at 3.548A., 4 S at 3.674A. and 4 S at 3.688A.); Ion chambers to detect the incident and transmitted the third shell by 12 Fe atoms at 4.26A. (not 4 Fe radiation were filled with Ar-He mixtures to absorb at 4.153 and 8 Fe at 4.311). Distances and coordi- 20% and 80% of the beam, respectively. nation numbers for initial fitting were calculated The spectrum for S was taken on Station 3.4 from the structural refinement of mackinawite pre- using a Ge(111) double crystal monochromator; a sented in Table 2. focussing mirror is used to remove higher harmon- The results of these iterative fits to the Fe K- ics. The sample of synthetic mackinawite was di- edge and S K-edge EXAFS spectra of mackinawite luted with graphite and coated as a slurry in acetone are consistent with the structure determined by onto aluminium stubs. Following evaporation of X-ray powder diffraction, and confirm both the tet- the acetone, the stubs were mounted in the spec- rahedral coordination of Fe by S, and the short Fe- trometer and the chamber evacuated. Spectra over Fe distances in the (001) plane of mackinawite. the S K-edge were taken using photo-electron yield AMORPHOUS IRON SULFIDE ("FeS") as the detection technique. In the case of the S spectrum, four separate scans were summed to im- The material initially formed when dissolved Fe prove the signal to noise ratio. and S react in solution is an extremely fine-grained Spectra were prepared for analysis by back- "X-ray amorphous" black precipitate. Such amor- ground subtraction of pre-edge and EXAFS regions phous iron sulfide precipitates, which are easily of the spectra, followed by normalisation using the prepared in the laboratory, form in nature by reac- Daresbury programs, EXCALIB and EXBACK tions taking place in Recent sediments, principally 3 (DAIKUN et al., 1984). Standard k weighting was during processes that take place just beneath the used for the EXAFS spectra. The data were ana- surface of certain fine sediments during early dia- lysed using single-scattering curved wave theory genesis. Here, the most common interactions in- in the program EXCURV92 (BINSTEDet al., 1991; volve sulfide S derived from reduction of seawater LEE and PENDRY, 1975; GURMAN et al., 1984, sulfate by bacteria, and Fe from detrital or other 1986). Phase shifts are derived from ab initio calcu- sources. lations 'within EXCURV92 using the default pa- These fine grained materials are difficult to study rameters for the muffin tin radii and exchange using techniques such as X-ray diffraction or elec- terms. The phase shifts were not further refined tron microprobe analysis. In our work, we have using model compounds. used X-ray Absorption Spectroscopy employing . A fit was produced by generating a theoretical synchrotron radiation to study the structure of these spectrum from a structure of several shells of back- precipitates. This work has involved attempts to scatterers around the central atom, and allowing study such precipitates by quenching in liquid N2 parameters refining radial distance from the central immediately following nucleation, after controlled scattering atom and Debye-Waller factors within periods of "ageing", and following buffering in a each shell to vary iteratively to give the best range of solution pH's. agreement with the experimental data. Coordina- tion numbers were kept fixed for analysis of syn- FeS Amorphous Precipitates; thetic mackinawite, and only those shells that made Ageing Experiments a significant improvement to the fit were included in the simulations. The goodness of fit of the theo- A series of FeS precipitates were prepared as retical spectrum to the experimental data is given follows. Equal volumes of an Fe acetate solution Iron sulfide formation 123

(a) (c) 15 12 10 ':1 8 >< 11\ ':1 CI) l' >< ~ CI) < '\,9 Yi ><: ~ \j \ Vir ----/ ~ >Il -5 \,_li ~ -: -10 -8'v -12 -15 i i i i 4 2 3 4 5 6 7 8 9 10 1 (b) (d) k(k ) 1.6

0) 1.6 'd -e0) ~ S i oj § ~ ~ '"oj ~ ~~'" l 10 5 6 7 8 9 10 r (A) rCA)

FIG. 3. (a) The k3-weighted S K-edge EXAFS spectrum of mackinawite (298 K). (b) The Fourier transform of the S K-edge EXAFS spectrum fitted with three shells (as listed in Table 4). (c) The 3 k -weighted Fe K-edge EXAFS spectrum of mackinawite (Iiq. N2 cooled). (d) The Fourier transform of the Fe K-edge EXAFS spectrum fitted with three shells (as listed in Table 4.). Solid lines, experimental data; dashes, data simulated in EXCURV92.

Table 4. Results of refinements of EXAFS S K-edge and Fe K-edge spectra for mackinawite

Central Atom Shell Element r(A) A: 20'2(A2) N rXRD(A) NXRD

S (298 K) 1 Fe 2.24 0.01 4 2.255 4(Fe) [R(min) = 14.9] 2 S 3.62 0.03 12 3.548 4(S) 3.674 4(S) 3.688 4(S) Fe 4.26 0.04 12 4.153 4(Fe) 4.311 8(Fe) Fe (298 K) 1 S 2.26 0.03 4 2.255 4(S) [R(min) = 7.7] 2 Fe 2.56 0.03 4 2.598 4(Fe)

Fe (liq N2) 1 S 2.24 0.01 4 2.255 4(S) [R(min) = 6.9] 2 Fe 2.59 0.02 4 2.598 4(Fe) 3 Fe 3.68 0.03 4 3.674 ·4(Fe)

r = radial distance of fitted shell from central atom; A = Debye-Waller factor N = coordination number; R(min) is the value of the fit index at predicted minimum rXRD= radial distance from central atom calculated from XRD data in Table 2 NXRD= coordination number of element (in brackets) calculated from XRD data in Table 2 124 A. R. Lennie and D. J. Vaughan

(0.05 molar) and a 1:1 mixture of IN NaOH and ity of data obtained for 10 and 20 seconds ageing a solution 0.21 molar in Na2S.9H20 were injected time is not sufficient to provide second shell infor- at the same time directly into polyethylene tubes mation for these samples. sealed at one end. The solution pH above the pre- cipitates thus formed was 7.5. After measured time FeS Amorphous Precipitates; pH Buffering periods (1, 5, 10, 20, 60, 300 and 1800 seconds), Experiments the tubes containing the precipitates were plunged A series of 'FeS' solids were precipitated, then into liquid nitrogen to quench the reaction. Rapid buffered as follows at different pH values. Iron quenching also limits the formation of ice-crystals acetate solution (1.6 mls, 0.05 M) was added to 3 which would give Bragg reflections in the absorp- mls of a solution 0.21 molar in Na2S.9H20. This tion experiments. Acetate was selected as the anion mixture gave a black amorphous FeS precipitate; in these experiments to prevent confusion with sul- the resulting solution pH was 4.5. Buffering to fide during EXAFS analysis. Cl as an anion was higher pH values, as determined by pH meter, was avoided as this has similar X-ray scattering proper- achieved by adding drops of a IN NaOH solution. ties to S, being close in atomic mass. The lower pH (3.9) was achieved by addition of At Daresbury, the tube containing the frozen pre- acetic acid. cipitate was cut to provide a 2.5 em length of tubing The precipitate, with buffering solution, was in- plus precipitate. The frozen precipitate contained jected into a polyethylene tube sealed at one end, within this length was removed and installed in an held for 15 mins. at 298 K, then plunged into liquid aluminium holder which was placed in a liquid N2 N2 to quench the reaction. Spectra over the Fe K- cryostat and evacuated to 10-4 mbar. Fe K-edge edge were then taken on Station 8.1 as described XAS measurements were made in fluorescence in the previous section. No S K-edge EXAFS spec- mode on Station 8.1 by detecting fluorescence us- tra have been taken of the pH buffered or quenched ing the Canberra solid state detector. amorphous precipitate experiments as the Station These spectra, the XANES region for which are 3.4 spectrometer design prevents analysis of frozen shown in Fig. 4, have been analysed using EX- precipitates. CUR V92 as described above. The coordination Data obtained from analysis of these Fe K-edge number has also been allowed to vary during itera- spectra using EXCURV92 are given in Table 6. tion. Data obtained from these fits are given in Again, the coordination number has been allowed Table 5. These show that a precipitate is rapidly to vary during iteration. The XANES region of formed which has Fe tetrahedrally coordinated by these spectra are shown in Fig. 5. These precipi- a first shell of S, and a second shell of Fe atoms tates show ordering after 15 minutes reaction suf- with short distances (2.59A) from the central Fe ficient to provide second shell information, and in- atom. These radial distances are corisistent with dicate formation of a mackinawite-like structure formation of a mackinawite-type structured phase for precipitates buffered at pH's of 4.5 and above. as early as 1 second after intial reaction. The qual- The results for the precipitate buffered to pH 3.9, which has been fitted with two S shells surrounding the Fe atom, may have been affected by partial oxidation of sulfide during addition of acetic acid to achieve the required pH. The data obtained from these pH buffered pre- cipitates do not show obvious changes in coordina- tion of Fe by S between ph 4.5 and 6.5; there is no evidence of 6-fold coordination of Fe by sulfur in these amorphous precipitates. We will refer to this result in the following discussion of a possible mechanism of formation for the phases Cubic FeS and troilite in the region of pH 4-5.

METASTABLE Fe-S PHASE RELATIONS -40 -20 20 40 60 80 100 Energy (eV) Formation of Cubic FeS and troilite from FIG. 4. Edge region Fe K-edge fluorescence spectra of aqueous solution iron sulfide precipitate ageing experiments. The energy Cubic FeS, a metastable synthetic iron sulfide scale (eV) is set so that the Fe K adsorption edge is at 0 with the sphalerite structure, first reported by eV. Iron sulfide formation 125

Table 5. Results of refinements of Fe K-edge EXAFS spectra for quenched reaction time iron sulfide precipitates FeS precipitate Shell Element r(A) A: 20-2 (A2) N 1 second 1 S 2.24 0.02 3.8 [R(min) = 44.3] 2 Fe 2.59 0.02 2.0 5 seconds 1 S 2.22 0.02 3.8 [R(min) = 38.1] 2 Fe 2.57 0.02 2.8 10seconds 1 S 2.26 0.01 [R(min) = 63.5] 3.0

20 seconds 1 S 2.25 0.01 [R(min) = 46.5] 3.4

60 seconds 1 S 2.20 0.02 3.9 [R(min) = 52.7] 2 Fe 2.59 0.03 3.5 300 seconds 1 S 2.23 0.01 4.0 [R(min) = 54.3] 2 Fe 2.68 0.02 2.5 1800 seconds 1 S 2.21 0.02 4.0 [R(min) = 39.7] 2 Fe 2.62 0.02 2.8

r = radial distance of fitted shell from central Fe atom; A = Debye-Waller factor N = coordination number; R(min) is the value of the fit index at predicted minimum

DE MEDICIS (1970 a, b), was synthesised by re- SHOESMITH et al. (1980) showed that formation K acting Fe metal at 298 with aqueous H2S solu- of both troilite and Cubic FeS from stainless steels tions having pH values between 4.0 and 4.5. TAK- corroded by aqueous sulfide solutions at 294 K ENO et al. (1970) synthesised Cubic FeS by reacting shows similar dependance on solution pH. Cubic Fe metal with H2S in solution at 323 K and report FeS formed in greater quantities than troilite with X-ray powder diffraction data for Cubic FeS syn- the quantity of troilite and Cubic FeS formed being thesised in solutions with a pH of 5.7. These au- greatest at pH's between 4 and 5. At higher pH, thors observed that euhedral troilite (and anhedral mackinawite is the predominant iron sulfide mackinawite) are also formed under the acidic re- formed while at lower pH, all these iron monosul- ducing conditions required for Cubic FeS fide phases readily dissolve. MUROWCHICK and formation. BARNES (l986a) found similar pH dependant distri-

Table 6. Results of refinements of Fe K-edge EXAFS spectra for pH buffered iron sulfide amorphous precipitates FeS precipitate Shell Element r(A) A: 20-2(A2) N pH 3.9 1 S 2.20 0.01 4 [R(min) = 42.6] 2 S 2.37 0.01 2 pH 4.5 1 S 2.25 0.01 4 [R(min) = 46.1 2 Fe 2.67 0.02 3 3 S 4.24 0.02 8 pH 5.4 1 S 2.19 om 4 [R(min) = 48.7] 2 Fe 2.61 0.02 4

pH 6.5 1 S 2.25 om 4 [R(min) = 40.3] 2 Fe 2.68 0.03 4 pH 7.4 1 S 2.23 0.01 4 [R(min) = 38.5] 2 Fe 2.65 0.03 2.7

r = radial distance of fitted shell from central Fe atom; A = Debye-Waller factor N = coordination number; R(min) is the value of the fit index at predicted minimum 126 A. R. Lennie and D. J. Vaughan

troilite formation. We first refer to a study by Tos- SELL et al. (1981) which accounted for pH de- 2.0 pendant formation of marcasite by suggesting that .~ interaction of surface bound H+ with electrons in .~ one of the 17r; molecular orbitals of S~- would ~ effectively remove 2 electrons from the M3dO'- ~ 1.0 A2l7r; system, distorting the S geometry around u: each Fe. Metastable marcasite would thus be formed in preference to stable pyrite. We propose that a similar electronic effect may

-40 -20 20 40 60 80 100 also be occurring in the formation of Cubic FeS Energy (eV) and troilite under acidic conditions. At pH's be- FIG. 5. Edge region Fe K-edge fluorescence spectra of tween 4-5, H+ ions bound to S at the surface of pH buffered iron sulfide precipitates. The energy scale the iron mono sulfide would act to withdraw elec- 6 (eV) is set so that the Fe K adsorption edge is at 0 eV. trons from Fe 3d orbitals. Ferrous ions (3d ) would assume 3ds character, thus making this Fe isoelec- tronic with Mn2+. It has been observed that in the Mn-S system, two metastable phases, /3-MnS and butions of troilite and Cubic FeS formation when y-MnS, form by precipitation with H2S gas from Fe metal was sulfidised by aqueous H2S in experi- aqueous manganous solutions (FURUSETH and ments carried out in solutions covering a pH range KTEKSHuS,1965). These phases have sphalerite and from 2 to 7, and between 328 K to 366 K. wurtzite structures respectively: there are no NiAs- Although troilite and above 413 K, stoichiomet- type or tetragonal PbO-type structures found in the ric hexagonal pyrrhotite, are considered stable phases in the Fe-S system, formation of the less S- MnS system. If Fe ions at the surface of a FeS crystal growing rich pyrrhotites from aqueous solution under equi- s librium conditions (hydrothermal synthesis) is pre- in acidic solution are assumed to have 3d character vented due to the unmanageably high H2 pressures as proposed above, formation of FeS under these conditions may be expected to result in iron mono- generated by dissociation of H20 at low S fugacity (KISSIN and SCOTT, 1982). Troilite is therefore sulfides having sphalerite and wurtzite structures, formed under non-equilibrium conditions in the as found for /3~MnS and y-MnS. Cubic FeS formed above experimental studies. under acidic conditions has the sphalerite structure, Cubic FeS spontaneously transforms to macki- in agreement with this theory. However, there are nawite at room temperature. SHOESMITHet al. no reports in the literature of wurtzite-type FeS (1980) measured the rate of this solid state transfor- formed under acidic conditions. Instead, it is troilite mation at 298 K using changing intensities of (with a distorted NiAs-type structure) which is ob- X-ray diffraction peaks, and proposed a mechanism served under these conditions. involving migration of Fe atoms for this transfor- We note however that both NiAs and wurtzite- mation. Although no measurements have been type structures have a hexagonal close-packed made of the temperature dependence of the rate of anion sublattice, as shown in Fig. 6. The arrange- this transformation, the observation that Cubic FeS ment of Fe (in hexagonal pyrrhotite/troilite) and does not persist above 366 K (MUROWCHICKand Zn (in wurtzite) are different; Fe occupies octahe- BARNES, 1986a) implies extremely rapid transfor- dral sites in troilite, whereas in wurtzite, Zn atoms mation at or above this temperature. There are are in tetrahedral sites. small deviations from a stoichiometric formula of By analogy with Cubic FeS, which transforms FeS reported in the literature for both Cubic FeS spontaneously to mackinawite by rearrangement of and mackinawite, which may indicate either iron Fe, a rapid solid-state transformation at the crystal excess or sulfur deficiency in these phases. How- surface from a (hypothetical) wurtzite-type FeS ever, the crystallographic origin of these composi- structure to FeS-troilite, after the removal of elec- tional variations has not been definitely established. tron withdrawing H+ ions from the surface, would explain both the absence of a wurtzite-type FeS Proposed mechanism of formation of Cubic FeS and the presence of troilite in the experiments re- and troilite from solution ferred to above. The degree of rearrangement of We now propose a mechanism to explain the Fe required for transformation of a wurtzite-type similar dependance on pH of both Cubic FeS and FeS to troilite would be much smaller than that Iron sulfide formation 127

ascribed to strong deviation of the S environment from cubic to pyramidal symmetry, and to direct S-S contact between Fe-carrying sheets. These (a) changes in electronic and crystal structure thus give mackinawite greater stability than Cubic FeS. For the purposes of electronic structure modeling, WELZ and ROSENBERG(1987) assumed stoichio- metric compositions for both Cubic FeS and mackinawite. BUfIDETT (1980) uses fragment formalism to view the structure of some simple ('AX') solids in terms of the packing of planar hexagonal sheets and their puckered analogues. The wurtzite struc- ture, often regarded as a metastable version of the sphalerite arrangement, is, in general, found for four-coordinate structures with the largest electro- (b) negativity difference (llx) between A and X. Bur- dett finds that the energy difference between these two AX structures decreases as the electronegativ- ity of X increases. As llx increases further, the rocksalt structure is produced, as shown by Pearson diagrams (PEARSON, 1962). FIG. 6. Crystal structure diagrams for (a) wurtzite, the proposed structure for the metastable hexagonal FeS pre- Solids having more than 8 valence electrons per cursor, and (b) the NiAs structure which forms the basis AX unit adopt structures that may be understood of troiJite and hexagonal pyrrhotite. In (a), open circles via bond-breaking processes of the electron-precise represent S while filled circles represent Zn (Fe). In (b), wurtzite Structure (BURDETT, 1980). Thus, Burdett open circles represent As (S), while filled circles represent Ni (Fe). argues that the NiAs-type structure can be gener- ated by considering the effect of adding electrons to an 'AX' puckered sheet structure. Following these arguments presented by Burdett, we propose that for transformation of Cubic FeS to mackinawite introduction of additional electrons into the 8 (see Fig. 6). This proposed transformation would electron structural framework of (hypothetical) therefore be much more rapid than the observed, 'wurtzite-type FeS ' is the driving force for trans- spontaneous, transformation of Cubic FeS to formation to a NiAs-type FeS. mackinawite. Thus our model for iron monosulfide formation Spontaneous transformation of Cubic FeS to can be used to explain the origin of all three differ- mackinawite (and of a wurtzite-type FeS to troilite) ent FeS crystal structures (troilite, Cubic FeS, indicates that electrons surplus to eight valence mackinawite) from Fe tetrahedrally coordinated by electrons per formula unit are accommodated by S. This proposed mechanism overcomes several structural rearrangements which increase electronic difficulties. Previous studies have shown that the stability. In sphalerite (ZnS), structural stability is initial amorphous FeS precipitate formed on reac- achieved because all low-energy molecular orbitals tion of aqueous Fe(U) and dissolved sulfide ages are occupied and a large energy gap is present to form mackinawite (BERNER, 1964; RICKARD, between highest occupied and lowest unoccupied 1989), and from EXAFS evidence (PARISE and molecular orbitals (TOSSELLand VAUGHAN,1992). SCHOONEN,1991; this study), we know that tetrahe- Electronic structure models for the PeS phases Cu- dral nucleation of Fe by S in reducing solutions bic FeS and mackinawite (WELZ and ROSENBERG, develops rapidly. Hence, we consider it unlikely 1987) show that mackinawite has a reduced density that direct troilite nucleation from solution species of states at the Fermi level when compared with with Fe coordinated by 6 S atoms occurs. By show- Cubic FeS. This increased band width of macki- ing that troilite could nucleate from solution via Fe nawite is due to contributions from both Fe d and S tetrahedrally coordinated by S, we avoid the need p orbitals. Considerable Fe-Fe interaction between for solution complexes, such as the trimeric Fe3S3 edge sharing FeS4 tetrahedra in mackinawite causes precursor proposed by TAYLOR (1980), as species a rearrangement and splitting of the Fe d-orbitals required for troilite formation. while the increased band width of S p orbitals is Our model may also be used to interpret the 128 A. R. Lennie and D. J. Vaughan

H

G

amorphous FeS

D .- marcasite

A B C Cubic FeS mackinawite greigite E pyrite

J K

o ....- marcasite M . <...:..:.:.:= hexagonal pyrrhotite .... !F •••• ~._ ?wurtzite FeS I Q ~ pyrite • N t •••••• - troilite

FIG. 7. Summary of known, and speculative phase transformations and formation reactions in the low temperature iron-sulfur system. Transformations that have not been directly demonstrated experimentally are shown by dotted lines. The key to the letters in this figure is given below: A. Formation from Fez+ or Fe, and HS- or HzS. pH 4-5 «368 K); MUROWCHICKand BARNES (l986a). B. Solid state transformation, kinetics only partially established, see SHOESMITHet al. (1980). C. Solid state transformation involving oxidation (of Fe), no kinetic data available; see TAYLORet al. (1979b). D. Inferred reaction, by sulfidation of greigite, at pH<5, T<523 K; MUROWCHICKand BARNES (1986b). E. Inferred reaction, by sulfidation of greigite, at pH>5, T<523 K; Munowcutcx and BARNES (1986b). For kinetics of pyrite formation from iron mono sulfide precursors, see RICKARD (1975). F. Solid state transformation, for kinetics of this reactions, see LENNIE and VAUGHAN(1992), structural aspects described by FLEET (1970). G. Sulfidation of Fe, by HS- or HzS, pH in range 2 3-10 (T<523 K). H. Precipitation from Fe +, by HS- or H2S, pH in range 3-10 (T<523 K). 1. Ageing of amorphous precipitate, see BERNER (1964). J. Solid state transformation; kinetics described in LENNIEet al. (l995a). K. Solid state transformation, kinetics not established. L. Precipitation from Fez+, by HS- or HzS, pH in range 4-6 wide range of temperatures, to above 573 K. Wurtzite-type FeS phase speculated, see this study. M & N. Formation of either hexagonal pyrrhotite (>413 K) M, or troilite «413 K) N from Fe2+ and HS- or HzS between pH 4-5. A (speculated) transformation from the proposed wurtzite-type FeS may also form these phases. O. Marcasite after pyrrhotite, see studies by MUROWCHICK(1992), FLEET (1978). P. Pyrite formed by sulfidation of pyrrhotite, see YUND and HALL (1970) for solid state kinetic study. This reaction is rapid above 573 K. Q. Troilite- hexagonal pyrrhotite reversible phase transition, see KRUSE (1992), KING and PREWITT (1982), HORWOODet al. (1976) and PUTNIS (1974) for further details on intermediate structures and transition mechanisms.

observation (SCHOONEN and BARNES, 1991c) that of mackinawite to pyrrhotite has been demon- the conversion rate of mackinawite to hexagonal strated to be slow below 530K (LENNIE et al., pyrrhotite is relatively slow in alkaline solutions at 1995a) and, because it is a solid state transforma- elevated temperatures (453-473 K) when com- tion, unlikely to be affected by pH. Instead, we pared to the conversion rate in acidic and neutral suggest that direct nucleation of hexagonal pyrrho- solutions. The rate of the solid-state transformation tite from solution is enhanced at elevated tempera- Iron sulfide formation 129

tures under acidic conditions, competing with for- The sequence of sulfidation of iron 'monosul- mation of mackinawite. In addition, under acid fides' is seen to follow two main paths, the first conditions, dissolution of mackinawite will be being that of sulfidation of iron sulfides with ccp more rapid than that of pyrrhotite allowing pyrrho- S sub-lattices, leading to formation of pyrite and tite to concentrate relative to mackinawite in acid marcasite (the Cubic FeS-mackinawite-greigite- solutions. marcasite/pyrite sequence). The second is that of sulfidation of pyrrhotite, again leading to pyrite, Sulfidation of iron 'monosulfides' forming marcasite. iron disulfides For iron sulfides FeS1+x. with x<0.33, the ther- The formation of pyrite and marcasite in sedi- modynamic driving force is in the direction of for- mentary and hydrothermal systems provides an im- mation of hcp S sub-lattices from ccp S sub- portant control on Fe and S in the environment. It lattices, i.e. mackinawite to hexagonal pyrrhotite, has been proposed that at temperatures below 523 greigite to ?hexagonal pyrrhotite (see Fig. I). The K or so, the direct nucleation of iron disulfides predominant sulfidation sequence occurring in na- from solution is extremely slow compared to for- ture at low temperatures «523 K), however, is mation as a result of sulfidation of the 'mono- the first sequence shown in Fig. 7. The abundant sulfide' precursors (SCHOONEN and BARNES, nucleation of pyrite or marcasite in sedimentary 1991a,b,c). systems thus depends on metastable precursors which, in turn, are prevented by kinetic factors Sulfidation of amorphous FeS by H2S or HS- proceeds at an insignificantly slow rate at 338 K from transforming to stable pyrrhotites. It is noted (SCHOONENand BARNES, 199Ib). However, at or that additional formation pathways resulting from dissolution or oxidation of iron sulfides, with sub- above 373 K, sulfidation by H2S proceeds at mea- surable rates (DROBNERet al., 1990; SCHOONENand sequent precipitation of iron sulfide phases, are not addressed in Fig. 7. BARNES, 1991c; TAYLORet al., 1979a,b); reduction of H ions forming H2 balances the redox require- This figure serves to show that the complex ments of this reaction. Sulfidation of iron 'mono- phase relations in the low temperature iron-sulfur sulfides' proceeds rapidly in the presence of S as system are now being clarified although further polysulfide and/or zerovalent S (RICKAfID, 1975). work is still required to clearly establish the rela- It is expected that mackinawite, being the pre- tionship of smythite to other phases in this system. dominant 'FeS' phase formed in reducing solu- Our study demonstrates again the importance of tions, would provide facile topotactic sites for py_ metastable phases in the iron-sulfur system. Further rite or marcasite nucleation. Because mackinawite studies are now required to clarify the role of sur- is so easily oxidised to greigite, greigite may also face structure and reactivity in formation and trans- form a suitable precursor for pyrite or marcasite formation of these phases. nucleation. To form a small pyrite (or marcasite) nucleus of unit-cell dimensions for further crystal Acknowledgernents- The support of the Natural Environ- ment Research Council is acknowledged. Thanks are due growth from the surface would require relatively to Drs. J. M. Charnock, K.E.R. England, J. F. W. Mossel- few bonds of the greigite structure to be broken and mans and P. F. Schofield for help with various aspects reformed. Although the role of the crystal surface is of the experimental work. Discussions with Professors thought to be of importance in these sulfidation George Helz and Jack Tossell were of great value in developing this work. reactions, the site(s) of nucleation of pyrite or mar- This paper is dedicated to the memory of Roger Burns casite crystallites on the iron mono sulfide precursor who inspired and encouraged the efforts of both authors remains unknown. Further progress in elucidating in studies of the crystal chemistry and geochemistry of mechanisms of S addition to Fe mono sulfides will iron sulfides. depend on carefully designed surface science stud- ies of these reactions. REFERENCES

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