Pyrrhotite Fe1–xS is the main sulfide component of platinum-group element (PGE) ores and commonly contains from a few tenths to a few dozen ppm of disseminated Pt. Here we report an X-ray absorption spectroscopy investigation into the state of Pt in synthetic pyrrhotite in combination with theoretical spectra modelling. The pyrrhotite crystals were obtained by means of the salt flux technique, using a eutectic mixture of alkali metal halides as the transport media. Analysis of the chemical composition of synthesised crystals showed that an increase of the temperature and sulfur fugacity yields higher concentrations of Pt in pyrrhotite. The Pt content reached 0.6 wt.% at the maximum temperature and sulfur fugacity (t = 720°C, log $f_{{\rm S}_ 2}$ = –0.1) achieved in a Pt-saturated system. X-ray absorption near-edge structure (XANES) analysis of Pt L3-edge spectra revealed that Pt is present in pyrrhotite in the 4+ and 2+ ‘formal’ oxidation states. Theoretical modelling of XANES and interpretation of extended X-ray absorption fine structure (EXAFS) spectra showed that Pt4+ substitutes for Fe in the crystal lattice of pyrrhotite, whereas Pt2+ forms PtS-like clusters disseminated in the pyrrhotite matrix. Atoms of isomorphous Pt4+ are surrounded by 6 S atoms at a distance of 2.39 ± 0.02 Å. According to theoretical simulations using the FDMNES program, the second coordination sphere of the solid-solution Pt contains one vacancy in the Fe sublattice within the Fe-layer. The Pt2+S-like clusters can be considered as a quench product. High sulfur fugacity stabilises the solid-solution Pt and prevents the formation of the PtS-like clusters during cooling. The maximum content of the solid-solution Pt in pyrrhotite is ca. 50 times lower than in pyrite and can be approximated by a straight line in the log C(Pt) vs. 1/T plot, it increases from 1 ppm at 350°C to 3 wt.% at 900°C.

To study thermal desulfurization of pyrite (FeS2), we conducted in situ neutron diffraction experiments in the temperature range 298–1073 K. On heating, pyrite remained stable up to 773 K, at which it started to decompose into pyrrhotite (Fe1−xS) and S2 gas. Rietveld analysis of the neutron data from 298 to 773 K allowed determination of the thermal expansion coefficient of pyrite (space group Pa$\bar 3$) to be αV = 3.7456 × 10−5 K−1, which largely results from the expansion of the Fe–S bond. With further increase in temperature to 1073 K, all the pyrite transformed to pyrrhotite (Fe1−xS) at 873 K. Unit-cell parameters of Fe1−xS (space group P63/mmc) increase on heating and decrease on cooling. However, the rates in cell expansion are larger than those in contraction. This hysteresis behavior can be attributed to continuous desulfurization of pyrrhotite (i.e., x in Fe1−xS decreases) with increasing temperature until the stoichiometric troilite (FeS) was formed at 1073 K. On cooling, troilite underwent a magnetic transition to an orthorhombic structure (space group Pnma) between 473 and 573 K. In addition, using differential thermal analysis (DTA) and thermogravimetric analysis (TGA) implemented with a differential scanning calorimeter, we performed kinetic measurements of pyrite decomposition. Detailed peak profile and Arrhenius (k = A exp(−Ea/RT)) analyses yielded an activation energy Ea of 302.3 ± 28.6 kJ/mol (based on DTA data) or 302.5 ± 26.4 kJ/mol (based on TGA data) and a ln(A) of 35.3 ± 0.1.

The thermal expansion coefficients for natural troilite, FeS, Ni-rich pyrrhotite, Fe0.84Ni0.11S, and Ni-poor pyrrhotite, Fe0.87Ni0.02S, were measured during cooling by in situ neutron powder diffraction over the temperature range 873–373 K. Between 873 and 573 K, the mean thermal expansion coefficients for the three compositions are 7.4(3)×10−5 {FeS}, 8.0(4)×10−5 {Fe0.84Ni0.11S} and 8.5(4)×10−5 K−1 {Fe0.87Ni0.02S}. Below 573 down to 373 K, the first two increase considerably to 14.1(7)×10−5 {FeS} and 9.3(5)×10−5 {Fe0.84Ni0.11S} while the latter sample shows no significant variation, 8.4(5)×10−5 K−1. Below 573 K, the thermal expansion is highly anisotropic, with Δa/100 K−1 ranging from 0.89(9)% {FeS} to 0.48(12)% {Fe0.87Ni0.02S} while Δc/100 K−1 ranges from −0.39(11)% {FeS} to −0.13(2)% {Fe0.87Ni0.02S}.

Upon cooling through 573 K, troilite and pyrrhotite undergo a transition where the FeS6 octahedra distort and in the case of pyrrhotite, cation-vacancy clustering occurs. The thermal expansion coefficients are bigger for low cation-vacancy concentrations and decrease as the pyrrhotites become less stoichiometric. This indicates that the thermal expansion in these minerals is damped by vacancy ordering or clustering. The thermal expansion coefficients for troilite and pyrrhotite are amongst the largest reported for sulphide minerals and their role in the formation of ore textures is discussed briefly.

Pyrite, the most widespread and abundant of sulphide minerals in the Earth's surficial rocks, commonly constitutes the primary opaque phase in ore deposits. Consequently, an understanding of the behaviour of pyrite and its relationships with coexisting phases during the metamorphism of pyritebearing rocks is vital to the interpretation of their genesis and post-depositional history. Metamorphism is commonly responsible for the obliteration of primary textures but recent studies have shown that the refractory nature of pyrite allows it to preserve some pre-metamorphic textures. Pyrrhotite in pyritic ores has often been attributed to the breakdown of pyrite during metamorphism. It is now clear that pyrrhotite can be primary and that the presence of pyrrhotite with the pyrite provides a buffer that constrains sulphur activity during metamorphism. Pyrite-pyrrhotite ratios change during metamorphism as prograde heating results in sulphur release from pyrite to form pyrrhotite and as retrograde cooling permits re-growth of pyrite as the pyrrhotite releases sulphur. Retrograde growth of pyrite may encapsulate textures developed during earlier stages as well as preserve evidence of retrograde events. Sulphur isotope exchange of pyrite with pyrrhotite tends to homogenise phases during prograde periods but leaves signatures of increasingly heavy sulphur in the pyrite during retrograde periods.

We have developed a cluster variation method (CVM) model based on cuboctahedral and octahedral basis clusters containing 13 and 6 atoms, respectively, and applied it to the analysis of the thermodynamic mixing properties of (Zn,Fe)S solid solutions. The model, in which the internal energy of the lattice is approximated by next to nearest neighbour (nnn) pair interactions and many-body interactions associated with nearest neighbour (nn) equilateral triangles, describes the FeS contents of sphalerites equilibrated with pyrrhotite and pyrite, and with pyrrhotite and iron metal within experimental uncertainties. The model predicts moderate deviations from ideality; the mean values of the Lewis and Randall activity coefficient of FeS and ZnS are, 1.48 and 1.03, respectively. Predictions of the model are in qualitative agreement with cell-edge data. The model also predicts that sphalerites undergo long-range ordering to lower-symmetry structures at temperatures only slightly below those investigated experimentally, a result in agreement with inferences from an existing Mössbauer investigation of synthetic sphalerites.

More realistic models in which interactions are ascribed to larger species (nn triangular and centred square species) predict that such long-range ordering occurs at even higher temperatures and underscore the need for better characterization of the structures of (Zn,Fe)S minerals.

Pyrrhotites in polished sections from more than twenty stratabound massive sulphide and magmatic nickel-copper deposits in Norway were studied under the microscope using the magnetic colloid method. In both types of deposits, two distinct styles of intergrowths between monoclinic and hexagonal pyrrhotites were found: crystallographically-controlled lamellar intergrowths and fissure-controlled irregular intergrowths.

Lamellar intergrowths consist of crystallographically oriented monoclinic lamellae occurring in a hexagonal matrix and were produced originally by exsolution from hexagonal pyrrhotite on cooling. Irregular intergrowths comprise blades and patches of monoclinic pyrrhotite occurring along fissures and grain boundaries of hexagonal pyrrhotite, and were formed by interactions between hexagonal grains and sulphur-rich hydrothermal solutions.

Increase in lamella thickness and spacing, development of lamella zonations, wedge-shaped composite ends, boxworks and composite lamellae were caused by progressive lamellae coarsening and maturation during natural annealing, which could have been promoted by anisotropic stress. Metamorphic recrystallization and annealing tend to homogenize pyrrhotite and erase preexisting exsolution lamellae.

Troilite is a sulfide that occurs commonly in meteorites. In the laboratory, it can be synthesized by gas-solid reaction of metallic Fe and H2S-H2 gas mixtures. In this research, the arc-discharge method was used to synthesize troilite. The resulting spherules are up to 2–3 mm in diameter and composed of troilite, pyrrhotite and metallic Fe. Nanometer-sized hollow carbon fibres and balls occur on the surface of the spherules. The features and coexisting relationships of the constituents of the spherules were studied by X-ray diffraction (XRD), energy-dispersive X-ray spectrometry (EDS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the phenomenon of transformation of pyrite (FeS2) into pyrrhotite (Fe1-xS), troilite (FeS) and metallic Fe under arc-discharge reducing conditions were examined.

A monoclinic form of Fe3S4, a polymorph of cubic greigite, occurs as precipitates in a sample of pyrrhotite collected from the Sudbury ore deposit. The nano-crystal precipitates are in a topotaxial relationship with the host pyrrhotite-4C (Fe7S8). The precipitate and the host pyrrhotite have a coherent (001) interface. Half of the octahedral layers in the crystal structure are fully occupied by Fe, while the other half of the octahedral layers are occupied by Fe atoms and vacancies in an ordered manner along the a axis. The crystal structure of the Fe3S4 nano-precipitates has monoclinic symmetry with a space group of I2/m. Its c dimension is 6% smaller than that of the host pyrrhotite due to the large number of vacancies in the structure. Fractional coordinates for S and Fe atoms within the unit cell are determined from Z-contrast images and density functional theory (DFT). The calculated results match the measured values very well. It is proposed that the monoclinic Fe3S4 nano-precipitates formed through ordering of vacancies in pyrrhotite with a low Fe/S ratio (i.e. <0.875) at low temperature.

The discovery in 1966 of Ni sulphide deposits at Kambalda, Western Australia resulted from the interpretation of siliceous limonitic material containing up to 1·0% Ni and 0·3% Cu as the gossanous expression of Ni sulphides. In many cases analysis for Ni, Cu, Zn, Co, Cr, Mn and Pb will distinguish Ni–Cu gossans from other oxidised sulphide assemblages and pseudo-gossans. However, most geochemical classification schemes cannot always distinguish strongly silicified or sulphide-poor Ni gossans from Fe sulphide gossans enriched in Ni and Cu or from some nickeliferous oxidised and lateritic products which have no sulphide source. Ambiguous cases may be resolved by analysis for Pd and Ir but this method is expensive and requires very sophisticated analytical techniques.

The use of reflected light microscopy to examine microtextures preserved in the surface gossans from a supergene pyrite-violarite-marcasite sulphide assemblage has allowed the refinement of geochemical classification schemes. The preservation of supergene zone textures by pseudomorphous Si–Fe replacement often permits the original sulphide proportions and Ni–Cu grade to be reliably assessed. Relict microtextures are usually preserved, even in pale coloured, highly siliceous gossans containing less than 0·01% Ni.

The massive pyrite-pyrrhotite-chalcopyrite-sphalerite deposit of Outokumpu, comprising the Keretti and Vuonos orebodies, is a deformed and metamorphosed strata-bound mass associated with mineralised stockworks. Mobilisation of much of the ore followed formation of large recumbent isoclinal folds that are the major structures of the surrounding rocks and associated with the modification of originally flat saucer-shaped ore lenses into elongate ruler-shaped masses. Further modification of shape took place at the mobilisation stage with much of the pyrrhotitic ore, particularly, now occupying the thickest parts of the orebodies in the form of breccia or microbreccia. In many parts gross original characters still exist and the pyritic and pyrrhotitic constituents of the ore have survived as separate entities while locally the pyritic ore retains pre-deformational characteristics and consistent stratigraphic position within a thin horizon.

Both ore and country rocks show evidence of extensive polyphase deformation with the effects of six fold phases shown in the ore. Mineral assemblages in the country rocks indicate a middle amphibolite facies peak of metamorphism. The serpentinite-black schist-carbonate-quartzite rock assemblage, with which the ore is associated, was tectonically incorporated within the regionally extensive mica schist by even earlier subhorizontal thrusting. This is related to the movement of a thrust nappe with the interdigitation of an ocean-floor ophiolite assemblage and flysch deposited during ocean closure associated with Svecokarelian tectonism.

The original formation of the Keretti and Vuonos sulphide masses took place in a marine exhalative environment with a pyritic layer overlying a pyrrhotitic layer in each of the two c. 4 km diameter irregularly oval-shaped depressions whose centres were c. 8 km apart. The mineralised stockwork below each mass represents the upper parts of the conduit for metalbearing fluids in a convective system.