Crystal structures pyrites

The first crystal structure to be detennined that had an adjustable position parameter was that of pyrite, FeS2 In this structure the iron atoms are at the comers and the face centres, but the sulphur atoms are further away than in zincblende along a different tln-eefold synnnetry axis for each of the four iron atoms, which makes the unit cell primitive. 

Fig. 1.2 Crystal structures of the major sulfides (metal atoms are shown as smaller or black spheres) (A) galena (PbS) structure (rock salt) (B) sphalerite (ZnS) structure (zinc blende) (C) wurtzite (ZnS) strucmre (D) pyrite structure and the linkage of metal-sulfur octahedra along the c-axis direction in (/) pyrite (FeSa) and (//) marcasite (FeSa) (E) niccolite (NiAs) structure (F) coveUite (CuS) structure (layered). (Adapted from Vaughan DJ (2005) Sulphides. In Selley RC, Robin L, Cocks M, Plimer IR (eds.) Encyclopedia of Geology, MINERALS, Elsevier p 574 (doi 10.1016/B0-12-369396-9/00276-8))...

The most controversial and contradicting problem is, perhaps, the natural and collectorless floatability of sulphide minerals. Gaudin (1957) classified the natural hydrophobicity of different minerals according to their crystal structure and showed that most sulphide minerals were naturally hydrophobic to some extent, which had been fiirther proved based on van der Waals theory by Chander (1988, 1999). Lepetic (1974) revealed the natural floatability of chalcopyrite in dry grinding. Finklestein (1975, 1977) demonstrated that orpiment, realgar and molybdenite were naturally floatable, and that pyrite and chalcopyrite had natural floatability at certain conditions due to the formation of surface elemental sulphur. Buckley and Woods (1990,1996) attributed the natural floatability of chalcopyrite... 

Figure 2.2 Crystal structure of pyrite (a) Pyrite (100) plane (b) Showing the coordination of ions...

The FeSa (100) surfaces are modeled using the supercell approximation. Surfaces are cleaved fi om a GGA optimized crystal structure of pyrite. A vacuum spacing of 1.5 nm is inserted in the z-direction to form a slab and mimic a 2D surface. This has been shown to be sufficient to eliminate the interactions between the mirror images in the z-direction due to the periodic boundary conditions. 

In the crystal structure of pyrite (Fig. 11.1), molecules with dumbbell form and... 

The pyrite (FeS2) structure (C2) consists of molecular ions (Fig. 1.8). The structure is closely related to NaCl from which it may be derived by replacing Na by Fe and Cl by S , with the centre of the S ion occupying the chloride position. Each iron is octahedrally coordinated but each sulphur is tetrahedrally surrounded (one S and three Fe). Several transition metal chalcogenides crystallize in this structure. 

Fig. 4. Computer-generated crystal structure models nop row. left to right) Cuprite, zinc-blende, rutile, perovskite. iridymite (second row) Cristobalite. potassium dihydrogen phosphate, diamond, pyrites, arsenic (third rowt Cesium chloride, sodium chloride, wurtzite. copper, niccolite (fourth row) Spinel, graphite, beryllium, carbon dioxide, alpha i uanz. [AT T Bel Laboratories ...

In the crystal structure of pyrite, which is modelled on the periclase structure (fig. 5.1), Fe2+ ions occupy Mg24- positions and the mid-points of (S-S)2- dimeric anions are located at the O2- positions. Each Fe2+ ion is in octahedral coordination with one sulphur atom belonging to six different (S-S)2- dimers, and the... 

Hence the elongation of the anion octahedron is such that the cation is in square-planar coordination while the anion is surrounded by a deformed tetrahedron of four cations (Fig. 51). This distortion of the crystal structure from NiS(h) to PtS exactly corresponds to the transition from high-spin ds pyrite NiSa to diamagnetic PdSa by elongation of the anion octahedra. 

TOPOLOGICAL PROPERTIES AND ELECTRONIC STRUCTURES OF COMPOUNDS WITH PYRITE-TYPE CRYSTAL STRUCTURES... 

AX2 compounds with crystal structures belonging to the pyrite type (C2) are well documented in the literature. Due to the constant interest in the electronic, optical, and magnetic properties of these materials, there exists extended experimental and theoretical work. Among the title compounds, synthetic or natural pyrites, FeS2, turn out to be very well studied, like other transition metal disulphides, in both respects [1-5], while the electronic structures of pyrite-type SiP2 [6,7] and AuSb2 [8] are only partially known, and no theoretical study exists for the latter. 

The extent of the reactions indicated by Equations 1 and 2 or the molar sulfate-to-sulfur ratio is 2.4 zb 0.2 when rock pyrite is used and 1.4 zb 0.4 for sedimentary pyrite found in the coals used in this work. Although both materials are FeS2 of the same crystal structure, differences in reacivity have been documented which are attributed to impurities and crystal defects peculiar to the various possible modes of formation (7). For coal, no significant variation in this ratio was found with ferric ion concentration, acid concentration, coal, or reaction time. The results for each coal are found in Table II. 

A number of important structure types are found in transition-metal sulphides which have no counterparts among oxide structures, notably the various layer structures and the pyrites, marcasite, and NiAs structures. Further, many sulphides, particularly of the transition metals, behave like alloys, the resemblance being shown by their formulae (in which the elements do not exhibit their normal chemical valences, as in 0983, Pd4S, TiSa), their variable composition, and their physical properties-metallic lustre, reflectivity, and conductivity. The crystal structures of many transition-metal sulphides show that in addition to M-S bonds there are metal-metal bonds as, for example, in monosulphides with the NiAs structure (see later), in chromium sulphides, and in many sub-sulphides such as Hf2S,... 

A mineral with the same composition as pyrite, or fools gold, but differing in crystal structure. 

Some relationships between the NaCl structure and materials with related structures such as pyrite are shown in Pigure 6.9. This schematic is one illustration of how a simple structure can be systematically distorted to produce a host of new crystal structures. 

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