The pyrites and marcasite structures

There are interesting changes in the S-S and M-S distances in the 3d disulphides with the pyrites structure, differences which have been correlated with the numbers of d electrons/  

The structures of the two forms of FeS2 (a) pyrites, and (b) marcasite. In (a) the S—S distance has been reduced to accentuate the resemblance of this structure to that of NaCl. In (b) the shaded circles represent Fe atoms, six of which surround each 2 group as is also the 

Note the abnormal distances in MnS2, attributed to the stability of the half-filled 3d shell. A cupric sulphide with the composition CuSj.g having the pyrites structure has been prepared from covellite (CuS) and S under high pressure at a temperature of 350°C or above.  

All three minerals, CoAsS, cobaltite, NiAsS, gersdorffite, and NiSbS, ullmannite, have structures which are obviously closely related to pyrites. There are three simple structural possibilities  

Structure (i) was originally assigned to all three compounds, but as the result of later studies the following structures have been proposed CoAsS, (ii), NiAsS, (iii), and NiSbS, (i). It would seem that the greatest reliance may be placed on the later work on CoAsS, which shows that the apparently cubic structure is a polysynthetic twin of a monoclinic structure, and it is not impossible that other structures in this family are in fact superstructures of lower symmetry. As a result of the difference between Ni-Sb, 2-57 A, and Ni-S, 2-34 A, the symmetry of NiSbS has dropped to the enantiomorphic crystal class 23 the absolute structure has been determined.  

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The pyrites and marcasite structures can be thought of as containing 82 units though the variability of the interatomic distance and other properties suggest substantial deviation from a purely ionic description. Numerous higher polysulfides S have been characterized, particularly for the more electropositive elements Na, K, Ba, etc. They are yellow at room temperature, turn dark red on being heated, and may be thought of as salts of the polysulfanes... 

The sulfur-bearing minerals that predominate in coal seams are the iron sulfide ores pyrite and marcasite. Both have the same ratio of sulfur to iron, but their crystallographic properties are quite different. Marcasite has an orthorhombic structure, while pyrite is isometric. Marcasite is less stable and more easily decomposed than pyrite. The latter is the most widespread of all sulfide minerals and, as a result of its greater abundance in the eastern United States, pyrite is recognized as the major source of acid mine drainage. FeS2(s) is used here as a symbolic representation of the crystalline pyritic agglomerates found in coal mines. 

The sulphides of iron include Fe3S4 (spinel structure), Fe7Sg (pyrrhotite), FeS (troilite), and FeS2 (pyrites and marcasite). Ferrous sulphide rarely has the Fe S ratio precisely equal to unity, though stoichiometric FeS can be prepared. A microcrystalline form prepared by precipitation has been examined by X-ray powder photography and also by electron diffraction. This form (mackinawite) has... 

The most abundant and widespread sulfides are pyrite and marcasite. They both have the formula FeS2 but the former crystallizes in a cubic structure whereas the latter is orthorhombic. They both contain divalent iron which is in a non-magnetic low spin state (Fe ). Consequently the Mossbauer spectra (Fig. 3.18.) consist of a doublet with low isomer shifts and moderate quadrupole splittings [130]. 

Microscopy with polarized light (see Section 3.2.4.2) can identify FeS2 (pyrite and marcasite), Si02 (quartz), and FeCOs (iron spar) by differentiation of the reflectance and color in a very easy way. But also change of light (intensity, wavelength), detection of the individual refraction index, and identification of crystalline structures (e.g., needles) can help to determine the types of minerals and their volume fraction. 

Finally, many disulfides have a quite different structure motif, being composed of infinite three-dimensional networks of M and discrete Sj units. The predominate structural types are pyrites, FeSa (also for M = Mn, Co, Ni, Ru, Os), and marcasite (known only for FeS2 among the disulfides). Pyrites can be described as a distorted NaCl-type structure in which the rodshaped S2 units (S-S 217 pm) are centred on the Cl positions but are oriented so that they are inclined away from the cubic axes. The marcasite structure is a variant of the rutile structure (Ti02,... 

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 prevalent modification of the disulfide, FeS2, is pyrite, which may be visualized as a distorted NaCl structure where the Fe atoms occupy sodium positions and S2 groups are placed with their centers at the chloride positions. Pyrite is a largely occurring crystal with semiconductor properties Eg = 0.95 eV). Another modification of FeS2 is the very similar to pyrite but somewhat less regular marcasite structure. 

Ru and Os alloys. A specimen of structure types observed in the alloys of these metals (very often as solid solution ranges) is shown in Table 5.48a. The formation of Laves-type phases, a phases and several CsCl-type phases can be underlined. Notice the formation of marcasite and pyrite type compounds with the semimetals and non-metals of the 15th and 16th groups. 

Sulphides. The partially ionic alkali metal sulphides Me2S have the anti-fluorite-type structure (each Me surrounded by a tetrahedron of S, and each S atom surrounded by a cube of Me). The NaCl-structure type (6/6 coordination) is adopted by several mono-sulphides (alkaline earth, rare earth metals), whereas for instance the cubic ZnS-type structure (coordination 4/4) is observed in BeS, ZnS, CdS, HgS, etc. The hexagonal NiAs-type structure, the characteristics of which are described in 7.4.2.4.2, is observed in several mono-sulphides (and mono-selenides and tellurides) of the first-row transition metals the related Cdl2 (NiAs defect-derivative) type is formed by various di-chalcogenides. Pyrite (cP 12-FeS2 type see in 7.4.3.13 its description, and a comparison with the NaCl type) and marcasite oP6-FeS2 are structural types frequently observed in several sulphides containing the S2 unit. 

Nickel, E. H. (1968a) Structural stability of of minerals with the pyrite, marcasite, arsenopyrite and lollingite structures. Canad. Mineral., 9, 311—21. 

It fits into this picture that the normal marcasites easily transform under pressure to the pyrite structure whereas the loellingites FeP2 and FeAs2 as well as the arsenopyrite-type representative C0AS2 retain their structure up to at least 65 kbar/400- 1100° C (16). 

We present here the preliminary results of our attempt to develop a new method for the analysis of pyrite in coal and lignite. It is well known that sulfur in coal is present in different forms. In particular, although the iron sulfide in coal is generally pyrite ( 1), other iron sulfides are frequently present. For example, iron disulfide occurs as marcasite, a rhombic crystalline form, as well as pyrite, a cubic crystalline form. Perhaps the term disulfide sulfur should be used to replace the pyritic sulfur more commonly quoted, as recently suggested by Youh (2). Since the chemical reactivity of these two disulfides of iron is similar, our method will record them equally well. Nonetheless, we will continue to refer to the pyrite determinations here, although we are really talking about the chemical species FeS2 rather than a particular crystalline structure. 

Another class of sulfides of considerable importance are the disulfides, represented by FeS2, CoS2, and others. All these contain discrete S2 units with an S—S distance almost exactly equal to that to be expected for an S—S single bond. These assume one of two closely related structures. First there is the pyrite structure named after the polymorph of FeS2 that exhibits it. This structure may be visualized as a distorted NaCl structure. The Fe atoms occupy Na positions and the S2 groups are placed with their centers at the Cl positions but turned in such a way that they are not parallel to any of the cube axes. The marcasite structure is very similar but somewhat less regular. 

Important representatives of P2" dumbbells are the pyrites (e.g. / -PtP2) and the marcasites (e.g. m-FeP2) with and without M-M bonds, for example C0P2. Dumbbells P2" and M-M chains characterize the structures of M0P2 and WP2 (see Figure (16)). CoPS is given as a representative of a phosphide sulfide with either (PS) dumbbells or mixtures of... 

Fig. 6.18, The linking of MX octahedra in the (a) pyrite (b) marcasite (c) JoeJ-lingite and (d) arsenopyrite structures.

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,... 

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