Pyrite interatomic distances

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... 

Data for which no reference is given are from the Slrukturbericht of P. P. Ewald and C. Hermann. 6 R. W. G. Wyckoff, Z. Krisl., 75,529 (1930). W. H. Zachariasen, ibid., 71, 501, 517 (1929). d The very small paramagnetic susceptibility of pyrite requires the presence of electron-pair bonds, eliminating an ionic structure Fe++S2. Angles are calculated for FeS2, for which the parameters have been most accurately determined. The parameter value (correct value = 0.371) and interatomic distances for molybdenite are incorrectly given in the Slrukturbericht. 

We have constructed a number of sets of atomic radii for use in compounds containing covalent bonds. These radii have been obtained from the study of observed interatomic distances. They are not necessarily applicable only to crystals containing pure covalent bonds (it is indeed probable that very few crystals of this type exist) but also to crystals and molecules in which the bonds approach the covalent type more closely than the ionic or metallic type. The crystals considered to belong to this class are tetrahedral crystals, pyrite and marcasite-type crystals, and others which have been found on application of the various criteria discussed in the preceding section to contain covalent bonds or bonds which approach this extreme. 

A set of values of tetrahedral covalent radii 7 for use in crystals of these types is given in Table 7-13 and represented graphically in Figure 7-7. These values were obtained from the observed interatomic distances in crystals of these tetrahedral types and of other types in which the atom of interest forms four covalent bonds with neighboring atoms which surround it tetrahedraliy. For example, in pyrite, FeS. each sulfur atom is surrounded tetrahedraliy by three iron atoms and one sulfur atom, with all of which it forms essentially covalent bonds (Fig. 7-8) the substance is a derivative of hydrogen disulfide, H2S2. That the Fe—S bonds are essentially covalent is shown by the magnetic eri-... 

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