Molybdenite structure

Molybdenite was the first substance found to contain a trigonal prism as ligation polyhedron for ligancy 6, instead of the more common octahedron (Dickinson and Pauling, 1923). An acceptable explanation of the stability of the molybdenite structure, rather than the cadmium iodide... 

Layered sulfides predominate in Groups IV to VII except for MnS, which is isotropic. The main structural feature of this class is a strong chemical anisotropy that greatly affects their catalytic properties. In the molybdenite structure (MoS2), the metal is in trigonal prismatic coordination. Other layered sulfides are octahedrally coordinated or distorted octahedrally coordinated (ReS2) with the metal surrounded by six sulfur atoms. It should also be noted that some TMS sulfides have structures that fall in between isotropic and layered sulfides. Rh2S3 is an example and others are completely amorphous, such as IrSt and OsSr. 

Fig. 47. Schematic diagram of molybdenite structure. Mo atoms (solid circles) are located at B sites, and the S atoms (open circles) at A sites which are above and below the plane in which the B sites lie. In hexagonal molybdenite the stacking sequence is AbBaAb. (In the rhombohedral variety the C sites assume importance.) (8 7)...

Several of the Mo and W chalcogenides adopt the molybdenite structure, but M0S2 is the most interesting phase and is an excellent (dry) lubricant. It is instructive to compare the M0S2 structure to the structure of graphite, which is shown for comparison in Figure 6.15. The unit... 

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. 

The arrangement found to account for X-ray data from molybdenite is shown in Fig. 2. This arrangement is derivable from the space group Dgh as well as from the space groups D, D , Cjh, and D. Considering the atoms as points or spheres, the structure then has holohedral hexagonal symmetry. 

T ike metals minerals also exhibit typical crystalline structures. As an example, the structure of molybdenite is shown in Figure 1.17. It is hexagonal with six-pole symmetry and contains two molecules per unit cell. Each sulfur atom is equidistant from three molybdenum atoms and each molybdenum atom is surrounded by six sulfur atoms located at the comers of a trigonal prism. There are two types of bonds that can be established between the atoms which constitute the molybdenite crystal stmcture. They are the covalent bonds between sulfur and molybdenum atoms and the Van der Waals bonds between sulfur-sulfur atoms. The Van der Waals bond is considerably weaker than the covalent sulfur-molybdenum bond. This causes the bonds of sulfur-sulfur to cleave easily, imparting to molybdenite the property of being a dry lubricant. Molybdenite adheres to metallic surfaces with the development of a molecular bond and the friction between metallic surfaces is replaced by easy friction between two layers of sulfur atoms. 

This is a layer structure similar to the Cdl2 structure except that the pairs of S layers are directly superimposed upon each other, so that the central cation is in trigonal prismatic coordination, not octahedral or tetrahedral. In the structures derived from this S-Mo-S unit, these layers can be stacked like the simple layers in atomic close-packed sequences. The molybdenite ((3-MoS2) structure has these layers in... 

Fuerstenau (1980) found that sulphide minerals are naturally floatable in the absence of oxygen. Yoon (1981) ever attributed the natural floatability of some sulphide minerals to their very low solubility. Finkelstein et al. (1975) considered that the natural floatability of sulphide minerals are due to the formation of elemental sulphur and related to the thickness of formation of elemental sulphur at the surface. Some authors reported that the hydrophobic entity in collectorless flotation of sulphide minerals were the metal-deficient poly sulphide (Buckley et al., 1985). No matter whichever mechanism, investigators increasingly concluded that most sulphide minerals are not naturally floatable and floated only under some suitable redox environment. Some authors considered that the natural floatability of sulphide minerals was restricted to some special sulphide minerals such as molybdenite, stibnite, orpiment etc. owing to the effects of crystal structure and the collectorless floatability of most sulphide minerals could be classified into self-induced and sulphur-induced floatability (Trahar, 1984 Heyes and Trahar, 1984 Hayes et al., 1987 Wang et al., 1991b, c Hu et al, 2000). 

The inherent hydrophobicity once thought to be typical of sulphides (Ravitz and Porter, 1933) is now thought to be restricted to sulphides such as molybdenite (Chander et al., 1975) and other minerals or compound with special structural feature (Gaudin et al, 1957b). Common commercial sulphide minerals, which are needed to recover in flotation, are normally composed of anion (S ) and heavy metal ions such as Cu, Cu, Pb, Zn, Hg, Sb, Bi transitive metal ion such as Fe, Co, Ni and noble and rare metal ions such as Ag, Au, Mo. On the basis of structural pattern or mode of linkage of the atoms or polyhedral imits in space, Povarennyk (1972) introduced a crystallochemical classification of sulphide minerals, which have six major patterns as shown in Table 1.1. 

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... 

Heyes and Trahar (1984) leached pyrite with cyclohexane and compared the extract with a sulphur-containing solution of cyclohexane in a UV spectra photometer as shown in Fig. 1.4, indicating that sulphur was present at the mineral surface. Therefore, the inherent hydrophobicity and natural floatability once thought to be typical of sulphides is now thought to be restricted to sulphides such as molybdenite and other minerals or compound with special structural features. The collectorless floatability that most sulphide minerals showed came from the self-induced or sulphur-induced flotation at certain pulp potential range and certain conditions. 

The comparison of the Re content of a molybdenite with its structural polytype has been proposed as a method for assessing the viability of samples for Re-Os dating. 3R molybdenite has been assumed to grow via a screw dislocation mechanism triggered by the incorporation of Re during crystallisation. Low Re 3R ... 

The molybdenite (M0S2) structure (C7) is a layer structure closely related to CdCl2 and Cdl2. Each layer of M0S2 consists of a sheet of metal atoms sandwiched between two sheets of nonmetal atoms (Fig. 1.8). These layers are stacked one upon the other along the hexagonal c-axis, the sequence of sheets of atoms being AbA BaB where the capital letters denote anions and the lower case letters cations. In this structure. 

Fig. 5-14.—The structure of the hexagonal crystal molybdenite, MoSt, showing the arrangement of sulfur atoms (large circles) at the corners of a trigonal prism about each molybdenum atom (small circles).

The 2 3TPT Layer Crystal Structure of Molybdenite [M0S2]... 

Figure 6.28. The 2-3TPT structure of molybdenite (M0S2). The Mo atoms are in P layers for an hep arrangement with S filling T layers above and below each P layer. The P layer between the sandwiches is empty.

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