Polymorphism Pyrite

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. 

Pyrite (FeS2> is by far the most abundant sulfide mineral, occurring in most types of geologic formations. Its less common polymorph, marcasite, usually forms in near-surface, low-temperature environments. At 25°C pyrite is more stable than marcasite by about -0.4 kcal/mol. The oxidative breakdown of these minerals as the result of exposure to aerobic conditions due to mining is the chief cause of acid mine waters. 

I. Pyrite—311 °C exotherm, polymorphic transformation 498 °C exotherm, oxidation and decomposition FejOS is obtained by liberation of SO 2 accompanied by a mass loss 637 °C... 

The structure of pyrite, FeS2 can be considered as a modified NaCl structure (cubic, Pa3) Fe atoms occupy fee lattice positions S-S covalently bonded pairs are also centered on fee positions but they alternate in directionality. This alternation destroys the overall face-centered symmetry. Pyrite is stable up to 743 °C at which temperature it breaks down to pyrrhotite and sulfur [387]. The phase relationship between pyrite and the orthorhombic (Pmnn) polymorph, marcasite, is still enigmatic [388]. In marcasite, S-S pairs point in the same direction in each layer. This causes a distortion from cubic to orthorhombic. The b dimension of the marcasite unit cell is almost identical with the lattice parameter of pyrite. In the transformation of marcasite to pyrite an orientation relationship 001 p// 101 m, with <100>p//[010]m is observed [389]. For the reverse transformation of pyrite to marcasite the similar atomic arrangement in pyrite 001 and marcasite 101 planes was mentioned [390]. 

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