Phase Equilibria at High Temperatures

The wide variety of metal sulfide structures and their accommodation of atomic substitution, non-stoichiometry and metal-metal (M-M) and ligand-ligand interactions allows for diverse physical, chemical and electronic properties. The energy band structure of 3d transition-metal sulfides, in particular, is strongly influenced by the covalence of metal-S bonds, which results in hybridization of S 3p and metal 3d bonding states and direct or indirect M-M bonding interactions in favorable cases. Differences in the phase relations of isostructural metal sulfides are often attributable to subtle changes in electronic states. The literature on metal sulfide phase relations relevant to the earth sciences is very extensive and could not possibly be summarized in a single chapter. Therefore, following Craig and Scott (1974), this chapter focuses on the base metal (Fe, Co, Ni, Cu, and Zn) sulfides, with the literature for other metal chalcogenides and pnictides and some sulfosalts summarized in a single table (Table 1⇓). The relevant phase relations of the platinum-group element (PGE) chalcogenides and pnictides have been comprehensively reviewed in Makovicky (2002). A section on the halite structure sulfides (niningerite, alabandite and oldhamite) is also included in this chapter. The material presented here relates closely to that discussed in other chapters, in particular the chapter on sulfide thermochemistry (Sack and Ebel 2006). The importance of understanding phase equilibria in the context of electronic and magnetic properties and, hence, electronic structure is also emphasized, thereby reinforcing material presented in several other chapters (Pearce et al. 2006; Vaughan and Rosso 2006).