Sulfur is the fifteenth most abundant element in the continental crust of the Earth (260 ppm), and the sixth most abundant element in seawater (885 ppm). Sulfur (atomic number 16) has the ground-state electronic structure [Ne]3 s 2 3p 4 , and is the first of the group VIB elements in the periodic table (S, Se, Te, Po). In minerals, sulfur can occur in the formal valence states S2−, S, S4+, and S6+, corresponding to the sulfide minerals, native sulfur , the sulfite minerals, and the sulfate minerals. In the sulfide minerals, S2− functions as a simple anion (e.g. CuFeS2, chalcopyrite) and as a compound S2 anion (e.g. FeS2, pyrite). In the sulfosalts, S2− functions as a component of a complex anion (e.g. AsS3 in tennantite, Cu12As4S13). In the sulfite minerals, S4+ has four valence electrons available for chemical bonding, and occurs in triangular pyramidal coordination with O. In the sulfate minerals, S6+ has six valence electrons available for bonding, and occurs in tetrahedral coordination with O. In addition, there are the thiosulfate minerals, in which S is in the hexavalent state, but is coordinated by three O2− anions and one S2− anion. Chemists frequently write the thiosulfate group as S2O3; however, we write it as SO3S to emphasize that one of the S atoms is an anion and is involved in a tetrahedral group. Although the focus of this chapter is the sulfate minerals, we will deal also with the sulfite and thiosulfate minerals, as they occur in the same types of geochemical environments. We adopt a pragmatic approach to matters involving chemical bonding. We use bond-valence theory …