In Chapter 22 we discussed the triangular plane CO32- group as a fundamental building block of carbonates. In phosphates and sulfates the fundamental building block group is either the PO43- or the SO42- tetrahedron, respectively. The crystal structures are rather complicated in detail, and we will not elaborate on them. However, it is noteworthy that several phosphate structures are identical with silicate structures, and these we will study in more depth in Chapters 26-29. For example, berlinite (AlPO4) is isostructural with quartz (SiO2), triphyline (LiFePO4) with olivine (Mg2SiO4), and xenotime (YPO4) with zircon (ZrSiO4). Mostly though, phosphate and sulfate coordination polyhedra are isolated, whereas in silicates, the tetrahedra are generally polymerized to form sheets, chains, and frameworks. Related to phosphates and sulfates are arsenates, vanadates, and tungstates, with AsO43-, and VO43- and WO42- tetrahedra, respectively.
The minerals in these groups are of considerable economic interest. Apatite (Ca5(PO4)3(F, OH, Cl)), is a major source of potassium used as fertilizer as well as the main constituent of bones and teeth. Gypsum (CaSO4‧2H2O) is used as a building material, and scheelite (CaWO4) is the major tungsten ore.
Phosphates, arsenates, and vanadates
Phosphates and related minerals are numerous, but they are rather rare minerals in the earth's crust. Some more common examples are listed in Table 23.1.
The vanadate vanadinite (Pb(VO4)3Cl) is isostructural with apatite and there are limited isomorphic substitutions of phosphor, vanadium, and arsenic. Vanadates may have also other coordination polyhedra such as VO55-, VO67-, and V2O86-.