VANADATE MINERAL

any of the many naturally occurring compounds of vanadium (V), oxygen (O), and various metals; most of these minerals are rare, having crystallized under very restricted conditions. Although vanadinite occasionally is mined as a vanadium ore and carnotite as a uranium ore, most vanadates have no economic importance; they are prized by mineral collectors, however, for their brilliant colours. The structures of the vanadate minerals are complex. Some vanadate minerals contain vanadate tetrahedra (VO4), in which four oxygen atoms occupy the corners of a tetrahedron surrounding a central vanadium atom. Each vanadate tetrahedron has a net charge of -3, which is neutralized by large, positively charged metal ions (e.g., calcium, manganese, or ferrous iron) outside the tetrahedron. Unlike the similar silicate tetrahedra, which link to form chains, sheets, rings, or frameworks, vanadate tetrahedra are insular. The vanadates containing these tetrahedra are structurally and chemically similar to the phosphate and arsenate minerals; indeed, some vanadium in many of these vanadates often is replaced by phosphorus or arsenic, forming solid-solution series with both the phosphates and the arsenates. Like the phosphate and sulfate minerals, many vanadates are complexes of transition metals, particularly of ferrous iron, manganese, and copper. Other vanadates, particularly those that contain uranium, contain V2O86- ions, in which two atoms of vanadium are surrounded by eight atoms of oxygen arranged in two square pyramids that share one edge. Very complex clusters also exist but are usually classed with the complex oxide minerals rather than with the vanadate minerals.