MOLYBDATE AND TUNGSTATE MINERALS

naturally occurring inorganic compounds that are salts of molybdic acid, H2MoO4, and tungstic acid, H2WO4. Minerals in these groups often are valuable ores. The structural unit of these minerals is a tetrahedral group formed by four oxygen atoms at the corners of a tetrahedron surrounding a molybdenum or tungsten atom. Each MoO4 or WO4 tetrahedron has a net charge of -2, which is neutralized by metal ions outside the tetrahedron. Unlike the silicate or borate minerals, which form chains, rings, sheets, or framework structures by sharing oxygen atoms between adjacent tetrahedra, the molybdate and tungstate minerals share none; they are similar in this respect to the phosphate, vanadate, arsenate, and chromate minerals. Because the molybdenum ion and the tungsten ion have similar radii, they may substitute for one another within the structure of any naturally occurring example; thus, they tend to form solid solution series. Among the molybdate and tungstate minerals, only the powellite-scheelite series (calcium-bearing molybdate/tungstates) and wulfenite (lead molybdate) are noteworthy. Scheelite is a valuable tungsten ore; wulfenite is a minor ore of lead. One other series of tungstates is important. Wolframite, another name for the hbnerite-ferberite series of manganese/iron tungstates, is perhaps the most important ore of tungsten. These minerals have a structure, unlike that of the other tungstates, based on WO6 octahedrai.e., each tungsten atom is surrounded by six oxygen atoms arranged at the corners of an octahedron. These minerals are classed with the complex oxides and are related to the niobates and tantalates.