Natural Tungstates

a group of minerals that are salts of tungstic acid. Only the Fe, Mn, Zn, Ca, Pb, and Al salts of tungstic acid (H₂WO₄) are found naturally; the most widely distributed of them are wolframite, (Fe,Mn)WO₄, and scheelite, CaWO₄. Other compounds include stolzite, PbWO4, and sanmartinite, (Zn,Fe)WO₄, which are rare.

Natural tungstates are formed under endogenic hydrothermal conditions. Aqueous solutions of basic tungstates—the minerals ferritungstite, Ca₂Fe₂²+Fe₂³+[WO₄]₇ ·9H2O, and anthoinite, A1(WO4)(OH)-H2O—are formed in an oxidation zone of tungsten deposits. Natural tungstates crystallize in monoclinic and tetragonal systems. The basic structure of monoclinic natural tungstates is formed by zigzag alternating chains of octahedrons of WO₆ and (Mn,Fe)O₆; W⁺⁶ is found in sextuple coordination in the structures of wolframite and sanmartinite. The structure of tetragonal natural tungstates is based on isolated (WO₄)^2- tetrahedrons joined by Ca²+ or Pb²+ ions; W⁶+ has quadruple coordination (the structures of scheelite and stolzite). This structure permits replacement of W6+ by a certain amount of Mo6+, in connection with which molybdenum-enriched scheelites (zeirigite) and the rare mineral chillagite, Pb(Mo,W)O4, are known. Wolframite and scheelite are the main industrial minerals from which tungsten is extracted.