Natural tungstates

NATURAL TUNGSTATES (EN: tungstates; DE: naturliche Wolframate; FR: tungstates naturels; ES: tungstatos naturales; RU: вольфраматы природные) is the class of the minerals, the salts of the tungstic acid H2WO4. For the majority of the natural tungstates, there is characteristic the complex tetrahedral radical [WO4]2-, which is stable within the compounds with the large cations Ca2+ and Pb2+. Such are the scheelite CaWO4 and stolzite PbWO4. This group of the natural tungstates crystallizes into the tetragonal crystal system, the structures are insular, the crystals are isometric or tabular with the square cross-sections, with the cleavage along the (101) plane. Other groups of the natural tungstates are characterized by the existence of the comparatively small cations (Fe2+, Mn2+, and Zn2+) within the composition. The tetrahedron WO42- transitions into the distorted octahedral grouping WO6, the crystal system decreases to monoclinic, the structures become chain-like, and are approaching the structures of the complex oxides. The crystals of this group of the natural tungstates are usually flat-elongated with the longitudinal cleavage along the (010) plane, the colouration is brown, and dark brown. The representatives of the monoclinic natural tungstates are the wolframite (Fe,Mn)WO4, and the sanmartinite (Zn,Fe)WO4. Within the nature, there are known only two aqueous natural tungstates, namely, the ferritungstite (Ca,Fe3+,Fe2+)2WO4(OH)4•4H2O, and the anthoinite AlWO4(OH)•H2O; both compounds have the poorly studied sub-layered structure. For the chemical composition of the natural tungstates, there are characteristic the isomorphic substitutions of the WO42- by the MoO42- (within the scheelite, there may be contained up to 20% of the MoO3), which are leading to the decreasing of the density of the molybdenum-containing phases, and to the change of the colour of the luminescence from pale bluish (of the scheelite) to yellow (of the molybdoscheelite). There are also characteristic the isomorphic substitutions of the Fe2+-Mn2+-Zn2+, which are inherent for the wolframite and sanmartinite, and which are influencing the physical properties of the minerals (density, transparency, colouration, colour of the streak).

The natural tungstates are formed prevalently during the hydrothermal and skarn processes (wolframite, scheelite), less often they are supergene, and are associated with the weathering of the endogenous natural tungstates (ferritungstite, anthoinite). The scheelite and wolframite are the major minerals of the tungsten ores.