Calcium Tungstate, CaWO₄

Calcium Tungstate, CaWO₄, occurs naturally as the mineral scheelite, in which some of the calcium is replaced by molybdenum. It may be obtained by fusing sodium tungstate with calcium chloride, or with a mixture of calcium and sodium chlorides. By the addition of a 20 per cent, solution of sodium tungstate to a boiling ammoniacal solution of calcium chloride, the calcium is quantitatively precipitated in a heavy crystalline form as calcium tungstate. It crystallises in octahedra of density about 6. The solubility is approximately 1 part in 500 of water at 15° C.

Crystallised calcium tungstate fluoresces under the influence of X-rays, and may be used for intensifying the photographic effect in X-ray photography.

The specific heat of scheelite is 0.0967.

Several acid calcium tungstates are known: CaO.2WO₃ and CaO.2WO₃.3H₂O, 3CaO.7WO₃.18H₂O, CaO.3WO₃.6H₂O, and CaO.4WO₃.10H₂O.

There are also complex phosphotungstates and silicotungstates, an arsenotungstate, and a borotungstate.

Certain compounds produced by the action of an electric current on a mixture of alkali tungstates with alkaline earth tungstates, and known as tungsten bronzes, have been described. Potassio-calcium bronze has the formula CaW₄O₁₂.5K₂W₄O₁₂, and the sodio-calcium bronzes, CaW₄O₁₂.5Na₂W₅O₁₅ and CaW₄O₁₂.10Na₂W₃O₉.

A calcium sodium paratungstate, 3Na₂O.2CaO.12WO₃.34H₂O, is also known.

Mazzucchelli and Inghilleri described a complex pertungstate to which they gave the name calcium ozo-tungsto-oxalate, CaC₂O₄.WO₄.H₂O. They obtained it by the action of hydrogen peroxide on calcium tungsto-oxalate which is prepared by the action of calcium chloride on the corresponding potassium salt.