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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.

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