Fluorescent Color and X-ray Powder Data of Synthesized Scheelite-Powellite Series as Guides to Determine its Composition

Abstract

The minerals of the scheelite-powellite series containing 0, 0.5, 1, 2, 4, 6, 10, 20, 40, 60, 80 and 100 mole % CaMoO₄ have been synthesized. The solid solution of this series was obtained by the slow addition of a Na₂WO₄-Na₂MoO₄ solution to a large volume of boiling CaCL₂ solution. The fluorescent colors of the precipitate change as follows: blue at the scheelite end, pale blue at 0.5 mole % CaMoO₄, white at 1 mole %, pale yellow at 2 mole %, and yellow at 4 mole %; and increase yellow tint up to 20 mole %, but do not show any remarkable change beyond that composition. Compared with the standard color card (Fig. 4), on which the precipitates are put, the composition of scheelite containing less than 10 mole % CaMoO₄ can be determined within the accuracy of 1 or 2 mole %.
The X-ray powder data show that the 2θ ₁₁₆ (CuKα)-2θ₂₂₀ (CuKα) values decrease linearly from 5.23° to 4.83° with the increasing amounts of powellite component in this solid solution. Using this value, the composition can be estimated within the accuracy of 10 mole % CaMoO₄. From the synthesis, fluorescent colors and X-ray data, it is inferred that the solid solution of the mineral series continues from the scheelite to powellite ends above a room temperature.