SOLUBILITY OF CASSITERITE IN HF + NAF SOLUTIONS AT 500°C AND 1000 ATMOSPHERES AT OXYGEN FUGACITY OF THE NI-NIO BUFFER

Abstract

We investigated the solubility of cassiterite and the forms of occurrence of tin in more complex acidic and near-neutral fluoride solutions at 500°C and 1000 atm with the oxygen fugacity controlled by the Ni-NiO buffer pair, as a function of the relative amounts of HF and NaF in them. After the experiment, the solutions were quenched, reacted qualitatively with silicomolybdic acid to detect divalent tin, and treated to remove fluorine. An atomic absorption analysis for tin was then made. The fact that the solubility of cassiterite in solutions of HF + NaF is considerably higher than that in HF alone indicates that previously unknown complexing reactions take place in these solutions. Our experiments with a granite-H2O-HF solution indicated that the generation of topaz requires an HF concentration of at least 0.3 mol/kg H2O (about 0.6 percent by weight). Our experimental data indicate that large amounts of tin may be liberated from solution in the process, crystallizing as cassiterite. The widespread deposits of quartz-cassiterite type associated with lithium-fluoride granites, ongonites or topaz rhyolites are likely to have been generated in this manner.