Weathering products of scheelite and wolframite as a function of environmental pH

Tungsten is considered to be inert to weathering and benign if released to the environment. The main tungsten ore minerals, scheelite and wolframite, seem to be weathering very slowly or not at all. In our research in the oxidation zone of the Ochtiná W-Mo deposit (eastern Slovakia), we observe complete weathering of the W minerals and release of W into the environment as a function of pH. The primary ores consist of quartz, pyrite, scheelite, and wolframite. The secondary minerals are iron oxides, identified as goethite by Raman spectroscopy. Some of the iron oxides have the morphology typical of schwertmannite. These Fe oxides contain 0-5 wt.% WO₃ and are products of weathering of the primary W minerals under mildly acidic conditions. Their textures change if the primary mineral was pyrite, wolframite, or scheelite. High porosity in the weathered W minerals suggest release of material, especially of W, into water percolating through the oxidation zone. X-ray absorption spectroscopy shows that W is adsorbed on the Fe oxides. In some samples, the weathering assemblage includes tungstite, hydrotungstite, W-bearing jarosite, goyazite, and baryte. Among these minerals, especially jarosite is an indicator of strongly acidic conditions. The occurrence of either Fe oxides or W oxides can be rationalized by calculation of their solubility curves as a function of pH. The solubility of Fe oxides attains its minimum at circumneutral conditions whereas that of the W oxides in strongly acidic media. This study explains the behavior of W in mining waste.