GeoPicture of the week: Crocoite from Tasmania

Crocoite is a fairly rare mineral in many parts of the world, consisting of lead chromate, PbCrO4. The relative rareness comes from the way it forms: it requires an oxidation zone of lead ore bed and presence of what are called ultramafic rocks, which act as a source of chromium. Ultramafic rocks are 90% mafic minerals (dark colored, high magnesium and iron content) – the mantle is composed of such rocks.