(named after the American geologist J. Powell), a mineral, calcium molybdate (CaMoO₄). Powellite contains 28.48 percent CaO and 71.52 percent M0O3; some varieties contain up to 10.3 percent WO₃. The mineral crystallizes in the tetragonal system. Sometimes it is found in the form of small crystals of a lamellar or dipyramidal habit. Most often it forms cryptocrystalline aggregates—products of alteration of molybdenite—which sometimes preserve the foliated structure typical of molybdenite. Its color ranges from white to dark green and brown. Powellite has a hardness of 3.5 on Mohs’ scale and a density of 4,250–4,520kg/m³. In ultraviolet light a yellow luminescence is usually detected. Powellite is a characteristic mineral of the oxidized zone of the ores of molybdenum deposits.