CHROMATE MINERAL

any member of a small group of rare inorganic compounds that have formed from the oxidation of copper-iron-lead sulfide ores containing minor amounts of chromium. A noteworthy occurrence is at Dundas, Tasmania, known for its large, brilliant orange prismatic crystals of crocoite; of trivial economic importance, crocoite is one of the most highly prized of minerals among collectors and museums. The basic structural unit of the chromate minerals is a tetrahedron formed from four oxygen atoms, each at one corner of a tetrahedron surrounding a central chromium atom; thus each CrO4 tetrahedron has a net electric charge of -2, which is neutralized by metal ions outside the tetrahedron.