Frankdicksonite
Frankdicksonite is a halide mineral with the chemical formula BaF2 which corresponds to the chemical compound barium fluoride. It occurs in the Carlin gold deposit of Eureka County, Nevada as cubic crystals sized between 0.1 and 4 mm, and is of hydrothermal origin. Its only associated mineral is quartz and the frankdicksonite crystals are always completely encapsulated in it. Frankdicksonite has fluorite crystal structure with a cubic symmetry and the lattice constant a = 619.64 pm. Its Vickers hardness on the {111} cleavage crystal faces varies between 88 and 94 kg/mm and is close to that of the synthetic barium fluoride (95 kg/mm). Its refractive index (1.475) is almost identical to that of BaF2 (1.474). Under electron irradiation, it emits strong blue cathodoluminescence. The major impurity in frankdicksonite is strontium with concentrations up to 0.5% by weight. Also present are silicon (0.02%) and magnesium (0.0015%); other impurities have concentrations below 0.0015%.