Powellite

 ( Calcium Minerals ) 

Powellite is a calcium molybdate mineral with formula CaMoO₄. Powellite crystallizes with tetragonal – dipyramidal crystal structure as transparent adamantine blue, greenish-brown, yellow-to-grey typically anhedral forms. It exhibits distinct cleavage, and has a brittle-to-conchoidal fracture. It has a Mohs hardness of 3.5 to 4 and a specific gravity of 4.25. It forms a solid solution series with scheelite (calcium tungstate, CaWO₄). It has refractive index values of n_ω=1.974 and n_ε=1.984.

Powellite was first described by William Harlow Melville in 1891 for an occurrence in the Peacock Mine, Adams County, Idaho, and named for American explorer and geologist, John Wesley Powell (1834–1902).

It occurs in hydrothermal ore deposits of molybdenum within the near-surface Redox zones. It also appears as a rare mineral phase in pegmatite, tactite and basalt. Minerals found in association with powellite include molybdenite, ferrimolybdite, stilbite, laumontite and apophyllite.

• Palache, C., H. Berman, and C. Frondel (1951) Dana’s System of Mineralogy, (7th edition), v. II, pp. 1079–1081.