Powellite

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.[2]

Powellite
General
Category	Molybdate minerals
Formula (repeating unit)	CaMoO4
Strunz classification	7.GA.05
Crystal system	Tetragonal
Crystal class	Dipyramidal (4/m) (same H-M symbol)
Space group	I41/a
Unit cell	a = 5.222 Å, c = 11.425 Å; Z = 4
Identification
Formula mass	200.02 g/mol
Color	Straw-yellow, greenish yellow, yellow-brown, brown, colorless, may show blue to black zones
Crystal habit	Flat tabular crystals often paper-thin on {001}, may be crusty to pulverulent or massive
Cleavage	Indistinct on {011}, {112} and {001}
Fracture	Conchoidal
Mohs scale hardness	3.5-4
Luster	Adamantine
Streak	light yellow
Diaphaneity	Transparent
Specific gravity	4.25
Optical properties	Uniaxial (+)
Refractive index	nω = 1.974 nε = 1.984
Birefringence	δ = 0.010
Pleochroism	O = blue; E = green
Ultraviolet fluorescence	Fluoresces bright yellow under shortwave ultraviolet light, dimmer under longwave
References	[1][2][3][4]

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).[2]

It occurs in hydrothermal ore deposits of molybdenum within the near surface oxidized 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.[4]