Keyite

Keyite is a mineral with the chemical formula Cu2+3Zn₄Cd₂(AsO₄)₆ · 2H₂O.^[2] The name comes from Charles Locke Key (born 1935), an American mineral dealer who furnished its first specimens.^[3] Keyite is monoclinic-prismatic, meaning its crystal form has three unequal axes, two of which have 90° angles between them and one with an angle less than 90°.^[3] Keyite belongs to the biaxial optical class, meaning it has more than one axis of anisotropy (optic axis), in which light travels with zero birefringence, and three indices of refraction, nα = 1.800, nβ, and nγ = 1.870.^[5][4] Being a very rare cadmium copper arsenate, keyite is only found in Tsumeb, Namibia in the Tsumeb mine, a world-famous copper mine known for its abundance of rare and unusual minerals.^[3]

Keyite
Keyite on adamite (greenish)
General
Category	Arsenate minerals
Formula	Cu2+3Zn4Cd2(AsO4)6 · 2H2O
IMA symbol	Key[1]
Strunz classification	8.CA.50
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	I2/a
Unit cell	a = 11.654(3) Å b = 12.780(5) Å c = 6.840(3) Å β = 99.11°; Z = 2
Identification
Formula mass	1,520.19 g/mol
Color	azure/ blue
Crystal habit	prismatic, tabular
Cleavage	good (001) cleavage
Mohs scale hardness	3.5 - 4
Streak	light blue
Diaphaneity	translucent
Density	5.106g/cm3
Optical properties	biaxial
Refractive index	nα = 1.800, nβ, and nγ = 1.870
Birefringence	0.070
Pleochroism	x: pale blue, y: greenish blue, z: deep blue
References	[2] [3] [4]