Annabergite

Annabergite is an arsenate mineral consisting of a hydrous nickel arsenate, Ni₃(AsO₄)₂·8H₂O, crystallizing in the monoclinic system and isomorphous with vivianite and erythrite. Crystals are minute and capillary and rarely met with, the mineral occurring usually as soft earthy masses and encrustations. A fine apple-green color is its characteristic feature. It was long known (since 1758) under the name nickel bloom; the name annabergite was proposed by H. J. Brooke and W H. Miller in 1852, from Annaberg in Saxony, one of the localities of the mineral. It occurs with ores of nickel, of which it is a product of alteration. A variety, from Creetown in Kirkcudbrightshire, in which a portion of the nickel is replaced by calcium, has been called dudgeonite, after P. Dudgeon, who found it.[5]

Annabergite
Annabergite (green) from Lavrion, Greece
General
Category	Arsenate minerals
Formula (repeating unit)	Ni3(AsO4)2·8H2O
IMA symbol	Anb[1]
Strunz classification	8.CE.40
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Unit cell	a = 10.179(2), b = 13.309(3) c = 4.725(1) [Å]; β = 105(1)°; Z = 2
Identification
Color	Apple-green, pale green, pale rose or pale pink, white, gray; may be zoned
Crystal habit	Usually as fibrous veinlets, crystalline crusts, or earthy; rare as well formed crystals
Cleavage	Perfect on {010}, indistinct on {100} and {102}
Tenacity	Sectile
Mohs scale hardness	1.5 – 2.5
Luster	Subadamantine, pearly on cleavages, may be dull or earthy
Streak	Pale green to white
Diaphaneity	Transparent to translucent
Specific gravity	3.07
Optical properties	Biaxial (-)
Refractive index	nα = 1.622 nβ = 1.658 nγ = 1.687
Birefringence	δ = 0.065
2V angle	Measured: 84°
References	[2][3][4]

Closely related is cabrerite wherein some of the nickel is replaced by magnesium. It is named for Sierra Cabrera in Spain where it was originally found.