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.[4]

Annabergite from Lavrion (Laurium), Greece

Annabergite
Annabergite, Dobšiná, Slovakia
General
Category	Arsenate minerals Vivianite group
Formula (repeating unit)	Ni3(AsO4)2·8H2O
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	[1][2][3]

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.

References

1. Handbook of Mineralogy
2. Mindat.org
3. Webmineral data
4.  One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Annabergite". Encyclopædia Britannica. 2 (11th ed.). Cambridge University Press. p. 59.