Annabergite

Formula:  Ni₃(AsO₄)₂^.8H₂O

Origin of name:  After locality; Annaberg, Saxony, Germany

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning:

Unit cell:   a = 10.05 Å, b = 13.30 Å, c = 4.72 Å, b = 102.1°

Colour:  Apple green, gray, pale green to intense yellow-green, white, may be zoned

Diaphaneity: Transparent to translucent

Luster:  Adamantine, pearly on cleavage, may be dull, earthy

Habit:  Crystals prismatic, flattened; striated; usually crystalline crusts, earthy masses

Hardness:  1.5 to 2.5

Specific gravity:  3.07

Cleavage:  3; {010} perfect, {100} and {-102} indistinct

Tenacity:  Flexible in thin laminae, sectile

Streak:  Lighter shade than colour

Synonyms/varieties: Cabrerite (Mg containing variety)

Comments:  Series with erythrite. Member of the vivianite group. An uncommon secondary mineral, formed by the alteration of Co-Ni bearing arsenides and sulphides, in the oxidised zone of hydrothermal mineral deposits.