Annabergite, Nickel Arsenate, [formula Ni3(AsO4)2.8H2O)]
Crystal system: Monoclinic (crystals are rare)
Annabergite is a rare secondary mineral formed by
oxidation of cobalt and nickel arsenides and
sulphides in hydrothermal deposits.
It is sometimes described as an apple green variant of Erythrite. Erythrite is a purple colour and is cobalt arsenate.
The mineral is named after Annaberg in Germany where it was originally found. The specimen in the picture came from Lavrion in Greece.