Erythrite

BlackErythrite is a hydrated cobalt arsenate mineral with the chemical composition cobalt arsenate octahydrate (Co₃(AsO₄)₂·8H₂O). It crystallizes in the monoclinic system and typically forms as prismatic to acicular (needle-like) crystals, fibrous or radiating aggregates, and sometimes as earthy or powdery coatings in the oxidized zones of cobalt-nickel-arsenic ore deposits. The vibrant crimson to peach-red or pale rose coloration of erythrite is directly linked to the presence of cobalt(II) ions in its structure, which absorb light in such a way as to give those distinct hues. Erythrite is primarily a secondary mineral, forming when primary cobalt-bearing arsenide or sulfide minerals (such as cobaltite or skutterudite) undergo oxidation and weathering.

Typical localities include the Schneeberg district in Saxony, Germany (its type locality at the Daniel Mine), the Bou Azzer District of Morocco, Cobalt, Ontario in Canada, and Sonora, Mexico. It often occurs in association with minerals like annabergite (its nickel analogue), malachite, scorodite, and other arsenates/oxides in the supergene oxidation zones of cobalt-nickel arsenide ore bodies. Because of its softness (Mohs hardness ≈ 1. 5–2. 5) and perfect cleavage on {010}, erythrite is fragile and must be handled with care.