What is Zalesiite?
Zalesiite is a rare hydrated calcium copper arsenate, usually forming green to blue-green crusts and radiating aggregates in oxidised copper-arsenic deposits.
Mineralogy and structure
Reference formula: CaCu6(AsO4)2(AsO3OH)(OH)6·3H2O. Zalesiite is hexagonal with variable hydrated copper-arsenate chemistry. The idealised formula is CaCu6(AsO4)2(AsO3OH)(OH)6·3H2O; a representative COD model is used and the arsenic content is made explicit for safe handling. Zalesiite is classified with arsenates and has hexagonal crystal symmetry. Typical hardness is 3–4 on the Mohs scale and specific gravity is approximately 3.5–4.0.
Colour, lustre and habit
Typical colours include pale green, blue-green, green. The usual botryoidal, crusty, radiating habit and vitreous, pearly, dull lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Zalesiite forms in or is associated with secondary oxidation of copper-arsenic sulfides and arsenide-bearing hydrothermal veins. Important documented localities include Czechia, Mexico, Austria, Japan and other oxidised copper-arsenic districts.
Blue-green oxidation colour with a rare-arsenate edge
Zalesiite sells to informed collectors through its colour and scarcity. A green crust with a documented copper-arsenic locality is a far stronger specimen than an unlabelled look-alike.
Collecting and care
Arsenic-bearing and soft: keep enclosed, avoid dust, acids and skin contact, and wash hands after handling. Do not use for jewellery or wellness claims.
