Formula: Cu4(SO4)(OH)6.H2O
Hydrated sulphate containing hydroxyl
Crystal System: Monoclinic
Specific gravity: 3.32 measured, 3.35 calculated
Hardness: 2 to 3
Streak: Pale blue
Colour: Sky-blue to dark-blue, may be slightly greenish-blue
Solubility: Insoluble in ammonia

Hydrothermal environments

Posnjakite is a secondary mineral formed in the oxidised zone of copper-bearing hydrothermal mineral deposits, commonly of post-mine origin; it may occur in slags. Associated minerals include brochantite, langite, devilline, serpierite, woodwardite, wroewolfeite, aurichalcite, azurite, malachite and chalcopyrite (HOM). It generally forms at low temperatures (Mindat).


At Canaveille, Pyrenees, France, posnjakite and langite pseudomorphs after calcite have been found (KL p193).

At Neudorf, Harz mountains, Saxony-Anhalt, Germany, posnjakite has been discovered on material from the dumps associated with serpierite, gypsum and aragonite (Minrec 43.1.40).

At the type locality, the Nura-Taldy tungsten deposit, Shet, Karaganda Region, Kazakhstan, posnjakite occurs in quartz veins in small grains, as small tabular crystals up to 0.5 mm and as films on fissures in quartz and fluorite. It is associated with aurichalcite and other secondary minerals associated with oxidising chalcopyrite (AM 52.1582-1583).

At Silver Gill, Caldbeck Fells, Cumbria, England, UK, posnjakite occurs with ramsbeckite, schulenbergite and minor dark blue linarite (JRS 8(2).92).

At Copper Creek, Bunker Hill District, Pinal county, Arizona, USA, posnjakite occurs as clusters of tiny crystals with orthoserpierite (R&M 96.2.155).

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