POSNJAKITE

    Class : Sulfates, chromates, molybdates
    Subclass : Hydrated sulfates
    Crystal system : Monoclinic
    Chemistry : Cu4SO4(OH)6 H2O
    Rarity : Very rare

Posnjakite is an extremely rare hydrated copper sulfate. It is a secondary mineral found in the oxidation zone of quartz copper veins. It was named in honor of Eugene Waldemar Posnjak, a geochemist at the Carnegie Institution of Washington. Posnjakite forms very small tabular crystals (0.5 mm at most), small grains and coatings, of a beautiful blue to dark blue color.

Main photo : Posnjakite from Palhal Mine, Branca, Portugal © Pedro Alves

Posnjakite from Clara Mine, Germany © Michael Förch
Posnjakite & agardite from Clara Mine, Germany © Michael Förch
Posnjakite from Torgal Mine, Beja, Portugal © Pedro Alves
Posnjakite de Lauriéras, Saint-Yrieix-la-Perche, Haute-Vienne, France © Thierry Lesénéchal
Posnjakite from Lauriéras, St-Yrieix-la-Perche, Haute-Vienne, France © Thierry Lesénéchal

Posnjakite in the World

Posnjakite is known in the chalcopyrite and wolframite veins of Nura-Talinsk (Kazakhstan), and in beautiful encrustations at the Drakewalls mine, near Gunnislake, as well as in the neighboring deposit of Fowley Consols (English Cornwall). Posnjakite is also reported, particularly in American and Canadian localities.

Posnjakite in France

In France, posnjakite is known in the La Verrière vein (Rhône), at Villefranche-de-Rouergue (Aveyron), at Faymont (Vosges), at Salsigne (Aude), at St-Yrieix-la-Perche (Haute-Vienne), at Mas Dieu and at St-Laurent-le-Minier (Gard) as well as in Cap Garonne (Var).

Twinning

Twins are common on (110).

Fakes and treatments

No fakes listed for this mineral species.


Hardness : 2 to 3
Density : 3.32
Fracture : Undetermined
Streak : Blue

TP : Translucent to transparent
RI : 1.625 to 1.706
Birefringence : 0.081
Optical character : Biaxial -
Pleochroism : Visible
Fluorescence : None

Solubility : -

Magnetism : NoneRadioactivity : None