PHARMACOSIDERITE

    Class : Phosphates, arsenates, vanadates
    Subclass : Hydrated arsenates
    Crystal system : Cubic
    Chemistry : KFe4(AsO4)3(OH)4 6-7H2O
    Rarity : Fairly common to rare

Pharmacosiderite is an uncommon arsenate, forming by alteration of iron arsenides and sulfides containing iron and arsenic (arsenopyrite, löllingite, tennantite). It is therefore frequently encountered in the gossans of arsenic, tin, tungsten hydrothermal veins, commonly associated with scorodite and limonite. Water is weakly bound to the crystal lattice, as in zeolites. Its name comes from the Greek pharmakon (remedy, poison) and sideros (iron), in allusion to its arsenic and iron content. Pharmacosiderite is translucent to transparent, its luster is greasy to adamantine. By exposure to light, its color very often changes from olive green or emerald green to yellow-green, sometimes it is orangy red. It occurs most of the time in small pseudocubic crystals reaching a maximum of 5 mm, more rarely in small tetrahedra, in coatings or in earthy encrustations.


Main photo : Pharmacosiderite from Vaulry, Haute-Vienne, France © Patrice Queneau

Pharmacosiderite from Wheal Gorland, Cornwall, England © Marko Burkhardt
Pharmacosiderite from Penberthy Croft Mine, Cornwall, England © Steve Rust
Pharmacosiderite from Lone Tree Mine, Nevada, USA © Tony Peterson
Pharmacosiderite from Broken Hill, Australia © John Haupt

Pharmacosiderite in the World

The mines of the Gwennap district in English Cornwall (Wheal Gorland, Wheal Unity and Wheal Muttrel) yielded the best pharmacosiderite crystals : 5 mm individuals implanted on limonite. The polymetallic veins of Saxony (Schneeberg, Schwarzenberg) and the Black Forest (Clara mine) gave small crystals rich in barium, as did the Greek mine of Kamariza in Lavrion. The largest known crystals are centimeter and come from Brazil Lake (Ontario, Canada), the Majuba Hill mine (Nevada) would also have provided 1 cm crystals, but this is not confirmed.

Pharmacosiderite in France

In France, pharmacosiderite is known in small veneers of a few square centimeters composed of submillimeter crystals in Cap Garonne (Var). It is also reported at Echassières (Allier), at the La Bessade mine (Haute-Loire), at Rompas and Villapourçon (Nièvre), at Vaulry, Bellac and Puy-les-Vignes (Haute-Vienne), at Kerarsaoz (Finistère), at the Tistoulet mine (Aude), at Villefranche-de-Rouergue and at Kaymar (Aveyron), at Franciman (Tarn), at Ste-Marie-aux-Mines (Haut-Rhin) and in many other localities.

Twinning

No twin known for this mineral species.

Fakes and treatments

No fakes listed for this mineral species.


Hardness : 2.5
Density : 2.797
Fracture : Irregular
Streak : White

TP : Translucent to transparent
RI : 1.660 to 1.704
Birefringence : 0,005 (anomalous)
Optical character : Biaxial (anomalous)
Pleochroism : None
Fluorescence : None

Solubility : Hydrochloric acid

Magnetism : NoneRadioactivity : None

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