Arseniosiderite - Encyclopedia

    Class : Phosphates, arsenates, vanadates
    Subclass : Hydrated Arsenates
    Crystal System : Monoclinic
    Chemistry : Ca2Fe3O2(AsO4)3 3H2O
    Rarity : Uncommon

Arseniosiderite is a hydrated arsenate of iron and calcium. It is a secondary mineral which is formed by oxidation of arsenic minerals, mainly arsenopyrite and scorodite. It owes its name to its chemical composition arsenic plus siderite. Flattened, acicular crystals are rare and do not exceed 2 mm. Arseniosiderite forms much more readily fibroradiated aggregates or concretionary masses ranging in color from golden yellow to orange-brown and black-brown.

Main photo : Arseniosiderite from Romanèche, Saône-et-Loire, France © Rob Lavinsky

Arseniosiderite from Falotta, Grisons, Switzerland © Matthias Reinhardt
Arseniosiderite and bariophar-
-macosiderite from Clara Mine, Germany © Franz Zöll
Arseniosiderite from Kamariza Mine, Lavrion, Greece © Gianandrea Ghirri
Arseniosiderite from Clara Mine, Germany © Volker Heck

Arseniosiderite in the World

Arseniosiderite is known from the metamorphic zinc deposits of Franklin and Ogdenbourg (New Jersey) and several other American localities. Beautiful specimens also come from the Ojuela mine (Mapimi, Mexico). Arseniosiderite also exists in several veins in the Black Forest and Saxony (Germany), Cumberland (England), as well as in Tsumeb (Namibia).
Photo : Arseniosiderite and bariopharmacosiderite from Clara Mine, Germany © Michael Förch

Arseniosiderite in France

In France, the discovery deposit, Romanèche (Saône-et-Loire) provided superb fibroradiated samples, perhaps the best known.

Twinning and special crystallizations

No known twin for this mineral species.

Fakes and treatments

No fake or treatments identified for this mineral species.

Hardness : 4.5
Density : 3.6
Fracture : Fibrous
Trace : Brown-yellow

TP : Opaque
RI : 1.792 to 1.898
Birefringence : 0.080
Optical character : Biaxial -
Pleochroism : Strong
Fluorescence : None

Solubility : Acids

Magnetism : None
Radioactivity : None