Cacoxenite - Encyclopedia

Class : Phosphates, arsenates, vanadates
Subclass : Hydrated phosphates
Crystal system : Hexagonal
Chemistry : Fe₂₄AlO₆(PO₄)₁₇(OH)₁₂ 75H₂O
Rarity : Quite common

Cacoxenite is a hydrated iron and aluminum phosphate. It is a secondary mineral that associates with other secondary phosphates and limonite in the surface oxidation zone of many iron deposits. It is present, but more discreet, always as a secondary mineral, in phosphate pegmatites. Its name comes from the Greek kakos (bad) and xenos (foreign), due to the phosphorus contained in this mineral which alters the quality of the silty iron ore which contains it. Its crystals are usually tiny, acicular, and cocoxenite presents itself in the very typical habit of radiated or concentric spherolites not exceeding 8 mm ; it appears more rarely in disordered sheaves of fibrous crystals. Its color is reddish orange, yellow to yellow-brown, sometimes brown.

Main photo : Cacoxenite from La Fumade, Tarn, France © Jean-Marc Johannet

Cacoxenite from La Fumade, Tarn, France © Serge Lavarde

Cacoxenite from the La Lande quarry, Morbihan, France © Luigi Chiappino

Cacoxenite from La Minas del Horcajo, Spain © Christian Rewitzer

Cacoxenite in the World

The most spectacular samples of cacoxenite come from the Rock Run Station mine (Alabama) which yielded groups of the largest known crystals (8 mm). It is a mineral also quite common in other iron mines in Arkansas, Pennsylvania, New York, etc. It is also known in the pegmatite of Palermo (New Hampshire). Cacoxenite is well represented in Germany (Hagendorf pegmatite and Amberg-Auerbach iron deposit in Bavaria, as well as in iron deposits in the region of Giessen, Hesse, etc.). It is also found in beautiful specimens at Minas del Horcajo and La Paloma (Spain), at Hrbek Mine as well as at the Trenice and Treskov quarries (Czech Republic) and in many other localities.

Cacoxenite in France

In France, we find cacoxenite in millimetric spherulites in very numerous occurrences, the best samples come from Echassières (Allier), La Fumade (Tarn), the La Lande quarry (Morbihan), Villefranche-de-Rouergue (Aveyron).

Twinning

No twin known for this mineral species.

Fakes and treatments

No fake identified for this mineral species.

Hardness : 3 to 4
Density : 2.2 to 2.6
Fracture : Irregular
Streak : White

TP : Translucent
RI : 1.575 to 1.656
Birefringence : 0.060
Optical character : Uniaxial +
Pleochroism : Visible
Fluorescence : None

Solubility : Acids

Magnetism : None
Radioactivity : None