Class : Phosphates, arsenates, vanadates
    Subclass : Hydrated phosphates
    Crystal system : Monoclinic
    Chemistry : FePO₄ 2H₂O
    Rarity : Fairly common

Phosphosiderite is a secondary phosphate of granitic pegmatites, more rarely a component of soils. It is the dimorph of strengite and probably forms a complete series with metavariscite. It owes its name to its chemical composition : phosphorus and iron (sideros in Greek). Phosphosiderite comes from the alteration of primary phosphates such as triphylite or lithiophyllite and usually accompanies other secondary phosphates (heterosite, leucophosphite, etc...). It forms tabular crystals with squat prismatic, more frequently encrustations or botryoidal masses with fibrous textures. The mineral is variable in color but often striking : purple, blue-violet to carmine red and pink, sometimes pinkish-orange or greenish, rarely colorless. It is a mineral that is cut into cabochons for jewelry, and used as an ornamental stone.

Main photo : Phosphosiderite from Bull Moose Mine, Custer, South Dakota, USA © Edgar Müller

In France, phosphosiderite is reported in the pegmatite of La Vilatte and Pecout (Haute-Vienne) but also in Echassières (Allier) and in the pegmatite field of Collioure (Pyrénées-Orientales).

Interpenetration twins on {101} are known.

No fake recorded for this mineral species.