Class : Silicates
    Subclass : Phyllosilicates
    Crystal system : Monoclinic
    Chemistry : (Fe,Mg)₅Al(Si₃Al)O₁₀ (OH,O)₈
    Rarity : Common

Chamosite is one of the four main species of the group of chlorites, common phyllosilicates of magnesium, iron and aluminum. It is the iron term of the group, which constitutes a continuous series with the magnesium term : clinochlore. Its name comes from its discovery site : Chamoson (Valais, Switzerland). Chamosite is mainly the chlorite of oolitic iron-bearing layers, of sedimentary origin, where it accompanies siderite and hematite in abundance. Chamosite is also common in low-gradient metamorphic rocks (chloritoschists, zeolite facies), as well as in altered magmatic rocks (basalts, spilites, gabbros, etc...) and alteration halos around hydrothermal deposits (alteration called "propylitic"). In these alteration contexts, chamosite comes from the transformation of ferromagnesian minerals (biotite, amphibole, pyroxene). Like other chlorites, chamosite occurs in chunky, pseudohexagonal lamellar crystals flattened on {001}, sometimes taking the appearance of barrels or prisms with horizontally striated faces. Sector twins are not rare. It also occurs in foliaceous, vermicular, or scaly aggregates and in compact, finely crystalline to cryptocrystalline masses. The color is usually dark green to greenish black, exceptionally dark gray. Chamosite, and chlorites in general, have only very marginal industrial use. Chlorite powder is sometimes used in decoration to enhance the shine of wallpaper.

Main photo : Chamosite from Huanuni Mine, Oruro, Bolivia © Uwe Haubenreisser

Twins are known.

Chamosite can completely replace almandine garnets (pseudomorph), notably at the Michigamme Mine (Michigan).

No fake identified for this mineral species.