Chloritoid - Encyclopedia

    Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Monoclinic
    Chemistry : Fe2Al4Si2O10(OH)4
    Rarity : Common


Chloritoid is a silicate typical of rocks of low to medium gradient regional metamorphism (schists, chlorite mica-schists, garnet, muscovite, staurolite, etc...). Less commonly it is found in contact metamorphism, as a product of hydrothermal alteration of lavas, and in certain hydrothermal veins with quartz and carbonates. Its name comes from the Greek khlôros (green) and eidos (appearance) because of its resemblance to chlorites. The chloritoid appears essentially in lamellae with a hexagonal outline, united in lamellar to scaly aggregates, clearly coarser and more brittle than the lamellae of micas : clear, tabular crystals are rare. The chloritoid is dark green to greenish black in color.

Main photo : Chloritoid from Dara-e-Pech, Kunar, Afghanistan © Rob Lavinsky

Chloritoid in the World

Among the many localities, Kosoy Brod in the Urals (Russia), Sverdlovsk (Urals), Gümüsh Dagh near Ephesus (Turkey), and to a lesser extent Chester (Massachussetts) have yielded good crystals. The largest known individualized crystals come from Dara-e-Pech (Afghanistan) and measure 4 cm.

Chloritoid in France

In France, the Île de Groix provided beautiful decimetric lamellar aggregates in quartz, the most spectacular of the national territory.

Right photo : Green chloroitoid and glaucophane from Pointe des Chats, Île de Groix, Morbihan, France © Michel Arliguie

Twinning

Numerous twins are known around the axes [100], [110] and [130] and polysynthetics on {001}.

Fakes and treatments

No fake identified for this mineral species.



Hardness : 6.5
Density : 3.4 to 3.8
Fracture : Irregular
Streak : White, gray, greenish



TP : Translucent
RI : 1.713 to 1.740
Birefringence : 0.010
Optical character : Biaxial +
Pleochroism : Strong
Fluorescence : None


Solubility : Sulfuric acid


Magnetism : Paramagnetic
Radioactivity : None

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