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
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