Chloritoid

nesosilicate

corundum

Formula: Fe2+Al2O(SiO4)(OH)2 nesosilicate (insular SiO4 groups)
Specific gravity: 3.4 to 3.8
Hardness: 6½
Streak: Colourless, green, grey
Colour: Dark green to green-gray or nearly black.
Solubility: Insoluble in water; soluble in sulphuric acid with decomposition
Environments:

Metamorphic environments

Chloritoid is a relatively common constituent of low to medium-grade regionally metamorphosed clay-rich rocks, particularly those rich in aluminium and ferric iron, and poor in calcium, magnesium, potassium and sodium, and it sometimes occurs due to contact metamorphism of marble.
Chloritoid may be found in phyllite, schist and marble.
In regional metamorphic environments it generally occurs as large crystals in a finer-grained matrix in association with muscovite, chlorite, staurolite, garnet and kyanite.
In contact metamorphism of marble, it may be associated with corundum and quartz, and also it sometimes occurs with corundum in emery deposits.
In metamorphic environments chloritoid first appears at the beginning of the greenschist facies, and it is also a mineral of the blueschist and eclogite facies.

Alteration

chloritoid and andalusite to staurolite and quartz and H2O
4Fe2+Al2O(SiO4)(OH)2 + 5Al2OSiO4 ⇌ 22Fe2+2Al9Si4O23(OH) + SiO2 + 3H2O
Increasing temperature favours the forward reaction. At higher pressure kyanite replaces andalusite in the above reaction (AM61.699-709).

chloritoid and quartz to staurolite, almandine and H2O
23Fe2+Al2O(SiO4)(OH)2 + 8SiO2 ⇌ 4Fe2+2Al9Si4O23(OH) + 5Fe2+3Al2(SiO4)3 + 21H2O
(DHZ 1A p844)

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