Formula: Fe2+Al2O(SiO4)(OH)2
Nesosilicate (insular SiO4 groups)
Crystal System: Monoclinic
Specific gravity: 3.4 to 3.8 measured, 3.56 calculated
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:
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 chloritoid generally occurs as large crystals in a finer-grained matrix in
association with
muscovite,
chlorite,
staurolite,
garnet and
kyanite.
In contact metamorphism of marble, chloritoid 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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