Vermiculite

minerals

muscovite

phlogopite

kaolinite

Formula: Mg0.7(Mg,Fe,Al)6(Si,Al)8O20(OH)4.8H2O
Phyllosilicate (sheet silicate)
Specific gravity: 2.3 to 2.7
Hardness: 1½ to 2
Streak: greenish white
Colour: brown, bronze-yellow
Common impurities: Ca, Na, K
Environments:

Carbonatites
Metamorphic environments (typical)
Hydrothermal environments

Vermiculite is an alteration product of iron-bearing phlogopite or biotite. It occurs at the interface between feldspar-rich and iron- and magnesium-rich igneous rocks, and it also occurs in carbonatites and metamorphosed limestone.
Primary vermiculite is unlikely to form at temperatures above 200 to 300oC (DHZ 3 p255).
In metamorphic environments it is associated with corundum, apatite, serpentine, chlorite and talc (DHZ 3 p255).

Alteration

In the zeolite facies clay minerals transform to muscovite variety illite, kaolinite and vermiculite.

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