Formula: Mg0.7(Mg,Fe,Al)6(Si,Al)8O20(OH)4.8H2O
Phyllosilicate (sheet silicate), smectite group
Crystal System: Monoclinic
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).
Localities
At lots 10 and 11 of concession 1, Bathurst Township, Lanark County, Ontario, Canada (DeWitts corner), the deposit is
located in the Grenville Geological Province, which consists mostly of
marble, gneiss, and
quartzite.
Syenite-migmatite was
also reported in the area where the vein-dikes are located. Characteristic features of the vein-dikes include the
fact that perfectly formed euhedral crystals of different minerals can often be found floating in
calcite with no points of contact with the walls. Sometimes these crystals
have inclusions of calcite, irregular or rounded in shape. It has been argued
that at least some of the vein-dikes were formed as a result of melting of Grenville
marble.
Vermiculite forms yellow 1-mm flakes replacing phlogopite
(R&M 97.6.556-564).
Alteration
In the zeolite facies
clay
minerals transform to muscovite variety illite,
kaolinite
and vermiculite.
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