MUSCOVITE

General Formula:
KAl2(Al,Si3O10)(OH)2
Sample: PT-51 System:
Monoclinic
Muscovite Flakes
Elongated, colourless flakes of muscovite, in a pelite from the Grenville Supergroup, Central Metasedimentary Belt of the Grenville Province.
Field of view = 2.7 mm, plane light
Muscovite Flakes
The flakes of muscovite display the characteristic second order blue interference colour. Note that the muscovite flakes define the fabric in the sample, with all exhibiting an EW orientation.
Field of view = 2.7 mm, crossed polars
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties

Colour
Pleochroism
colourless
non pleochroic
Form found as micaceous flakes or tablets with irregular outlines
Relief
RI
moderate positive
nalpha = 1.552-1.580
nbeta = 1.582-1.620
ngamma = 1.587-1.623
Cleavage perfect on {001}
Birefringence
Interference Colours
0.036-0.049
vivid second order blues and greens
Twinning rare
Interference Figure
Optic Sign
2V
biaxial
negative
30-47°
Optic Orientation parallel extinction, cleavage traces are length slow
Composition highly variable Alteration not generally altered
Occurrence common in a wide variety of metamorphic rocks, felsic igneous rocks and as detrital grains in sedimentary rocks Distinguishing Features colourless, parallel extinction, "birds-eye" extinction


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