Muscovite KAl2(OH)2(AlSi3O10)
Optical Properties:
Biaxial negative, small 2V (30-40 degrees); clear in plane polarized
light; high birefringence, producing up to third order colors; 1 excellent
cleavage; parallel extinction. Muscovite is a distinctive mineral, because
of its high birefringence, parallel extinction, and excellent basal cleavage.
The two photographs above show muscovite in crossed polarizers (left photo)
and uncrossed polarizers (right photo) in granite. In the left picture,
muscovite shows yellow to blue interference colors; it is colorless and
clear in uncrossed polarizers. Microcline is
in the lower right and at the right edges of the photographs, and biotite
is the pleochroic (colored) mineral most clearly visible in the right photo.
In the left photo below (crossed polarizers), a muscovite grain shows high
birefringence and bent cleavage planes. In the plane polarized light view
on the right, clear, subhedral muscovite cuts diagonally across the picture
and appears to penetrate the dark biotite grain.
Occurrence:
Common in pegmatites, low to medium grade metamorphic rocks,
and present in some granites.
Petrography
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