Muscovite (white mica)



Occurrence
Muscovite is widespread and common in a wide variety of igneous and metamorphic rocks and, to a much lesser extent, in some immature sedimentary rocks. Muscovite is the commonest white mica. Distinguishing it from other white micas and britle micas in thin section can be difficult or impossible. So, many petrologists use the generic phrase "white mica" when describing what they see in thin section, rather than trying to get more specific. A fine grained variety of muscovite, often occurring as an alteration product, is termed sericite.

Composition
Muscovite's chemistry is somewhat variable but generally close to KAl2AlSi3O10(OH)2.

Identification

In general, keys to identifying muscovite are its low relief, lack of color, mica habit and cleavage, and second order interference colors.

Important properties
 ·Color - usually colorless, less commonly pale green and slightly pleochroic.
 ·Habit and cleavage - often appears as tabs or long skinngy flakes which may be bent. It may show one excellent cleavage, depending on grain orientation. Hexagonal tabular crystals are rare.
 ·Relief is low to moderate; often appears as a white transparent mineral with few features (PP).
 ·Second-order interference colors (up to second order red), often with birds-eye or pebbly extinction or wavey pattern of interference colors, especially when looking down on flat side of flakes.
 ·Extinction angle is 0 to a few degrees from cleavage.
 ·Biaxial (-), 2V= 35-50o .

Similar minerals
 ·Biotite, except for the most Mg-rich phlogopite is much more strongly colored. Phlogopite has a smaller 2V than muscovite.
 ·Talc is is very similar but has a smaller 2V.
 ·Very pale chlorite is similar but has lower birefringence and often anomalous interference colors.
 ·Paragonite and margarite often cannot be distinguished from muscovite in thin section. Both, however, are much rarer than muscovite.
 ·Pyrophyllite is very similar but has a higher 2V.


Muscovite in a Muscovite-Biotite-Kyanite Schist

The views above show mostly muscovite (clear in PP) and biotite (brown in PP). Several small grains of kyanite (high relief, near center and lower left of photo) are also present. The long flakey habit is typical of micas, as are the mottled extinction and interference colors (XP view).

The rock shown is a mica schist from Manhattan, New York. Field of view is 2 mm.


 Muscovite in a Mica-Kyanite Schist

The PP view above shows clear muscovite, light brown biotite, and a few grains of clear, high relief, kyanite near the top center. Although the flakey habit, typical of micas, can be seen here, it is not as pronounced as in the previous photos because this view looks down on the flat surface of muscovite flakes instead of at flake edges.

The rock shown is a mica schist from Manhattan, New York. Field of view is 2 mm.


Muscovite in a Biotite Granite

The view above shows a large muscovite grain, about 1.5 mm in longest dimension. Also present are clear (PP light), low birefringence (XP light) plagioclase (upper left), microcline (lower right) and quartz, and a few dark colored flakes of biotite. The quartz, plagioclase, and microcline are best distinguished in the XP view, due to twinning in the feldspars. Note the variable interference colors of the muscovite, due to varying thickness of the grain which results in varying amounts of retardation.

The rock shown is a granite from near Barre, Vermont. The field of view is about 2 mm.


Muscovite in a Granite

The view above shows large muscovite grains surrounded mostly by quartz. The muscovite here shows typical interference colors, but the large extinct grain (black in XP) at the lower right is also muscovite. Note how the muscovite flakes appear to be a bit bent, and also the way quartz appears to have intruded along cleavages.

The rock shown is a granite from near Concord, New Hampshire. The field of view is about 2 mm.