Muscovite

Muscovite
Property	Value	Comments
Formula	KAl2(AlSi3O10)(OH)2	Insoluble in acids.
Crystal System	Monoclinic	Beta = 95 degrees
Crystal Habit	Well formed tabular crystals, micaceous flakes or tablets with irregular outlines (diamond or hexagonal).
Cleavage	{001} perfect	Well displayed, controls fragment orientation.
Color/Pleochroism	Colorless or shades of light green, red, or brown; colorless in thin section
Optic Sign	Biaxial (-)
2V	30-47^°
Optic Orientation	Z=Y X^c = 0^o to 5^o Y^a = 1^o to 3^o O.A.P. perp to (010)
Refractive Indices alpha = beta = gamma =	1.560-1.572 1.593-1.611 1.599-1.615
Birefringence	0.027-0.055 (high)	3rd order, vivid colors of 2nd order
Elongation	yes
Extinction	Parallel to cleavage in all orientations, BIRD'S EYE	The maximum extinction angle is less than 3 degrees
Dispersion
Distinguishing Features	Biotite (dark in color), Talc (smaller 2V), Pyrophyllite (larger 2V)
Occurrence	Muscovite has a widespread occurance and is characteristic of granites and granite pegmatites. Muscovite is very common in large variety of metamorphic rocks including slate, schist, phyllite, gneiss, hornfels and quartzite. Igneous occurances include granite, grandorite, aplite, pegmatite and related felsic rocks.
Editors	Priscilla Delano (02), Sarah Clifthorne (02), Marit Gamberg (01), Jenny McNicholas (11)

	This picture of Muscovite shows the characteristic small cleavage lines as well as the differences between Muscovite and Biotite. In crossed polarized light, the Muscovite shows the bright 2nd order colors against the darker feldspar crystal beside it. Click on the thumbnail for a larger, rollover version.
	This picture of Muscovite shows the characteristic bird's eye extinction. The bright 2nd order colors also contrast against the darker feldspar crystal beside it
	Photomicrograph of muscovite showing 2nd and 3rd order interference colors.
	Another image of muscovite in crossed polarized light.
	Muscovite sample W-12 in cross-polarized light. Click the thumbnail to see a rollover image of the bird's eye texture when the crystal is close to extinction. Bird's eye texture could be be due to damage during slide preperation, but it is also one method of identifying muscovite under the microscope.
	A muscovite crystal from sample W-12 in cross-polarized light. Note the bird's eye texture and feather-like lines running across it. The feather-like lines could be due to deformation and is often seen in muscovite crystals found in granite such as this one. The c-axis of this crystal is nearly vertical. Click the thumbnail to view a rollover of the interference figure.
WWW Images	Brock University - Muscovite U.C.L.A Petrographic Workshop - Muscovite