Smith College - Geology 222b - Petrology

Petrographic Data File

Formula KAl2(AlSi3O10)(OH)2 Solid solution mineral; substitutions can occur for K, [VI]Al, (OH) and in tetrahedral sites.
Crystal System Monoclinic (2/m) TOT + c crystal structure
Crystal Habit Well formed crystals are tabular and have pseudohexagonal outlines. More often found as micaceous flakes or tablets with irregular outlines. Well-formed crystals are sometimes called "books" in reference to their flaky nature.
Cleavage {001} perfect Well displayed, controls fragment orientation.
Color/Pleochroism Colorless or shades of light green, red, or brown in hand sample; colorless in thin section. No pleochroism.  
Optic Sign Biaxial (-) Muscovite is length slow
2V 28-47o  
Optic Orientation Z=b 
X^c = +1o to +4o
Y^a = +1o to +3o
O.A.P. perp to (010)
Refractive Indices
alpha = 
beta = 
gamma = 

Birefringence  0.036-0.049 (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 
Distinguishing Features Biotite (darker in color and is pleochroic in ppl), Talc (smaller 2V), Pyrophyllite (larger 2V), birds-eye extinction and cleavage are also distinctive.

Muscovite has a widespread occurance and is characteristic of sedimentary, igneous and metamorphic rocks.

Sediments eroded from igneous and metamorphic rocks often carry muscovite, accounting for its presence in sedimentary rocks.

Igneous occurances include granite, grandorite, aplite, pegmatite and related felsic rocks. is common in granites and granitic pegmatites.

Muscovite is very common in large variety of metamorphic rocks including slate, schist, phyllite, gneiss, hornfels and quartzite.

Editors Priscilla Delano ('02), Sarah Clifthorne ('02), Marit Gamberg ('01), Jenny McNicholas ('11), Kathryn Durkin ('12), Theo Sweezy ('14)

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.

Rollover image showing muscovite from sample W-11 as length slow.

This property is observed when the stage is rotated until the crystal is exinct. The stage is then rotated another 45 degrees so the crystal is lying NE-SW and the gypsum plate is inserted. If the interference colors compound the sample is length slow, if they subtract the sample is length fast.

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.


Muscovite from sample DR-4 in plane polarized light. Click on the thumbnail to see a rollover image in cross-polars.

PE-19 Muscovite in thin section from sample PE-19. The green muscovite in the center of the image displays twinning, as does the piece to its left. Click to see a rollover image that shows the twinning as the thin section is rotated and gives a better view of the "birds-eye" extinction.
Another view of PE-19 showing textures at extinction.
WWW Images

Return to Petrography Index