Muscovite | ||
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Value |
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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) |
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Refractive Indices
alpha = beta = gamma = |
1.552-1.576 1.582-1.615 1.587-1.618 |
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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 |
Dispersion | ||
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. | |
Occurrence | 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. |
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Editors | Priscilla Delano ('02), Sarah Clifthorne ('02), Marit Gamberg ('01), Jenny McNicholas ('11), Kathryn Durkin ('12), Theo Sweezy ('14) |