Muscovite - K2Al4[Si6Al2O20](OH,F)4

The name muscovite comes from Muscovy-glass, a name given to the mineral in Elizabethan England due to its use in medieval Russia as a cheaper alternative to glass in windows. Muscovite in the basic aluminium-potash mica with basic formula K2Al4[Si6Al2O20](OH,F)4. Na may replaces K in Muscovite to form a distinct minerals called paragonite with similar optical properties.

Muscovite is the most common of the Mica Group minerals, it is typically found as massively crystalline material in "books" or in flaky grains as a constituent of many rock types. It is clear with a pearly luster on cleavage faces, often having a sparkly look in rocks. It can form a continuous series with celadonite and aluminoceladonite; intermediates are known as the variety phengite and K-deficient variants as illite.

Large alkalis (Rb and Cs) and some alkaline earths (Ca, Sr, Ba) may appears as minor impurities replacing K, Ba-rich muscovite is called Oellacherite. In Octahedral coordination, Vanadium (V3+) may appear as a very major constituent to form Roscoelite. Li+ and Cr3+ are often significant, forming Lithian muscovite and Fuchsite respectively.

Muscovite is more characteristic of metamorphic rocks than of igneous rocks, where Aluminia content seldom exeeds that required by feldspar. It appears however in granitic rocks associated with biotite and microcline, where it may crystallize at high pressures directly from the magma.

Optical properties:
Color: Colourless
Habit: Micaceous
Cleavage: (001) perfect
Interference colors: Moderate to strong in all section normal to cleavage, weak in basal section
Relief: Moderate

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Red garnet and silver-gray muscovite crystals in a granitic pegmatite. From Sand Atlas




Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc


Photo
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Muscovite crystals in a Granite. XPL image. 2x (Field of view = 7mm)
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Muscovite crystals in a Granite. XPL image. 2x (Field of view = 7mm)
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Muscovite crystals in a Granite. XPL image. 10x (Field of view = 2mm)
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Muscovite crystals in a Granite. XPL image. 2x (Field of view = 7mm)
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Muscovite crystals in a Granite. XPL image. 2x (Field of view = 7mm)
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Muscovite crystals in a Granite. XPL image. 10x (Field of view = 2mm)
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Muscovite crystals in a Granite. XPL image. 2x (Field of view = 7mm)