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 my 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. Metamorphic rocks of nearly all kind and grades contain muscovite. In zones of regional metamorphism, it is one of the first minerals to appear; in high grade rocks, such as greenschist and anfibolite, muscovite tends to dissociate in favor of microcline and sillimanite.

Optical properties:

• Form: Micaceous crystals
• Color: Colourless
• Relief: Moderate
• Cleavage: (001) perfect
• Interference colors: Moderate to strong in all section normal to cleavage, weak in basal section

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Photo
Muscovite crystals in a gneiss. XPL image, 2x (Field of view = 7mm)	Muscovite crystals in a gneiss. XPL image, 2x (Field of view = 7mm)	Muscovite crystals in a gneiss. XPL image, 2x (Field of view = 7mm)
Muscovite crystals. XPL image, 2x (Field of view = 7mm)	Muscovite crystals. XPL image, 2x (Field of view = 7mm)	Muscovite crystals. XPL image, 2x (Field of view = 7mm)
Muscovite crystals. XPL image, 2x (Field of view = 7mm)	Muscovite crystals. XPL image, 2x (Field of view = 7mm)	Muscovite crystals. XPL image, 2x (Field of view = 7mm)
Lepidoblastic muscovite layers. Italian Alps. XPL image, 2x (Field of view = 7mm)	Lepidoblastic muscovite layers. Italian Alps. XPL image, 2x (Field of view = 7mm)	Lepidoblastic muscovite layers. Italian Alps. XPL image, 2x (Field of view = 7mm)
Lepidoblastic muscovite layers. Italian Alps. XPL image, 2x (Field of view = 7mm)	Lepidoblastic muscovite layers. Italian Alps. XPL image, 2x (Field of view = 7mm)	Lepidoblastic muscovite layers. Italian Alps. XPL image, 2x (Field of view = 7mm)
Lepidoblastic muscovite layers. Italian Alps. XPL image, 2x (Field of view = 7mm)	Lepidoblastic muscovite layers. Italian Alps. XPL image, 2x (Field of view = 7mm)	Lepidoblastic muscovite layers and granoblastic quartz layers. Apuan Alps, Italy. XPL image, 2x (Field of view = 7mm)
Lepidoblastic muscovite layers and granoblastic quartz layers. Apuan Alps, Italy. XPL image, 2x (Field of view = 7mm)	Lepidoblastic muscovite layers and granoblastic quartz layers. Apuan Alps, Italy. XPL image, 2x (Field of view = 7mm)	Lepidoblastic Muscovite layer. Dora Maira massif, Italy. XPL image, 2x (Field of view = 7mm)
Lepidoblastic Muscovite layer. Dora Maira massif, Italy. XPL image, 2x (Field of view = 7mm)	Lepidoblastic Muscovite layer. Dora Maira massif, Italy. XPL image, 2x (Field of view = 7mm)	Lepidoblastic Muscovite layer. Dora Maira massif, Italy. XPL image, 2x (Field of view = 7mm)
Lepidoblastic Muscovite layer. Dora Maira massif, Italy. XPL image, 2x (Field of view = 7mm)	Lepidoblastic Muscovite layer. Dora Maira massif, Italy. XPL image, 2x (Field of view = 7mm)	Lepidoblastic Muscovite layer. Dora Maira massif, Italy. XPL image, 2x (Field of view = 7mm)
Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)	Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)	Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)
Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)	Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)	Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)
Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)	Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)	Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)
Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)	Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)	Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)
Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)	Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)	Kink folds in lepidoblastic muscovite layers. XPL image, 2x (Field of view = 7mm)

Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

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

Muscovite - K₂Al₄[Si₆Al₂O₂₀](OH,F)₄