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Muscovite

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Muscovite is the most common mineral of the mica own family. It is an essential rock-forming mineral present in igneous, metamorphic, and sedimentary rocks. Like other micas it with no trouble cleaves into skinny transparent sheets. Muscovite sheets have a pearly to vitreous luster on their floor. If they’re held up to the light, they’re obvious and almost colorless, however maximum have a mild brown, yellow, inexperienced, or rose-color tint.

Name: From \Muscovy glass,” for an occurrence in the old province of Muscovy, Russia.

Polymorphism & Series: 2M1 ; 1M, 3A polytypes; interstrati¯es with vermiculite, paragonite, montmorillonite.

Mineral Group: Mica group

Association: Quartz, plagioclase, potassic feldspar, biotite, tourmaline, topaz

Diagnostic Features: Characterized by its highly perfect cleavage and light color. Distinguished from phlogopite by not being decomposed in sulfuric acid and from lepidolite by not giving a crimson flame.

Physical Properties of Muscovite

Chemical ClassificationSilicate
ColorThick specimens often appear to be black, brown, or silver in color; however, when split into thin sheets muscovite is colorless, sometimes with a tint of brown, yellow, green, or rose
StreakWhite, often sheds tiny flakes
LusterPearly to vitreous
DiaphaneityTransparent to translucent
CleavagePerfect
Mohs Hardness2.5 to 3
Specific Gravity2.8 to 2.9
Diagnostic PropertiesCleavage, color, transparency
Chemical CompositionKAl2(Si3AlO10)(OH)2
Crystal SystemMonoclinic
UsesUsed in the manufacturing of paint, joint compound, plastics rubber, asphalt roofing, cosmetics, drilling mud.

Muscovite Chemical Composition

Muscovite is a potassium-rich mica with the following generalized composition…

KAl2(AlSi3O10)(OH)2

In this formula potassium is sometimes replaced by other ions with a single positive charge such as sodium, rubidium, or cesium. Aluminum is sometimes replaced by magnesium, iron, lithium, chromium, or vanadium.

Uses of Muscovite

Used chiefly as an insulating material in the manufacture of electrical apparatus. Many of the small parts used for electrical insulation are built up of thin sheets of mica cemented together. They may thus be pressed into shape before the cement hardens. Most of the mica used for this purpose in the United States is imported from India. Used as a transparent material, isinglass, for stove doors, lanterns, etc. Scrap mica, or the waste material in the manufacture of sheet mica, is used in many ways, as in the manufacture of wall papers to’give them a shiny luster; as a lubricant when mixed with oils; as a nonconductor of heat; and as a fireproofing material.

Optical Properties of Muscovite

Muscovite clear under PPL but with perfect cleavage and bright interference colours.

 

Muscovite
Property
Value
Comments
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 =
1.552-1.576
1.582-1.615
1.587-1.618
 
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.

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

 

Occurrence

Muscovite is a widespread and very common rockforming mineral. Characteristic of deep-seated siliceous igneous rocks as granite and syenite. Especially characteristic of pegmatite dikes, and found lining cavities in granites, where it has evidently been formed by the action of mineralizing vapors during the last stages of the formation of the rock. Also very common in metamorphic rocks, as gneiss and schist, forming the chief constituent in certain mica-schists. In some schistose rocks it occurs in the form of fibrous aggregates of minute scales which have a silky luster, but which do not show plainly the true nature of the mineral. This variety, known as sencite, is usually the product of alteration of feldspar. Muscovite also originates as the alteration product of several other minerals, as topaz, kyanite, spodumene, andalusite, scapolite. Finite is a name given to the micaceous alteration product of various minerals, which corresponds in composition more or less closely to muscovite.

In the pegmatite dikes, muscovite occurs associated with quartz and feldspar, with tourmaline, beryl, garnet, apatite, and fluorite. It is found often in these veins in large crystals, called books, which in some localities are several feet across

Distribution

Uncommon in euhedral crystals.

  • From Slyudyanka, near Lake Baikal, Siberia, and at Mursinka, Ural Mountains, Russia.
  • At Kammerfors, KragerÄo, and Bamble, Norway.
  • In the USA, at Mt. Mica, near Paris, Oxford Co., and elsewhere in Maine; large crystals from Pennsbury, Chester Co., Pennsylvania; at Amelia, Amelia Co., Virginia; crystals from near Shelby, Cleveland Co., North Carolina; in the Black Hills, in Pennington, Lawrence, and Custer Cos., South Dakota; and from the Harding mine, Dixon, Taos Co., New Mexico.
  • In Methuen and Calvin Townships, Ontario, Canada.
  • In Brazil, in Minas Gerais, at Urucum, from the Taquaral mine. Around Nellore, Andhra Pradesh, India, in commercial deposits which may contain huge crystals.
Cite this article as: Geology Science. (2019). Muscovite. [online] Available at: http://geologyscience.com/minerals/muscovite/ [12th November 2019 ]
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