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The Mineral muscovite

Triangular Muscovite Crystal Grouping

Muscovite is the most common form of mica. Its name is derived from "Muscovy Glass", which describes thick sheets of transparent mica that were once used as a glass substitute in Russia. Because of Muscovite's abundance, its presence is usually lacking in collections except for it being an accessory mineral to other minerals. However, there are certain interesting formations and colors which are very aesthetic, and those forms are well-represented in collections. Muscovite can come in enormous crystal groupings that can weigh several hundred pounds. Thin sheets can be peeled off as layers, and the thinner a layer is peeled the greater its transparency becomes.

Except for large and resistant specimens, Muscovite is very hard to clean because if washed it will absorb water internally and start to break apart. The best way to wash Muscovite and other Micas is with a dry electric toothbrush.

Chemical Formula

KAl3Si3O10(OH)2

Color

Colorless, white, beige, yellow, brown, gray, green, pink, purple, red, black; occasionally multicolored

Crystal System

Monoclinic

Properties

Streak
Colorless
Hardness
2 - 2.5
Transparency
Transparent to translucent
Specific Gravity
2.7 - 3.0
Luster
Pearly
Cleavage
1,1
Fracture
Uneven
Tenacity
Sectile, Elastic
Other ID Marks
Tendency for small pieces or flakes or peel off.

Crystal Habits

Crystals are in thick flakes, micaceous masses and groupings, and in tabular, foliated, flaky, and scaly forms. Crystals may also be elongated with one dimension flat, or stubby triangular or hexagonally shaped crystals. Muscovite also forms interesting aggregates of dense bladed crystals, thick rosettes, uniquely twinned star-shaped formations, and rounded botryoidal and globular masses of dense flakes.

Muscovite may also form pseudomorphs after other minerals, assuming the original minerals crystal shape.

3D Crystal Atlas

Additional Information

Composition
Basic potassium aluminum silicate, sometimes with some chromium or manganese replacing the aluminum
In Group
Silicates; Phyllosilicates; Mica Group
Striking Features
Flaky habit, crystals, sectility, and mode of occurence.
Environment
Muscovite is a very common rock-forming mineral and is an important constituent in many environments. Its presence is noted especially in granite pegmatites, in contact metamorphic rocks, in metamorphic schists, and in hydrothermal veins. Important Muscovite deposits where large significant crystals occur are almost exclusively from granite pegmatites.
Rock Type
Igneous, Metamorphic

Other Names

Common Mica
Isinglass Old name for mica, especially Muscovite. May also describe mica in the form of large, thin sheets.
Potash Mica

Varieties

 -   Manganese-rich, pink to red variety variety of Muscovite.
 -   Dark green, chromium-rich variety of Muscovite. Named in honor of German professor and mineralogist Johann Nepomuk von Fuchs (1774-1856).
 -   Green form of Muscovite mica in small dense flake groups found in Mariposa (and Tuolumne) County, California. Mariposite forms in metamorphasized Dolomite and Quartz, and these are usually present as veins or as a base material. A combination of the green mica and the veins or base material forms a rock which is also called Mariposite, and it is sometimes used as an ornamental stone.
 -   A light pink form of Muscovite.
 -   A fine-grained form form of mica, usually Muscovite, that is somewhat silky in appearance.
 -   Describes twinned Muscovite crystals in pointed star-shaped sections.

Uses

Muscovite is a very poor conductor of heat and electricity, and is thus used as an insulator for various electrical products and semiconductors. It is also used in the production of automotive tires and cosmetics. Large Muscovite sheets were also once used for oven windows ("isinglass") due to their ability to withstand high temperatures and keep the heat inside.

Noteworthy Localities

Muscovite is an extremely common mineral, nevertheless there are some exceptional localities that need to be mentioned. Outstanding Muscovite crystals, some bright yellow and others with perfect star formations, come from the famous pegmatite localites in the Doce and Jequitinhonha Valleys in Minas Gerais Brazil. Specific localities include Divino das Laranjeiras ("stars"),Galileia, Governador Valadares, Conselheiro Pena, Jaguaracu, Aracuai, and Coronel Murta. Beautiful yellow Muscovite comes from the Davib Ost Farm, Karibib, Erongo, Namibia. Fine Muscovite plates, often associated with valuable pegmatite gemstones, come from the Shigar Valley and Skardu District in Northern Pakistan. Nellore, India, is famous for its Muscovite mines which have produced some of the worlds largest mica sheets. Xuebaoding Mountain, Pingwu, in Sichuan Province, China, provides excellent Aquamarine and Scheelite that are associated with beautiful Muscovite plates.

In the U.S., Muscovite is fairly common in the pegmatites of San Diego county, including Pala and Ramona. Rare Muscovite pseudomorphs after Tourmaline come from the Willow Spring Ranch, Oracle, Pinal Co., Arizona. A bright pink, lithium-rich variety comes from the Harding Mine, Taos Co., New Mexico; and gemmy green crystals from Lincoln Co., North Carolina. Other important localties include Mt Antero, Chaffee Co., Colorado; the Diamond Mica Mine, Keystone, Pennington Co., South Dakota; Shelby, Cleveland Co., North Carolina; and Bedford, Westchester Co., New York.

New England contains some important Muscovite in its famous pegmatites, specifically at Middlesex Co., Connecticut (East Hampton, Haddam, and Portland); Grafton Co., New Hampshire (the Palermo No. 1 Mine, Groton; and the Ruggles Mine, Grafton); Mt. Apatite, Auburn, Androscoggin Co., Maine; and Greenwood and Newry, Oxford Co., Maine.

Common Mineral Associations

Albite, Quartz, Microcline, Almandine, Spodumene

Distingushing Similar Minerals

Chlorite and Margarite - Not as elastic as Muscovite.
Biotite, Phlogopite - Usually darker in color, otherwise very difficult to distinguish.
Lepidolite - Very difficult to distinguish from pink Muscovite.
Gypsum - Cannot be peeled into micaceous sheets, crystals usually differently shaped.


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