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Triangular Muscovite Crystal Grouping
SILICATES; PHYLLOSILICATES; MICA GROUP · MONOCLINIC

Muscovite

KAl3Si3O10(OH)2

Hardness
2–2.5
Streak
Colorless
Spec. Gravity
2.7–3.0
System
Monoclinic

About

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.

Crystal Forms & Aggregates

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.

Other ID Marks

Tendency for small pieces or flakes or peel off.

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.

Varieties

✓ common  · ✗ uncommon

  • Alurgite— Manganese-rich, pink to red variety variety of Muscovite.
  • Fuchsite— Dark green, chromium-rich variety of Muscovite. Named in honor of German professor and mineralogist Johann Nepomuk von Fuchs (1774-1856).
  • Mariposite— 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.
  • Sericite— A fine-grained form form of mica, usually Muscovite, that is somewhat silky in appearance.
  • Schernikite— A light pink form of Muscovite.
  • Star Muscovite— 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

Distinguishing 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.

Photos

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Crystal forms

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Hexagonally Shaped
Modified Hexagonally Shaped