Muscovite

Mineral Name: Muscovite

Chemical Composition: KAl2(Si3AlO10)(OH)2

Color: White, gray, silver white, brownish white, greenish white

Streak: White/ Clear

Hardness: 2.5

Cleavage/Fracture: Muscovite has a single plane of cleavage that allows it to be broken into sheets.

Crystal Form: Muscovite forms in sheets, micaceous masses, tabular, scaly, flaky, and foliated crystals due to the strongly bonded aluminum silicate sheets that have bonded weakly to either potassium or sodium. Muscovite can form rosettes, twinned star-shaped formations, bladed crystals, or spherical masses. It is often found as pseudohexagonal crystals in granite. (7)

Luster: Pearly to Vitreous

Special Features: It is the only common mineral that breaks into thin and flexible transparent sheets. It has perfect cleavage.

Varieties: Adamsite, Al-illite-hydromica, Alurgite, Ammersooite, Astrolite, Avalite, Barian Muscovite, Barian-Chromian Muscovite, Barium-Vanadium Muscovite, Batchelorite, Brammallite, Chacaltaite, Chromian Sericite, Damourite, Ferrimuscovite, Fuchsite, Gieseckite, Gilbertite, Illite, Illite Jade, Leverrierite, Liebenerite, Lithian Muscovite, Lithium Muscovite, Oellacherite, Persbergite, Pig’s Egg, Rubidian Muscovite, Schernikite, Sericite, Star Muscovite, Vanadian Muscovite, Verdite, Wilsonite, Zincian Muscovite (6).

Mineral Group: Muscovite is a member of the Silicates group due to its aluminum silicate composition. It is also a member of the Phyllosilicates because of its sheeted nature. Finally, it belongs to the the Mica Group because it falls into the category of hydrous potassium, aluminum silicate minerals (1).

Environment: Muscovite is mostly found in granitic igneous rocks, combined with other Mica group members in both low and high grade metamorphic rocks, and a part of the undifferentiated clay material in fine-grained sedimentary rocks.

Associated Rock Types: Muscovite is a key mineral in most granites, such as granite pegmatites, and is also found in many hydrothermal deposits. Muscovite is common in metamorphic schist and phyllite, giving them their pearly luster. This mineral can also give some types of gneiss its shine its colored bands. Muscovite can be found in some shales, sandstones and siltstones because the mineral is so fragile and can break into small pieces easily.

Occurrence in North America: In the United States, muscovite can be found many places, for instance in Middlesex County, Connecticut; San Diego County, California; and many more counties in New England where it is present in pegmatite form (7).

Economic Uses: Because muscovite mineral can break into flakes so easily, muscovite is used for many economic purposes. It is grounded into flakes and used in electric insulators. Muscovite is also grounded into paints, used to join cement, and as a dusting agent. The largest net profit, however, is from the big commodities that the U.S. produces and uses, such as plastic and rubber.

Industrial Uses: Muscovite has been used for many electrical and insulating purposes due to its resistance to heat and electrical transference. Most notably, however, it was used for furnace and microwave windows before glass became readily available (3).

First Notable Identification: Muscovite comes from the word “Muscovy-glass”, which was used to describe the glass windowpanes Russians used for their homes, churches, and businesses.

How We Identified It: We were able to identify Muscovite based on its level of hardness (2.5) and more importantly based on its ability to be peeled into sheets. Another identifier is its pearly sheen.

Don’t Confuse It With: People often confuse muscovite with biotite. Muscovite is lighter in color and biotite is usually darker (5).