Subscribe to receive an email notification when a publication is added to this page. Skip to main content. Search Search. National Minerals Information Center. John, CC BY 2. The mica mineral group includes 34 phyllosilicate minerals, all with a layered, platy texture. The mineral has been known for millennia: Mica was first mined in India about 4, years ago, where it was used primarily in medicines.
The Mayans used it for decorative effect in stucco to make their temples sparkle in the sun. Today it is used in everything from electrical products to makeup. Mica is highly reflective, and its excellent cleavage allows it to split into thin sheets; these tough, flexible sheets are the distinguishing characteristic of the mineral group.
Thin sheets are transparent or, if deeply colored, translucent. Micas are stable when exposed to electricity, light, moisture and extreme temperatures. Crosscuts and raises are developed to follow promising exposures of mica.
When a pocket of mica is found, extreme care is exercised in the removal to minimize damage to the crystals. The charge is just sufficient to shake the mica free from the host rock. After blasting, the mica is hand-picked and placed in boxes or bags for transporting to the trimming shed where it is graded, split, and cut to various specified sizes for sale.
Sheet mica is no longer mined in the U. Most sheet mica is mined in India, where labor costs are comparatively low. The flake mica produced in the U. It is mined by conventional open-pit methods. Scrap, flake, and ground muscovite are used as fillers and extenders in a variety of paints, surface treatments, and manufactured products.
The pearlescent luster of muscovite makes it an important ingredient that adds "glitter" to paints, ceramic glazes, and cosmetics. Muscovite is easily identified because its perfect cleavage allows it to be split into thin, flexible, elastic, colorless, transparent sheets with a pearly to vitreous luster.
It is the only common mineral with these properties. The best way to learn about minerals is to study with a collection of small specimens that you can handle, examine, and observe their properties. Inexpensive mineral collections are available in the Geology. Muscovite: Muscovite from Stoneham, Maine. Specimen is approximately 4 inches 10 centimeters across. Hand specimens of this size and thickness often appear to have a black, brown, or silver color; however, when they are split into thin sheets, the clear transparent nature of muscovite is revealed.
Thin sheets often have a slight tint of brown, green, yellow, or rose. Muscovite is found in igneous, metamorphic, and sedimentary rocks.
In igneous rocks, it is a primary mineral that is especially common in granitic rocks. In granite pegmatites, muscovite is often found in large crystals with a pseudohexagonal outline. These crystals are called "books" because they can be split into paper-thin sheets. Muscovite rarely occurs in igneous rocks of intermediate, mafic, and ultramafic composition.
Muscovite can form during the regional metamorphism of argillaceous rocks. The heat and pressure of metamorphism transforms clay minerals into tiny grains of mica which enlarge as metamorphism progresses.
Muscovite can occur as isolated grains in schist and gneiss , or it can be abundant enough that the rocks are called "mica schist" or "micaceous gneiss.
Muscovite is not especially resistant to chemical weathering. It is quickly transformed into clay minerals. Tiny flakes of muscovite sometimes survive long enough to be incorporated into sediments and immature sedimentary rocks. It is evidence that these sediments and rocks have not been subjected to severe weathering. Muscovite schist: A specimen of muscovite schist.
Muscovite is formed during the metamorphism of argillaceous rocks. Specimen shown is about two inches five centimeters across. In this formula potassium is sometimes replaced by other ions with a single positive charge such as sodium, rubidium, or cesium.
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