Mica
Sheet silicate group, e.g. KAl2(AlSi3O10)(OH)2 (muscovite)
A family of sheet silicate minerals that split into thin, flexible, shiny sheets, giving rocks a sparkling, layered appearance.
- Mohs hardness
- 2-3
- Color
- colorless, silvery, brown, black, green, or pink depending on species
- Type
- mineral
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Overview
Mica is a group of related sheet (phyllosilicate) minerals defined by their perfect basal cleavage, which lets them split into extremely thin, flexible, elastic sheets. The most common members are silvery muscovite, dark brown-black biotite, and the pale brown phlogopite; lithium-rich lepidolite is lilac-pink.
Micas are extremely widespread, occurring as glittering flakes in granites, schists, gneisses, and many sandstones. Their high luster gives many rocks their characteristic sparkle.
Because sheets are transparent, heat-resistant, and electrically insulating, mica has been used since ancient times as window panes ('isinglass') and remains an important industrial mineral.
Formation & geology
Micas crystallize in a wide range of settings. Muscovite and biotite form in granitic and pegmatitic magmas and are major minerals in metamorphic schists and gneisses, where they grow aligned by pressure to create foliation. Phlogopite forms in metamorphosed magnesium-rich limestones and ultramafic rocks. Lepidolite forms in lithium-rich granite pegmatites.
Giant 'books' of mica several feet across can crystallize slowly in pegmatites, which are the main source of sheet mica.
Fine mica flakes also survive weathering and accumulate in sediments, sparkling in micaceous sandstones and shales.
How to identify it
The diagnostic feature is perfect one-directional cleavage into thin, flexible, elastic sheets that can be peeled apart with a fingernail. Sheets are translucent to transparent with a pearly to vitreous luster, and the mineral is soft (Mohs 2-3).
Color separates the species: silvery/colorless = muscovite, dark brown-black = biotite, lilac-pink = lepidolite. Streak is white to colorless.
Look-alikes: Chlorite cleaves into sheets too but the flakes are non-elastic (they bend but stay bent) and usually green. Graphite is soft and flaky but greasy, black, and marks paper. Selenite gypsum splits into sheets but is not elastic.
Uses & significance
Sheet mica is valued as a transparent, heat- and electricity-resistant material for furnace windows, electrical insulators, and capacitors. Ground mica is used as a filler and shimmer agent in paints, cosmetics (the sparkle in eyeshadow and lipstick), plastics, and drilling muds.
Lepidolite is an ore of lithium, and green chromium-rich muscovite (fuchsite) is sold as a decorative and metaphysical stone.
Mica's pearlescent flakes are also used in pearlescent pigments and the 'mica' shimmer in many products. Collectors prize large crystal books and well-formed pseudo-hexagonal plates.
Frequently asked questions
What makes mica sparkle?
Mica's flat, reflective cleavage sheets act like tiny mirrors, reflecting light and giving micaceous rocks their characteristic glitter.
Is mica the same as glitter?
Cosmetic and craft glitter often uses ground natural or synthetic mica coated with pigments, so mica is a common base material for shimmer products.
What is the difference between muscovite and biotite?
Muscovite is the silvery, light-colored, aluminum-rich mica; biotite is the dark brown-to-black, iron- and magnesium-rich mica.
Is mica safe to handle?
Solid mica is safe to handle, but inhaling fine mica dust over long periods can irritate the lungs, so cutting or grinding should be done with protection.
Mica guides
In-depth guides for identifying, valuing, and understanding Mica.
Mica identified by the community
Real specimens identified with Rock Identifier.
