Mica

Mica in sheet form.

Mica flakes.

Mica is an important group of rock-forming silicate minerals, belonging to the subgroup called phyllosilicates. Members of the mica group are made up of sheets of silicate tetrahedra. The group consists of more than 30 members, the most common among them being biotite, lepidolite, and muscovite.

This group includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency toward pseudo-hexagonal crystals and are similar in chemical composition. The highly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal, sheet-like arrangement of its atoms.

The word "mica" is thought to be derived from the Latin word micare, meaning to shine, in reference to the brilliant appearance of this mineral (especially when observed in small scales).

Mica in ancient times

The shape of a hand carved from mica, from the Hopewell culture.

Mica was known to ancient Egyptian, Greek and Roman civilizations, as well as the Aztec civilization of the New World. Yet, the earliest use of mica has been found in cave paintings created during the Upper Paleolithic period (40,000 to 10,000 B.C.E.). The first colors were red (from iron oxide, hematite, a form of red ochre) and black (manganese dioxide), though black was also produced from bridget or pine carbons. White from kaolin or mica was used occasionally.

A few kilometers (km) northeast of Mexico City stands the ancient site of Teotihuacan. The most striking structure here is the towering pyramid of the Sun. This pyramid contained considerable amounts of locally mined mica, in layers up to one-foot thick.

Throughout the ages, fine powders of mica have been used for various purposes, including decorative purposes. The colored Gulal and Abeer used by Hindus of north India during holi festival contain fine crystals of mica. The majestic Padmanabhapuram palace, 65 km from Trivandrum, India, has colored mica windows.

Occurrence

A rock with mica.

Micas are widely distributed and occur in igneous, metamorphic and sedimentary regimes. Large crystals of mica used for various applications are typically mined from granitic pegmatites.

Until the nineteenth century, large crystals of mica were quite rare and expensive as a result of the limited supply in Europe. However, its price dramatically dropped when large reserves were found and mined in Africa and South America after the early nineteenth century.

Scrap and flake mica is produced all over the world. The flake mica comes from several sources: the metamorphic rock called schist as a by-product of processing feldspar and kaolin resources, from placer deposits, and from pegmatites. Major producers of flake mica are Canada, India, Finland, and Japan. Sheet mica is considerably less abundant than flake and scrap mica. Sheet mica is occasionally recovered from mining scrap and flake mica. The most important sources of sheet mica are the pegmatite deposits. Major sheet mica mining countries are India, Belgium, Germany and China.

Classification of mica

Chemically, micas can be given the general formula:

X₂Y_4-6Z₈O₂₀(OH,F)₄

where X is K, Na, or Ca or less commonly Ba, Rb, or Cs

Y is Al, Mg or Fe, or less commonly Mn, Cr, Ti, Li, or other metal ions

Z is chiefly Si or Al but also may include Fe³⁺ or Ti.

Structurally, the micas can be classed as di-octahedral (Y = 4) and tri-octahedral (Y = 6). Also if the X ion is K or Na the mica is a common mica whereas if the X ion is Ca the mica is classed as a brittle mica.

Di-octahedral micas

Common micas:

• Muscovite
• Paragonite
• Glauconite

Brittle micas:

• Margarite

Tri-octahedral micas

Common micas:

• Phlogopite
• Biotite
• Zinnwaldite
• Lepidolite

Brittle micas

• Clintonite

Interlayer deficient micas

Very fine-grained micas with typically greater variation in ion and water content are informally termed clay micas. They include:

• Hydro-muscovite, with H₃⁺ along with K in the X site.
• Illite, with a K deficiency in the X site and correspondingly more Si in the Z site.
• Phengite, with Mg or Fe²⁺ substituting for Al in the Y site and a corresponding increase in Si in the Z site.

Properties and uses

Mica has a high dielectric strength and excellent chemical stability, making it a favored material for manufacturing capacitors for radio-frequency applications. In addition, it has been used as an insulator in high-voltage electrical equipment. It is also birefringent and is commonly used to make quarter-and-half wave plates.

Given its heat resistance, mica is used instead of glass in windows for stoves and kerosene heaters.

Illites or clay micas have a low cation exchange capacity for 2:1 clays. K⁺ ions between layers of mica prevent swelling by blocking water molecules.

Aventurine is a variety of quartz with mica inclusions used as a gemstone.

Pressed Mica sheets are often used in place of glass in greenhouses.

Muscovite mica is the most common substrate for sample preparation for the atomic force microscope.

See also

• Mineral
• Silicate

References

ISBN links support NWE through referral fees

• Deer, W.A., R.A. Howie, and J. Zussman. 1996. An Introduction to the Rock-Forming Minerals. 2nd ed. Upper Saddle River, NJ: Prentice Hall. ISBN 0582300940.

• Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412.

• Klein, Cornelis, and Barbara Dutrow. 2007. Manual of Mineral Science. 23rd ed. New York: John Wiley. ISBN 978-0471721574.

• Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.

• Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.

• Mineral Gallery. 2006. The Mica Group of Minerals. Amethyst Galleries. Retrieved April 11, 2007.

External links

• Mica Group Mindat.org. Retrieved April 11, 2007.

• The Mica Group of Minerals. minerals-n-more.com. Retrieved April 11, 2007.

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