Mica

      The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic with a tendency towards 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, for glitteren, in reference to the brilliant appearance of this mineral (especially when in small scales). Mica has a lamellar form with a shiny lustre.

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Mica classification

Chemically, micas can be given the general formula:^[1]

X₂Y_4-6Z₈O₂₀(OH,F)₄

in which 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, etc

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). 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.

Tri-octahedral micas

Common micas:

• Phlogopite
• Biotite
• Zinnwaldite
• Lepidolite

Brittle micas:

• Clintonite

Interlayer deficient micas

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

• Hydro-muscovite with H₃O⁺ 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

Occurrence

  The British Geological Survery reports that as of 2005, India had the largest deposits of mica in world. China was the top producer of mica with almost a third of the global share, closely followed by the USA, South Korea and Canada.

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

Until the 19th 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 during the early 1800s.

Scrap and flake mica is produced all over the world. 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. 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 pegmatite deposits.

Properties and uses

Mica has a high dielectric strength and excellent chemical stability, making it a favoured material for manufacturing capacitors for radio frequency applications. It has also 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.

Because mica is resistant to heat it is used instead of glass in windows for stoves and kerosene heaters. It is also used to separate electrical conductors in cables that are designed to have a fire-resistance rating in order to provide circuit integrity. The idea is to keep the metal conductors from fusing in order to prevent a short-circuit so that the cables remain operational during a fire, which can be important for applications such as emergency lighting.

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.

Some brands of toothpaste include powdered white mica. This acts as a mild abrasive to aid polishing of the tooth surface, and also adds a cosmetically-pleasing glittery shimmer to the paste.

Mica sheets are used to provide structure for heating wire (like Kanthal, Nichrome, etc..) in heating elements and can withstand up to 900 °C. ^{[2] [3] [4]}

Another use of Mica is in the production of ultraflat thin film surfaces (e.g. gold surfaces) using mica as substrate. Although the deposited film surface is still rough due to deposition kinetics, the back side of the film at mica-film interface provides ultraflatness, when the film is removed from the substrate.

Role in primitive life

Helen Hansma, a research scientist affiliated with the University of California, has proposed that the unique properties of Mica enabled the formation of life in the oceans of the distant past.^{[5] [6]}

Mica in ancient times

  Human use of mica dates back to pre-historic times. Mica was known to ancient Egyptian, Greek and Roman civilizations, Chinese civilization, as well as the Aztec civilization of the New World.

The earliest use of mica has been found in cave paintings created during the Upper Paleolithic period (40,000 BC to 10,000 BC). The first colours were red, iron oxide (hematite, a form of red ochre) and black (manganese dioxide), though black from juniper or pine carbons has also been discovered. White from kaolin or mica was used occasionally.

A few kilometeres northeast of Mexico City stands the ancient site of Teotihuacan. The most striking visual and striking structure of Teotihuacan is the towering pyramid of the sun. The pyramid contained considerable amounts of locally mined mica in layers up to 30cm thick.^[7]

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

References

• Mineral Galleries data
• Mindat

See also

• Mica insulator