mica

 

(mī'kə) , general term for a large group of minerals, hydrous silicates of aluminum and potassium, often containing magnesium, ferrous iron, ferric iron, sodium, and lithium and more rarely containing barium, chromium, and fluorine. All crystallize in the monoclinic system, but mica is most commonly found in the form of scales and sheets. All the micas have an excellent basal cleavage, splitting into very thin, elastic laminae. Some varieties are transparent; resistance to heat is high. Commercially, the most important micas are muscovite (potassium mica) and phlogopite (magnesium mica). Muscovite, the commoner variety, is usually colorless, but it may be red, yellow, green, brown, or gray, with a vitreous to pearly luster. It occurs in granites, syenites, mica schists, and gneisses, but is most common in pegmatite dikes. It is widely distributed. Phlogopite varies in color from yellow to brown, some specimens having a coppery tint and others being greenish. It occurs in crystalline limestones, dolomites, and serpentines in Canada, New York, New Jersey, and Finland. Mica mining, because of the necessity of keeping the crystals intact, is a delicate operation; drills and blasting powder must be used carefully, if at all. The mined crystals are first “cobbed,” i.e., roughly trimmed of rock and cut, then split with a hammer into plates, and further split into sheets with a knife. Sheet mica is used as an insulating material and as a resonant diaphragm in certain acoustical devices. Scrap and ground mica is used in wallpaper, fancy paint, ornamental tile, roofing, lubricating oil, and Christmas-tree snow. Ground mica is sometimes pressed into sheets (micanite) that can be used as sheet mica. Most of the sheet mica used in the United States is imported, chiefly from India and also from Brazil. Synthetic mica was produced in the United States after intensive government-sponsored research began in 1946.

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 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). 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.

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 and 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

In 2005, India has the largest deposits of mica in world. China was the top producer of mica with almost one-third global share closely followed by the USA, South Korea and Canada, reports the British Geological Survey.

Mica is 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 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 since early 19th 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. 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.

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. 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 such things 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 in ancient times

Human use of mica dates back to pre-historic times, and 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 colors 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 locally mined mica in layers up to one foot thick.^[2]

Through out 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, Padmanabhapuram, 65 km from Trivandrum, India, has coloured mica windows.

The medieval Chinese mechanical engineer, horologist, pharmacologist, and statesman Su Song (1020-1101 AD) of the Song Dynasty outlined in his book Ben Cao Tu Jing the medicinal properties and uses of the then every known type of mica (which was 8 in all, refer to article on mineralogy).

References

1. ^ Deer, W. A., R. A. Howie and J. Zussman (1966) An Introduction to the Rock Forming Minerals, Longman, ISBN 0-582-44210-9
2. ^ Garrett G. Fagan, ed., Archaeological Fantasies: How Pseudoarchaeology Misrepresents the Past and Misleads the Public, p. 102, ISBN 0415305934

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