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Mica
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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, to glitter, in reference to the brilliant appearance of this mineral (especially when in small scales).
Mica has a lamellar form with a shiny luster.
Mica classificationChemically, micas can be given the general formula:
- X2Y4-6Z8O20(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 micasCommon micas:
Brittle micas:
Interlayer deficient micasVery 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 Survey 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. Large Deposits of Sheet Mica were mined in New England from the 19th Century to the 1960's. Large mines existed in CT, NH and Maine.
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. The shimmer from mica is also used in makeup, as it gives a translucent "glow" to the skin or helps to mask imperfections.
Mica sheets are used to provide structure for heating wire (like Kanthal, Nichrome, etc..) in heating elements and can withstand up to 900 °C.
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.
Mica slices are used in electronics to provide electric insulation between a heat generating component and the heat sink used to cool it . The same word is sometimes used by technicians to designate a synthetised gum (usually blue or grey) which is used for the same purpose, but which does not actually consist of silicate mineral (language abuse).
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.
In atomic force microscopy, biological samples are placed on mica because it is atomically flat. Even bare DNA molecules may be seen as small ridges. Inspecting mica under the microscope, bits of algae and other organic materials suggested to her the possibility early life molecules could have evolved within mica sheets in a communal fashion eons before the evolution of cell membranes or lipid vesicles. Mica might have provided a secure place with time and space and protection from the open ocean. Further research might provide additional predictions about energy and entropy for life. Mica is old rock, some of earliest evidence for life's most primitive cells is in Akilia Island, Greenland, where biotite, a common mica, is also found. Potassium ions, which hold the sheets of mica together, are is also required by cells. Primordial soup with amino acids and simple building blocks of life might have seeped between the water-loving mica sheets. The large planer area between sheets might have facilitated the building of long chain molecules. Negative spaces holding the potassium ions on mica are 0.5 nm apart, as are the single stranded DNA molecules (letters of genetic code), as well as amino acids in proteins. Clay also provides spacing that might encourage this process, but the planer area might better encourage the process. Expansion and contraction caused by temperature changes and ocean currents might provide mechanical energy to help rearrange the molecules and trigger the formation of chemical bonds.
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.
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.
See also
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