Mica
From MedBib.com - Medicine & Nature
The mica group of sheet silicate (phyllosilicate) 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, meaning "to glitter", in reference to the brilliant appearance of this mineral (especially when in small scales).
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Mica classification
Chemically, micas can be given the general formula[1]
- 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, micas can be classed as dioctahedral (Y = 4) and trioctahedral (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.
Trioctahedral micas
Common micas:
Brittle micas:
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 Survey reports that as of 2005, Kodarma district in Jharkhand state in 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 1960s. Large mines existed in Connecticut, New Hampshire, 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, their price dramatically dropped when large reserves were found and mined in Africa and South America during the early 1800s. The largest sheet of mica ever mined in the world came from a mine in Denholm, Quebec, Canada.[2]
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
Electronics
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, and between the bars of commutators in Direct Current motors and generators. Mica is used because it can be split into very thin slices, and this keeps its thermal resistance low while retaining sufficient dielectric strength to prevent current from flowing across it at moderate voltages. The insulation is usually necessary when the heat sink is earthed (grounded) while the electronic component's metal surfaces will be connected to a power supply or signal line. If they were in direct contact this could form a short circuit. Heat sink insulation can also be necessary to prevent the heat sink from acting like an antenna if the component is connected to a rapidly varying signal.
Isinglass
Thin transparent sheets of mica called "isinglass" were used for peepholes in boilers and lanterns because they were less likely to shatter compared to glass when exposed to extreme temperature gradients. Such peepholes were also used in "isinglass curtains" in horse-drawn carriages[3] and early 20th century cars. A book about a journey in a Model T Ford car describes isinglass curtains as follows:
"Oiled canvas side curtains were put up over the windows for wind, rain, and cold (there were no heaters) and were held in place with rods that fit into the doors and twisting button snaps around the perimeter... 'Isinglass' peepholes in the curtains allowed limited visibility. Isinglass was made of thin sheets of cracked mica."[4]
Other
Resin-bonded mica or micanite or built up micanite from splittings or agglomerated micanite made from pulped mica powder are called micanites. All these products have additives, like resins, and the resultant products, while having inferior insulation properties to natural "mica insulators" are called micanites or micafoliums.
Mica is also birefringent and is commonly used to make quarter and half wave plates.
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.
Because mica can be pressed into a thin film, it is often used on Geiger-Müller tubes to detect low penetrating alpha particles.
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 is used in the production of pearlescent pigments. Many metallic looking pigments are composed of a substrate of mica coated with another mineral, usually titanium dioxide (TiO2). The resultant pigment produces a reflective color depending on the thickness of the coating. These products are used to produce automobile paint, shimmery plastic containers, high quality inks used in advertising and security applications.
Mica sheets are used to provide structure for heating wire (such as in Kanthal or Nichrome) in heating elements and can withstand up to 900 °C (1,650 °F). Because mica is resistant to heat, it is used instead of glass in windows for stoves and kerosene heaters.
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.
It is also used as an insulator in concrete block, home attics, and can be poured into walls (usually in retrofitting uninsulated open top walls).
May also be used as a soil conditioner, especially in potting soil mixes and in gardening plots.
Muscovite mica is the most common substrate for sample preparation for the atomic force microscope. Freshly-cleaved mica surfaces have been used as clean imaging substrates in atomic force microscopy, enabling for example the imaging of bismuth films,[5] plasma glycoproteins,[6] membrane bilayers,[7] and DNA molecules.[8]
Greases used for axles are composed of a compound of fatty oils to which mica, tar or graphite is added to increase the durability of the grease and give it a better surface.
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 hues were red (iron oxide, hematite, or red ochre) and black (manganese dioxide, pyrolusite), though black from juniper or pine carbons has also been discovered. White from kaolin or mica was used occasionally.
A few kilometers 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 mica in layers up to 30 cm (12 in) thick. The mica found in the pyramid of the sun has been identified as mined in Brazil, that is 3400 km from the pyramid complex.[9]
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 small crystals of mica. The majestic Padmanabhapuram palace, 65 km (40 mi) from Trivandrum in India, has colored mica windows.
References
- ^ Deer, W. A., R. A. Howie and J. Zussman (1966) An Introduction to the Rock Forming Minerals, Longman, ISBN 0-582-44210-9
- ^ "Denholm" (in French). MRC de La Vallée-de-la-Gatineau. http://www.mrcvg.qc.ca/textes/mun-denholm.shtml. Retrieved 2008-08-27.
- ^ Isinglass curtains are referred to in the 1955 musical Oklahoma's song The Surrey with the Fringe on Top.
- ^ Joanne Wilke, Eight Women, Two Model Ts and the American West, University of Nebraska Press, (2007), ISBN 0803260199
- ^ Weisenhorn, A. L.; et al. (1991). "Atomically resolved images of bismuth films on mica with an atomic force microscope". Journal of Vacuum Science & Technology, B: Microelectronics and Nanometer Structures 9 (2): 1333–1335. doi:.
- ^ Marchant, Roger E.; Lea, A. Scott; Andrade, Joseph D.; Bockenstedt, Paula (1992). "Interactions of von Willebrand factor on mica studied by atomic force microscopy". Journal of Colloid and Interface Science 148 (1): 261–272. doi:.
- ^ Singh, Seema; Keller, David J. (1991). "Atomic force microscopy of supported planar membrane bilayers". Biophysical Journal 60 (6): 1401–1410. doi:.
- ^ Thundat, T.; Allison, D. P.; Warmack, R. J.; Brown, G. M.; Jacobson, K. B.; Schrick, J. J.; Ferrell, T. L. (1992). Scanning Microscopy 6: 911. PMID 1295085.
- ^ Fagan, Garrett G. (2006). Archaeological Fantasies: How Pseudoarchaeology Misrepresents the Past and Misleads the Public. New York: Routledge. pp. 102. ISBN 0415305934. http://books.google.com/books?id=sIYpx9mzd4gC&pg=PA102.
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