Mica is a generic name for a group of silicate minerals which, have chemical as well as physical traits. They are also referred to as sheet silicates since they occur in layers, each layer separate and distinct from the other. Mica is a considerably light mineral which is comparatively soft and flexible when it occurs in sheets or as flakes. Mica has very low or negligible thermal as well as electrical conductivity. As many as 37 varieties of mica are known, among them, the most commonly known and used include clear Muscovite, black butt, purple Lepidolite and brown phlogomite.
The use of mica by humans can be traced back to prehistoric times. Mica was known to ancient Indian, Egyptian, Greek and Roman and Chinese civilizations, 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 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.
Natural mica was and still is used by the Taos and Picuris Pueblos Indians in north-central New Mexico to make pottery. The pottery is made from weathered Precambrian mica schist, and has flecks of mica throughout the vessels. Tewa Pueblo pottery is made by coating the clay with mica to provide a dense, glittery micaceous (graphitic) finish over the entire object.
Mica flakes (called abrak in Urdu) are also used in Pakistan to embellish women's summer clothes, especially dupattas (long light-weight scarves, often colorful and matching the dress) Thin mica flakes are added to a hot starch water solution, and the dupatta is dipped in this water mixture for 3–5 minutes. Then it is hung to air dry
Origin and occurrence
Micas may originate as the result of diverse processes under several different conditions. Their occurrences, listed below, include crystallization from consolidating magmas, deposition by fluids derived from or directly associated with magmatic activities, deposition by fluids circulating during both contact and regional metamorphism, and formation as the result of alteration processes—perhaps even those caused by weathering—that involve minerals such as feldspars. The stability ranges of micas have been investigated in the laboratory, and in some associations their presence (as opposed to absence) or some aspect of their chemical composition may serve as geothermometers or geobarometers
For commercial and industrial purpose natural mica are mainly divided into three categories, viz:
1. Processed Mica 2. Fabricated Mica 3. Manufactured Mica
Processed Mica is fundamentally natural sheet mica of an irregular size and polygonal shapes that is relatively flat and free from physical defects and structural imperfections having a minimum usable area of 4.8 sq. cm. (0.75 sq. inch), suitable to be cut, punched or stamped into specific size and shape mainly for use by the electronic and electrical industries.
Fabricated Mica is basically a natural sheet mica, cut, stamped or punched to specified size, shape and thickness used for industrial, such as; discs, washers, cut-films or sheets, joints, backing plates, plates, spacers, mica formers for irons, toasters, rice-cookers, etc.
Manufactured Mica means mica-based products manufactured from natural mica, such as, Micanite or built-up mica, mica paper, mica heating elements, mica capacitors, mica flakes and powder, mica bricks, etc.
Chemical formula: Any of a group of hydrous potassium, aluminum silicate minerals.
Color(s): white, yellowish,green, gray
Luster: vitreous to pearly
Transparency: transparent, translucent, opaque
Crystal system: monoclinic
Hardness (Mohs): 2.5 - 3
Uses: As insulators in electronics
Mica is found in many rocks around the world. Notable deposits are found in India, South Dakota, Russia, and Brazil.
The general formula for minerals of the mica group is XY2–3Z4O10(OH, F)2 with X = K, Na, Ba, Ca, Cs, (H3O), (NH4); Y = Al, Mg, Fe2+, Li, Cr, Mn, V, Zn; and Z = Si, Al, Fe3+, Be, Ti. Compositions of the common rock-forming micas are given in the table.
Methods of grinding
Based on its application, mica in the form of scrap pieces can be ground by dry or wet grinding or Micronizing (Most cost effective and efficient way to attain particle size) The grinding process determines the quality and other properties such as color, bulk density and mesh size of the product. Dry-ground mica powder is prepared by passing the scrap pieces through high-speed hammer mills, which crush them into a fine powder like flour. In the case of wet-ground mica, mica scrap is ground with water, which enables the selective dissemination of the mica flakes. Wet grinding is carried out in churn mills equipped with rollers or large wheels that move through horizontal shafts.( Micronizing).
Production of scrap as well as flake mica occurs all over the world. Based on relevant data obtained in 2010, Russia was the major producer (100,000 tonnes), followed by Finland, the United States of America (53,000), South Korea (50,000), France (20,000) and Canada (15,000). Although there are no reliable and accurate data for China, the total world production was estimated to be 350,000 tonnes. The majority of sheet mica production occurred in India (3,500 tonnes) followed by Russia (1,500). There are many sources of mica flakes, among them, schist, (a type of metamorphic rock), during the processing of materials that yield feldspar and kaolin (as a by-product), extracted from deposits containing placer and also from pegmatites. Sheet mica in comparison occurs less abundantly than a scrap or flake mica. Occasional recovery of sheet mica from scrap and flake mica set apart for mining is also seen. Pegmatite deposits, however, are the main sources of mica sheets. The price of sheet mica can vary between $1 per kilogram for the low grade variety to a little over $2,000 per kilogram for the mica of the finest grade.
Two varieties of mica minerals, namely Muscovite and Phlogopite, are the most widely used of the micas. Their widespread usage is due to their fine properties that include low electrical as well as thermal conductivity, large dielectric constant value, difficulty in fusing, perfect cleavage, elasticity and flexibility. ‘Sheet mica’ of the Muscovite and Phlogopite varieties are mostly used in heating elements or in commutator segments as an insulating sheet or as an electrical insulator. Muscovite sheets that are of specific thickness also find application in optical instruments. Powdered mica has multi-purpose applications that include its use as dusting material such as in the case of asphalt tiles to prevent them from sticking to one another, its use in the wallpaper manufacture to impart a sheen to the material, and its use as a lubricant, absorbent and filler. Lepidolite, another mineral variety that is mined as a lithium ore, finds application as a component of glass that is resistant to heat. In the Unites States of America, particularly on the plains of coastal New Jersey, Glauconite-Rich Greensands are used as fertilizer material. Glauconite has a rapid rate of regeneration and a large capacity to switch base salts. These properties are found as an application in its use as a water softener.