The mica group is composed of 37 phyllosilicate minerals. All crystallize in the monoclinic system, with a tendency towards pseudohexagonal crystals, and are similar in structure but vary in chemical composition. Micas are translucent to opaque with a distinct vitreous or pearly luster, and different mica minerals display colors ranging from white to green or red to black. Deposits of mica tend to have a flaky or platy appearance.
The crystal structure of mica is described as TOT-c, meaning that it is composed of parallel TOT layers weakly bonded to each other by cations (c). The TOT layers in turn consist of two tetrahedral sheets (T) strongly bonded to the two faces of a single octahedral sheet (O). It is the relatively weak ionic bonding between TOT layers that gives mica its perfect basal cleavage.
The tetrahedral sheets consist of silica tetrahedra, each silicon ion surrounded by four oxygen ions. In most micas, one in four silicon ions is replaced by an aluminium ion, while half the silicon ions are replaced by aluminium ions in brittle micas. The tetrahedra each share three of their four oxygen ions with neighboring tetrahedra to produce a hexagonal sheet. The remaining oxygen ion (the apical oxygen ion) is available to bond with the octahedral sheet.
The octahedral sheet can be dioctahedral or trioctahedral. A trioctahedral sheet has the structure of a sheet of the mineral brucite, with magnesium or ferrous iron being the most common cation. A dioctahedral sheet has the structure and (typically) the composition of a gibbsite sheet, with aluminium being the cation. Apical oxygens take the place of some of the hydroxyl ions that would be present in a brucite or gibbsite sheet, bonding the tetrahedral sheets tightly to the octahedral sheet.
Tetrahedral sheets have a strong negative charge, since their bulk composition is AlSi3O105-. The octahedral sheet has a positive charge, since its bulk composition is Al(OH)2+ (for a dioctahedral sheet with the apical sites vacant) or M3(OH)24+ (for a trioctahedral site with the apical sites vacant; M represents a divalent ion such as ferrous iron or magnesium) The combined TOT layer has a residual negative charge, since its bulk composition is Al2(AlSi3O10)(OH)2− or M3(AlSi3O10)(OH)2−. The remaining negative charge of the TOT layer is neutralized by the interlayer cations (typically sodium, potassium, or calcium ions).
Because the hexagons in the T and O sheets are slightly different in size, the sheets are slightly distorted when they bond into a TOT layer. This breaks the hexagonal symmetry and reduces it to monoclinic symmetry. However, the original hexahedral symmetry is discernible in the pseudohexagonal character of mica crystals. The short-range order of K+ ions on cleaved muscovite mica has been resolved.
Sheet mica is recovered by either sinking a shaft along the strike and dip of a pegmatite or by open-pit surface mining of semi-hard pegmatite ore. In either case, it is a very economically risky mining procedure because of the cost involved in locating the vein and the unpredictability of the quality and quantity of the mica that might be recovered once the vein is located and worked.
In underground mining, the main shaft is driven through the pegmatite at suitable angles to the dip and strike using air drills, hoists and explosives. Crosscuts and raises are developed to follow promising exposures of mica. When a pocket of mica is found, extreme care is exercised in the removal to minimize damage to the crystals. Small explosive charges of 40% to 60% strength are carefully placed around the pocket and care is exercised with the drilling procedure so the mica will not be penetrated. The charge is just sufficient to shake the mica free from the host rock. After blasting, the mica is hand-picked and placed in boxes or bags for transporting to the trimming shed where it is graded, split, and cut to various specified sizes for sale.
Sheet mica is no longer mined in the U.S. because of the high cost of mining, the small market, and the high capital risk. Most sheet mica is mined in India, where labor costs are comparatively low.
The flake mica produced in the U.S. 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. It is mined by conventional open-pit methods. In soft residual material, dozers, shovels, scrapers and front-end loaders are used in the mining process. North Carolina's production accounts for half of total U.S. mica production. Hard-rock mining of mica-bearing ore requires drilling and blasting. After blasting, the ore is reduced in size with drop balls and loaded on the trucks with shovels for transport to the processing plant, where mica, quartz and feldspar are extracted.
The principal use of ground mica is in gypsum wallboard joint compound, where it acts as a filler and extender, provides a smoother consistency, improves workability, and prevents cracking. In the paint industry, ground mica is used as a pigment extender that also facilitates suspension due to its light weight and platy morphology. The ground mica also reduces checking and chalking, prevents shrinkage and shearing of the paint film, provides increased resistance to water penetration and weathering, and brightens the tone of colored pigments. Ground mica also is used in the well-drilling industry as an additive to drilling "muds."
The plastic industry used ground mica as an extender and filler and also as a reinforcing agent. The rubber industry uses ground mica as an inert filler and as a mold lubricant in the manufacture of molded rubber products, including tires.
Sheet mica is used principally in the electronic and electrical industries. The major uses of sheet and block mica are as electrical insulators in electronic equipment, thermal insulation, gauge "glass", windows in stove and kerosene heaters, dielectrics in capacitors, decorative panels in lamps and windows, insulation in electric motors and generator armatures, field coil insulation, and magnet and commutator core insulation.
Of the 28 known species of the mica group, only 6 are common rock-forming minerals. Muscovite, the common light-coloured mica, and biotite, which is typically black or nearly so, are the most abundant. Phlogopite, typically brown, and paragonite, which is macroscopically indistinguishable from muscovite, also are fairly common. Lepidolite, generally pinkish to lilac in colour, occurs in lithium-bearing pegmatites. Glauconite, a green species that does not have the same general macroscopic characteristics as the other micas, occurs sporadically in many marine sedimentary sequences. All of these micas except glauconite exhibit easily observable perfect cleavage into flexible sheets. Glauconite, which most often occurs as pelletlike grains, has no apparent cleavage.
The names of the rock-forming micas constitute a good example of the diverse bases used in naming minerals: Biotite was named for a person—Jean-Baptiste Biot, a 19th-century French physicist who studied the optical properties of micas; muscovite was named, albeit indirectly, for a place—it was originally called "Muscovy glass" because it came from the Muscovy province of Russia; glauconite, although typically green, was named for the Greek word for blue; lepidolite, from the Greek word meaning "scale," was based on the appearance of the mineral's cleavage plates; phlogopite, from the Greek word for firelike, was chosen because of the reddish glow (colour and lustre) of some specimens; paragonite, from the Greek "to mislead," was so named because it was originally mistaken for another mineral, talc.
Micas have sheet structures whose basic units consist of two polymerized sheets of silica (SiO4) tetrahedrons. Two such sheets are juxtaposed with the vertices of their tetrahedrons pointing toward each other; the sheets are cross-linked with cations—for example, aluminum in muscovite—and hydroxyl pairs complete the coordination of these cations (see figure). Thus, the cross-linked double layer is bound firmly, has the bases of silica tetrahedrons on both of its outer sides, and has a negative charge. The charge is balanced by singly charged large cations—for example, potassium in muscovite—that join the cross-linked double layers to form the complete structure. The differences among mica species depend upon differences in the X and Y cations.
Although the micas are generally considered to be monoclinic (pseudohexagonal), there also are hexagonal, orthorhombic, and triclinic forms generally referred to as polytypes. The polytypes are based on the sequences and number of layers of the basic structure in the unit cell and the symmetry thus produced. Most biotites are 1M and most muscovites are 2M; however, more than one polytype is commonly present in individual specimens. This feature cannot, however, be determined macroscopically; polytypes are distinguished by relatively sophisticated techniques such as those employing X-rays.
The micas other than glauconite tend to crystallize as short pseudohexagonal prisms. The side faces of these prisms are typically rough, some appearing striated and dull, whereas the flat ends tend to be smooth and shiny. The end faces are parallel to the perfect cleavage that characterizes the group.
The rock-forming micas (other than glauconite) can be divided into two groups: those that are light-coloured (muscovite, paragonite, and lepidolite) and those that are dark-coloured (biotite and phlogopite). Most of the properties of the mica group of minerals, other than those of glauconite, can be described together; here they are described as pertaining simply to micas, meaning the micas other than glauconite. Properties of the latter are described separately later in the discussion.
The perfect cleavage into thin elastic sheets is probably the most widely recognized characteristic of the micas. The cleavage is a manifestation of the sheet structure described above. (The elasticity of the thin sheets distinguishes the micas from similarly appearing thin sheets of chlorite and talc.) The rock-forming micas exhibit certain characteristic colours. Muscovites range from colourless, greenish to blue-green to emerald-green, pinkish, and brownish to cinnamon-tan. Paragonites are colourless to white; biotites may be black, brown, red to red-brown, greenish brown, and blue-green. Phlogopites resemble biotites but are honey brown. Lepidolites are nearly colourless, pink, lavender, or tan. Biotites and phlogopites also exhibit the property termed pleochroism (or, more properly for these minerals, dichroism): When viewed along different crystallographic directions, especially using transmitted polarized light, they exhibit different colours or different absorption of light or both.
The lustre of the micas is usually described as splendent, but some cleavage faces appear pearly. The minutely crystalline variety consisting of muscovite or paragonite (or both), generally referred to as sericite, is silky.
Mohs hardness of the micas is approximately 21/2 on cleavage flakes and 4 across cleavage. Consequently, micas can be scratched in either direction with a knife blade or geologic pick. Hardness is used to distinguish micas from chloritoid, which also occurs rather commonly as platy masses in some metamorphic rocks; chloritoid, with a Mohs hardness of 61/2, cannot be scratched with a knife blade or geologic pick.
Specific gravity for the micas varies with composition. The overall range is from 2.76 for muscovite to 3.2 for iron-rich biotite.
Glauconite occurs most commonly as earthy to dull, subtranslucent, green to nearly black granules generally referred to as pellets. It is attacked readily by hydrochloric acid. The colour and occurrence of this mineral in sediments and sedimentary rocks formed from those sediments generally are sufficient for identification.
Uses of Mica Mineral
The world’s largest mica deposits are found in India in igneous, metamorphic, and sedimentary regions of Bihar and Nellore district of Madras. Mainly commercially important micas are muscovite and phlogopite. The unique properties of mica are very useful in various fields.
The main applications of mica are listed below：
Uses of Mica in Everyday Life - Today, mica is used in almost everything - from the construction of buildings to makeup. 37 phyllosilicate minerals of the mica group possess platy texture and are used in fields. It is used as a pigment extender. Mica disc is used in breathing apparatus, communication devices, lenses, broadband waveplates etc. Mica is used in microwave ovens as well. Not only this, eyeliner or lip gloss that most women use on a daily basis also contains mica.
Uses of Mica Powder – We are using mica powder for various purposes especially for decorations for ages. Mica powder is used in clay pots, traditional Pueblo pottery, coloured powders, printing techniques or woodblock printmaking. It is also used in the decoration of windows of the buildings and to brighten the coloured pigments. It is widely used in cosmetics.
Uses of Mica Sheets – Mica sheets are mainly used as window sheets. Small pieces of mica sheets are used in toys as well. Sheet mica is used in electronics, microscopy, diaphragms for oxygen-breathing equipment, navigation compasses, thermal regulators, optical fibres, pyrometers (a type of thermometer used to measure the temperature of distant objects), stove or kerosene heater windows, mica thermic heaters etc.
As mica shows a refractive index that depends on the polarization and propagation direction of light, it is commonly used to make quarter and half-wave plates. The specialized use of mica is found in aircraft components and sea-launched missile systems. Apart from these, it is used in laser devices, radar systems and Geiger Muller tubes etc.
Uses of Mica in Cosmetics – Reflective and refractive properties of mica make it an important ingredient of cosmetic products. Mica is used in blushes, lipsticks, lip gloss, eyeliner, eye shadow, foundation, glitters, mascara, nail polish, moisturizing lotions etc. Some teeth whitening agents also contain mica. Mica creates a natural shimmery finish on the skin. It helps to give a more youthful and shinier, wrinkle-free look. Apart from these, mica does not react with skin and is suitable for all skin types.
Uses of Mica Paper – Mainly, mica paper is used in mica plates and mica tapes. Mica is an excellent electrical insulator while a good thermal conductor and high-temperature resistant (up to 1000℃). Due to these properties, mica tape is used in electrical and thermal appliances. It can also be used as a substitute for sheet mica. It is used for decorative purposes.
Uses of Mica in Medicines – We use mica in Ayurveda (ancient medicine prevalent in India). It is used in the preparation of various medicines for the treatment of respiratory and digestion-related diseases.
Other Uses of Mica – Thin and transparent sheets of mica are used in peepholes in lanterns, boilers, stoves etc. It is used to make capacitors for calibration standards. It is also used in transistors and high-pressure steam boilers.
Micas that form common rock are found all throughout the world. The following are the more significant events:
Biotite is found in many igneous rocks (such as granites and granodiorites), as well as numerous pegmatite masses and metamorphic rocks (e.g., gneisses, schists, and hornfelsed). It is scarce in sediments and sedimentary rocks because it changes readily during chemical weathering. The weathering of biotite has caused some uncertainty at one point. Biotite loses its flexibility and turns into silvery grey flakes as a result of chemical weathering. Weathered biotite is golden yellow with a bronzy sheen in an intermediate stage that can be mistaken for gold flakes by novice observers.
Phlogopite is uncommon in igneous rocks, however, it can be found in ultramafic (silica-poor) rocks. It can be found in some peridotites, particularly those known as kimberlites, which are the rocks that contain diamonds. Some magnesium-rich pegmatites contain phlogopite, which is an uncommon component.
Muscovite is found in metamorphic gneisses, schists, and phyllites in particular. Muscovite occurs as minute grains (sericite) in fine-grained foliated rocks like phyllites, giving these rocks their silky luster. Muscovite is also found in various granitic rocks. It is abundant in complicated granitic pegmatites and miarolitic druses. Much of the muscovite in igneous rocks are assumed to have originated late in the parent magma's consolidation, or shortly afterwards. Muscovite is a weather-resistant mineral that can be found in various soils formed over muscovite-bearing rocks, as well as clastic deposits and sedimentary rocks produced from them.
Only a few gneisses, schists, and phyllites have been confirmed to contain paragonite, which appears to play a similar role to muscovite. It's possible, however, that it's a lot more prevalent than people think. It is because all light-coloured micas in rocks were mistakenly labelled as muscovites until recently without examining their potassium to sodium ratios, some paragonites may have been mistakenly identified as muscovites. It weathers in much the same way that muscovite does. Lepidolite is nearly primarily found in complicated lithium-bearing pegmatites, while it has also been found in a few granites.
As previously stated, glauconite is developing in several modern-day marine settings. It's also a prevalent component of sedimentary rocks, whose precursor sediments are thought to have been deposited on old continental shelves' deeper sections. Greensand is a term used to describe glauconite-rich sediments. The most common form of glauconite is granules, which are sometimes known as pellets. It's also available as a pigment, usually in the form of films that coat a variety of substrates like fossils, faeces pellets, and clastic debris.
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