| Mica | |
|---|---|
.jpg) | |
| General | |
| Category | Phyllosilicates |
| Formula (repeating unit) | AB2–3(X, Si)4O10(O, F, OH)2 |
| Identification | |
| Color | purple, rosy, silver, gray (lepidolite); dark green, brown, black (biotite); yellowish-brown, green-white (phlogopite); colorless, transparent (muscovite) |
| Cleavage | Almost perfect |
| Fracture | flaky |
| Mohs scale hardness | 2.5–4 (lepidolite); 2.5–3 biotite; 2.5–3 phlogopite; 2–2.5 muscovite |
| Luster | pearly, vitreous |
| Streak | White, colorless |
| Specific gravity | 2.8–3.0 |
| Diagnostic features | cleavage |
Sheets of mica
Photomicrographs of a thin section containing phlogopite. In cross-polarized light on left, plane-polarized light on right.
Dark mica from Eastern Ontario
The mica group of sheet silicate (phyllosilicate) minerals includes several closely related materials having nearly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals,
and are similar in chemical composition. The nearly 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 derived from the Latin word mica, meaning a crumb, and probably influenced by micare, to glitter.
Classification
Chemically, micas can be given the general formula
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.
Dioctahedral micas
Trioctahedral micas
Common micas:
Brittle micas:
Interlayer-deficient micas
Very
fine-grained micas, which typically show 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 and production
Mica is widely distributed and occurs in igneous, metamorphic and sedimentary regimes. Large crystals of mica used for various applications are typically mined from graniticpegmatites.
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 19th century. The
largest documented single crystal of mica (phlogopite) was found in Lacey Mine, Ontario, Canada; it measured 10 m × 4.3 m × 4.3 m (33 ft × 14 ft × 14 ft) and weighed about 330 tonnes (320 long tons; 360 short tons).[8] Similar-sized crystals were also found in Karelia, Russia.
The British Geological Survey reported that as of 2005, Koderma district in Jharkhand
state in India had the largest deposits of mica in the world. China was
the top producer of mica with almost a third of the global share,
closely followed by the US, South Korea and Canada. Large deposits of
sheet mica were mined in New England from the 19th century to the 1970s.
Large mines existed in Connecticut, New Hampshire, and Maine.
Scrap and flake mica is produced all over the world. In 2010, the
major producers were Russia (100,000 tonnes), Finland (68,000 t),
United States (53,000 t), South Korea (50,000 t), France (20,000 t) and
Canada (15,000 t). The total global production was 350,000 t, although
no reliable data were available for China. Most sheet mica was produced
in India (3,500 t) and Russia (1,500 t). Flake mica comes from several sources: the metamorphic rock called schist
as a byproduct of processing feldspar and kaolin resources, from placer
deposits, and from pegmatites. Sheet mica is considerably less abundant
than flake and scrap mica, and is occasionally recovered from mining
scrap and flake mica. The most important sources of sheet mica are
pegmatite deposits. Sheet mica prices vary with grade and can range from
less than $1 per kilogram for low-quality mica to more than $2,000 per
kilogram for the highest quality.
Properties and uses
The mica group represents 37 phyllosilicate minerals
that have a layered or platy texture. The commercially important micas
are muscovite and phlogopite, which are used in a variety of
applications. Mica's value is based on several of its unique physical
properties. The crystalline structure of mica forms layers that can be
split or delaminated into thin sheets usually causing foliation in rocks. These sheets are chemically inert, dielectric,
elastic, flexible, hydrophilic, insulating, lightweight, platy,
reflective, refractive, resilient, and range in opacity from transparent
to opaque. Mica is stable when exposed to electricity, light, moisture,
and extreme temperatures. It has superior electrical properties as an
insulator and as a dielectric, and can support an electrostatic field
while dissipating minimal energy in the form of heat; it can be split
very thin (0.025 to 0.125 millimeters or thinner) while maintaining its
electrical properties, has a high dielectric breakdown, is thermally
stable to 500 °C (932 °F), and is resistant to corona discharge.
Muscovite, the principal mica used by the electrical industry, is used
in capacitors that are ideal for high frequency and radio frequency.
Phlogopite mica remains stable at higher temperatures (to 900 °C
(1,650 °F)) and is used in applications in which a combination of
high-heat stability and electrical properties is required. Muscovite and
phlogopite are used in sheet and ground forms.
Ground mica
The leading use of dry-ground mica in the US is in the joint compound for filling and finishing seams and blemishes in gypsum wallboard (drywall).
The mica acts as a filler and extender, provides a smooth consistency,
improves the workability of the compound, and provides resistance to
cracking. In 2008, joint compound accounted for 54% of dry-ground mica
consumption. In the paint industry, ground mica is used as a pigment
extender that also facilitates suspension, reduces chalking, prevents
shrinking and shearing of the paint film, increases the resistance of
the paint film to water penetration and weathering and brightens the
tone of colored pigments. Mica also promotes paint adhesion in aqueous
and oleoresinous formulations. Consumption of dry-ground mica in paint,
the second-ranked use, accounted for 22% of the dry-ground mica used in
2008.
Ground mica is used in the well-drilling industry as an additive to drilling fluids.
The coarsely ground mica flakes help prevent the loss of circulation by
sealing porous sections of the drill hole. Well drilling muds accounted
for 15% of dry-ground mica use in 2008. The plastics industry
used dry-ground mica as an extender and filler, especially in parts for
automobiles as lightweight insulation to suppress sound and vibration.
Mica is used in plastic automobile fascia and fenders
as a reinforcing material, providing improved mechanical properties and
increased dimensional stability, stiffness, and strength.
Mica-reinforced plastics also have high-heat dimensional stability,
reduced warpage, and the best surface properties of any filled plastic
composite. In 2008, consumption of dry-ground mica in plastic
applications accounted for 2% of the market. The rubber industry used
ground mica as an inert filler and mold release compound in the
manufacture of molded rubber products such as tires and roofing. The
platy texture acts as an anti-blocking, anti-sticking agent. Rubber mold
lubricant accounted for 1.5% of the dry-ground mica used in 2008. As a
rubber additive, mica reduces gas permeation and improves resiliency.
Dry-ground mica is used in the production of rolled roofing and asphalt shingles,
where it serves as a surface coating to prevent sticking of adjacent
surfaces. The coating is not absorbed by freshly manufactured roofing
because mica's platy structure is unaffected by the acid in asphalt or
by weather conditions. Mica is used in decorative coatings on wallpaper,
concrete, stucco,
and tile surfaces. It also is used as an ingredient in flux coatings on
welding rods, in some special greases, and as coatings for core and
mold release compounds, facing agents, and mold washes in foundry
applications. Dry-ground phlogopite mica is used in automotive brake
linings and clutch plates to reduce noise and vibration (asbestos substitute); as sound-absorbing insulation for coatings and polymer
systems; in reinforcing additives for polymers to increase strength and
stiffness and to improve stability to heat, chemicals, and ultraviolet
(UV) radiation; in heat shields and temperature insulation; in
industrial coating additive to decrease the permeability of moisture and
hydrocarbons; and in polar polymer formulations to increase the
strength of epoxies, nylons, and polyesters.
Mica flakes embedded in a fresco for glitter
Wet-ground mica, which retains the brilliance of its cleavage faces,
is used primarily in pearlescent paints by the automotive industry. 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. In the cosmetics industry, its
reflective and refractive properties make mica an important ingredient
in blushes, eye liner, eye shadow, foundation, hair and body glitter, lipstick, lip gloss, mascara,
moisturizing lotions, and nail polish. 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 is added to latex balloons to
provide a colored shiny surface.
Mica is also used as an insulator in concrete block and home
attics and can be poured into walls (usually in retrofitting uninsulated
open top walls). Mica may also be used as a soil conditioner,
especially in potting soil mixes and in gardening plots. 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.
Built-up mica
Muscovite
and phlogopite splittings can be fabricated into various built-up mica
products. Produced by mechanized or hand setting of overlapping
splittings and alternate layers of binders and splittings, built-up mica
is used primarily as an electrical insulation material. Mica insulation
is used in high-temperature and fire-resistant power cables in
aluminium plants, blast furnaces,
critical wiring circuits (for example, defense systems, fire and
security alarm systems, and surveillance systems), heaters and boilers,
lumber kilns,
metal smelters, and tanks and furnace wiring. Specific high-temperature
mica-insulated wire and cable is rated to work for up to 15 minutes in
molten aluminium, glass, and steel. Major products are bonding
materials; flexible, heater, molding, and segment plates; mica paper;
and tape.
Flexible plate is used in electric motor and generator armatures, field coil insulation, and magnet and commutator
core insulation. Mica consumption in flexible plate was about 21 tonnes
in 2008 in the US. Heater plate is used where high-temperature
insulation is required. Molding plate is sheet mica from which V-rings
are cut and stamped for use in insulating the copper segments from the
steel shaft ends of a commutator. Molding plate is also fabricated into
tubes and rings for insulation in armatures, motor starters,
and transformers. Segment plate acts as insulation between the copper
commutator segments of direct-current universal motors and generators.
Phlogopite built-up mica is preferred because it wears at the same rate
as the copper segments. Although muscovite has a greater resistance to
wear, it causes uneven ridges that may interfere with the operation of a
motor or generator. Consumption of segment plate was about 149 t in
2008 in the US. Some types of built-up mica have the bonded splittings
reinforced with cloth, glass, linen, muslin,
plastic, silk, or special paper. These products are very flexible and
are produced in wide, continuous sheets that are either shipped, rolled,
or cut into ribbons or tapes, or trimmed to specified dimensions.
Built-up mica products may also be corrugated or reinforced by multiple
layering. In 2008, about 351 t of built-up mica was consumed in the US,
mostly for molding plates (19%) and segment plates (42%).
Sheet mica
Mica insulator items
Muscovite windows
Technical grade sheet mica is used in electrical components,
electronics, in atomic force microscopy and as window sheets. Other uses
include diaphragms for oxygen-breathing equipment, marker dials for
navigation compasses, optical filters, pyrometers, thermal regulators, stove and kerosene heater windows, radiation aperture covers for microwave ovens, and micathermic heater elements. Mica is birefringent and is therefore commonly used to make quarter and half wave plates. Specialized applications for sheet mica are found in aerospace components in air-, ground-, and sea-launched missile systems, laser
devices, medical electronics and radar systems. Mica is mechanically
stable in micrometer-thin sheets which are relatively transparent to
radiation (such as alpha particles) while being impervious to most gases. It is therefore used as a window on radiation detectors such as Geiger-Müller tubes.
In 2008, mica splittings represented the largest part of the
sheet mica industry in the United States. Consumption of muscovite and
phlogopite splittings was about 308 t in 2008. Muscovite splittings from
India accounted for essentially all US consumption. The remainder was
primarily imported from Madagascar.
Small squared pieces of sheet mica are also used in the traditional Japanese Kodo
ceremony to burn incense: A burning piece of coal is placed inside a
cone made of white ash. The sheet of mica is placed on top, acting as a
separator between the heat source and the incense, in order to spread
the fragrance without burning it.
Electrical and electronic
Sheet
mica is used principally in the electronic and electrical industries.
Its usefulness in these applications is derived from its unique
electrical and thermal properties and its mechanical properties, which
allow it to be cut, punched, stamped, and machined to close tolerances.
Specifically, mica is unusual in that it is a good electrical insulator
at the same time as being a good thermal conductor. The leading use of
block mica is as an electrical insulator in electronic equipment.
High-quality block mica is processed to line the gauge glasses of
high-pressure steam boilers because of its flexibility, transparency,
and resistance to heat and chemical attack. Only high-quality muscovite
film mica, which is variously called India ruby mica or ruby muscovite
mica, is used as a dielectric in capacitors. The highest quality mica film is used to manufacture capacitors for calibration standards. The next lower grade is used in transmitting capacitors. Receiving capacitors use a slightly lower grade of high-quality muscovite.
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).
Peepholes
Thin transparent sheets of mica were used for peepholes in boilers, lanterns, stoves, and kerosene heaters
because they were less likely to shatter than glass when exposed to
extreme temperature gradients. Such peepholes were also used in "isinglass curtains" in horse-drawn carriages and early 20th-century cars.
Atomic force microscopy
Another
use of mica is as a substrate in the production of ultraflat, thin-film
surfaces, e.g. gold surfaces. Although the deposited film surface is
still rough due to deposition kinetics, the back side of the film at the
mica-film interface is ultraflat once the film is removed from the
substrate. Freshly-cleaved mica surfaces have been used as clean
imaging substrates in atomic force microscopy, enabling for example the imaging of bismuth films, plasma glycoproteins, membrane bilayers, and DNA molecules.
Early history
Hand carved from mica from the Hopewell tradition
Human use of mica dates 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 finish over the entire object.
Mica flakes (called abrak in Urdu and written as ابرک) 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.
Mica powder
Throughout the ages, fine powders of mica have been used for various
purposes, including decorations. Powdered mica glitter is used to
decorate traditional water clay pots in India, Pakistan and Bangladesh;
it is also used on traditional Pueblo pottery, though not restricted to use on water pots in this case. The gulal and abir (colored powders) used by North Indian Hindus during the festive season of Holi contain fine crystals of mica to create a sparkling effect. The majestic Padmanabhapuram Palace, 65 km (40 mi) from Trivandrum in India, has colored mica windows. Mica powder is also used as a decoration in traditional Japanese woodblock printmaking, as when applied to wet ink and allowed to dry it sparkles and reflects light.
Medicine
Ayurveda,
the Hindu system of ancient medicine prevalent in India, includes the
purification and processing of mica in preparing Abhraka bhasma, which is employed in treating diseases of the respiratory and digestive tracts.
Health impact
Mica dust in the workplace is regarded as a hazardous substance for respiratory exposure above certain concentrations.
United States
The Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for mica exposure in the workplace as 20 mppcf over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 3 mg/m3 respiratory exposure over an 8-hour workday. At levels of 1,500 mg/m3, mica is immediately dangerous to life and health.
Substitutes
Some lightweight aggregates, such as diatomite, perlite, and vermiculite, may be substituted for ground mica when used as filler. Ground synthetic fluorophlogopite,
a fluorine-rich mica, may replace natural ground mica for uses that
require thermal and electrical properties of mica. Many materials can be
substituted for mica in numerous electrical, electronic, and insulation
uses. Substitutes include acrylate polymers, cellulose acetate, fiberglass, fishpaper, nylon, phenolics, polycarbonate, polyester, styrene, vinyl-PVC, and vulcanized fiber. Mica paper made from scrap mica can be substituted for sheet mica in electrical and insulation applications.
