The mica group of sheet silicate
( phyllosilicate ) minerals includes several
closely related materials having close to
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. [1]
Classification
Chemically, micas can be given the general
formula [2]
X2 Y 4–6 Z 8O 20 ( 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
Fe 3+ 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:
Biotite
Lepidolite
Muscovite
Phlogopite
Zinnwaldite
Brittle micas:
Clintonite
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 H3 O + 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 Fe 2+ substituting for Al
in the Y site and a corresponding increase in Si
in the Z site.
Occurrence and production
Mica embedded in metamorphic rock.
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 19th century. The
largest documented single crystal of mica
( phlogopite ) was found in Lacey mine, Ontario,
Canada ; it measured 10×4.3×4.3 m and
weighed about 330 tonnes .[3] Similar-sized
crystals were also found in Karelia , Russia .[4]
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 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). [5] 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. [6]
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, 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) 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. [7]
Ground mica
The leading use of dry-ground mica in the US
is in 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 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. [6]
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 antiblocking,
antisticking 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. [6]
( phyllosilicate ) minerals includes several
closely related materials having close to
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. [1]
Classification
Chemically, micas can be given the general
formula [2]
X2 Y 4–6 Z 8O 20 ( 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
Fe 3+ 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:
Biotite
Lepidolite
Muscovite
Phlogopite
Zinnwaldite
Brittle micas:
Clintonite
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 H3 O + 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 Fe 2+ substituting for Al
in the Y site and a corresponding increase in Si
in the Z site.
Occurrence and production
Mica embedded in metamorphic rock.
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 19th century. The
largest documented single crystal of mica
( phlogopite ) was found in Lacey mine, Ontario,
Canada ; it measured 10×4.3×4.3 m and
weighed about 330 tonnes .[3] Similar-sized
crystals were also found in Karelia , Russia .[4]
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 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). [5] 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. [6]
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, 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) 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. [7]
Ground mica
The leading use of dry-ground mica in the US
is in 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 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. [6]
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 antiblocking,
antisticking 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. [6]
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