Any of a group of chemically and physically
related aluminum silicate minerals, common in igneous and metamorphic
rocks, characteristically splitting into flexible sheets used in
insulation and electrical equipment. Mica is a mineral that can be
split into very thin sheets. These sheets can be so thin that 1000 can
be layered into mica 1 inch high. Mica can be clear, black, green, red,
yellow, and brown.
Mica is commonly found as small flakes or lamellar
plates without a crystal outline. Muscovite and biotite sometimes occur
in thick books, tabular prisms with a hexagonal outline that can be up
to several feet across. The prominent basal cleavage is a consequence
of the layered crystal structure. Thin cleavage sheets of micas,
particularly muscovite and phlogopite, are flexible, elastic, tough,
and translucent to transparent (isinglass). They have low electrical
and thermal conductivity and high dielectric strength.
The laboureres engaged in mica mines and factories
where mica is hand-dressed are experts in dressing the crude mica into
block, films and splittings. They do it by visual aid only. During the
course of processing a considerable portion of mica goea waste. It is
termed as scrap mica. The ratio of scrap mica to crude may vary from 60
to 80% depending on the defective portion in the crude mica.
Micas have Mohs hardnesses of 2¨C3 and specific
gravities of 2.8¨C3.2. Upon heating in a closed tube, they evolve water.
They have a vitreous-to-pearly luster. Muscovite is colorless to pale
shades of brown, green, or gray. Paragonite is colorless to pale
yellow. Phlogopite is pale yellow to brown. Biotite is dark green,
brown, or black. Lepidolite is most often pale lilac, but it can also
be colorless, pale yellow, or pale gray.
Material Notes:
Silicates of alumina with hydrogen (hydrous
alumino silicate) and potash (Muscovite). Mica is mined in Brazil,
India, many parts of Africa, Canada, and the United States. It is used
in the manufacturing of electronic and electrical devices. Mica is
invaluable in the electrical industry because of its unique combination
of physical, chemical and thermal properties, low power loss factor,
dielectric constant and dielectric strength.
Sheet mica is used in a number of electrical and
electronic appliances in different shapes and sizes. As an insulating
material it is used in equipment like condensers, transformers,
sheostats, radio and electronic tubes and radar circuits. It is used in
the form of washers, discs, tubes and plates.
Nowadays mica is finding increasing use in
equipment that encounters very high temperatures like rockets, missiles
and jet engine ignition system. It is reported that in the manufacture
of Telestar transmission satellites by teh USA, good use of mica has
been made.
In the stove manufacturing industry, mica is being
replaced by refractory glass, Pyrex and Iena, owing to the fact that
while mica sheet cracks under heat, pyrex does not. In the electrical
equipment industry numerous substitutes, usually made of plastics e.g.,
teflon, nylon, stratified fibres and araldite or silicon varnishes, are
coming into use. Mica is, however, still in use in precision work where
substitutes have so far failed.
Mica minerals make some rocks sparkle! They are
often found in igneous rocks such as granite and metamorphic rocks such
as schist. They sparkle because light is reflected on their flat
surfaces, which are where the mineral breaks along its plane of
cleavage. These minerals break so easily along their cleavage that some
crystals have broken into many thin layers that look like the pages of
a little book. Colonial Americans used the ¡°pages¡± of large mica
crystals as glass for windows.
