Cochise College Student Papers in Geology
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Roger Weller, geology instructor
wellerr@cochise.edu
Mica
Tedina Allen
Physical Geology
Fall 2007
Mica
What is Mica?
Mica is a generic term applied to a group of complex aluminosilicate
minerals having a sheet or plate like structure. It forms in flat, six sided
monoclinic crystals that have perfect basal cleavage and large surfaces. It is
capable of easily splitting into thin, flat films. Chemically, mica is a
complex silicate of aluminum and alkalis with hydroxyl. Some varieties may
contain impurities of iron, magnesium, lithium, fluorine, barium, manganese and
vanadium.
It is because of its perfect basal cleavage and its chemical and physical
properties, that mica is a very valuable and desired mineral in many ways. No
other substance found in nature possesses all the properties of mica.
Properties of Mica
Mica possesses some of the most outstanding combinations of physical,
electrical, thermal and mechanical properties which are not found in any other
product. The crystalline structure forms the layers that are so easily
delaminated into sheets. Mica is chemically inert, a non-conductor of
electricity ( dielectric ), elastic, flexible, readily absorbs water (
hydrophilic ), insulating, lightweight, reflective, flat and broad ( platy ),
refractive, resilient, and can range from transparent to opaque as needed.
Mica is stable when exposed to light, moisture, and extreme temperature.
It is inert to the action of water, most acids, oil, and is virtually unaffected
by atmospheric action.
Types of Mica
There are several different types of Mica that can vary in composition due to geologic occurrence. There are more than 20 chemically distinct mica species. All micas have the same essential crystal structure and are easily recognizable as mica.
There are six main important mica minerals, these are:
Muscovite ( pure potassium mica )
Paragonite ( sodium mica )
Lepidolite ( lithium mica )
Phlogopite ( magnesium mica )
Biotite ( magnesium, iron mica )
Zinnwaldite (lithium, iron mica)
Out
of all of the mica minerals, muscovite and biotite are the most common.
Whenever the word mica is used, it is generally thought to be muscovite that is
being referred to.
Uses for Mica
Mica
is extremely valued by industry due to its unique properties. Industry
seems to have found use mostly from three different types of mica.
Lepidolite is the source of mica from which lithium is obtained. That is the
only way Lepidolite is utilized.
Phlogopite is able to remain stable at a very high temperature, usually
about 1000 degrees Celsius. It is because of this incredible durability, it is
often used in applications in which a combination of high heat stability and
electrical properties are required. However, phlogopite does not possess as much
flexibility and split ability as other micas.
The
most widely used type of mica is Muscovite. Muscovite has greater
dielectric strength than any other insulating material. It surpasses all
other micas in dielectric strength, thermal endurance, mechanical strength,
moisture resistance, transparency, flexibility, resilience, toughness, and
perfection of cleavage. It is recognized as the best mica for electrical
and electronics devices. It is the principal mica used by industry.
It is used in many ways, but is invaluable to the electrical industry.
Electrically, mica not only has dielectric strength but also a uniform
dielectric constant and capacitance stability, low power loss, high electrical
resistivity and low temperature coefficient and capacitance. It is noted for its
resistance to arcing.
Thermally, mica is fireproof and non-flammable, infusible, and
incombustible. Most micas can resists temperatures of about 900 degrees Celsius
with no noticeable effects. It has low heat conductivity, and excellent
stability.
Mechanically, mica is relatively soft, flexible, and elastic and yet it is
still tough. It can be hand cut, machined or die punched.
Mica
is usually used in two forms for most things. It is used in sheet form or ground
up. It is the sheet form that is used in so many electrical apparatus.
It is used in heating appliances, electrical control, lighting, industrial
electrical needs, radios, radars, televisions, motors, generators, and
transformers. It is used to line the gauge glass of high pressure steam
boilers, in optical filters, in diaphragms for oxygen- breathing equipment,
missile systems, laser devices, and much more.
There are two different types of ground Mica, dry ground and wet ground:
Dry
ground mica is prepared by grinding it in high speed hammer mills until it is
the consistency of flour. Dry ground mica can be found in joint compound
that is used for finishing and filling in seams and blemishes that are found in
drywall. It is a filler and an extender. It has a smooth consistency
and it improves the workability of the compound used. It has great
resistance to cracking. Joint compound accounts for about 51% of dry
ground mica use.
It is
also very popular in the paint industry. It is used as a pigment extender.
It helps to facilitate suspension and cuts down on chalking. It prevents
shrinking and shearing of paint film. It also increases resistance to
water penetration and weathering. Mica also brightens the tone of the
pigments. The use of mica in paint is thought to account for about 24%.
The
rubber industry also utilizes mica as an inert filler and mold release compound
in the manufacturing of molded rubber products such as tires and roofing
supplies. It makes a very good anti-sticking agent.
Mica
is also used by the plastic industry as an extender and filler, especially in
the manufacturing of automobile parts such as fascia and fenders. Sometimes it
is used as a lightweight insulation to suppress sound and vibration. It is
a reinforcing material that provides improved mechanical properties and
increases strength, stiffness, and dimensional stability.
Wet
ground mica is often used in industry; it is produced by grinding the mica in
water involving preferential delamination of flakes. Churn mills are used
with large wheels or rollers that rotate on horizontal shafts to grind the mica
until it is reduced to a fine scale. Wet ground mica is more costly than
dry ground mica. It is used predominantly in paint, rubber, plastics,
lubricants, as wallpaper coating and cosmetics. It is often used wherever
its attractive, shiny luster is desired. It is responsible for the bright,
metallic colors on automobiles as well as on a woman’s face. Its
reflective and refractive properties make it an important ingredient in blushes,
eye shadows, body and hair glitter, nail polish, and foundation.
Where is Mica found?
Mica is found to occur in thick book forms in pegmatites. Pegmatites are
very coarse grained igneous rocks that have grain size of 20 mm or more.
It is still a subject of research to determine the exact mode of formation of
mica, which is found in the form of small flakes to big slabs. However, it
seems to be fairly clear that the degree of presence or absence of orthoclase
feldspar indicates the possibility of finding mica within the pegmatite.
It is thought that mica forms at the expense of orthoclase feldspar.
India is the leading producer of muscovite mica, followed by Brazil and Western Africa in Tanzania.
Leading producers of phlogopite are the Malagasy Republic and Tanzania. Small amounts are also found in Canada and India.
References:
Photo References:
http://skywalker.cochise.edu/wellerr/aawellerweb.htm
www.mineralminers.com/html/zinmins.stm
Text References:
http://www.britannica.com/eb/article-80070/mica
http://www.icrmica.com/icrmica_mica_introduction.html
http://www.mineralszone.com/minerals/mica.html
http://minerals.usgs.gov/minerals/pubs/commodity/mica/440497.pdf
http://www.ima-na.org/about_industrial_minerals/mica.asp
http://www.mineralminers.com/html/mcaminfo.htm
