Micas

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Jason Cole

The identification of a sheet of mica is never much of a problem even for an untrained eye. All forms of mica have perfect basal cleavage, unique hexagonal shaped crystals, and an unmistakable "snap" of cleavage flakes when they are bent together and then released. There are several varieties of mica, but the three primary varieties are, muscovite, a hydrated silicate of aluminum and potassium; phlogopite, which is a magnesium bearing mica; and biotite, a ferro-magnesium variety. The world's chief deposits of mica are found in India at Bihar and in the Nellore district of Madras. Over 50% of the mica used today comes from these two regions. Other major producers are Belgium, Brazil and China. In industry, all types of mica are used in two different ways. It can either be used in long sheets or it can be ground up into tiny bits, depending upon the specific use for the mica.

Muscovite mica Muscovite in economic quantities is confined to coarse-grained granite pegmatite dykes consisting largely of quartz, feldspar and mica. The dykes are of acidic igneous origin, and the minerals probably crystallized directly from magma.

When it comes to modern technology, sheet muscovite is an indispensable resource. It is used in almost every electronic device sold today as an insulator. Its high resistance to the passage of electricity and heat are so great that no substitute, artificial or natural, has proved to be economically suitable to replace it. No other mineral has better cleavage, flexibility or elasticity. It is possible to roll a sheet of muscovite less than 0.1mm thick into a cylinder 6mm thick and its elasticity would enable the sheet to flatten out again quite easily. Sheet mica is just as important to the electrical and electronic industries as copper wire and now ranks as one of the essential minerals of modern life.

Muscovite Rose with Apatite. Aldea Velha Mine, Conselherio, Pena, Minas Gerais, Brazil. University of Waterloo Earth Sciences Museum Collection.

Ground muscovite is used for entirely different products, than is sheet muscovite. Muscovite can be transparent to translucent in colour, but will always maintain its lustrous cleavage faces. Because of this property, muscovite is used in many different beauty products to make them sparkle. Products such as glitter, lip stick, and mascara all can contain small ground up pieces of muscovite mica. Other industrial products use mica as a non-stick agent in moulds. Many rubber and plastic companies use it as a filler and as a dusting medium to prevent their products from adhering to the moulds in which they were shaped.

Phlogopite

Phlogopite mica can be easily recognized by its distinct red-brown colour. The word phlogopite comes from the Greek word phlogopos, which means "fire-like", in allusion to its red-brown colour. Phlogopite mica is more suitable than other types of mica for separating the segments of a commutator, because it wears more evenly and at about the same rate as the metal bars. Some of the highest grade phlogopite is found in Canada, in an area within 150 kilometres Ottawa, where large hexagonal crystals, none exceeding 30 centimetres in diameter, can be found. This resource is not being mined at present.

Phlogopite is associated with basic igneous rocks, especially pyroxenites, which can often be found as veins/fissures in metamorphosed limestones and gneisses. However, these deposits tend to form sporadically and therefore are difficult to mine economically.

Muscovite mica cannot be used in temperatures that exceed 550 degrees Celsius, whereas, phlogopite can be utilized at temperatures up to 1000 degrees Celsius. Muscovite is still used more commonly in insulators because the price of phlogopite is quite high in comparison to other micas. Still for special circumstances where heat and/or wear might be a problem, phlogopite is always used.

Phlogopite. Portland, Quebec. University of Waterloo Earth Sciences Museum Collection.

Lepidolite

Lepidolite mica is an uncommon mica and has only in the past decade became available on the world markets in large quantities. It is an ore of lithium and typically forms in granitic masses that contain high amounts of lithium. This mineral can be found in such places as Brazil, Russia and the west coast of the United States.

Lithium as an element is often obtained from lepidolite mica to be used in such applications as lasers, T.V. and radio tubes, fireworks, as well as in medications for treating bipolar disorders. Its primary use is in glass making. When added to the melted glass, lepidolite acts as an opacifier in opalescent and white glass that is widely used for table tops and jars. It also enhances the strength of the glass. Lepidolite is not normally mined because of its properties as a mica, rather it is mined for its lithium content. However, lepidolite has an attractive violet colour that does make it valuable as an ornamental stone or as a mineral specimen.