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About Minerals

The Mica Group

By Chuck DeFlorin, Minnesota Mineral Club Member

Mica is the generic term that applies to a group of complex aluminum silicate minerals. The Mica group is a very large group containing thirty members, but only a very few are common to most collectors. Those few members make up a large percentage of the earth's crust. Some of the more common Mica minerals are: Biotite, Muscovite, Lepolite, Fuchsite, Phlogopite and Zinnwaldite. Most mineral collectors are sure to have these six minerals.

The name Mica is believed to have been derived from the Latin word micare, which means to shine or glitter. Evidence indicates that the ancient Hindus mined Mica as far back as 2000 B.C. They used it in medicines, window glazing and also as a painting surface for pictures.

All Mica members form a flat six-sided mono-clinical crystal, but are most commonly found in the form of scales and sheets. Mica is characterized by perfect basal cleavage that causes them to split into optically flat films. When split into sheets (films) they remain tough and elastic (able to return to its original shape after being bent) even at high temperatures. A piece of Mica that is one inch thick can be split into nearly a thousand sheets, each as thin as tissue paper. Mica ranges in hardness from 2 to 4 and a specific gravity of 2.7 to 3.2 Practically inert and very stable, Mica is virtually unaffected by the action of water and weather. It can be found in sands that have undergone a lot of erosion because of its durability. Mica is also fire proof, infusible, incombustible and nonflammable, resisting temperatures of 600 degrees C to 900 degrees C depending on the type.

There are many uses for Mica because of its many properties. Small dry ground Mica flakes are used as a thin coating on rubber surfaces to overcome tackiness and sticking. In exterior house paints, dry ground Mica adds body, reduces running and improves weatherability. The addition of mica to all types of sealers for porous surfaces (such as wallboard, masonry and concrete blocks) greatly reduces penetration and improves holdout. Wet ground Mica is used to coat wallpaper, because it imparts an attractive silky and pearly luster. Sheet Mica is used as a resonant diaphragm in certain acoustical devices. Mica is also used as heat and electrical insulators for industrial purposes and also dry powder fire extinguishers. Lastly, Mica is used for spraying Christmas trees, giving them a fresh snowed on appearance.

The most useful of the Micas is Muscovite (Potassium Mica) and is sometimes referred to as white or common Mica. Muscovite's name was derived from Muscovia, a district in Russia, where it was identified in 1609. Muscovite is usually colorless (in thin films), but may be red, yellow, green, brown, or silver gray, with a vitreous to pearly luster. No matter what the color it is still referred to as white Mica. There are a number of distinct varieties containing chromium (Fuchsite), Iron (ferrianmuscovite) and manganese (Alurgite). Fuchsite is a dark emerald green color that can have a beautiful shine if the crystals are small. Ferrianmuscovite is a very dark green color and Alurgite is a pinkish red color. Muscovite is one of the most common mineral in rocks, especially platonic igneous rocks rich in silica and aluminum (pegmatites and granites).

Biotite, sometimes called black Mica is named after the French physicist, professor Jean Baptiste Biot. Biotite, which contains iron, has a splendid shiny luster and is usually dark green, dark brown or black in color. Because of the iron content, it isn't used industrially as electrical insulators as is muscovite. Biotite is found in many intrusive igneous rocks, pegmatites, lavas and also metamorphic rocks. Fine crystals can be found in Alaska as well as the U.S.S.R, Greenland and Brazil.

Via the Rock Rustlers News 5/03 and the Southwest Gem 7/03