Friday, January 14, 2011
Mica: a Shining Star
Micas are chemically similar and all possess similar properties. Their crystals are made up of many layers of thin sheets that resemble the pages of a book and can be split very easily. Mica crystals are technically monoclinic, which, without the scientific definition, means they have ten faces, are very thin, and look somewhat like two keys from a desktop-computer’s keyboard fused at the base to make a single shape. These crystals usually have six sides. In rare cases people have found mica with twinned crystals that looked like five-pointed stars. However, micas are so soft that their six-sided, monoclinic shape is often distorted, or at least all of the specimens that I found had been. Individual mica crystals can be very large, the largest ever found measured thirteen feet across! The color of different mica minerals depends on their chemical composition. While there are currently more than twenty micas identified, I wall only cover the most common: muscovite is usually colorless, but is sometimes pale green; biotite is jet-black, phlogopite is colorless or brown, and lepidolite is violet-pink. Mica also has a very reflective surface. I learned this first-hand while driving through a campground at night in a place where small fragments of mica littered the ground. As the light from the car headlights struck the pieces of mica they seemed to glow. At the time I did not know a great deal about mica and it wasn’t until the next morning that I found out what the little glowing things were!
In industry large sheets of mica are called sheet mica, and small flakes are called flake mica or scrap mica. Sheet mica was once used to make machine parts since it is not flammable, but I think that other substances have replaced it in that application by now. In today’s industry sheet mica is used as an insulator in electrical applications because it does not conduct electricity. Given mica’s shiny surface, flake mica is ground and used to give wallpaper a shiner luster. Perhaps the most interesting use that I have seen for mica comes from the Muscovy province in Russia. There, people once used large sheets of muscovite mica (which was named for the province, by the way) as a substitute for glass in the windows of their homes!
Since they are rock-forming minerals, micas are found all over the world in igneous, sedimentary, and metamorphic rocks. Some good sources of sheet mica include: India, Brazil, and Madagascar. Some good sources of flake mica include: North Carolina, New Mexico, and South Dakota.
Because it is used in today’s industry and is one of the major components in many rocks, the micas are certainly important minerals. Some are quite literally shining stars!