Silicate minerals comprise over 95% of the Earth's crust (by volume), distinguished by a common, basic building block, the Silicon-Oxygen tetrahedron. The various structural arrangements of these tetrahedra create very different families of mineral types; olivine (isolated), pyroxene (single chains), amphiboles (double chains), two dimensional sheets and three dimensional frameworks.
One of these silicate minerals, Mica, has a two-dimensional sheet structure. Mica is rich in silicon, potassium and aluminum, and thus belongs to the Sialic group of silicates. One of the most prominent characteristics of Mica is its perfect cleavage. The one-dimensional cleavage planes cause Mica to cleave into thin, bendable flakes. This distinctive flat flaking is possible because of the sheet structure of the mineral.
There are two main varieties of Mica; muscovite and biotite. Muscovite, KAl2 (AlSi3) O10 (OH)2, is a sialic mineral, often either white or colorless although its color can range to pale brown. Biotite, K(Mg, Fe+2)3(Al,Fe+3) Si3O10(OH)2, is an iron and magnesium-rich mineral that belongs to the mafic mineral group. It can be brown or green in thin section and has perfect basal cleavage (i.e. cleavage between two parallel faces). In addition to color variations, muscovite is also less dense than biotite at 2.8g/cm3 and 3 g/cm3 respectively.
Mica is abundant in granites and sandstone, and thus is found all over the world in association with batholiths and other metamorphic features of the continental crust. Because of its attainability, Mica has many uses outside the Geology classroom. Sheet muscovite is used in electric insulators and ground mica can be found not only in paint, but also as a dusting agent. Mica has even been used to make windows, due to its excellent transparency.
But does Mica have any cultural or artistic merit beyond its role in geologic studies? When Mica appears with other silicates in rocks we often get commonplace granites. Other configurations of silicates are far more appealing to the aesthetic sense, aventurine for example. Aventurine, a transparent variety of quartz, contains tiny inclusions of other minerals, such as green mica. Green aventurine is often thought to be a talisman of independence. Also, this pale green stone flecked with green and gold is told to assure good health to those who own it.
Mica is only one example out of the amazing variety of silicate minerals that exist in nature. Its distinctive structure and appearance ensure its easy identification, and it has proven itself a valuable and useful resource. It also has cultural significance; it is used as a healing stone and its presence in other minerals often creates beautiful crystals and stones. Mica, while seeming to be an ordinary mineral, has not only geologic function, but cultural as well.