Muscovite

This is the mineral Muscovite. It has a vitreous, or glassy, luster that is especially visible on its crystal faces. A crystal face is a geometric plane that develops as a crystal grows. Muscovite’s crystal faces are flat, sheet-like planes that look glassy under reflected light. The mineral exhibits perfect basal cleavage, meaning that it splits parallel to other cleavage planes. It ranks between 2.5 and 3 on Mohs hardness scale, making it a soft and easily scratched mineral. Its specific gravity is 2.7 to 3, thus it feels relatively light in your hand. Muscovite produces a white streak, and many flakes are often shed when a specimen is drawn across a streak plate. Like many other rock-forming minerals, Muscovite belongs to the Silicate chemical family because it contains repeating chains of linked Silicon and Oxygen tetrahedra. These tetrahedra consist of a single, central Silicon atom surrounded by 4 Oxygen atoms. A distinctive characteristic of Muscovite and other mica minerals such as Biotite, Phlogopite, and Lepidolite, is its ability to flake into thin, pliable, and transparent sheets. In fact, Muscovite, which is often clear or exhibits a light brown color, was used around Moscow during the 18th century as a substitute for window glass. Muscovite is commonly seen as visible crystals embedded within igneous rocks, including Granite, Diorite, and Pegmatite. While Muscovite crystals embedded within Granite are often quite small, requiring a hand lens to be seen, those in Pegmatite are large and often measure over a centimeter in diameter. Muscovite’s high melting point, approximately 1,300 degrees Celsius, make it an ideal thermal insulator. This contrasts with another common mica mineral, Biotite, which has a much lower melting point of approximately 900 degrees Celsius. Many early ovens and furnaces thus used sheets of Muscovite as windows into their fiery depths. Since it lacks Iron it’s also an ideal insulator and thus used to manufacturing electronics. You may have unknowingly used Muscovite recently because it’s in many common products, including dry lubricants, paints, nail polishes, lipsticks, toothpastes, and eye shadows.