How Is Muscovite Formed?

Muscovite Chemistry

• Muscovite forms when potassium, aluminum, silicon, oxygen, and hydrogen are bound together to form the mineral. Sometimes iodine and fluorine also occur in muscovite. The general chemical formula is H2KAl3(SiO4)3. Muscovite is one type of mica and is distinguished from the others by the amount of potassium. It is also the most common of the micas.

Molecular Structure

• Muscovite and other micas forms very thin parallel layers, which is one of their most striking features. These layers form because the aluminum, silicon, oxygen and hydrogen bind together to form thin molecular sheets called aluminum phyllosilicates. The other chemicals in muscovite bind these layers together like the frosting between cake layers. Muscovite splits into sheets when the chemical bonds separating the aluminum phyllosilicate layers break. These sheets can be thin enough for light to pass, and Russians (who gave muscovite its name) used them for windows. Large deposits of muscovite can be mined, and it is possible to find sheets of mica that are more than a meter across.

Igneous Muscovite

• Muscovite forms in rocks known as pegmatites (most commonly granite), which occur when magma wells up from deep within the earth's mantle and then cools and hardens before reaching the earth's surface. Because the magma cools slowly, the mineral grains in pegmatites can grow to be very large. Muscovite can also form in pegmatites when feldspar weathers, but this type is often called sericite. It can also form in extrusive igneous rocks (such as basalt) that form when magma reaches the surface of the earth before cooling, but this type of magma tends to have less silicon in it so micas are rare.

Muscovite Deposits

• Muscovite forms in pegmatites as tiny flecks, a millimeter or two across, and as large veins, called books. Sometime several parallel books occur in the same rock. Books of muscovite can extend for up to 200 meters before pinching out and can be easily mined.

Metamorphic Muscovite

• Muscovite also forms when minerals (usually feldspar, topaz and kyanite) are transformed (metamorphosized) when subjected to great heat or pressure. It usually forms in metamorphic rocks called schist and gneiss.

Sedimentary Muscovite

• Muscovite does not form in sedimentary rocks, but grains that weather out of igneous and metamorphic rock are often deposited along with other the mineral grains that are then cemented together to form these rocks.