Muscovite is potassium aluminum silicate and is the most common member of the mica family. Micas occur in many kinds of igneous and metamorphic rocks and all are characterized by a layered structure. The chemical bonds within the layers are strong, and between layers very weak. As a result micas display perfect cleavage (the tendency to break along a flat plane) and can be split into microscopically thin sheets. These sheets are usually transparent and are quite flexible. Muscovite is usually light brown or colorless and can occur in large crystals, known as 'books'. Mica sheets have many uses, including as insulators in electrical apparatus; at times large transparent cleavage sheets have even been used as window panes. This application of muscovite led to its name, from 'Muscovy glass', in allusion to its use in windows in Moscow.