muscovite

Abundant silicate mineral that contains potassium and aluminum and has a layered atomic structure. It is the most common member of the mica group. Because it occurs in thin, transparent sheets, it was used in Russia for window panes and became known as Muscovy glass (isinglass), hence its name. Muscovite is usually colourless but may be light gray, brown, pale green, or rose red. Its low iron content makes it a good electrical and thermal insulator.