An aluminium silicate which forms thin layered sheets. Mica is a family of minerals with two main groups, both of which form solid solution series. The two main groups are biotite and muscovite. Biotite tends to be dark in colour while muscovite is gnerally white or even clear.
Mica can withstand changes in temperature without cracking and since some forms are translucent it was used extensively as a replacement for glass in furnaces and lanterns. This has now been superceded by modern forms of glass which do the same job and are easier to source and make. Mica also makes a good capacitor, its values remain the same even if the device overheats.