Any of a group of silicate minerals with a layered structure. Micas are composed of linked SiO4 tetrahedra with cations and hydroxyl groupings between the layers. The cations between the layers link the layers. The general formula is X2Y4–6Z8O20(OH,F)4, where X = K,Na,Ca; Y = Al,Mg,Fe,Li; and Z = Si,Al. The three main mica minerals are:
muscovite, KA12 (A1Si3)O10 (OH)2;
biotite, K(Mg,Fe)3(AlSi3O10) (OH)2;
lepidolite, KLi2Al (Si4O10) (F,OH)2.
Micas have perfect basal cleavage and the thin cleavage flakes are flexible and elastic. Flakes of mica are used as electrical insulators and as the dielectric in capacitors.