Mica

The mica group of minerals are aluminosilicates with various metals. They split into fine sheets, which puts them in the phyllosilicate group of silicate minerals.
Mica includes several closely related materials having perfect basal cleavage. 
All are monoclinic which means their crystals are shaped like one kind of prism. They tend towards pseudo-hexagonal crystals and are similar in chemical composition. The highly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.
Being excellent electrical insulators, they are used in electronic equipment including capacitors.