The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic with a tendency 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.
Chemically, micas can be given the general formula:
in which X is K, Na, or Ca or less commonly Ba, Rb, or Cs
Y is Al, Mg or Fe or less commonly Mn, Cr, Ti, Li, etc
Z is chiefly Si or Al but also may include Fe3+ or Ti
Structurally the micas can be classed as di-octahedral (Y = 4) and tri-octahedral (Y = 6). If the X ion is K or Na the mica is a common mica whereas if the X ion is Ca the mica is classed as a brittle mica.