10 Å layer silicates which cleave into parallel sheets. They may be represented by an infinitely thick I-beam. Micas contain cations between layers. They are divided into dioctahedral and trioctahedral groups. Micas are also characterized into elastic and brittle micas. Micas may form in various polytypes, including 1M (phlogillite), 2O (not found in nature), 6H (not found in nature), and 2M (muscovite).