They form most of schists.
Under cross-polarized light, these minerals exhibit motted and bird's-eye patterns as the light nears extinction. These minerals, with the exception of the Ba-Fe brittle type anandite, exhibit parallel extinction. These minerals are used to prevent shrinking in paint. They form most of schists. This family of minerals exhibits pseudo-hexagonal twinning. The bottom of the discontinuous branch of Bowen's reaction series consists of one mineral of this class, while another can be found above quartz. These phyllosilicates often have monoclinic crystalline structures and exhibit perfect basal cleavage. For 10 points, name this class of minerals that includes biotite and muscovite.
Under cross-polarized light, these minerals exhibit motted and bird's-eye patterns as the light nears extinction.
micas [prompt on phyllosilicate; prompt on sheet silicate minerals]