Lozenge-shaped “fish” of this material are a common component of mylonitic rocks. Potassium or sodium characterizes the common type of this material contrasted with a calcium-containing brittle type. In a solid-solution series of this material, annite sits opposite the magnesium endmember phlogopite. Domains named for this material separate quartz and feldspar layers in (*) slate. A typical chemical formula for this material contains one atom of potassium, two to three of a Y element, four of silicon, ten oxygen, and two fluorine or OH. Books of this material are often found in pegmatites. This material, which has perfect basal cleavage, includes biotite and muscovite types. (-5[1])For 10 points, name this group of silicate minerals known for easily flaking and having a shimmering appearance. ■END■ (0[1])