Lozenge-shaped “fish” of this material are a common component of mylonitic rocks. Potassium or sodium characterizes the common type of this material contrasted (-5[1])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, (-5[1])and two fluorine or OH. Books of this material are often found in pegmatites. This material, which has perfect basal cleavage, (10[2])includes biotite and (10[1])muscovite types. (10[1])For 10 points, name this group of silicate minerals known for easily flaking and having a shimmering (10[1]-5[1])appearance. ■END■ (10[1]0[3])