Occuring in pegmatites, some lesser-known varieties include zinwaldite, phlogopite, and lepidolite.
One type is a main source of rubidium and cesium, and an important source of lithium. Their typical composition is of many T-O-T layers sandwiched between interlayers of cations such as potassium and calcium. The "black" type has a magnesium-to-iron ratio of less than 2 to 1, and is used in potassium-argon dating. Another type, known as isinglass, is colorless and was used in window panes. Occuring in pegmatites, some lesser-known varieties include zinwaldite, phlogopite, and lepidolite. A class of phyllosilitcates, they have perfect basal cleavage, allowing them to be split into very thin laminae, or sheets. For 10 points, name this class of minerals containing biotite and muscovite.
One type is a main source of rubidium and cesium, and an important source of lithium.
{mica} (prompt on “{silicate}”, “{sheet silicate}”, “{phyllosilicate}”, “{minerals}”, etc. until mentioned)