Mica is a family of minerals primarily composed of silicon, oxygen and aluminum, with varied other metals depending on the specific mica in question. Mica molecules are based on the basic form of the silica sheet, which is a mass of silicon and oxygen arranged in flat, friable plates.
Silicate is a molecule made up of a central silicon atom and four oxygen atoms arranged around it like the corners of a three-sided pyramid. In silica sheets, the oxygen corners are shared, and the molecules are arranged in flat, scaly planes. Mica is a variation on this structure, where aluminum replaces some of the silicon atoms in the array. The various kinds of mica have other metals in their arrays in addition to aluminum. For example, the white micas contain sodium or potassium. The pink mica lepidolite contains potassium and lithium. Other micas may contain iron or magnesium.