Mica is the mineral group that includes muscovite and biotite and splits into thin sheets.
Micas have a layered atomic structure. Strong bonds hold atoms within broad sheets, while weaker bonds occur between those sheets, producing perfect basal cleavage. As a result, a mica crystal can often be separated into flexible, transparent or translucent flakes.
Muscovite is a light-colored, potassium-rich mica, whereas biotite is usually dark because it contains iron and magnesium. Micas occur in igneous, metamorphic, and sedimentary rocks. Large muscovite sheets were once valuable as window material and remain useful as electrical and thermal insulators.
Mica is a group, not one chemical substance. This matters because its varieties differ in composition, color, and industrial use. Mica’s sheet-like cleavage also distinguishes it from feldspar, which commonly breaks along two cleavage directions that intersect at roughly right angles.