A dioctahedral mica with the general composition Ca₂Al₄(Si₄Al₄O₂₀)(OH,F)₄, found in fine-grained, low-grade, regional metamorphic rocks (see metamorphic grade). Calcium is the main cation found between the individual silicate sheets of the mica structure. Margarite has the typical appearance of mica, but is much harder than common mica types. Cleavage sheets are also less elastic than those of the common micas and hence margarite is a member of the ‘brittle mica’ group. At one time this mineral was thought to be a rare component of metamorphic rocks but recent X-ray diffraction work has demonstrated that it can be abundant in low-grade metapelites (see meta-). It occurs with corundum in emery deposits and in mica schists along with tourmaline and staurolite.