Carbonate rocks dominantly consist of carbonate minerals such as calcite, aragonite or dolomite. Calcite, aragonite and vaterite are pure calcium carbonate minerals. Albite, calcite, and pyrite often show polysynthetic twinning. These limestone towers consist primarily of calcium carbonate minerals such as calcite (CaCO). Minerals associated with graphite include quartz, calcite, micas and tourmaline. It is often associated with the minerals sphalerite, calcite and fluorite. Many are intergrown with other minerals, such as calcite and dolomite. In other taxa, alternate layers of calcite and aragonite are laid down. Chalk is a kind of limestone, made of the mineral calcite, or calcium carbonate. Common calcium-containing minerals are calcite and gypsum. Other linear polarizers exploit the birefringent properties of crystals such as quartz and calcite. The faults are of limited width, filled with calcite, pyrite and remoulded clay. It occurs in association with orpiment, arsenolite, calcite and barite. Lenses of trilobites' eyes were made of calcite (calcium carbonate, CaCO). Some important minerals are carbonates, notably calcite.