grayish silicate mineral in the olivine family, calcium and magnesium silicate (CaMgSiO₄), that occurs as small crystals or grains in metamorphosed siliceous dolomites, in contact skarn zones (of contact-metamorphic rock rich in iron), and, more rarely, in igneous rocks such as periodotite or nephelinite. For detailed physical properties, see olivine (table).

Minerals in which manganese and iron replace magnesium in the crystal structure are called glaucochroite and kirschsteinite, respectively; these minerals have physical properties similar to monticellite and vary only slightly from their ideal compositions. Glaucochroite has been reported in ore deposits at Franklin, N.J., in the United States; kirschsteinite occurs in slags.