Monticellite and kirschsteinite are gray silicate minerals of the olivine group with composition CaMgSiO4 and CaFeSiO4, respectively. Most monticellites have the pure magnesium end-member composition but rare ferroan monticellites and magnesio-kirschsteinite are found with between 30 and 75 mol.% of the iron end member. Monticellite is named after Teodoro Monticelli Italian mineralogist (1759-1845).
Like other members of the group monticellite and kirschsteinite have orthorhombic unit cells (space group Pbnm). iron and magnesium ions are located on the M1 inversion sites and calcium ions occupy the M2 site on mirror planes. The unit cell is somewhat larger than for the calcium free olivines forsterite and Fayalite.