Monticellite and kirschsteinite are gray silicate minerals of the olivine group with compositions 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. Pure kirschsteinite is only found in synthetic systems. Monticellite is named after Monticello.
Like other members of the group monticellite and kirschsteinite have orthorhombic unit cells (space group Pbnm) shown in Figure 1. 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 with a = 4.815 angstroms (Å) or 0.4815 nanometer (nm), b = 11.08 Å (1.108 nm) and c = 6.37 Å (0.637 nm), and for monticellite a = 4.875 Å (0.4875 nm), b = 11.155 Å (1.1155 nm) and c = 6.438 Å (0.6438 nm).