Monticellite

(repeating unit)Ca(Mg,Fe)SiO₄IMA symbolMtc – Kir^[1]Strunz classification9.AC.10Crystal systemOrthorhombicCrystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)Space groupPbnmIdentificationColourcolourless or greyCleavage{010}Mohs scale hardness5.5Specific gravity3.05 – 3.27Refractive indexα = 1.638 – 1.654,
β = 1.646 – 1.664,
γ = 1.650 – 1.674Melting point1503 °CReferences^[2]

Monticellite and kirschsteinite (commonly also spelled kirschteinite^[3]) 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 Teodoro Monticelli, an Italian mineralogist (1759–1845). Kirschsteinite is named after Egon Kirschstein, a German geologist.

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 = 0.4815 nm,

b = 1.108 nm and

c = 0.637 nm,

and for monticellite

a = 0.4875 nm,

b = 1.1155 nm and

c = 0.6438 nm.

Gallery

• 
  The atomic scale structure of olivine looking along the a axis. Oxygen is shown in red, silicon in pink, and Mg, Fe and Ca in blue. A projection of the unit cell is shown by the black rectangle.
• 
  Colourless crystals of kirschsteinite

References

• Deer, W. A., Howie, R. A., and Zussman, J. (1992). An introduction to the rock-forming minerals (2nd ed.). Harlow: Longman ISBN 0-582-30094-0
• Mindat.org
• Webmineral.com
• Handbook of Mineralogy