Electrostatic energies of mantle minerals in the MgO-SiO2 system, and their crystal-chemical properties and cation distributions.

Abstract

Electrostatic energies of nine mantle minerals in the MgO–SiO2 system (forsterite, modified spinel, spinel, orthoenstatite, clinoenstatite, ilmenite, perovskite, periclase and stishovite) were calculated by the method of Bertaut (1952) with error estimations.It is noted that the structure of ortho- and clinoenstatites have atomic geometries which are skillfully arranged to gain low electrostatic energies compared with ilmenite and perovskite, although the former minerals have lower densities than the latter. This low energy results from the low values of Mg1, O3A and O3B ions in these pyroxenes but mainly from the fact that O2A and O2B oxygens, which are “undersaturated” with Pauling’s bond-strength sums, do not have so high values as O1 of modified spinel which are also “undersaturated” with them.Periclase and hence, Mg and O of the mineral exhibit an extremely high same value, showing a clear contrast with other minerals even in the high pressure. This indicates that Fe2+ ion prefers magnesiowustite (which is isostructural with periclase) in the cation distributions of Fe2+ and Mg2+ between perovskite and magnesiowustite, and spinel and magnesiowustite which might coexist in the mantle transition zone. This is qualitatively consistent with the high pressure experiments studied by high pressure workers.