Crystal-chemical study of cation disordering in Al-rich and Al-poor orthopyroxenes from spinel lherzolite xenoliths

Structural refinements of natural and heated orthopyroxenes from two spinel lherzolite xenoliths were used to investigate the extent of disordering of Mg and Fe2+, and of the trivalent cations, in the orthopyroxene crystal structures. The orthopyroxenes were studied both before and after heating. Although disordering of Mg and Fe2+ between M1 and M2 sites was increased at high temperatures (1050-1150°C), trivalent cations continued to be restricted to M1 sites. Disordering of Mg and Fe2+ was more easily achieved when contents of trivalent cations in the orthopyroxene are low. -Authors