Temperature-dependent disorder in a natural Mg-Al-Fe2+-Fe3+-spinel

A natural Mg-Al-Fe spinel from the Balmuccia peridotite (Italian Western Alps) was annealed at T between 650 and 1150 degrees C, under controlled oxygen activity, and quenched in H2O. Twenty-three cation distributions were calculated from XRD structural refinements in tandem with microprobe analysis, and verified by Mossbauer spectroscopy in the case of unheated samples. Unheated crystals showed essentially ordered distribution of Fe3+ in octahedral and Fe2+ in tetrahedral sites, the only intracrystalline disorder being represented by similar to 0.12 atoms per formula unit of Al-[4] and Mg-[6]. Thermal runs and quenching maintained substantially ordered distribution of Fe2+ and Fe3+ up to similar to 990 degrees C and produced continuous Mg-[4]-Al-[6] exchange. Between 990 and 1150 degrees C, the previous order of Fe2+-Fe3+ appeared to change slightly, Fe-[6](2+) reaching similar to 0.04 afu and Mg-[6] congruent to Al-[4] congruent to 0.25 afu at the highest T. After quenching from this temperature, Fe2+ still resided mainly in the T site. Some previously heated crystals underwent reordering on lowering of the temperature. Experimental data, integrated with existing literature, enabled cation-oxygen distance in this structure to be improved. Results from annealed samples allowed the formulation of an experimental thermometric function based on Mg-Al intracrystalline disorder.