Abstract: A natural Mg-Al-Fe spinel from the Balmuccia peridotite (Italian Western Alps) was annealed at T between 650 and 1150°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 Mössbauer 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 ∼0.12 atoms per formula unit of ^[4]Al and ^[6]Mg. Thermal runs and quenching maintained substantially ordered distribution of Fe²⁺ and Fe³⁺ up to ∼990°C and produced continuous ^[4]Mg-^[6]Al exchange. Between 990 and 1150°C, the previous order of Fe²⁺-Fe³⁺ appeared to change slightly, ^[6]Fe²⁺ reaching ∼0.04 afu and ^[6]Mg ≅ ^[4]Al ≅ 0.24 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.