Crystal chemistry of a highly disordered Mg-Al natural spinel

A Mg-Al spinel from a xenolith found in the Sacrofano volcanic area (Latium, Italy) was investigated by X-ray single crystal diffraction and electron microprobe. Its main structural parameters were: a(0)=8.0922(3) Angstrom and the oxygen coordinate u = 0.26213(4), which gave to tetrahedral and octahedral bond distances of 1.9220(6) Angstrom and 1.9299(3) Angstrom respectively. Cation distribution, obtained from structural and chemical data, showed a very high Mg-Al disorder, with an Al(T) content of 0.205 and an inversion parameter, i, of about 0.23. Comparison with the cation distribution of a spinel with similar chemical composition but occurring in chlorite schists (Shishimsk Mountains, Urals, Russia) with a(o) = 8.1003(3) Angstrom and u = 0.26330(3), and with data from the literature concerning spinels from metamorphosed limestones (Kalkbro, Sweden) and websteritic dikes (Balmuccia, Italy), all characterized by much lower Mg-Al disorder (i about 0.13-0.12), suggests a completely different thermal history, with rapid cooling rates and consequent high closure temperature of intracrystalline exchange, consistent with the Sacrofano geological environment.