CRYSTAL-CHEMISTRY OF A HIGHLY DISORDERED MG-AL NATURAL SPINEL

A Mg-Al spinel from a xenolith found in the Sacrofano volcanic area (L atium, 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 t etrahedral and octahedral bond distances of 1.9220(6) Angstrom and 1.9 299(3) Angstrom respectively. Cation distribution, obtained from struc tural 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. Com parison with the cation distribution of a spinel with similar chemical composition but occurring in chlorite schists (Shishimsk Mountains, U rals, Russia) with a(o) = 8.1003(3) Angstrom and u = 0.26330(3), and w ith data from the literature concerning spinels from metamorphosed lim estones (Kalkbro, Sweden) and websteritic dikes (Balmuccia, Italy), al l characterized by much lower Mg-Al disorder (i about 0.13-0.12), sugg ests a completely different thermal history, with rapid cooling rates and consequent high closure temperature of intracrystalline exchange, consistent with the Sacrofano geological environment.