Temperature-Dependent Disorder in a Nautral Mg-Al-Fe2+-Fe3+-Spinel

Antonio Della Giusta, Susanna Carbonin and Giulio Ottonello
Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi 37, 35122 Padova, Italy
Dipartimento di Scienze della Terra, Università di Genova, Corso Europa 26, 16132 Genova, Italy

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.

Keywords: spinel • disorder • temperature dependence • peridotite • Italian Alps

Mineralogical Magazine; August 1996 v. 60; no. 401; p. 603-616; DOI: 10.1180/minmag.1996.060.401.06
© 1996, The Mineralogical Society
Mineralogical Society (www.minersoc.org)