CALIBRATION AND APPLICATIONS OF SPINEL EQUILIBRIA IN THE SYSTEM FEO-AL2O3-SIO2

Abstract

A new thermobarometer, based on the equilibrium: 3 \hspace{0.17em} FeAl 2 O 4+5 \hspace{0.17em} Sio 2= Fe 3 Al 2 Si 3 O 12+2 \hspace{0.17em} Al 2 SiO 5 hercynite \hspace{0.17em} quartz \hspace{0.17em} almadine \hspace{0.17em} sillimanite has been calibrated with experiments carried out in the piston-cylinder apparatus. Reversed equilibria were obtained using well-calibrated 2.54 cm NaCl furnace assemblies and Ag80Pd20capsules with fO2buffered at or near iron-wustite. The equilibrium is located between 5.2–5.4, 6.6–6.8, and 8.6–8.8 kb at 880, 940, and 1020°C, respectively, and at 5.2 and 8.8 kb between 865–880 and 1020–1030°C, respectively. X-ray refinement data indicate that the hercynite (a = 8.15546 ?) has approximately 18 per cent inverse character. M?ssbauer spectra reveal that 4 mol per cent of the Fe is ferric (2 per cent magnetite component). Broad Mossbauer lines and a Fe2+ energy level splitting of 3.7 kJ mol?1 calculated from the Mossbauer spectra are consistent with the X-ray determined degree of inversion, although no separate octahedral Fe2+ spectral doublet is resolved. Calibration of this equation allows calculation of the equilibrium: 3 \hspace{0.17em} FeAl 2 O 4+5 \hspace{0.17em} Sio 2= Fe 3 Al 2 Si 3 O 12+2 \hspace{0.17em} Al 2 O 3 hercynite \hspace{0.17em} sillimanite \hspace{0.17em} almadine \hspace{0.17em} corundum Thermobarometers based on the above equilibria are widely applicable in granulite fades rocks and yield pressure/temperature data that are consistent with other well-calibrated barometers and thermometers.