Electrochemical measurement of the free energy of almandine (Fe 3Al 2Si 3O 12) garnet

Abstract

The free energy of the reaction (GISQ): 2 Fe + 2 3 Al 2SiO 5 + 4 3 SiO 2 + O 2 = 2 3 Fe 3Al 2Si 3O 12 metal sillimanite quartz almandine has been measured electrochemically between 1073 and 1173 K at 1 atmosphere pressure. Relative to quartz-fayalite-iron (QFI) equilibrium we obtain: log ƒ O 2GISQ − log ƒ O 2QFI (± 0.1) = −2.23 + 2400 T (1073–1173 K). In the experimental temperature range this result is in excellent agreement with data obtained from studies of garnet-rutile-sillimanite-ilmenite-quartz (GRAIL) equilibrium and for Fe-Mg partitioning between garnet and olivine. It is also in good agreement with recently published estimates based on other phase equilibrium data (Berman, 1988; Anovitz and Essene, 1987). The measured ƒ O 2 for GISQ equilibrium was found to increase with increasing cell edge of the synthetic almandine. The above value is for almandine with a o = 11.527 (±0.001) A ̊ . Almandine with a cell edge of 11.534 Å gave ƒ o 2 values about 0.3 log units more oxidized. This increase in ƒ O 2 correlates with increasing Fe +3 of the synthetic garnet. 57Fe Mössbauer spectroscopy indicates that between 3 and 6 percent of the iron is Fe +3 in the high cell-edge material, whereas almandine with a o = 11.527 A ̊ is essentially ferric-free