Characterization and thermal behaviour of garnets from almandine-pyrope series at 1200 degrees C

Abstract

The natural garnets from almandine (Fe3Al2Si3O12) -pyrope (Mg3Al2Si3O12) series with the iron to magnesium atomic ratio ranging from 0.2 to 1 were characterised and their thermal behaviour at 1200degreesC studied by Fe-57 Mossbauer spectroscopy, X-ray powder diffraction, X-ray fluorescence, DTA, TG and electron microprobe analysis. The pyrope-type samples with a dominant magnesium content at position 24c in the cubic garnet structure undergo oxidative decomposition at 1200degreesC resulting in the formation of the paramagnetic spinel Mg(Al, Fe)(2)O-4 structure with a low iron content, enstatite (Mg, Fe) SiO3 and anorthite CaAl2Si2O8 as the host compound for calcium. Contrary to pyropes, the iron-rich garnets exhibit complete oxidation at 1200degreesC conforming to the formation of magnetically ordered nanocrystalline gamma-Fe2O3 or Mg(Fe, Al)(2)O-4 spinels depending on the initial chemical composition of the garnets. In the reaction products of iron-rich garnets, cordierite (Mg2Al4Si5O18) and anorthite were identified as non-ferrous phases.