Characterization and Thermal Behaviour of Garnets from Almandine–Pyrope Series at 1200°C
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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 1200°C studied by 57Fe Mössbauer 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 1200°C resulting in the formation of the paramagnetic spinel Mg(Al,Fe)2O4 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 1200°C conforming to the formation of magnetically ordered nanocrystalline γ-Fe2O3 or Mg(Fe,Al)2O4 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.
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