Thermal behavior of almandine at temperatures up to 1,200°C in hydrogen
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The thermally induced reductive decomposition of a natural near end-member almandine [VIII(Fe2.85Mg0.11Ca0.05Mn0.02)VI(Al1.99)IV(Si2.99)O12] and possible hydrogen diffusion into its structure have been carried out at temperatures up to 1,200°C, monitored by simultaneous thermogravimetric analysis and differential scanning calorimetry (DSC), infrared and 57Fe Mössbauer spectroscopy and X-ray powder diffraction. Below 1,000°C, evidence for hydrogen diffusion into almandine structure was not observed. At temperatures above 1,000°C, reductive decomposition sets in, as displayed by a sharp endothermic peak at 1,055°C on the DSC curve accompanied by a total mass loss of 3.51%. We observe the following decomposition mechanism: almandine + hydrogen → α-Fe + cristobalite + hercynite + water. At higher temperatures, fayalite and sekaninaite are formed by consecutive reaction of α-Fe with cristobalite and water, and cristobalite with hercynite, respectively. The metallic α-Fe phase forms spherical and isolated particles (~1 μm).
KeywordsFe-bearing garnet Almandine Thermal treatment Hydrogen diffusion Reductive decomposition Iron particles
This work has been supported by research projects of the Academy of Sciences of the Czech Republic (grant no. KAN115600801) and the Ministry of Education, Youth and Sports of the Czech Republic (grant no. MSM6198959218; and the Operational Program Research and Development for Innovations—European Regional Development Fund, project no. CZ.1.05/2.1.00/03.0058). Part of this work has been done during a stay of JF at the Department of Mineralogy, Swedish Museum of Natural History, Stockholm (financed by the European Community—Research Infrastructure Action under the FP6 Program within SYNTHESYS; Project SE-TAF-4065). We wish to thank R. Škoda for EMPA measurements, M. Heřmánek, K Šafářová and J. Ševčíková for technical assistance and J. Tuček for language corrections. Samples for this study were kindly provided by the Moravian Museum in Brno, Czech Republic.
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