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[SCG58-10] Phase transition of sillimanite with Al/Si-disordering at high temperature
Keywords:sillimanite, mullite, Al/Si-disordering, synchrotron X-ray experiment
We heated sillimanite (Al2.00Si0.99Fe0.01O5) crystals in Rundvagshetta, Lutzow-Holm, Antarctica.sillimanite in the range of 0.8-2.5GPa, 1000-1500℃ for 10-1751 hours and then quenched. Experimental products (46 samples) were examined by Synchrotron powder XRD experiment at BL-4B2 in photon factory, KEK and observed TEM (JEOL JEM-2100F) from the viewpoint of l=odd reflections (which distinct with Al/Si-disordering) and chemical composition.
As a result of XRD experiments, the diffraction patterns of mullite were observed in many samples. Moreover, in four samples heated at 1300-1400℃, 1GPa, the peaks of unknown phase were observed in addition to that of sillimanite and mullite. This phase have an intermediate feature between peaks of sillimanite and mullite. (So, we will call it “intermediate-phase”.) As a result of TEM observation of these four samples, grains of the intermediate-phase did not give l = odd reflections, which are characteristic of sillimanite, but distinct with Al/Si-disordering. By chemical analysis (TEM-EDS), the intermediate-phase was slightly Al-rich more than sillimanite but less than mullite. So, It was revealed that the intermediate-phase has the structure which has disordered distribution of Al/Si on the tetrahedral sites, with releasing very little SiO2 from Al2SiO5. This phase can be stable at high temperature and 1GPa. The intermediate-phase is probably not same as “completely Al/Si-disordered sillimanite” by Greenwood (1972) etc., but may be same as the new phase founded by Fischer et al. (2014). The sillimanite with APB and fine mullite inclusion by Miyake et al. (2008) can be explained as that sillimanite transformed from intermediate phase with mullite. APB was made by Al/Si-ordering when intermediate phase transformed to sillimanite.
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