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Sillimanite mullitization: Atem investigation and point defect model

A polycrystal sample of Fe-bearing prismatic sillimanite was annealed experimentally at T= 1675°C and P = 20 kbar for 12 min. ATEM investigation of the sample reveals that the starting material partly transformed into mullite during the annealing, and that this process was assisted by partial meltin...

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Bibliographic Details
Published in:Phase transitions 1999-01, Vol.68 (3), p.481-500
Main Authors: Raterron, Paul, Carpenter, Michael, Doukhan, Jean-Claude
Format: Article
Language:English
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Summary:A polycrystal sample of Fe-bearing prismatic sillimanite was annealed experimentally at T= 1675°C and P = 20 kbar for 12 min. ATEM investigation of the sample reveals that the starting material partly transformed into mullite during the annealing, and that this process was assisted by partial melting. The exsolved partial melt (now a glass), observable at triple edges and under the form small (< 100nm) precipitates that exsolved within the primary sillimanite grains, consists of about 80 wt.%SiO 2 ,12wt.% Al 2 O 3 ,5wt.%Fe 2 O 3 with some amount of K 2 O. Composition profiles throughout the sillimanite residual matrix reveal that sillimanite mullitization was still going on at the end of the run. The equilibrium sillimanite composition corresponding to the annealing conditions, and measured at the contact with the exsolved SiO 2 -rich melt, corresponds to the formula (Al, Fe) 433 Si 1·67 O 9·83 . From the equilibrium melt and matrix compositions and the size of the largest isolated precipitates, a rough estimate of the Si-Al interdiffusion coefficient in sillimanite (D) (associated to the mullitization at run conditions) is D ≈ 2 × 10 −17 m 2 /s. From the ATEM results, a point defect model is proposed to explain sillimanite mullitization. In the framework of this model, the majority point defects are the oxygen vacancies and the aluminum cations substituted for silicon (on T* sites). No distinction is made between sillimanite and mullite which are considered as one unique phase with different non-stoichiometry (i.e. different oxygen vacancy concentrations). The model, which accounts for the compositions of nonstoichiometric sillimanite measured at various temperatures and pressures, quantifies explicitly (with respect to P and T) the solid-solution composition of the sillimanite-mullite joint.
ISSN:0141-1594
1029-0338
DOI:10.1080/01411599908224529