Phase formation and crystal-chemical characterization of low- and high-alumina mullite-type compounds

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It is the aim of this project to understand the complete structural evolution of the phases in the alumino-silicate system of mullite-type compounds with the endmembers sillimanite (Al2O3 . SiO2, x=0) and jota-alumina (Al2O3, x=1) in the compositional series Al4+2xSi2-2xO10-x. While sillimanite and synthetic mullite with its most abundant phases 2:1-mullite (2Al2O3 . SiO2, x=0.4) and 3:2-mullite (3Al2O3 . 2SiO2, x=0.25) have been studied extensively within the last decades, there is essentially no information on the crystal structures of these compounds at the low (x < 0.2) and high-alumina (x > 0.8) side of the series. Minerals (usually containing some Fe) are known to exist in the low-alumina range, but there are no crystal-structure investigations available. The existence or nonexistence of jota-alumina is still a matter of debate. In hypothetical models derived from DFT calculations a highly disordered structure was proposed (Aryal et al., Phys. Rev. B84, 2011, 174123) with AlO6 and AlO5 polyhedra forming the polyhedral chain crosslinked by di-, tri-, and tetraclusters of AlO4 tetrahedra. Instead, we suggest that it could consist of AlO6 octahedra and of AlO4 forming tetrahedral tetraclusters only. However, experimental proof is still missing. We propose to study a series of natural mullites already available and to be collected in the Eifel area as members of the low-alumina compounds. Since we have shown just recently (Fischer et al., Amer. Mineral. 2015, in press) that a mineral exists which has structural features from both sillimanite (Si/Al ordering) and mullite (O vacancies) it will be especially interesting to characterize these Eifel minerals structurally and microchemically. It is also planned to synthesize these intermediate low-alumina phases by annealing sillimanite at high temperatures and pressures in a piston cylinder apparatus. High-alumina compounds including jota-alumina will be synthesized in a microwave system following known routes. It will be the first time that crystal-structure investigations are performed on these compounds. Since mullite-type materials are most important ceramic constituents it will be extremely important to extend our knowledge on these compounds towards the hitherto unknown crystal structures of the low- (x<0.2) and high- (x>0.8) alumina mullites and mullite-type phases.