Phase formation and crystal-chemical characterization of low- and high-alumina mullite-type compounds
Final Report Abstract
This work covered the characterization of natural and synthetic mullites in the high and low alumina region of the idealized compositional series Al4+2xSi2-2xO10-x. Synthetic mullites have outstanding properties, such as extraordinary high-temperature stability, low thermal expansion and conductivity, high creep resistance, and mechanical strength. Thus, it is an important component in ceramic materials. In contrast to the synthetic product, the mineral mullite is only poorly investigated. Here, we present the first crystal-structure description of the natural mineral. Based on crystal-chemical aspects, we have set up a standardized chemical formula for mullite minerals according to M+yMg2+zM2+wM3+4+2x+y+2w-2zM4+2-2x-y-2w+zO10-x with Mg residing on an octahedral site and other divalent cations in vacant O3 positions. That way, more than 50 chemical compositions from mullite minerals known in the literature are directly comparable. Chemical analyses of the mullite minerals studied here revealed an incorporation of Fe and Ti cations and yielded the following compositions: Al4.34Fe0.12Si1.49Ti0.05O9.77 for one specimen from the Eifel area. The second one from the Eifel consists of two different parts with compositions of Al4.406Fe0.103Si1.455Ti0.037O9.746 and Al4.15Fe0.107Si1.68Ti0.06O9.872, respectively. The third crystal from Kladno, Czech Republic has the composition Al4.40Fe0.018Si1.58O9.791. The crystal-structure refinements showed that Fe predominantly enters the octahedral sites, which is an explanation for the slight increase in lattice parameters c. It could be demonstrated that the incorporation of foreign cations increases the refractive indices. Our results confirmed previous findings, that mullite minerals are often formed at the SiO2-rich side (x < 0.25) of the Al2O3 – SiO2 system with compositions not yet observed for synthetic mullites, which usually range between about 0.25 ≤ x ≤ 0.4. Another mineral was found to have properties intermediate between sillimanite (ordered Si/Al distribution) and mullite (oxygen vacancies). It is described here as a polytype of mullite with doubled c lattice parameter with respect to mullite, designated mullte-2c. The reproduction of synthesis routes from literature to produce pure alumina with a mullite-type crystal structure named iota-alumina (ι-Al2O3) showed that the resulting phase corresponds to Na-aluminate mullite and not to pure alumina. We discussed all published work on the ι-Al2O3 phase and concluded that this phase most probably does not exist or has not been obtained so far.
Publications
- (2020) Mullite-type Na0.67Al6O9.33 and a discussion of iota-alumina. Journal of the European Ceramic Society 40 (12) 4276–4280
Lenz, Stephan; Schneider, Hartmut; Fischer, Reinhard X.
(See online at https://doi.org/10.1016/j.jeurceramsoc.2020.03.038) - Natural mullites: chemical composition, crystal structure, and optical properties. European Journal of Mineralogy 31 (2019) 353-367
Lenz, S., Birkenstock, J., Fischer, L.A., Schüller, W., Schneider, H., Fischer, R.X.
(See online at https://doi.org/10.1127/ejm/2019/0031-2812) - Mullite-2c – a natural polytype of mullite. European Journal of Mineralogy 32 (2020) 235-249
Lenz, S., Birkenstock, J., Fischer, L.A., Schneider, H., Fischer, R.X.
(See online at https://doi.org/10.5194/ejm-32-235-2020)
