Articles | Volume 32, issue 2
Research article
11 Mar 2020
Research article |  | 11 Mar 2020

Mullite-2c – a natural polytype of mullite

Stephan Lenz, Johannes Birkenstock, Lennart A. Fischer, Hartmut Schneider, and Reinhard X. Fischer

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Cited articles

Agrell, S. O. and Smith, J. V.: Cell dimensions, solid solution, polymorphism, and identification of mullite and sillimanite, J. Am. Ceram. Soc., 43, 69–76, 1960. 
Aramaki, S.: Sillimanite and cordierite from volcanic xenoliths, Am. Mineral., 46, 1154–1165, 1961. 
Aramaki, S. and Roy, R.: Revised phase diagram for the system Al2O3SiO2, J. Am. Ceram. Soc., 45, 229–242, 1962. 
Becker, P. J. and Coppens, P.: Extinction within the limit of validity of the Darwin transfer equations. I. General formalism for primary and secondary extinction and their applications to spherical crystals, Acta Crystallogr. A, 30, 129–147,, 1974. 
Beger, R. M., Burnham, C. W., and Hays, J. F.: Structural changes in sillimanite at high temperature, in: Abstracts with Programs of the Annual Meeting of the Geological Society of America, Milwaukee, USA, 11–13 November 1970, 490–491, 1970. 
Short summary
A mineral from Ettringer Bellerberg (Eifel, Germany) proved to be a polytype of the important ceramic-phase mullite termed mullite-2c, with – similar to sillimanite – doubling of the c lattice parameter due to strong (Si,Al) order in tetrahedral diclusters and – similar to mullite – presence of oxygen vacancies and tetrahedral triclusters due to Al / Si > 1 in diclusters. Crystals were characterised using single-crystal XRD, electron microprobe analysis (EMPA) and spindle-stage optical methods.