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Figure 1
A model of the main structural elements of mullite. Chains of AlO6 octahedra along the c axis are interconnected via a network of dicluster and tricluster tetrahedra, which also form chains along the c axis. The Al/Si ratio determines the concentration of vacancies which in turn determines the concentration of triclusters, as each vacancy has a characteristic environment of two triclusters. Subsequent AB layers are shown with decreasing opacity to indicate that the vacancies do not form channels but are completely enclosed by the network of tetrahedra. In this model, below each vacancy there is a dicluster that consists of two Si cations.

IUCrJ
Volume 5| Part 4| July 2018| Pages 497-509
ISSN: 2052-2525
https://doi.org/10.1107/S2052252518007467