Synthetic mullite (Al2[Al2+2xSi2-2x]O10-x) is a very well characterised material. Due to its outstanding properties it is used for a vast range of technical applications representing one of the most important groups of ceramic materials. In contrast to the synthetic product, the mineral mullite is only poorly investigated. In this study crystal-structure refinements of natural mullites are presented for the first time. Three mullite single-crystals from the Ettringer Bellerberg, Eifel area, Germany (ME1 & ME2) and from Kladno, Czech Republic (MK1) have been examined by single-crystal X-ray diffraction methods, Electron Microprobe Analyses (EMPA), and the spindle-stage method to determine the refractive indices. Chemical analyses revealed an incorporation of Fe and Ti cations and yielded the following compositions per formula unit with Z = 1: Al4.34Fe0.12Si1.49Ti0.05O9.77 for ME1 corresponding to an x-value of x = 0.23(4); ME2 consists of two different parts with compositions of Al4.406Fe0.103Si1.455Ti0.037O9.746 and Al4.15Fe0.107Si1.68Ti0.06O9.872 corresponding to x = 0.254(2) and x = 0.128(14), respectively; MK1 has a composition of Al4.40Fe0.018Si1.58O9.791 with x = 0.209(15). The crystal structure of natural mullite is similar to its synthetic counterpart with chains of edge-sharing (Al, Fe)O6 octahedra parallel c crosslinked by T2O7 groups with T = Al,Si and a number of oxygen vacancies corresponding to the x-value in the general composition. Mullite crystallises in the orthorhombic space group Pbam with a = 7.5542(3) Å, b = 7.6998(3) Å, c = 2.8935(1) Å, V = 168.31(2) Å3 for ME1; a = 7.5353(3) Å, b = 7.7018(3) Å, c = 2.8928(3) Å, V = 167.89(4) Å3 for ME2 representing average values for the two parts; a = 7.5439(3) Å, b = 7.6958(3) Å, c = 2.8879(1) Å, V = 167.66(1) Å3 for MK1. The refinements give clear evidence that iron 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 (nx ≈ ny = 1.654(2) and nz = 1.668(2) for ME1; nx ≈ ny = 1.648(4) and nz = 1.662(4) for MK1). Our results confirm previous findings, that mullite minerals are commonly formed on 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.

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