Crystal chemical studies on incommensurately modulated mullite
Final Report Abstract
Single crystals of 2:1 mullite were examined by X-ray and neutron diffraction. From the 2D detector data undistorted reciprocal sections were calculated which confirmed the perfect reproducibility of previous data. This indicates that the disorder and the modulations are not arbitrary but well repeatable. The 1D incommensurately modulated crystal structure with q-vector (0.3137(2) 0 ½) was refined for the first time using the superspace approach. The results are discussed with respect to possible arrangements of the structural units in real space. The findings clearly indicate conflicting principles between a hypothetical need to form an n·5-fold superstructure, which is required by the chemical composition, and a uniform energy distribution in the structure which results in favourable motifs of structural arrangements. On average these motifs form a modulated structure along the above modulation direction and at least in two more directions which are associated with the q-vectors (0 0.4021 0.1834) and (0 0.4021 -0.1834). The principles found here for 2:1 mullite are identically applicable to 3:2 mullite, both with respect to the chemically ideal superstructure as well with respect to the impossibility of its realization. All possible combinations of oxygen-vacancy distributions within 2×2×2 and 3×3×2 enlarged unit cells were calculated to get further information on the partial ordering schemes. A new mineral intermediate between sillimanite and mullite was studied by electron microprobe analyses and single-crystal X-ray diffraction methods. The chemical compositions derived from the microprobe results and the crystal-structure refinements are Al7.84Fe0.18Ti0.03Mg0.03Si3.92O19.96 and Al8.28Fe0.20Si3.52O19.76, respectively. The first composition is close to a stoichiometric sillimanite (with Fe,Ti, and Mg incorporated) and the second composition is between sillimanite and mullite. The deviation is assumed to be caused by exsolution of amorphous SiO2 from the crystal structure detected by the microprobe but not affecting the diffraction data. It crystallizes in the orthorhombic space group Pnam with a = 7.5127(4) Å, b = 7.6823(4) Å, c = 5.785(3) Å, V = 333.88(4) Å3, Z = 1. It has a complete Si/Al ordering like sillimanite but with neighboring double chains of SiO4 and AlO4 tetrahedra having an offset of ½ parallel c relative to each other causing the change of the space-group setting from Pbnm (sillimanite) to Pnam. Difference Fourier calculations and refinements with anisotropic displacement parameters revealed the formation of oxygen vacancies and triclusters as known in the crystal structures of mullite. Final refinements converged at R1 = 5.9% for 1024 unique reflections with Fo > 4σ (Fo). Fe was found to reside predominantly in the octahedral site and with minor amounts in one of the T* sites. Mg and Ti were not considered in the refinements. Therefore, the crystal studied here could be considered to represent a new mineral intermediate between sillimanite and mullite, named sillimullite. For the first time, single crystals of naturally occurring mullites were investigated by X- ray diffraction methods and microprobe analyses. The system of crystal structures in the mullite-type family was reevaluated and extended with the new compounds and findings from this project.
Publications
- News on mullite and mullite-type compounds. XXII Congress and General Assembly of the International Union of Crystallography at Madrid, Spain
R.X. Fischer, J. Birkenstock, A. Gaede-Köhler, H. Schneider
- Mullite and mullite-type crystal structures. Int. J. Mater. Res. 103 (2012) 1-6
R.X. Fischer, A. Gaede-Köhler, J. Birkenstock, H. Schneider
(See online at https://doi.org/10.3139/146.110713) - Structural examinations of synthetic and natural single crystals of mullite. European Crystallographic Meeting (ECM-28) at Warwick University, England (2013)
R.X. Fischer , J. Birkenstock, V. Petrícek, B. Pedersen, H. Schneider
- The crystal structure of modulated 2:1 mullite (Al4.8Si1.2O9.6) revisited. Jahrestagung der Deutschen Gesellschaft für Kristallographie in Freiberg (2013)
J. Birkenstock, V. Petrícek, B. Pedersen, H. Schneider, R.X. Fischer
