Structure description of the thermic phase transformation sillimanite–mullite

The thermic phase transformation of sillimanite to 3:2 mullite was structurally examined with the main focus on the initial stage of this transfomation. Single crystals of very pure sillimanite were annealed for different time periods (1600 °C; 45 min–96 h) and then analyzed employing X-ray and electron microscopic investigations. Electron diffraction patterns showed decreasing intensities of the 〈101〉 reflections depending on the duration of thermal treatment. Dynamic effects causing the decrease of the 101 reflection could be excluded, because calculations of the dynamic electron diffraction intensities showed an intensity increase of the 101 reflection with increasing crystal thickness. Using the videographic reconstruction method the initial stage of the transformation (2–6 h, 1600 °C) of sillimanite to 3:2 mullite is characterized as follows: (1) Splitting of the oxygen sites, resulting in tetrahedra- and octahedra-tilts and/or rotations; (2) statistical distribution of Si and Al on the tetrahedral sites. The results of the videographic reconstruction were confirmed via videographic simulations. A prolonged period of thermal treatment (24 h, 1600 °C) leads to the complete transformation of sillimanite to 3:2 mullite with precipitations of amorphous SiO2. This precipitation of partial melt appears in HRTEM images as white rectangles (approximately 20×20 nm) with edges parallel to 〈110〉.