Structure description of the thermic phase transformation sillimanite-mullite

The thermic phase transformation of sillimanite to 3:2 mullite was structur ally examined with the main focus on the initial stage of this transformati on. Single crystals of very pure sillimanite were annealed for different ti me periods (1600 degreesC; 45 min-96 h) and then analyzed employing X-ray a nd electron microscopic investigations. Electron diffraction patterns showe d decreasing intensities of the (101) reflections depending on the duration of thermal treatment. Dynamic effects causing the decrease of the 101 refl ection could be excluded, because calculations of the dynamic electron diff raction 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 degreesC) of silliman ite 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 simulat ions. A prolonged period of thermal treatment (24 h, 1600 degreesC) leads t o the complete transformation of sillimanite to 3:2 mullite with precipitat ions of amorphous SiO2. This precipitation of partial melt appears in HRTEM images as white rectangles (approximately 20 x 20 nm) with edges parallel to (110). (C) 2001 Elsevier Science Ltd. All rights reserved.