The transformation of andalusite to mullite and silica: Part I. Transformation mechanism in [001](A) direction

The high-temperature transformation of andalusite (Al2O3. SiO2 = A) single crystal to 3:2-mullite (3Al(2)O(3). 2SiO(2) = M) and non-crystalline silica (SiO2) taking place between 1500 and 1600 degrees C was investigated by sc anning electron microscopy (SEM) and transmission electron microscopy (TEM) . Electron diffraction studies confirmed the topotactical orientation relat ionship ([100](A) parallel to [010](M), [010](A). parallel to [100](M), [00 1](A) parallel to [001](M)) between the primary product (A) and the product phase (M), which was predicted by Pannhorst and Schneider (1978) on the ba sis of single-crystal X-ray diffraction studies. The transformation starts at the (001)(A) surface and proceeds rapidly along to the c(A,M)-axis. The overall reaction front is parallel to (001) of andalusite and mullite, thou gh on the microscopic scale it exhibits a zigzag shape with facets parallel to (011)(A) and (201)(M). The development of (011)(A) and (201), microface ts is favorable, because both lattice planes exhibit a small misfit, which is close to zero at the transformation temperature. The undistorted and dir ect transition from andalusite to mullite and the high velocity of the reac tion along [001](A) suggests that the AlO6 octahedra which occur in both st ructures are being preserved during the transformation. Other structural un its, however, have to be decomposed and reconstructed. The newly formed mul lite crystallites display an acicular shape elongated to the c-axis with an average size of about 250 nm along their small dimension. Excess non-cryst alline SiO2 is exsolved between the mullite crystallites in small channels running parallel to [001](M). The major amount of the exsolved silica liqui d is transported to the (001) surface of the former andalusite crystal by m eans of capillary forces.