Lattice parameter determination of mullite by energy-filtered needle-texture electron diffraction pattern.
The thermal transformations of pyrophyllite to mullite by heating were re-examined using mainly energy-filtering transmission electron microscopy and, for the first time, the texture electron diffraction pattern of the mullite was completely interpreted. Through a temperature range in which pyrophyllite dehydroxylate maintained a long-range order with a fluctuation of approximately 1% in d-spacings of (100) and (010) planes at 1000 degrees C, without prominent exothermic feature, pyrophyllite dehydroxylate was gradually decomposed and transformed into mullite through topotaxy. Pyrophyllite dehydroxylate did not collapse completely until 1100 degrees C, which promoted the rapid growth of mullite in random orientation at 1200 degrees C and the crystallization of amorphous silica to cristobalite at 1300 degrees C. The mullite needles, having their c-axis (texture axis) parallel to the elongation direction, lined up along the b(*)-axis of the pyrophyllite dehydroxylate in the needle-texture electron diffraction patterns. The mullite needles had monoclinic symmetry with lattice parameters of 7.27 A (a), 7.75 A (b), 2.90 A (c), 90 degrees (alpha), 90 degrees (beta) and 88.41 degrees (gamma), which, because of the structural affiliation to the parent pyrophyllite dehydroxylate, differ to the orthorhombic 3/2-mullite.