New Electron‐Optical Data on the Kaolinite‐Mullite Transformation

A technique is described for preparing fired kaolinites for examination by electron diffraction and electron microscopy. Specimens fired on platinum foil at 850° and 950°C. were etched while on the foil in 10% sodium hydroxide for 1 week and were then removed by a two‐stage collodion‐carbon replica. The etching is believed to have removed free silica which is formed during the transition from metakaolin to the cubic spinel‐type phase. Electron diffraction patterns show the (111) orientation of the spinel‐type structure on the a‐b plane of the metakaolin. Patterns of specimens at 950° C. show that the mullite phase is oriented with the c axis parallel to the [110] axes of the spinel‐type phase as proposed by Brindley and Nakahira. Particle size of the spinel‐type phase estimated from electron micrographs appears to fall within 75 to 125 a.u. Kaolinites varying in degree of crystallinity are being studied by this method.