Mullite Development from Fibrous Kaolin Mineral

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Abstract
The development of mullite from kaolin minerals was studied using transmission electron microscopy and X‐ray powder diffraction methods on the unusual long‐fibered kaolin from Piedade, Sao Paulo, Brazil. The mullite crystals observed at ≥ 1200°C initially exhibited an elliptical shape and were randomly oriented inside the tubes; at 1250°C their shape changed to laths oriented either parallel to the tube length or at 120° to the adjacent lath. At 1300°C, the shape changed to a bipointed lath and the crystals developed a hexagonal configuration with preferred orientation at 120°, similar to mullite developed from kaolinite plates. Thus, the temperature at which mullite begins to develop from this tubular kaolin is ∼200°C above that normally observed in well‐crystallized kaolinites and is 100°C above b axis disordered kaolinite. Cristobalite was not observed in the electron microscope in the range 1000° to 1300°C.

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