The KaoIinite‐MuIIite Reaction Series: Ill, The High‐Temperature Phases

X‐ray diffraction data for the high‐temperature phases show that a spinel‐type structure develops with marked orientation at about 925°C. This phase is considered to be an aluminum silicon spinel with vacant cation sites. Mullite is thought to be formed by the decomposition of the spinel. Silica is eliminated progressively as metakaolin transforms to the spinel phase and thence to mullite. The X‐ray data show variation in the mullite parameters between 1200° and 1400°C.; at 1400°C., the composition probably approximates 3A1 2 O 3 ·2SiO 2 . The nature of the intermediate spinel‐type phase is discussed in relation to the crystal chemistry of spinels.