Reactions in Silica‐Alumina Mixtures

Examination of mixtures of extremely pure silica and alumina shows that the greatest reactivity is not encountered with stoichiometric ratios to form mullite with the formula 3Al 2 O 3 . 2SiO 2 but rather with the formula 2A1 2 O 3 ‐SiO 2 , and that reactivity also depends on the crystalline modification of alumina. A sharp exothermic differential thermal peak at 980°C. is attributed to three simultaneous reactions dependent on the silica‐alumina ratio of the mixture: (1) the crystallization of gamma alumina, (2) the crystallization of a hydrogen aluminum spinel (HAl 5 O 5 ), and (3) the reaction of silica with the hydrogen aluminum spinel to form mullite.