Spinel

As metakaolin is heated, it undergoes a structural transformation around 980°C, a temperature of significant interest in the synthesis of mullite ceramics [35]. Brindley, among others, has observed the formation of spinel, an amorphous siliceous phase, and a small amount of nanocrystalline mullite at 980°C [33]. This process is exother­mic with no accompanying weight loss. The observed heat of reaction comes mainly from spinel formation [33]. Investigators are in general agreement that the spinel phase is similar in structure to the cubic transitional alumina y-Al2O3 and that it con­tains most of the alumina from the original kaolin. The amorphous phase is mainly silica, but it also contains a small amount of alumina plus most of the impurities from the original clay. The mullite phase makes up only a small volume fraction of the total volume after heating to 980°C and is composed of submicrometer needle-like mullite grains. Questions remain regarding the composition of the spinel phase, with pro­posed compositions ranging from pure y-Al2O3 to 2Al2O33SiO2, which includes the mullite composition, 3Al2O32SiO2 [33,36]. It seems unlikely that spinel is pure y-Al2O3, since mixtures of y-Al2O3 and silica prepared from colloidal particles form a-Al2O3 and amorphous silica around 1200°C prior to mullite formation at higher temperatures [37]. It also seems unlikely that spinel is poorly crystalline mullite, at least after heating to 980°C, since a second mullite crystallization event is recorded at higher temperatures [37]. Recent studies using nuclear magnetic resonance spectros­copy indicate that the spinel phase formed at 980°C may contain just a few weight percent silica [38]. Logically, the composition of the spinel phase probably lies between that of metakaolin (Al2O32SiO2) and mullite (3Al2O32SiO2) and is part of a phase separation process that leads to the eventual formation of mullite and an amor­phous silica-rich phase [33].