THE THERMAL REACTIONS AND THE CONTROL OF PHASE CHANGES OF KAOLINITE

It is well known that,when heated,kaolin minerals will undergo dehy-droxylization and show an intense endothermic reaction within the tem-perature range of 400-700℃.The anhydrite thus produced is generallycalled metakaolin.In the case of halloysite,adsorbed water is removed inan initial endothermic reaction at about 120℃.At about 950-1050℃ theseminerals experience a marked abrupt exothermic effect.A second smallexotherm is observed at about 1250℃.It is generally correlated to the for-marion and/or crystal growth of mullite,though some workers inclinedto attribute it to the conversion of amorphous silica to cristobalite.