Differential Thermal Analysis of Kaolin Group Minerals Under Controlled Partial Pressures of H 2 O

Differential thermal analyses of samples of Kamec (N. C.) kaolin, Tako (Ala.) kaolin, an English kaolin, dickite (Ouray, Colo.), halloysite (Eureka, Utah), and diaspore (Chester, Mass.) were run under dynamic conditions in atmospheres containing water vapor of known partial pressures ranging from 5.0 to 760 mm. The differential thermal analysis curves of all kaolins showed the first indication of dehydration (that is, the beginning of the endothermic peak) at about 400°C., the halloysite at 320°C., the dickite at 500°C., and the diaspore at 465°C. in dry air. In steam atmosphere, the starting temperatures of the endothermic peaks were 470°, 485°, 585° and 510°C., respectively. The peak temperatures of the endothermic peaks were only slightly affected by the atmospheres, hence the peaks were more intense and covered a narrower temperature spread in the steam atmosphere. All kaolin‐group minerals showed the exothermic peak at 1000°C. in dry air and 970° to 980°C. in steam atmosphere. The data on In p H 2 O vs. 1/ T for each mineral were plotted according to the van't Hoff equation. The straight line of dickite above In p H 2 O = 5.30 was used to calculate the heat of dehydration. The value is 137 cal. Per gm. The van't Hoff lines for the kaolinites and for diaspore, although straight lines, are in error since the recorded temperatures are considerably above the equilibrium temperatures because the rates of reaction are too slow at or near equilibrium. Actually kaolinite will decompose extremely slowly in the dry air at about 250°C., but in the steam atmosphere it will not decompose until about 475°C.