252 Subsurface Geologic Methods 



permit absorption of some water between the lattice layers. This accounts : 

 for the minor endothermic peak at about 150° C. This peak is greatly en- , 

 larged in the case of endellite, the more hydrous form of halloysite. En- 

 dellite yields water to form halloysite under 100° C. Allophane has been 

 considered in the kaolin group ^*^ presumably because of the sharp 980° C. 

 exothermic peak corresponding to the formation of the gamma alumina 

 and a rough agreement of chemical analyses. The kaolin group is best 

 delimited by the unique lattice type, which is observed in nacrite, dickite, 

 kaolinite, and halloysite. 



Figure 105 contains a series of kaolinite curves. The variations in 

 the shape of the endothermic peaks are probably due to differences in clay- 

 mineral particle-size distribution. The samples with a narrow range of 

 particle size appear to give the sharpest peaks. Since the total heat evolved 

 is dependent only on the concentration of reactive molecules present 

 around the thermocouple, the area under the curve should be roughly 

 constant. It is evident from the set of curves that the differences in the 

 shapes of the curves are not great, and, hence, the amplitudes are essen- 

 tially the same. The specimens from Dry Branch, Georgia; Cornwall, 

 England; Newman Pit, California; Franklin, North Carolina; and Santa 

 Rita, New Mexico, give characteristic kaolinite X-ray patterns. The 

 Georgia material was used as a standard for comparison. 



The samples were prepared by passing the kaolinite through a 50- 

 mesh screen. One specimen (not shown) of Georgia kaolin fines ob- 

 tained by gravity separation exhibits a kaolinite curve with an endother- 

 mic peak depressed slightly and lowered in temperature about 10°. The 

 980° C. exothermic peak of this material also shows a slight shift to lower 

 temperatures. This probably is because of the finer-particle-size mate- 

 rial, which is in a less stable state with a correspondingly lower tempera- 

 ture of recrystallization, 



A private communication from R. E. Grim indicates that some kao- 

 linite samples give curves with upward swings in the thermal record be- 

 tween the large endothermic reaction and the final exothermic reaction. 

 These same kaolinite samples also give a slight endothermic dip just be- 

 fore the final exothermic peak. Of the few kaolinite curves shown in figure 

 105, only the Marysville, Utah, and Northwest, New Mexico, samples in- 

 dicate the slight endothermic peak mentioned above. 



Figure 106 contains typical thermal curves of halloysite, endellite, 

 and allophane. The first four halloysite samples are believed to be of 

 high purity. X-ray-dij6Fraction photographs have been obtained from 

 these four specimens and display the lines of halloysite. The thermal 

 curves are similar to those of kaolinite with two significant differences, 

 the small endothermic peak at 150° C. due to adsorbed water, and the shift 

 in the main endothermic peak to about 570° C. Grim "^^ claims that halloy- 

 site does not have a lower temperature for the main endothermic peak than 



'" Speil, Sidney, Berkelhamer, L. H., Pask, J. A., and Davies, Ben, op. cit. 



"Grim, R. E., Modern Concepts of Clay Minerals: Jour. Geology, vol. 50, pp. 225-275, 1942. 



