254 Subsurface Geologic Methods 



kaolinite. Other workers '^- ^^ show evidence of the lower temperature of 

 the halloysite peak. In this laboratory eight typical halloysite specimens 

 yield endothermic peaks at 575° ±10° C. On the other hand, the kao- 

 linite samples examined show endothermic peaks at 600° C. or above. 

 This difference is well above the limits of experimental error. One pos- 

 sible complication is noted in the case of very fine kaolinite. Here the 

 605° C. peak is shifted downward toward the halloysite peak. However, 

 the fine kaolinite does not have so low an endothermic peak as does 

 halloysite, the amplitude and the shape of the 600° C. peak is altered, 

 and the 980° C. exothermic peak is shifted down the temperature scale. 



These data are not sufficiently conclusive to establish the range of 

 the endothermic peak of kaolinite. The structure variations in halloysite 

 and kaolinite and the correlation with the thermal phenomena require 

 further investigation. 



The halloysite from Bedford, Indiana, contains a small amount of 

 gibbsite. The endellite from Bedford is typical, showing the halloysite 

 curve with a greatly increased low-temperature endothermic peak. The 

 last two curves in figure 106 are typical of allophane. 



Figure 107 contains the thermal curves of certain three-layer lattice 

 minerals. Montmorillonite furnishes a broad classification for a certain 

 crystal structure, but with wide substitution possibilities in the lattice. 

 An excellent paper by Ross and Hendricks '^^ has contributed to the clari- 

 fication of this group. The thermal curve of the Polkville, Mississippi, 

 material exhibits the previously recognized low-temperature doublet, the 

 two high-temperature endothermic peaks, and the final high-temperature 

 exothermic peak. The amplitude of the doublet is dependent to a large 

 extent on the humidity conditions before thermal analysis. Hendricks and 

 others "^^ pointed out that the shape of these peaks was due to the quantity 

 of adsorbed water and the type of adsorbed cation between the three-layer 

 units. The high-temperature endothermic peaks occur variably between 

 the limits of 550° C. and 1,000° C. This is probably to be attributed 

 to substitution within the layer itself. The temperature of the peaks has 

 not as yet been correlated with chemical analysis. This is now being 

 investigated. The high-temperature exothermic peak is dependent in part 

 on the substitution of iron for aluminum within the layer. The substitu- 

 tions in the montmorillonite lattice are perhaps more apparent from the 

 shifts in the thermal-curve peaks than the shifts in the lines of the diffuse 

 X-ray-diffraction patterns. 



The curves of specimens from Ventura, California; Wisconsin; 

 Texas; and Rideout, Utah, are typical of montmorillonite. The "meta- 



'^ Spiel, Sidney, Berkelhamer, L. H., Pask, J. A., and Davies, Ben, op. cit. 



" Norton, F. H., Analysis of High-Alumina Clays by the Thermal Method: Am. Ceram. Soc. Jour., 

 vol. 23, pp. 281-282, 1940. 



'* Ross, C. S., and Hendricks, S. B., Minerals of the Montmorillonite Group: U. S. Geol. Survey 

 Prof. Paper 205-B, 1945. 



'^ Hendricks, St B., Nelson, R. A., and Alexander, L. T., Hydration Mechanism of the Clay Mineral 

 Montmorillonite Saturated with Various Cations: Am. Chem. Soc. Jour., vol. 62, pp. 1457-1464, 1940. 



