250 Subsurface Geologic Methods 



suits obtainable. It is believed that they agree substantially with those 

 sample. This forms the basis for the quantitative use of differential ther- 

 mal analysis. The linear relationship holds reasonably well. More exact 

 determinations of comparatively simple systems can be made by running 

 known mixtures and preparing a calibration curve of area versus per- 

 centage of each component. 



The derivation above neglects the differential terms and the tempera- 

 ture gradient in the sample. It shows that the area under the curve is a 

 measure of the total heat effect. The area is also considered independent 

 of the specific heat. This factor, however, actually does affect the shape 

 of the peak and may change the area slightly. For many purposes the 

 approximate relationships are sufl&cient. 



Qualitative Applications 



The various clay minerals yield suJG&ciently different peaks to make 

 the differential-thermal-analysis method particularly useful. When a speci- 

 men is relatively pure, preliminary identification by thermal curves is 

 frequently comparatively simple. In addition, two-component mixtures 

 are often resolved and at times even three-component mixtures. If, how- 

 ever, mixtures become too complex, only one or possibly two of the major 

 components may be identified. 



The multiple-thermal-analysis apparatus makes possible a rapid, 

 widespread survey of the groups of minerals that can be identified by this 

 procedure. The thermal curves given here are representative of the re- 

 of the other workers on record. Illustrative curves are shown for the 

 kaolinite and montmorillonite groups along with the hydrous oxides of 

 aluminum and iron and some sulphates and carbonates. 



The kaolin minerals (figs. 104, 105, and 106) are characterized by a 

 large endothermic peak ranging from 550 to 700° C, owing to the 

 decomposition of the kaolinite lattice into amorphous silica and alumina 

 and a sharp exothermic peak of 980° C. caused by the recrystallization 

 of amorphous alumina to gamma alumina. 



Thermal curves of dickite from several localities are shown in figure 

 104. A number of the samples illustrated have been studied in connection 

 with other investigations to such an extent that they may be considered 

 representative of this clay mineral. The sample from Red Mountain, 

 Colorado, was ground, and curves were run to compare 100-200-mesh, 

 200-300-mesh, and smaller than 300-mesh material. It is interesting to 

 note that an ordinary specimen from St. Peter's dome yields a curve 

 similar to Red Mountain dickite ground to minus-300 mesh. It is evident 

 that particle size is a factor to be considered. 



As the degree of orderliness in the superposition of the kaolin layers 

 decreases from dickite through kaolinite to halloysite, the endothermic 

 peak shifts downward in temperature, indicating less stability of the lat- 

 tice. The disorder reaches a sufficient extent in the case of halloysite to 



