262 Subsurface Geologic Methods 



be attributed to the conductance of the heat away from the particles in the 

 endothermic reaction by the foreign inert neighbors. The 980° C. exo- 

 thermic peak is not shifted appreciably. This is probably a result of the 

 narrow temperature range of the reaction. Below a certain temperature, 

 under these conditions of molecular structure, amorphous alumina will 

 not change over to gamma alumina. At 980° C, however, crystallization 

 occurs almost instantaneously. Hence, the mixture of 50-mesh inert mate- 

 rial with 50-mesh kaolinite does not appreciably shift this peak. 



Figure 112 illustrates the effect of mixing quartz with kaolinite. The 

 quartz curve is a straight line aside from a minor peak at the inversion 

 point. The kaolinite curve is depressed by the admixture of quartz, but 

 comparison with figure 105 indicates that otherwise there does not appear 

 to be any substantial change. 



Figure 113 represents mixtures of sericite and kaolinite. Sericite 

 shows little noticeable differential effect. On the other hand, even as little 

 as ten percent kaolinite in a mixture with sericite may be detected. Since 

 both minerals are common in zones of hydrothermal alteration, this fea- 

 ture is of interest. 



Figure 114 contains curves of kaolinite and alunite, which represent 

 a mixture of two thermally active minerals that may occur together in 

 the same deposits. Both minerals yield sharp and distinctive thermal peaks. 



Figure 115 represents a sequence of thermal curves for alunite and 

 jarosite where the samples are artificial mixtures. Both alunite (fig. 109) 

 and kaolinite (fig. 105) are illustrated elsewhere. Where curves show 

 such prominent peaks, mixtures may be studied with reasonable facility. 

 A proportional decrease in the amplitude as well as a downward shift 

 of peak temperatures occurs with an increase in foreign constituents. 



A common problem in the study of zones of argillic alteration con- 

 cerns the estimation of the relative amounts of kaolinite and dickite present 

 in a natural mixture. Figure 116 illustrates a series of artificial mixtures 

 of the two minerals. 



Kaolinite-montmorillonite mixtures are illustrated in figure 117. Evi- 

 dently the apparatus as normally employed is less sensitive for the de- 

 tection of montmorillonite in a mixture than it is for distinguishing min- 

 erals with higher temperatures and more distinctive thermal effects. 



Montmorillonite-sericite mixtures are indicated in figure 118. While 

 montmorillonite would be detected in such mixtures, it seems likely that 

 sericite would escape detection. It is evident that the effect of shifting 

 the peaks with the percentage of impurity must be determined for each 

 mineral properly to identify minerals in mixtures. The carbonates appear 

 particularly sensitive to this effect. 



The curves above have been used effectively in the semiquantitative 

 determination of the argillic constituents of an altered mineralized area. 

 The application of the technique to this form of problem offers signifi- 



