Subsurface Laboratory Methods 261 



position. Cuthbert and Rowland ^^ published thermal curves of several 

 carbonate minerals. 



In these curves the carbonate peaks are low because of the admixture 

 of inert material. The curves from figure 110 closely approximate the car- 

 bonate specimens run by other workers.^° ^^ 



Artificial Mixtures 



Figures 111 to 118 show sets of thermal curves of predetermined mix- 

 tures ground to 50-mesh.*^ Although theoretically the area under the 

 curve should be proportional to the percentage of the mineral present, 

 this does not strictly hold experimentally. It has been found, however, 

 that for known mixtures the amplitude of the peak plotted against the 

 percentage of the mineral present gives a smooth curve. Moreover, it 

 has been found that this "calibration curve" is not particularly affected by 

 the chemical nature of the other components. Using figures for artificial 

 mixtures containing kaolinite to furnish data, the graph in figure 119 was 

 prepared. The amplitude of the endothermic 605° C. peak for kaolinite 

 is plotted against the percentage of kaolinite in the particular mixture. A 

 different symbol is used for each mixture. The area within the two smooth 

 curves indicates the possible error to be expected from a mixture of 

 kaolinite with an unknown aggregation, as indicated by artificial mix- 

 tures. Clay minerals that give such distinctive peaks as kaolinite may 

 be quantitatively estimated with reasonable certainty for simple mixtures 

 within ten or twenty percent. The variation may be due in part to minor 

 differences in the heat conductivity of the foreign constituent. 



The necessary assumption to render valid the application of the cali- 

 bration curve to an unknown mixture is that the clay minerals in the un- 

 known must be in roughly the same physical and chemical condition as in 

 the artificial mixtures. This is probably a good approximation in many 

 cases, particularly in the case of hydrothermal clays formed in situ. Grim ^^ 

 has already pointed out the need for great caution in making such an 

 assumption for certain sedimentary-clay mixtures. 



Figure 111 shows a suite of kaolinite-goethite mixtures. The endo- 

 thermic decomposition peaks for both minerals are shifted down in tem- 

 perature with increasing percentage of the other mineral. This shift is to 



™ Cuthbert, F. L., and Rowland, R. A., Differential Thermal Analysis of Some Carbonate Minerals: 

 Am. Mineralogist, vol. 32, p. Ill, 1947. 



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

 ^ Faust, unpublished. (No reference given in original paper.) 



'- The samples used in these artificial mixtures were essentially uncontaminated materials from well- 

 known localities and were checked both optically and by means of X-ray diffraction. The alunite sample 

 was analyzed chemically by Ledoux and Company. 



Mineral Locality Mineral Locality 



Alunite. Santa Rita, New Mexico Quartz 



Jarosite Santa Maria mine, Sericite A.merican Canyon, Nevada 



Jelardena, Durango, Mexico Dickite _ Cusihuirachic, Mexico 



Kaolinite Dry Branch, Georgia GoethJte ._ „..Lake Superior 



Montmorillomte..Po'kville, Mississippi 

 '^ Grim, R. E., Differential Thermal Curves of Prepared Mixtures of Clay Minerals: Am* Mineralogist. 

 vol. 32, p. 493, 1947. 



