256 Subsurface Geologic Methods 



bentonite" from Highbridge, Kentucky, contains potash, but the thermal 

 curve indicates a material more like montmorillonite than hydromica or 

 illite, in which the first high-temperature endothermic peak occurs at 600° 

 C. or lower. The specimen from Candelaria, Nevada, shows a thermal 

 curve more characteristic of hydromica, while that from Transylvania 

 appears to be a mixture of montmorillonite and hydromica (illite) . 



The last three curves of this set are from specimens labelled "sapon- 

 ite," the high-magnesium montmorillonite clay. This mineral shows a 

 distinct double peak in the neighborhood of 800 to 850° C. All of these 

 specimens give saponite X-ray patterns. 



Figure 108 shows thermal curves of gibbsite, diaspore, brucite, and 

 goethite. All of these specimens were checked by X-ray diffraction. The 

 curves for gibbsite agree with those in the literature, which show the main 

 endothermic peak to occur from 330 to 350° C. Although Speil's 

 sample "^^ does not show the lower-temperature minor endothermic peak, 

 the others do. This may be assumed as due to the high purity of Speil's 

 sample. Pack and Davies '^^ ascribe the initial minor endothermic peak to 

 cliachite. The specimen from P050S de Caldos appears to contain a small 

 amount of kaolinite. 



The diaspore labelled "white bauxite" from China is apparently un- 

 usually uniform in grain size. It powders readily on crushing the sample, 

 making grinding unnecessary. Conversely, the coarsely crystalline diaspore 

 from Chester, Massachusetts, requires considerable grinding. The result- 

 ing material evidently has a large grain-size distribution, as indicated by 

 the shape of the curve. Apparently both of these specimens are of high 

 purity. 



Typical well-crystallized brucite specimens from Texas, Pennsylvania, 

 and Gabbs, Nevada, give thermal curves which correspond. Similarly 

 curves from specimens of goethite from the Lake Superior copper district 

 and Roxbury, Connecticut, agree with each other. 



Curves for alunite and jarosite are shown in figure 109. These miner- 

 als display prominent peaks that are distinctive and can be detected in the 

 presence of foreign materials. The specimens from Bulledehah and Bar- 

 ranca probably contain inert impurities such as sericite that have depressed 

 the peaks. Those from Santa Rita, Hyagoken, Los Lamentos, and Tintic 

 have been checked by means of X rays. 



Figure 110 shows some preliminary carbonate curves. These are con- 

 sistent with themselves and indicate the possible use of thermal analysis 

 for quantitative studies of carbonate rocks. Siderite ^^ and rhodochrosite 

 yield exothermic oxidation "domes" owing to the reaction with the oxygen 

 of the air of the lower-valence oxide produced in the carbonate decom- 



'* Spoil, Sidney, Berkelhamer, L. H., Pask, J. A., and Davies, Ben, op. cit. 

 " Speil, Sidney, Berkelhamer, L. H., Pask, J. A., and Davies, Ben, idem. 



" Kerr, P. F., and Kulp, J. L., Difierential Thermal Analysis of Siderite: Am. Mineralogist, vol. 32, 

 p. 678, 1947. 



