Subsurface Laboratory Methods 241 



test. With a constant heating rate a thermal reaction in the sample will be 

 recorded as a deviation from the straight-line plot of temperature differ- 

 ence against temperature. This deviation is dependent upon the nature 

 of the heat change for its direction and amplitude. Peaks may be due to 

 loss of either absorbed or lattice water, decomposition, or changes in 

 crystal structure. They are characteristic for most thermally active min- 

 erals. Mixtures show a composite curve of the effects of the individual 

 components in their proper proportion. 



Although the original work on thermal analysis was done by Le 

 Chatelier in 1887, it was not until the late 1930's that the method began 

 to be used for semiquantitative study of clay minerals. In recent years 

 studies have been made at the National Bureau of Standards,^'^ the Massa- 

 chusetts Institute of Technology,^^ the United States Geological Survey,^^ 

 the Illinois Geological Survey,'^^ the Bureau of Plant Industry,*'^ and 

 various United States Bureau of Mines research laboratories.^^ 



Publications resulting from these studies emphasize the value of dif- 

 ferential thermal analysis as a supplementary method coordinated with 

 chemical, optical, and X-ray methods in studying clay minerals. X-ray 

 data may have certain advantages in indicating a general clay-mineral 

 group. Thermal-analysis curves, on the other hand, may contribute quan- 

 titative data on mixtures not readily available from X-ray-dififraction 

 studies. Also, substitution in the clay-mineral lattice is frequently more 

 apparent in the peak shifts of thermal curves than in X-ray patterns that 

 frequently lack suitable definition. In combination, the two methods offer 

 a solution to many complex problems in the study of clays. 



The authors wish to acknowledge the helpful criticisms of the manu- 

 script received from R. E. Grim, the Illinois Geological Survey; Ben B. 

 Cox and Duncan McConnell, the Gulf Research and Development Labora- 

 tories; M. L. Fuller, T. L. Hurst, L. D. Fetterolf, and D. G. Brubaker, the 

 New Jersey Zinc Company; Robert Rowan and R. H. Sherman, the Creole 

 Petroleum Corporation; and Parke A. Dickey, the Carter Oil Company. 



The Apparatus 



The use of thermal analysis in the study of argillic alteration of a 

 mineralized area or a stratigraphic-correlation problem requires the test- 

 ing of hundreds of samples. This has involved a tedious laboratory pro- 

 cedure in the forms of apparatus described in the literature,^^ ^^ where a 



^' Insley, H., and EweO, R. H., Thermal Behavior of Kaolin Minerals: Nat. Bur. Standards Jour. 

 Research, vol. 14, pp. 615-627, 193S. 



^^ Norton, F. H., Critical Study of the Differential Thermal Method for the Identification of the Clay 

 Minerals: Am. Ceram. Soc. Jour., vol. ?2, pp. 54-63, 1939. 



^^ Alexander, L. T., et al.. Relationship of the Clay Minerals Halloysite and Endellite: Am. Min- 

 eralogist, vol. 28, pp. 1-18. 1943. 



*" Grim, R. E., and Rowland, R. A., Differential Thermal Analysis of Clay Minerals and Other 

 Hydrous Materials: Am. Mineralogist, vol. 27, pp. 746-761; 801-818, 1942. 



'^ Hendricks, S. B., Goldrich, S. S., and Nelson, R. A., On a Portable Differential Thermal Outfit: 

 Econ. Geology, vol. 41, p. 41, 1946. , 



*- Speil, Sidney, Berkelhamer, L. H., Pask, J. A., and Davies, Ben, Differential Thermal Analysis, 

 Its Application to Clays a-'d other Aluminus Minerals: U. S. Bur. Mines Tech. Paper 664, 81 pp., 1945. 



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



'■^ Norton, F. H., op. cit. 



