240 Subsurface Geologic Methods 



increase the reliability of geologic logging. The method supplements 

 petrographic techniques of logging drill core, making possible quick and 

 precise identification of even exceedingly fine-grained types and complex 

 mixtures. In addition, the method can supply basic data on petrography 

 and mineralogy necessary to interpret completely the electric and gamma 

 radiation logs of drill holes. Both engineers and geologists are finding 

 that the X-ray method of analysis is a powerful tool in the identification 

 of potentially unsound materials in foundation strata or construction 

 materials proposed for use in dams, powerhouses, buildings, and other 

 large engineering works. 



Finally, X-ray-diffraction analysis, both of geologic materials col- 

 lected in the field from outcrops and cores and of synthetic materials in 

 the laboratory, will yield detailed knowledge of processes involved in 

 deposition, consolidation, and induration of sediments. The methods of 

 X-ray-diffraction analysis are unsurpassed in effectiveness and efficiency 

 in the tracing of progressive changes in mineralogy and structure of 

 materials. Application of these methods will demonstrate the process 

 of recrystallization during consolidation and induration, such as may 

 occur in unstable minerals like clays, and the formation of new minerals, 

 such as feldspar, mica, and zeolites. Only when these and related pro- 

 cesses are understood will the conditions of petroleum formation, migra- 

 tion, accumulation, and production be understood fully. 



The versatility and adaptability of X-ray-diffraction methods have 

 justified recognition by the petroleum geologist, engineer, and chemist. 

 For one problem the methods may afford merely a valued supplement 

 to other techniques; for another problem the methods may be indispens- 

 able to a successful solution. Consequently, the supervisor of subsurface in- 

 vestigations should be cognizant of the potentialities of the X-ray-diffrac- 

 tion methods so that they will be used when and as required by the 

 nature of the problems to be solved. 



MULTIPLE-DIFFERENTIAL THERMAL ANALYSIS 

 PAUL F. KERR and J. L. KULP 



Differential thermal analysis provides a useful technique for the study 

 of specific minerals or mineral groups with distinctive heating curves. 

 The method is suitable for both qualitative and semiquantitative studies 

 of the clay minerals, the hydrous oxides of iron, aluminum and manganese, 

 the carbonates, the zeolites, and a goodly number of other minerals. In 

 general, the method applies to substances that yield characteristic peaks 

 in the differential thermal curves. 



In this technique a dual-terminal thermocouple is employed. One 

 terminal is inserted in an inert material which does not undergo exo- 

 thermic or endothermic reaction through the temperature interval to be 

 studied. The other is placed in the mineral or mixtures of minerals under 



