High-Pressure Differential Thermal Analysis: Application to Clay Minerals
Thermal analysis of clay and other substances at atmospheric pressure has developed into a major analytical method, beginning with the pioneering studies of Le Chatelier (1887) on kaolinite, more than a century ago. Thermal analytical methods differ from chemical or structural methods in that they rely on a phenomenological approach by investigating the response of material with respect to a change in temperature. If a material is investigated at constant pressure, but at different temperatures, the results represent observations using one independent variable (i.e., temperature). If successive experiments are carried out at different pressures as well, a two-dimensional pressure-temperature grid can be created, from which important information may be derived.
The advantage of using pressure as an additional variable has not gone un-noticed, and different types of apparatus that allow high-pressure studies to be made have been developed (Wendlandt, 1986). The paucity of thermal analytical research at elevated pressures, however, suggests that these earlier methods are cumbersome. Recently, a relatively convenient high-pressure (≥ 10 kbar) differential thermal analysis method was developed in the authors’ laboratory (Koster van Groos, 1979). Because the apparatus is expensive to build and maintain, a simplified computercontrolled version is being developed for routine studies at moderate pressures < 2 kbar), which should facilitate wider usage of pressure in thermal analyses.
Rather than review the literature extensively or describe the technique in detail, the purpose of this chapter is to illustrate that high-pressure differential thermal analysis (HP-DTA) is an important tool in solving geologic problems. One
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Thermal Analysis in Clay Science
Thermal analysis involves the observation of a physical property of a sample and how that property changes in response to a change in temperature. Thus, the essence of this group of techniques includes the measurement of a physical property, e.g. mass, temperature, and volume, and the control of temperature. Inasmuch as heating objects is a very ancient practice, one should not be surprised that the first observations of the response of certain materials to heat were made quite some time ago. Such observations might be considered as a form of thermal analysis (Mackenzie, 1981), but serious investigations required that the temperature be known with reasonable accuracy. Temperature measurements, especially of a solid material that is being heated rapidly, was first ccomplished with a thermocouple. Two events, then, mark the beginning of thermal analysis. The first was the invention of the thermocouple. This led directly to the study of the thermal properties of a group of clay minerals. In fact, thermal analysis, in the modern sense, started with a simple description: “Si I 'on echauffe rapidement unepetite quantite d'argile, il seproduit, au moment de la deshydratation, un relentissement dans l'elevation de temperature...” (if one heats rapidly a small quantity of clay, there is, at the point of dehydration, a slowing in the increase in temperature...) (Le Chatelier, 1887). The temperature at which the dehydration occurred was determined for each of the clay minerals examined by Le Chatelier, and he pointed out that the temperature at which dehydration occurred could be used to distinguish between and to identify different clay minerals.