Thermogravimetric Analysis of Minerals
Thermogravimetric analysis (TGA), in which the mass of a sample is monitored as a function of temperature, is one of the oldest analytical techniques used in clay mineralogy. One of the first applications to the study of minerals was reported in 1903 by Nernst and Riesenfeld. Since then, TGA has been used to obtain a variety of information on minerals, particularly hydrous phases, such as clays and zeolites. TGA is currently widely used in many other disciplines as well, including polymer chemistry, pharmaceuticals, and inorganic analysis (Wendlandt, 1986). Despite the active interest in TGA in other areas of solid characterization and despite the many studies of clay minerals that have been conducted using TGA, the method has not enjoyed significant popularity in recent years in the mineral sciences. The technique appears to be viewed as somewhat qualitative in a field that is demanding more and more quantitative results. Fortunately, however, as a result of the large amount of research in other fields employing TGA, the application of TGA has grown in recent years from a simple technique often used as a fingerprint or water-analysis method to one that can provide quantitative information on many types of thermal processes in minerals. Data available through TGA include, but are not limited to, the kinetics of dehydration and dehydroxylation reactions, the analysis of solids for non-water volatiles, the determination of equilibrium dehydration behavior of hydrous minerals, the quantitative analysis of multicomponent mixtures, and the separation of overlapping dehydration reactions.
Figures & Tables
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.