Recent developments and the future of low‐T calorimetric investigations in the Earth sciences: Consequences for thermodynamic calculations and databases |
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| Authors: | Charles A Geiger Edgar Dachs |
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| Institution: | 1. Department of Chemistry and Physics of Materials, Section 2. Materials Science and Mineralogy, Salzburg University, Salzburg, Austria |
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| Abstract: | Low‐T calorimetry is an experimental science that measures the thermodynamic function heat capacity, Cp(T), from which the standard third‐law entropy (298.15 K), S°, is calculated. The recent technological development of relaxation calorimetry allows both new experimental strategies and types of Cp investigations to be made, which were previously not possible. The Cp measurements are fast and automated and can be made on mg‐sized mineralogical samples between 2 and 400 K. These advantages, when careful measurement procedures are used, permit better determinations of Cp(T) behaviour. The Cp of synthetic single‐crystal MgO was measured between 5 and 302 K, and S° calculated using relaxation calorimetry to further investigate the method's precision and accuracy. A number of synthetic and natural end‐member or nearly end‐member compositions of silicate garnet were investigated in the past via adiabatic calorimetry, an old and established technique, and more recently and extensively with the relaxation method. First Cp(T) and S° results, using relaxation calorimetry, have been obtained on spessartine (Mn3Al2Si3O12) and knorringite (Mg3Cr2Si3O12). Furthermore, reinvestigations on pyrope (Mg3Al2Si3O12), almandine (Fe3Al2Si3O12), grossular (Ca3Al2Si3O12) and andradite (Ca3Al2Si3O12), often on multiple samples, have resolved uncertainties and certain problems with published thermodynamic data. S° can be affected by various low‐T physical phenomena, such as cooperative magnetic phase transitions or Schottky anomalies at temperatures of <15 K, which were not described fully in some older adiabatic calorimetric studies. New Cp results show that small differences in the thermodynamic behaviour between some natural and synthetic silicates may exist as demonstrated by extensive work on grossular. Important and “new” research questions on the thermodynamic behaviour of minerals are coming to light and are being investigated. The Cp behaviour and S° values for six silicate garnet end‐members are analysed and the latter are compared to the “best fit or optimized” S° values given in various internally consistent thermodynamic databases. Conclusions are drawn on what types and directions of calorimetric study are required in order to obtain better thermodynamic property determinations of minerals, as well as achieving a better understanding of the underlying microscopic physical behaviour that determines the macroscopic Cp and S° functions. |
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| Keywords: | calorimetry garnet heat capacity MgO standard entropy thermodynamic databases |
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