Low-temperature heat capacities and entropies of minerals

Heat capacity and entropy are key-thermodynamic quantities and play an important role in several scientific branches like physics, chemistry, earth- and material-science. A major obstacle for a better understanding of the thermodynamic properties of rock-forming and industrial relevant minerals is the lack of low-temperature heat capacity data (5 - 300 K). This is especially true for silicate solid solutions, because excess heat capacities can occur in this low temperature range, and for iron-rich solids which exhibit low-temperature heat capacity anomalies due to magnetic ordering. There are new technological developments in the field of calorimetry which allow it for the first time to measure low-temperature heat capacities on milligram-sized samples. It is the aim of the planned project to establish such a microcalorimeter laboratory and to measure low-temperature heat capacities on a number of well characterised synthetic phases relevant in nature and industrial processes (e.g. rock-forming minerals: biotites, chlorites, pyroxenes; industrial-relevant minerals: CSH-phases). This opens new scientific ground for many substances that can not be synthesized in large amounts and promises considerable scientific progress in the field of directly measured thermodynamic quantities.

Heat capacity and entropy are key-thermodynamic quantities that play an important role in several sciences such as physics, chemistry, earth and material science, and also various industrial processes. During the course of this project a calorimetry laboratory was established at Salzburg University, which permitted the measurement of low- temperature heat capacities on samples weighing milligrams. This is a technological innovation developed by Quantum Design and commercially available as the so called Physical Properties Measurement System (PPMS). The main objects of the calorimetric measurements were well characterised synthetic solid solutions relevant in nature: pyroxenes, olivines, biotites and chlorites. For these, excess heat capacities and entropies were quantified. These are a measure for the deviation of solid solutions from ideal mixing behaviour. The information obtained allow activity-composition relations in the relevant mixtures to be determined. Another focus of the project was the measurement of low-temperature heat capacities of rock-forming mineral end- members, such as Fe-biotite (annite), Fe-chlorite (chamosite), Fe-serpentine, Fe-cordierite, Fe- and Mg-carpholite. In addition, ultra-high pressure phases like "hollandite-type" KAlSi3 O8 , Si-wadeite, wadsleyite und -Fe2 SiO4 that are relevant for the Earth`s mantle were investigated. The low-temperature heat capacities have also been measured for industrially relevant CSH-phases like xonotlite. From these calorimetric data, the standard entropy, a key thermodynamic quantity of each substance, could be directly determined. The calorimetric measurements and subsequent data evaluation performed during the course of this project contributed to a better understanding and a more sound thermodynamic description of various solid solutions in nature, the possibility that relevant phase diagrams can be calculated more reliably, which is relevant especially for the ultra-high pressure phases studied and their stability in the Earth`s mantle, new knowledge concerning the thermal behaviour of industrial-relevant CSH-phases.