Thermodynamic mixing properties of binary oxide and silicate solid solutions determined by direct measurements: The role of strain
Published:January 01, 2001
Charles A. Geiger, 2001. "Thermodynamic mixing properties of binary oxide and silicate solid solutions determined by direct measurements: The role of strain", Solid Solutions in Silicate and Oxide Systems, Charles A. Geiger
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Most rock-forming silicates and many oxides are substitutional solid solutions that are capable of exchanging different cations on crystallographically defined structural sites. The mixing of cations often results in measurable changes in the macroscopic properties such as the thermodynamic functions, G, the Gibbs free energy, H the enthalpy, S the entropy and V the volume. Measurements of these functions, either directly (e.g. calorimetry, X-ray diffraction measurements) or indirectly (e.g. phase diagrams), constitute an important part of mineralogy, petrology and geochemistry. Because of the experimental difficulties often involved in a determination of these quantities and, in addition, considering the large compositional ranges that must be investigated, effort has been made in trying to estimate or predict, with microscopic-based models, the thermodynamic functions. Studies have also been made to try to understand the nature of the microscopic/mesoscopic structural properties (e.g. strain fields, polyhedral distortion, bonding changes, etc.) associated with substitutional solid solutions and, in addition, on how they control the macroscopic properties.
In the field of mineralogy/petrology phase equilibrium experiments have played a central role in determining phase diagrams and in extracting thermodynamic data for rock-forming silicates and oxides. Indeed, Zen (1977) spoke of the phase-equilibrium calorimeter. Presently, there are several internally consistent thermodynamic data bases that contain standard entropies, enthalpies of formation and volumes of end-member oxides and silicates that are to a large degree based on such experiments (e.g. Berman, 1988; Chatterjee et al., 1998). Thermodynamic data on solid solutions, derived from phase-equilibrium studies, are generally fewer and are not as well constrained.
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Solid Solutions in Silicate and Oxide Systems
The EMU book series or notes, as they are called, were introduced to provide university teachers with up-to-date reviews in important, rapidly evolving areas of mineralogy, petrology and geochemistry. They are also meant to introduce scientists into special and often interdisciplinary fields of research. In this regard, a volume on solid solutions is current and sorely needed. The solid Earth, as well as many meteorites and the other solid planets, consists for the most part of mineral solid solutions. Research on solid solutions is extremely broad encompassing work in physics and chemistry, metallurgy, materials science and, last but not least, mineralogy and petrology. Hence, because the theme is so strongly interdisciplinary in nature, the workshop was organised to include solid state physicists, physical chemists, crystallographers, mineralogists and petrologists. The various chapters reflect some of this diversity and show what mineralogy has become. Experimental investigations in mineralogy now routinely include different types of spectroscopies along with more traditional phase equilibrium, X-ray diffraction, calorimetry, and TEM methods. There have also been new and impressive developments in theory and computation. Many computational approaches relating to the study of solid solutions, for example, the Cluster Variation Method or Monte Carlo simulations, have been brought in from materials science, chemistry and physics. It can be concluded that the traditional or historical, and perhaps artificial, boundaries between the various disciplines are disappearing. Many current research efforts in mineralogy are similar to those in chemistry, materials science and physics.