Structure-Based Thermodynamic Properties of Feldspars

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Feldspar minerals comprise 60% of the Earth's crust. Therefore knowledge of their thermodynamic and elastic properties is essential for understanding petrological and geochemical processes at all scales from the microscopic to the scales of mountain building. With the support of a previous NSF-EAR grant, our research group has determined the elastic properties of one group of the feldspar minerals, the plagioclase feldspars, whose compositions range from NaAlSi3O8 to CaAl2Si2O8. These plagioclase feldspars comprise about half of the feldspars found in the Earth's crust. The other half are the alkali feldspars (NaAlSi3O8 to KAlSi3O8), for which there is very little equation of state or elasticity data. What little data that does exist is mostly old, contradictory, and exhibits no clear trends with either composition or structural state. We will to address this critical gap in our knowledge base by determining high-precision equation of state and structural data on alkali and ternary feldspars. This new data, combined with the crystal-chemical concepts developed in our previous research, will allow us to build the first structure-based thermodynamic model for feldspars that will unite the description of their bulk solid solutions and that of their dilute (trace element) regimes into a single, consistent description. The resulting model will predict cation site sizes and framework configuration as a function of pressure, temperature and cation content. This in turn will allow elasticity, thermal expansion and the volumes of mixing to be calculated for any pressure, temperature and composition. The successful development of this model will not only allow thermodynamic properties of feldspars to be predicted, but will also provide insights in to the atomic-scale mechanisms that control cation partitioning between feldspars, melts and fluids, and key constraints on seismic wave velocities and anisotropy in the Earth's crust. Such a model will therefore be of great use and interest to a broad range of earth scientists, including mineralogists, petrologists, geochemists and geophysicists alike. The crystal-chemical principles that we are developing to describe the response of feldspars to P,T, and X should be equally applicable to other framework minerals, including the industrially important zeolite minerals and technologically important perovskites.