EQUATIONS OF STATE AND STRUCTURAL EVOLUTION OF ALKALI FELDSPARS AT HIGH PRESSURE

Feldspars are the dominant minerals in the Earth’s crust and therefore the relationship between their elastic properties and structure is crucial for understanding petrological and geochemical processes at all scales from the microscopic to the scales of mountain building. Surprisingly there is very little equation of state or elasticity data for the alkali feldspars (NaAlSi₃O₈ to KAlSi₃O₈) and what little data that does exist is mostly old, contradictory, and exhibits no clear trends with either composition or structural state. We will present newly-measured high-precision equation of state and structural data for three well-characterized alkali feldspars, ranging in composition from Or99 (sanidine), Or89 and Or64, determined from high-pressure single-crystal X-ray diffraction experiments. Analysis of the structural data will provide calibration of the crystal-chemical model recently developed by Angel et al. (2012) who used the concepts of Helen Megaw to describe the changes in the conformation of the tetrahedral framework of feldspars in terms of four distinct tilt systems of the four tetrahedra that comprise the 4-membered rings that lie parallel to (010). The ultimate goal is to develop a 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.

Angel, R.J., Sochalski-Kolbus, L. , Tribaudino, M. (2012) American Mineralogist, 97: 765-778.