Diffusion-diffusive phase transformations in alkali feldspar

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Alkali feldspar, a binary solid-solution between albite (NaAlSi3O8) and K-feldspar (KAlSi3O8), pertains to the most abundant mineral group in the Earth’s crust. During changes in pressure and temperature it undergoes several phase transformations producing microstructures such as twinning, braid texture, or exsolution lamellas, which bear genetic information. At high temperatures alkali feldspar shows continuous miscibility. Below about 600°C a miscibility gap opens. During cooling from super-solvus temperatures, intermediate alkali feldspar exsolves forming an intergrowth of Na-rich and K-rich lamellas, known as perthite. With time, the lamellas undergo coarsening and increasing chemical separation, which are both time- and temperature dependent and have been calibrated for application in geo-speedometry.So far, the accuracy at which thermal histories can be reconstructed from perthites is limited, because calibration is largely based on exsolution experiments, which integrate over all underlying processes including Na-K diffusion, formation of new phase boundaries, and evolution of coherency stress. For an improved quantification of exsolution each of these processes must be calibrated independently, and their coupling must be understood. We will develop a model for Na-K-interdiffusion based on the determination of Na- and K tracer diffusion coefficients including temperature-, composition-, direction-, and strain-dependence using a novel tracer/interdiffusion technique. This will be complemented by ab-initio calculations of defect- and migration energies to be used in molecular dynamics and Monte Carlo simulations. Finally, exsolution experiments will be done and the nm scale exsolution lamellas will be analyzed using atom probe tomography. This will allow, for the first time, direct determination of the coherent solvus and of interdiffusion in strained alkali feldspar.The combination of the broad spectrum of theoretical, experimental and analytical methods make this project unique and will allow for substantial improvement of geo-speedometry applications based on diffusion and diffusive phase transformations in alkali feldspar. The consortium of researchers involved combines expertise in all relevant fields including the mineralogical insight and background in thermodynamics and diffusive phase transformations (R. Abart), in atomistic modeling (C. Dellago), diffusion theory, experimentation, and Monte Carlo simulation (S. Divinski).