Diffusion-diffusive phase transformations in alkali feldspar

In this project, the exsolution of alkali feldspar will be investigated experimentally and by means of thermodynamic and atomistic modelling. Alkali feldspar is one of the most abundant minerals in the Earths crust. At temperatures above about 600C it forms a continuous solid-solution series between a sodium- and a potassium end-member. An alkali feldspar of intermediate composition that was formed at high temperatures tends to unmix during cooling, whereby typically a lamellar intergrowth of more sodium-rich and more potassium-rich feldspar is formed. The extent of chemical segregation between the lamellae and the characteristic lamellar width depend on the cooling rate and thus bear valuable information on the thermal history of the host rock. For a quantitative extraction of the information stored in the lamellar microstructure of exsolved alkali feldspar and in the chemical compositions of the lamellae, the mechanisms underlying the exsolution must be understood and need to be calibrated experimentally. In nature, the cooling of rock units occurs over time spans of millions of years, which makes a direct experimental calibration difficult, because laboratory experiments cannot be run over geological time spans. The only feasible experimental approach is the investigation of small, nanometer-scaled exsolution microstructures, which can be produced in laboratory experiments over well amenable run durations on the order of several weeks or months. We plan exsolution- and diffusion experiments, which will address the mobilities of sodium and potassium within alkali feldspar, on the one hand, and the thermodynamics and kinetics underlying the exsolution process, on the other hand. The experimental run products will be analyzed for their microstructures and chemical compositions on the nanometer scale using analytical techniques with high spatial resolution including scanning- and transmission electron microscopy and atom probe tomography. The analytical data will be integrated into thermodynamic and kinetic model calculations and atomistic simulations to quantify the relevant model parameters such as diffusion coefficients, interfacial energies, and the mechanical stress induced in coherent lamellar intergrowth. Based on an improved understanding of the exsolution process, a comprehensive model will be developed and calibrated, which adequately describes the exsolution of alkali feldspar in a qualitative and in a quantitative sense and which allows for the extraction of cooling rates from natural exsolved alkali feldspars. Considering the ubiquitous occurrence of alkali feldspar, such a geo-speedometer will substantially contribute to an improved understanding of geodynamic processes in the Earths crust.

In this project, the exsolution of alkali feldspar will be investigated experimentally and by means of thermodynamic and atomistic modelling. Alkali feldspar is one of the most abundant minerals in the Earth's crust. At temperatures above about 600C it forms a continuous solid-solution series between a sodium- and a potassium end-member. An alkali feldspar of intermediate composition that was formed at high temperatures tends to unmix during cooling, whereby typically a lamellar intergrowth of more sodium-rich and more potassium-rich feldspar is formed. The extent of chemical segregation between the lamellae and the characteristic lamellar width depend on the cooling rate and thus bear valuable information on the thermal history of the host rock. For a quantitative extraction of the information stored in the lamellar microstructure of exsolved alkali feldspar and in the chemical compositions of the lamellae, the mechanisms underlying the exsolution must be understood and need to be calibrated experimentally. In nature, the cooling of rock units occurs over time spans of millions of years, which makes a direct experimental calibration difficult, because laboratory experiments cannot be run over geological time spans. The only feasible experimental approach is the investigation of small, nanometer-scaled exsolution microstructures, which can be produced in laboratory experiments over well amenable run durations on the order of several weeks or months. We plan exsolution- and diffusion experiments, which will address the mobilities of sodium and potassium within alkali feldspar, on the one hand, and the thermodynamics and kinetics underlying the exsolution process, on the other hand. The experimental run products will be analyzed for their microstructures and chemical compositions on the nanometer scale using analytical techniques with high spatial resolution including scanning- and transmission electron microscopy and atom probe tomography. The analytical data will be integrated into thermodynamic and kinetic model calculations and atomistic simulations to quantify the relevant model parameters such as diffusion coefficients, interfacial energies, and the mechanical stress induced in coherent lamellar intergrowth. Based on an improved understanding of the exsolution process, a comprehensive model will be developed and calibrated, which adequately describes the exsolution of alkali feldspar in a qualitative and in a quantitative sense and which allows for the extraction of cooling rates from natural exsolved alkali feldspars. Considering the ubiquitous occurrence of alkali feldspar, such a geo-speedometer will substantially contribute to an improved understanding of geodynamic processes in the Earth's crust.