Na-K diffusion in alkali feldspar: interdiffusion model
Primary Investigator: Dr. Elena Petrishcheva
Funding Organisation: FWF (Austrian Science Fund)
Abstract: The project „Na-K diffusion in alkali feldspar: interdiffusion model“ deals with the motion of Na and K ions in the crystal structure of alkali feldspar. With 50 volume percent feldspar is the most abundant mineral in the Earth’s crust. Its crystal structure is comprised of an aluminum-silicon-oxygen framework with cavities, which are occupied by alkali and alkali earth cations, primarily Na, K, and Ca-ions. In alkali feldspar only the monovalent Na and K ions are present, which are mobile at the high temperatures prevailing in the Earth’s crust. They may thus be effectively re-distributed in the crystal by diffusion, they may also be removed from or introduced into the crystal. The chemical composition a feldspar has obtained at a certain set of pressure-temperature conditions tends to re-equilibrate under changing environmental conditions, and the “memory” of the feldspar for earlier stages and thus for the conditions of rock formation may be changed or entirely lost. In this context, the interdiffusion of Na and K in alkali feldspar is the most important process. Only when the temperature- and pressure dependence of the interdiffusion rate is known, conditions of rock formation and possibly their evolution in time may be determined from the chemical patterns observed in feldspar.
In the literature there is considerable discrepancy between direct experimental determinations of Na-K interdiffusion rates in alkali feldspar and calculations that are based on theoretical interdiffusion models and independently determined rates of Na and K self-diffusion. It is hypothesized that this is due to inadequate quality of Na- and K tracer diffusion coefficients previously determined with out-dated methods, with which it was difficult to unravel underlying diffusion mechanisms. Moreover, due to the composition dependence of the lattice parameters, Na-K interdiffusion induces mechanical stress, which feeds back into diffusion via thermodynamic effects and lattice distortion. Neither the effect of different diffusion vehicles nor the feedback between self-induced mechanical stress and diffusion has been accounted for in interdiffusion models for alkali feldspar so far.
With the intended project new data on the self-diffusion will be generated by doing dedicated tracer diffusion experiments and with specific 22Na diffusion experiments on previously applied sharp Na-K composition gradients the feedback between mechanical stress and diffusion will be calibrated. The experimental results will then be used to develop a comprehensive interdiffusion model, which accounts for different diffusion mechanisms and for the feedback between self-induced mechanical stress and diffusion. This broad, combined experimental and theoretical approach has not been applied to alkali feldspar so far and to the knowledge of the proponent is novel in mineralogy. The expected outcomes will yield substantial improvement in the kinetic modelling of mineral reactions involving alkali feldspar. Due to its ubiquitous occurrence in the Earth’s crust and the considerable level of knowledge about basic thermodynamic and mechanical properties alkali feldspar is best suited for this project.