Radiotracer diffusion and conductivity of sodium and potassium in alkali feldspar

点击翻译按钮获取中文摘要

The proposal is an interdisciplinary, bilateral initiative between a material science group (Münster, Germany) and a geoscience group (Vienna, Austria), which aims at investigating alkali transport in alkali feldspar crystals of different composition and in different orientations as a function of temperature. The ultimate goal is to identify the underlying diffusion mechanisms and to characterize the point defects involved. Achievement of this goal will greatly help to understand the retention of major- and trace-element composition and isotope record in alkali feldspar, one of the most abundant phases in the Earth´s crust, over geological time scales. Moreover, evaluation of the experimental data is expected to shed light on fundamental issues of materials science not yet sufficiently dealt with, such as Frenkel pair formation and diffusion via interstitialcy defects on a binary (sub)lattice, diffusion correlation factors and their relevance to mixed alkali effects, and a possible composition-dependent changeover from vacancy- to interstitialcy-controlled diffusion. The project goals will be achieved by performing 22Na and 43K radiotracer diffusion experiments on gem quality alkali feldspars from the Eifel and Madagascar differing in composition. Additional compositions can be prepared by cation exchange, so that a wide range can be investigated covering relative K contents XOr between 0.65 and 1.00. The resulting tracer diffusion coefficients will be compared with ion conductivity data obtained by frequency-dependent impedance measurements on samples of the same composition and crystallographic direction using the Nernst-Einstein equation. Data analysis will involve a continuum approach based on the numerical solution of coupled diffusion-reaction equations. Monte Carlo simulations will be performed for testing theoretical models of alkali transport in the monocline feldspar structure.