Radiotracer diffusion and conductivity of sodium and potassium in alkali feldspar

• Applicant: Professor Dr.-Ing. Gerhard Wilde, since 6/2017
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials ; Mineralogy, Petrology and Geochemistry
• Term: from 2015 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 275110401

• Final Report Year: 2018

Final Report Abstract

This project aims at a comprehensive understanding of the diffusion processes taking place on the alkali sublattice of alkali feldspar. The diffusion of Na and K in alkali feldspar determines its potential for preserving the composition patterns it obtains during crystallization at the high pressures and temperatures prevailing during mineral and rock formation in the deep crust. Main achievements of this project are the accurate calibration of Na and K selfdiffusion coefficients and the combination of these data with corresponding data on ionic conductivity. Based on the experimental results a coherent interpretation of the underlying diffusion mechanism including the point defects involved is reported. A prerequisite for the realization and for the success of this project was the unique availability of three pertinent experimental methods: radiotracer diffusion, electrochemical impedance spectroscopy and Monte Carlo simulation of correlation effects pertaining to a given microscopic diffusion mechanism. The achieved results can be summarized as follows: • A substantial extension of the data on Na tracer diffusion in alkali feldspar has been provided. In particular, Na tracer diffusion coefficients for the compositions CK = 0.71, 0.83 and 0.95 have been quantified for two directions, i.e. normal to (001) and normal to (010). • The diffusivity ratio D*Na / D*K has been accurately determined using the same gemquality VF feldspar mineral with CK = 0.83 in the temperature range from 1021 K to 1169 K. The data show that D*Na exceeds D*K by more than three orders of magnitude. • We do, for the first time, a MC simulation of diffusion in the monoclinic feldspar structure. The results obtained for a binary alkali sublattice provide evidence that the available experimental data cannot be reconciled with the vacancy diffusion mechanism. • A microscopic model for an interstitialcy diffusion mechanism with additional doubleinterstitialcy jumps and short-range interactions between neighboring ions has been developed and analyzed by MC simulation. It has been demonstrated that this model is compatible with data from tracer diffusion and ion conductivity measurements. • The proposed interstitialcy diffusion model for alkali feldspar is related to a critical concentration in the narrow range from CK = 0.88 to CK = 0.90 for site-percolation of the faster moving Na atoms. This finding perfectly fits to our experimental observations and supports the hypothesis of an interstitialcy diffusion mechanism of Na selfdiffusion. In conclusion, it can be stated that in the course of this project our understanding of Na-K diffusion in alkali feldspar and its implications for re-equilibration phenomena, microstructure and composition evolution as well as for chemically induced elastic and plastic deformation has been significantly improved.

Publications

• Monte Carlo simulation of diffusion and ionic conductivity in a simple cubic random alloy via the interstitialcy mechanism. J. Phys. Condens. Matter 27.50 (2015) 505401
  F. Wilangowski and N. A. Stolwijk
  
• Radiotracer experiments and Monte Carlo simulation of sodium diffusion in alkali feldspar: Evidence against the vacancy mechanism. Defect Diffus. Forum 363.79 (2015) 79-84
  F. Wilangowski, S. V. Divinski, R. Abart and N. A. Stolwijk
  
• Vacancy-related diffusion correlation effects in a simple cubic random alloy and on the Na-K sublattice of alkali feldspar. Phil. Mag. 95.21 (2015) 2277–2293
  F. Wilangowski and N. A. Stolwijk
  
• Ionic conductivity in gem-quality single-crystal alkali feldspar from the Eifel: temperature, orientation and composition dependence. Phys. Chem. Minerals 43.5 (2016) 327–340
  H. El Maanaoui, F. Wilangowski, A. Maheshwari, H. D. Wiemhöfer, R. Abart, N. A. Stolwijk
  
• A Monte Carlo study of ionic transport in a simple cubic random alloy via the interstitialcy mechanism: Effects of non-collinear and direct interstitial jumps. Phil. Mag. 97.2 (2017) 108–127
  F. Wilangowski and N. A. Stolwijk
  
• Potassium self-diffusion in a K-rich single-crystal alkali feldspar. Phys. Chem. Minerals 44.5 (2017) 345–351
  F. Hergemöller, M. Wegner, M. Deicher, H. Wolf, F. Brenner, H. Hutter, R. Abart, N. A. Stolwijk