Kinetics of Al-Si exchange in low and high quartz: calculation of Al diffusion coefficients

Abstract

Non-uniform distribution of Al between the three symmetrically equivalent Si sites in low quartz is converted to a random distribution by dry or hydrothermal annealing above 400°C due to Al-Si exchange, as shown previously by EPR measurements. Rate equations for the Al-Si exchange are derived from the experimental results presented here. It is shown that the kinetics of the Al-Si exchange reaction are strongly influenced by the type of Si substitution, whether by Al+Na ([AlO4/Na]4-) or Al+Li ([AlO4/Li]4-). It is concluded that Na and Li are involved in the rate determining step of the Al-Si exchange reaction. From the experimental results obtained on samples from different growth sectors of the same crystal, it is concluded that a vacancy mechanism is responsible for the Al-Si exchange. Based on the "random walk' theory, equations are derived which allow calculation of the diffusion coefficients of Al from rate constants of the Al-Si exchange reaction. -from Authors