Substitution of ^[4]Al in layer silicates: Calculation of the Al-Si configurational entropy according to ²⁹Si NMR Spectra

Abstract

A new constructive statistical method was used to simulate the Al-Si distributions in tetrahedral layer of mica according to short range and long range order restrictions and to calculate values of configurational entropy which correspond to simulated distributions. The simulations and the calculated entropy values were constrained by the existing ²⁹Si NMR data for natural and synthetic mica samples. The results of simulation confirm previous conclusions on the importance of short range order restrictions such as Al-avoidance and homogeneous dispersion of charges (HDC) in the tetrahedral layer. The results suggest, however, that the Al-Si distribution in mica closely follows the HDC restriction only for the samples with the Al/(Si+Al) ratios in the range of 0.11–0.3. At higher values of the ratio (0.3–0.37) the degree of the HDC-type ordering decreases towards pure Al-avoidance. In the range of 0.37–0.5 two alternative models namely the HDC model and the model with partial long range order can be used to explain the observed NMR intensities.