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

Abstract

Abstract.A new constructive statistical method was used to simulate the A1-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 A1-Si distribution in mica closely follows the HDC restriction only for the samples with the A1/(Si +A1) 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.

Thermodynamic properties of illite, smectite and beidellite by calorimetric methods: Enthalpies of formation, heat capacities, entropies and Gibbs free energies of formation

“…The entropic term for the disorder in the tetrahedral layer was calculated according to the statistical method of Vinograd (1995), based on simulations of Si-Al distribution in tetrahedral layers constrained by 29 Si NMR results on natural and synthetic mica samples. In particular, in the case of our smectite and illite samples, the Al/(Si + Al) ratio criterion being lower than 0.3, the configurational entropic term was calculated by considering the homogeneous dispersion of charge model.…”

Analysis of Fe segregation in the octahedral sheet of bentonitic illite-smectite by means of FTIR, 27 Al MAS NMR and reverse Monte Carlo simulations

“…Such studies have been developed in aluminosilicates (for example: Herrero and Ramirez, 1992;Herrero, 1993;Dove and others, 1996), but few of them were conducted on 2:1 phyllosilicate structures, and they have focused mainly on the tetrahedral sheet (Herrero, Sanz, and Serratosa, 1986;Herrero and others, 1987;Vinograd, 1995). Simulations of cation distribution in the octahedral sheet present more difficulties due to the wider variety of possible isomorphous substitutions.…”

Computer simulations of cations order‐disorder in 2:1 dioctahedral phyllosilicates using cation‐exchange potentials and monte carlo methods

“…Several computational and theoretical methods for the study of cation ordering have been developed in recent years. These include the cluster variation method [25,26] and approaches based on configurational averaging. [27] This review is concerned with developing models of the energetics of ordering, and using Monte Carlo (MC) methods to study the variation of equilibrium ordered states (both long-and shortrange) with temperature.…”