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Kinetic rate laws derived from order parameter theory IV: Kinetics of Al,Si disordering in Na-feldspars

Abstract

The kinetic rate laws of Al-Si disordering under dry conditions (T = 1353K, 1253 K, 1223 K, 1183 K) and in the presence of water (p = 1 kbar, T = 1023 K, 1073 K, 1103 K) were studied both experimentally and theoretically. A gradual change of the degree of order was found under dry conditions. For intermediate degrees of order broad distributions of the order parameter Q od occur. The variations of Q od are correlated with structural modulations as observed in the transmission electron microscope. The time evolution of the mean value of Q od can be well described by the rate law:

$$\frac{{dQ_{od} }}{{dt}} = - \frac{\gamma }{{RT}}\exp \sum\limits_{i = 1}^n {X_i^2 } \left[ {\frac{{ - (G_a^0 + \varepsilon (\Delta Q_{od} )^2 )}}{{RT}}} \right]\frac{{dG}}{{dQ_{od} }}$$

with the excess Gibbs energy G and G a 0 = 433.8 kJ/mol, ɛ= -27.4 kJ/mol, γ = 1.687 · 1014 h −1.

Under wet conditions, two processes were found which occur simultaneously. Firstly, some material renucleated with the equilibrium degree of order. Secondly, the bulk of the material transformed following the same rate law as under dry conditions but with the reduced activation energy G a 0 = 332.0 kJ/mol and ɛ = -43.0 kJ/ mol, γ = 1.047 · 1013 h−1. The applicability of the kinetic theory is discussed and some ideas for the analysis of geological observations are evolved.

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Wruck, B., Salje, E.K.H. & Graeme-Barber, A. Kinetic rate laws derived from order parameter theory IV: Kinetics of Al,Si disordering in Na-feldspars. Phys Chem Minerals 17, 700–710 (1991). https://doi.org/10.1007/BF00202240

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Keywords

  • Transmission Electron Microscope
  • Activation Energy
  • Gibbs Energy
  • Mineral Resource
  • Material Processing