Thermodynamic modelling of clay dehydration, stability and compositional evolution with temperature, pressure and H2O activity

Vidal, O. and Dubacq, B. (2009) Thermodynamic modelling of clay dehydration, stability and compositional evolution with temperature, pressure and H2O activity. Geochimica et Cosmochimica Acta, 73 (21). pp. 6544-6564. DOI 10.1016/j.gca.2009.07.035

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Abstract

<p><br/>We propose a thermodynamic approach to model the stepwise dehydration with increasing temperature or decreasing H2O activity of K, Na, Ca and Mg-smectite. The approach relies on the relative stability of the different solid-solutions that describe the hydration of di- or trioctahedral-smectites containing 0, 1, 2 or 3 interlayer water layers. The inclusion of anhydrous mica end-members makes it possible to cover, with the same solid-solution model, the entire range of composition from low-charge smectite to mica, through high-charge smectite and illite. Non-ideal Margules parameters were used to describe the non-ideality of the solid solutions between the hydrated and dehydrated smectite end-members. Standard state properties of all smectite end-members as well as Ca- and Mg-muscovite and -phlogopite were initially estimated by oxide summation. These values were then refined and the other non-ideal interactions were estimated on the basis of different experimental data. The stepwise dehydration of smectite, and its stability and compatibility relations were calculated by Gibbs free energy minimising. Our results account for the progressive evolution of smectite to interlayered illite/smectite and then to mica, as observed in nature and experiments, and our model provides an explanation for the thermodynamic stability of smectite and illite/smectite compared to mica + kaolinite or pyrophyllite assemblages. The results suggest that the enthalpic contribution of interlayer water is a function of the ionic potential of the interlayer cation and the number of interlayer water molecules. This evolution makes possible to estimate the standard-state thermodynamic parameters and hydration-temperature behaviour of smectite of virtually all possible compositions. For the four-interlayer cations considered in the study, our model reproduces the 3 --> 2 --> 1 water-layer transitions that accompany a reduction of water activity or an increase of temperature at ambient pressure. The range of water content and interlayer distance calculated for the 3w, 2w and 1w states are also in fair agreement with the experimental values at ambient pressure.</p>

Item Type: Article
Uncontrolled Keywords: NIL AREP;
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Journal or Publication Title: Geochimica et Cosmochimica Acta
Volume: 73
Page Range: pp. 6544-6564
Identification Number: 10.1016/j.gca.2009.07.035
Depositing User: Sarah Humbert
Date Deposited: 04 Jun 2011 14:21
Last Modified: 23 Jul 2013 10:02
URI: http://eprints.esc.cam.ac.uk/id/eprint/2074

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