A chemical and thermodynamic model of aluminous dioctahedral 2:1 layer clay minerals in diagenetic processes; regular solution representation of interlayer dehydration in smectite

Thermodynamics of Hydration in Minerals: How to Predict These Entities

“…As the standard entropy of hydration is assumed constant in [66] and [68] models, the integral hydration enthalpies provided by these two previous models are nearly linear. Those obtained by [68] exhibit three sections of curves corresponding to the three states of hydration (1, 2 and 3 water layers).…”

“…As indicated in the enthalpy section, integral hydration entropies modelled by [66] (yellow dotted line, N°1) and [68] (green dotted line, N°2a, 2b and 2c corresponding to 1 st , 2 nd and 3 rd layer) are linear and merge together. In the model proposed by Vieillard et al [63], three calculated integral entropies of SWy1-Na -water system were plotted in Figure 5A and correspond to the adsorption (red line), the desorption (blue line) and the theoretical equilibrium water -Swy1-Na (black line, N° 3), whose equation is given: …”

“…The number of external surface water molecules (expressed in mmol/g dry clay) can be expressed as a function of relative humidity [63]. There are numerous papers about the adsorption-desorption isotherms performed on various clays minerals, but few are devoted to the acquisition of enthalpy of All recent predictive models of the hydration of smectites are based on an approach that uses the solid solution initiated by Ransom & Helgeson [66]. The hydration of a smectite is considered through the following reaction between hydrated and dehydrated end-members:…”

“…The amount of interlayer water in a smectite is proportional to the mole fraction of hydrated end-member: x*H2O= nm*xhs=(1-nm)*xas; to the mole fraction of the hydrous end-member, xhs or to the anhydrous end-members xas. Ransom & Helgeson [66] considered the system hydrated smectite -anhydrous smectite + bulk water as a strictly regular binary solid solution by considering the free energy only. Thus, the excess free energy of mixing can be expressed as follows:…”

“…The curve 1 in Figure 4A (yellow dotted line) comes from model of Ransom & Helgeson [66], is linear and is obtained from the following equation of integral enthalpy of hydration: ()°Δ given in Table 7. The curves 2b, 2c and 2d (green dotted lines) displayed in Figure 4A come together from the model of Vidal et al [68] …”