A New Method for the Prediction of Gibbs Free Energies of Formation of Hydrated Clay Minerals Based on the Electronegativity Scale

Abstract

Abstract--A new method for the prediction of Gibbs free energies of formation for hydrated clay minerals is proposed based on the parameter AGO-Mz+(clay) characterizing the oxygen affinity of the cation M ~ § The Gibbs free energy of formation from constituent oxides is considered as the sum of the products of the molar fraction of an oxygen atom bound to any two cations multiplied by the electronegativity difference defined by the A60 = MZ+(clay) between any two consecutive cations. The A60 = Mz+(clay) value, using a weighting scheme involving the electronegativity of a cation in a specific site (interlayer, octahedral, or tetrahedral) is assumed to be constant and can be calculated by minimization of the difference between experimental Gibbs free energies (determined from solubility measurements) and calculated Gibbs free energies of formation from constituent oxides. Results indicate that this prediction method compared to other determinations, gives values within 0.5% of the experimentally estimated values. The relationships between Act-Mz+(clay) corresponding to the electronegativity of a cation in either interlayer or octahedral sites and known Act-Mz+(aq) were determined, thereby allowing the prediction of the electronegativity of transition metal ions and trivalent ions in hydrated interlayer sites and octahedral sites. Prediction of Gibbs free energies of formation of any clay mineral with various ions located in the interlayer and with different cations in octahedral sites is possible. Examples are given for Al-rich montmorillonite from Aberdeen, transition element-exchanged montmorillonite, and Ni-rich stevensite, and the results appear excellent when compared to experimental values.

Do clay mineral dissolution rates reach steady state?

“…were computed from equations and parameters reported by Vieillard (2000). The logarithm of the equilibrium constant of reaction 1 at 25°C obtained from the average structural formula presented in reaction 1 is 6.57.…”

Differential alteration of basaltic lava flows and hyaloclastites in Icelandic hydrothermal systems

“…For instance, smectites, when only identified as "smectites", are not reported in this table because smectite is not a single mineral, but a mineral family which includes montmorillonites, nontronites, beidellites, saponites, and hectorites. Thermodynamic properties of them can be very different from one category to another one (Vieillard 2000). There is a lack of accurate smectite identification.…”

A new simple approach to evaluate pedogenic clay transformation in a Vertic Calcisol

“…The K sp (theoretic solubility product) for the different clay minerals was obtained from their predicted Gibbs free energies DG f 8 according to Vieillard (2000). Log Q/K ratio was calculated to characterize the real equilibrium condition.…”

“…From DG f 8 of ions given by Vieillard (2000), the log K sp of smectite and illite are equal to À 37.89 and À 39.32, respectively.…”