Philippe Vieillard

A NEW METHOD FOR THE PREDICTION OF GIBBS FREE ENERGIES OF FORMATION OF HYDRATED CLAY MINERALS BASED ON THE ELECTRONEGATIVITY SCALE

A new method for the prediction of Gibbs free energies of formation for hydrated clay minerals is proposed based on the parameter ΔGO= Mz+(clay) characterizing the oxygen affinity of the cation Mz+. 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 ΔGO= Mz+(clay) between any two consecutive cations. The ΔGO= 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 ΔGO= Mz+(clay) corresponding to the electronegativity of a cation in either interlayer or octahedral sites and known ΔGO= 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.

Behavior of gold in the lateritic equatorial environment: weathering and surface dispersion of residual gold particles, at Dondo Mobi, Gabon

Abstract A supergene gold dispersion halo has developed at Dondo Mobi, southern Gabon, from the weathering of an auriferous lisvenite under equatorial humid rain forest conditions. Morphological and chemical studies were performed on gold particles extracted from both fresh and weathered rocks from different parts of the dispersion halo. The results demonstrate that the gold particles are residual and are subjected to increasing weathering, upward through the profile, from the protore to the surface in the central part of the halo, and at the surface both laterally upslope and downslope. Weathering continues downward into the formerly barren saprolites which were derived from Proterozoic schist and Archean gneiss (rim of the halo). Thus, the Au content progressively decreases as does the Ag content of gold particles with distance from the center of the ore body. Gold particles subjected to increased weathering are smaller with more rounded shapes and surfaces pitted by dissolution. The dispersion pattern is controlled by Au dissolution and translocation of residual gold particles. These processes involve an autochthonous evolution in the central part of the halo and a para-autochthonous evolution at the rim of the halo. At Dondo Mobi, chloride and humic substances are the only ligands present in significant concentrations that are capable of complexing Au. Thermodynamic calculations demonstrate that under such acidic conditions (pH ranges from 4 to 6), gold stability increases with increasing silver content. The dominant Au complexes formed have been calculated to be an aurous hydroxichloride complex and an organometallic complex, both of which are stable at surficial conditions. Thus, both chemical and translocation processes control the mobility of Au and gold particles in equatorial lateritic weathering profiles.

A NEW METHOD FOR THE PREDICTION OF GIBBS FREE ENERGIES OF FORMATION OF PHYLLOSILICATES (10 Å AND 14 Å ) BASED ON THE ELECTRONEGATIVITY SCALE

The method for prediction of Gibbs free energies of formation, based on the parameter ΔGO=Mz+(clay) characterizing the oxygen affinity of the cation Mz+, on the smectites, considered as hydrated clay minerals, has been used for micas and brittle micas, and yielded underestimated values. This method of prediction can be improved by a new set of parameters ΔGO=Mz+(clay), characterizing the electronegativity of a cation in a specific site (interlayer, octahedral, tetrahedral in the 10 Å minerals), determined by minimizing the difference between experimental Gibbs free energies and calculated Gibbs free energies of formation from constituent oxides. By considering the crystal structure of 10 Å and 14 Å minerals, and assuming the same electronegativity of cations, ΔGO=Mz+(o), in the octahedral sheets, an attempt is made to determine the electronegativity of cations in the brucitic sheet, ΔGO=Mz+(b). The results indicate that this prediction method compared to other determinations, gives values within 0.25% of the experimentally-estim ated values. The relationships between ΔGO=Mz+(clay) corresponding to the electronegativity of a cation in the interlayer, octahedral, tetrahedral or brucitic sites and known ΔGO=Mz+(aq) were thus determined, allowing the determination of the electronegativity of transition metal ions and trivalent ions in each of the four sites and consequently contribute to the prediction of Gibbs free energies of formation of different micas and chlorites. Examples are given for low-Fe clinochlore whose solubility is measured experimentally and the results appear excellent when compared with experimental values.

Differences in the dehydration-rehydration behavior of halloysites : New evidence and interpretations

Two reference halloysites from New Zealand (Te Puke and Opotiki) were studied by X-ray diffraction under (1) various levels of relative humidity (RH) from 95 to 0% (dehydration), and (2) various temperatures increasing from 25 to 120°C (dehydration). They were also studied by differential thermal and thermogravimetric analyses at 40 and 0.2% RH. The impact of freeze drying along with the influence of cation saturation (Ca and K) on halloysite hydration were studied. The dehydration of the two halloysite samples upon decrease in RH started below 70% RH. However, the dehydration of Opotiki was still incomplete at ∼0% RH regardless of the saturation cation whereas Te Puke was completely dehydrated at ∼10% RH. For each sample, the decrease in RH and the increase in temperature induce similar dehydration behavior, but the dehydration processes of the Opotiki and Te Puke samples are different. The dehydration of Te Puke proceeds with one intermediate hydration state reacting as a separate phase due to the presence of ‘hole’ water molecules. The dehydration of the fully hydrated Opotiki halloysite gives a dehydrated phase and no 8.6 Å phase. The results suggest the presence of different types of water molecule, the ‘associated’ and the ‘hole’ water, controlling the dehydration behavior of halloysites. Freeze-dried halloysite samples are essentially dehydrated and the size of their coherent scattering domains is strongly reduced. Rehydration experiments performed after dehydration either at 95% RH or by immersing the sample in water for 3 months result in their partial rehydration. Calcium saturation promotes the rehydration process. The results suggest the presence of interlayer cations in the Opotiki sample, Ca ions being associated with the strongly held ‘hole’ water. As a result of this study, we assert that the (de)hydration behavior of halloysite is highly heterogeneous and cannot be generalized a priori.

A predictive model for the entropies and heat capacities of zeolites

The classical predictive models for thermodynamic entities are based on summation techniques. At the time these predictive methods were designed, no or few calorimetric data on zeolites were available. The number of calorimetric measurements (third-law entropy and heat capacities) performed since 1990 on various zeolitic minerals allows a quick verification of all algorithms. It shows that the predicted values display a discrepancy above 4 % relative to the measured values. Recent predictive methods for entropy and for heat capacity also give very high differences between predicted and experimental values. With a limited number of thermodynamic values of zeolites and their dehydrated forms, the entropy and heat capacity predictions have been improved by using the thermodynamic properties of zeolite-like silica polymorphs and of zeolitic water obtained from the difference between the thermodynamic parameters of anhydrous and hydrated zeolites. The improvements introduced in this work reduce the prediction errors to about 2.95 % for entropy and 2.86 % for heat capacities.

A generalized model for predicting the thermodynamic properties of clay minerals

A set of models for estimating the enthalpy of formation, the entropy, the heat capacity and the volume of dehydrated phyllosilicates is presented. The model for entropy and heat capacity estimation is essentially based on a method of decomposition into polyhedral units, similar to that published by Holland (1989). The model for predicting the enthalpy of formation is based on the electronegativity scale, as previously developed by Vieillard (1994a, 1994b). For the sake of consistency, the models are parameterized using the same critical selection of thermodynamic properties from the literature. This includes a set of direct measurements especially dedicated to clay minerals that had not been taken into account in previous calculation methods. The accuracy of the predictions is tested for each property. The verification tests are also carried out for minerals that include different chemical elements than the phases used to derive the model constants, especially lithium-bearing micas. Verification tests also concern the Gibbs energy function that combines contributions from both models. Finally, the models are used in order to propose a complete thermodynamic database for clay mineral end-members. The consistency of the stability domains calculated on the basis of these thermodynamic properties is investigated by drawing relevant predominance diagrams for some chemical systems of interest. The models proposed represent a significant improvement with respect to previous works as regards the global accuracy of the estimates and because the developments were realized and tested using the same set of minerals, whose properties had been collected through a critical selection of the literature.

A predictive model for the enthalpies of hydration of zeolites

Abstract A compilation of the average hydration enthalpies per mole of water of 145 diversely originating zeolites measured using different technical methods [76 data from transposed-temperature drop calorimetry (TTDC), 57 data from immersion calorimetry (IC), 6 data from phase equilibria (PE), 5 data from gas-adsorption calorimetry (GAC), and 3 data from hydrofluoric acid solution calorimetry (HF)] was generated. Statistical regressions between three parameters involving the average hydration enthalpy per mole of water {ΔHhyd-W, ΔHhyd-W/(Al/Si), ln[-ΔHhyd-W/(Al/Si)]}and six parameters namely: (1) the charge defined by the Al/Si ratio; (2) the ratio of the framework charge to the number of H2O molecules (Al/H2O); (3) the framework density (FD) calculated from the molecular volume of the anhydrous zeolite, FDanh, and hydrated zeolite, FDhyd; (4) the average cation electronegativity in the exchange site characterized by parameter ΔHO=(site A)aq; and (5) the intracrystalline water porosity (WP) determined from the volume of liquid water that can be recovered upon thorough outgassing of the hydrated zeolite. The regressions were performed by taking into account either the nature of the measurement technique, or the nature of the zeolite family. Within the zeolites from the TTDC and IC populations (133 data), the best results were obtained with ln[-ΔHhyd-W/(Al/Si)] and Al/(Al + Si). Whatever the measurement technique, considering the nature of the zeolite family having a constant framework density of the anhydrous form (129 data), the Al/(Al + Si) ratio remains the best parameter and the enthalpy of hydration can be expressed as follows: ΔHhyd-w = -(Al/Si)*e{5.491 - 4.674*[Al/(Al + Si)]} This general relationship can be improved by considering the following parameters: FDanh, ΔHO=(site A)aq, WP and a new parameter that is the product of three parameters Al/Si, ΔHO=(site A)aq, and WP weighing the variation of the water porosity related to the nature of the cation and to the total charge of the exchange site. Therefore, an understanding of the chemical formulae and unit-cell volumes of anhydrous and hydrated zeolites is required to evaluate the enthalpy of hydration with an accuracy of ±3.25 kJ/mol H2O.

Thermodynamic properties of saponite, nontronite, and vermiculite derived from calorimetric measurements

Abstract The stability of clay minerals is an important factor in assessing the durability of containment barriers for deep waste storage. In that context, the complete thermodynamic data set of three 2:1 ferro-magnesian clay minerals have been determined at 1 bar and from 2 to 520 K, using calorimetric methods. The studied clay samples were, respectively, the Na-saturated saponite Sap-Ca-1, Na0.394 K0.021Ca0.038(Si3.569Al0.397Fe3+0.034)(Mg2.948Fe2+0.021Mn0.001)O10(OH)2, the Ca-saturated nontronite NAu-1, Ca0.247K0.020(Si3.458Al0.542)(Mg0.066Fe3+1.688Al0.268Ti0.007)O10(OH)2, and the Ca-saturated Santa Olalla vermiculite, Ca0.445(Si2.778Al1.222)(Al0.192Mg2.468Fe3+0.226Fe2+0.028Ti0.018Mn0.007)O10(OH)2. The standard enthalpies of formation were obtained by solution-reaction calorimetry at 298.15 K. The heat capacities were measured between 2 and 520 K, using low-temperature adiabatic calorimetry, heat-pulse calorimetry, and differential scanning calorimetry. The standard entropies and the Gibbs free energies of formation at 298.15 K have been calculated from these values. Finally, the equilibrium constants at 298.15 K have been determined. A comparison between these experimental data and estimated values obtained from prediction models available in the literature enabled the most usual calculation methods available to date to be assessed for each thermodynamic property.

Prediction of enthalpies of formation of hydrous sulfates

Abstract A method for the prediction of the enthalpies of formation ΔHOf for minerals of hydrous sulfates is proposed and is decomposed in the following two steps: (1) an evaluation of ΔHOf for anhydrous sulfates based on the differences in the empirical electronegativity parameter ΔHO= Mz+(c) characterizing the oxygen affinity of the cation Mz+; and (2) a prediction of the enthalpy of hydration based on the knowledge of the enthalpy of dissolution for anhydrous sulfates. The enthalpy of formation of sulfate minerals from constituent oxides is correlated to the molar fraction of oxygen atoms bound to each cation and to the difference of the oxygen affinity ΔHO= Mz+(c) between any two consecutive cations. The ΔHO= Mz+(c) value, using a weighing scheme involving the electronegativity of a cation in a given anhydrous sulfate, is assumed to be constant. This value can be calculated by minimizing the difference between the experimental enthalpies and calculated enthalpies of formation of sulfate minerals from constituent oxides. The enthalpy of hydration is closely related to the nature of the cation in the anhydrous salt, to the number of water molecules in the chemical formula and to the enthalpy of dissolution for the anhydrous salt. The results indicate that this prediction method gives an average value within 0.55% of the experimentally measured values for anhydrous sulfates and 0.21% of the enthalpies of hydration or hydrous sulfates. The relationship between ΔHO= Mz+(sulfate), which corresponds to the electronegativity of a cation in a sulfate compound, and known parameter ΔHO= Mz+(aq) was determined. These determinations allowed the prediction of the electronegativity of some anhydrous transition metal double sulfate and contributed to the prediction of the enthalpy of formation for any hydrous double sulfate. With a simplified prediction of the entropy of a hydrous sulfate, calculations of Gibbs free energy of formation can be evaluated and contribute to the knowledge of the stability of some hydrous sulfates in different environmental conditions such as temperature or air moiety. Therefore, to check the reliability of the predictive model, stability fields for some hydrous ferric sulfates such as pentahydrate ferric sulfate, lawsonite, kornelite, coquimbite, and quenstedtite vs. temperature and relative humidity were studied and compared with experimental measurements.

MINENT: a FORTRAN program for prediction of enthalpy of formation from elements of minerals with known crystal refinements

Abstract The computer code MINENT uses the crystallographic properties (bond length, cation oxygen and its estimated standard deviation, nature of cation occupying a site, molecular volume, space group) and the optical properties of any mineral to calculate its enthalpy of formation from elements and its accuracy. This method of computation is applied to all minerals belonging to the following system: Li 2 O-Na 2 O-K 2 O-BeO-MgO-CaO-SrO-BaO-MnO-FeO-CoO-NiO-ZnO-Al 2 O 3 -Cr 2 O 3 -Fe 2 O 3 -Mn 2 O 3 -SiO 2 -ZrO 2 -H 2 O (20 different cations). This method of computation takes into consideration the following properties: • hypothesis of a unique effective ionic radii for oxygen; • polarizability additivity assumption; • electronegativity based on the parameter Δ H O 2− cation; • difference in electronegativity between the oxide state and the compound is a function of the surrounding of cation (bond length, shortest bond length, polarization of the cation and of oxygens); • presence or absence of bridging oxygen atoms between any two consecutive sites; • presence or absence of hydrogen bondings; • provides a prediction error related to the estimated standard deviation of mean bond length. Two minerals, ferrobustamite, and natrolite, have been selected to show the different steps in the computations.