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Smectites are able to retain molecular tightly bound water (TBW) at temperatures above 100 °C, even after prolonged drying. The presence of TBW affects the stable isotope ratios, the dehydroxylation behavior of smectites and smectite-rich samples and also has implications in measuring various properties of clay-rich rocks. Five reference smectites, in Mg-, Ca-, Na-, and Cs-exchanged forms were subjected to different drying protocols followed by the determination of TBW contents using precise thermogravimetric (TG) analysis. Activation energies (Ea) of the removal of different water fractions at temperatures up to 1000 °C were determined in non-isothermal TG experiments using model-independent methods. Additionally, 4A and 13X zeolites were examined in both cases as apparent OH-free references. After drying at 110 °C, all smectites still contained up to 3 water molecules per interlayer cation. The TBW contents in smectites were found to be primarily dependent on the isothermal drying temperature. For a given temperature, TBW contents decreased with respect to the type of interlayer cation in the following order: Mg > Ca > Na > Cs. The influence of the time of drying and the smectite layer charge were found to be negligible. The Ea of dehydration below 100 °C, as determined by the Friedman method, was quite constant within the 45-60 kJ/mol range. The Ea of TBW removal increased along with the degree of reaction from 90 to 180 kJ/mol, while the Ea of dehydroxylation was found in the 159-249 kJ/mol range, highly depending on the sample's octahedral sheet structure and the interlayer cation. The Mg2+ cation can hold H2O molecules even beyond 550 °C, making it available during dehydroxylation or--for geologic-scale reactions--pass H2O to metamorphic conditions. High similarities between the TBW contents and the Ea of dehydration for smectites and cationic (low Si/Al-) zeolites lead to the conclusion that TBW in smectites is remarkably similar to zeolitic water in terms of cation bonding and diffusion characteristics. The optimal drying protocol for smectites is to substitute interlayer cations with cations of a low-hydration enthalpy, such as Cs, and to dry a sample at 300 °C, provided that the sample is Fe-poor. Fe-rich smectites should be dried at 200 °C to avoid dehydroxylation that occurs below 300 °C. [ABSTRACT FROM AUTHOR]

Dehydroxylation-rehydroxylation experiments were performed on various dioctahedral aluminous 2:1 layer clay minerals using the precise thermogravimetry (TG) system equipped with a continuous N 2 gas flow enriched with H 2 O vapor of controlled concentration. The setup and rehydroxylation conditions applied are specifically sensitive to study in-situ rehydroxylation of smectites with various octahedral compositions and vacancy, saturated with counterions in various sizes and valences. Under the conditions applied, the pyrophyllite structure does not rehydroxylate, illite shows a minor ability to rehydroxylate, and smectites present an entire range of potential rehydroxylation, from remaining in the completely dehydroxylated state to an almost complete rebuild of the octahedral sheet. The mass gained during single- or multi-isotherm rehydroxylation matches the mass lost during the subsequent (secondary) dehydroxylation. In smectites, the Al-pyrophyllite-like arrangement is preferentially rehydroxylated, but at higher degrees of rehydroxylation, OH groups in other octahedral arrangements are also rebuilt. Among the temperatures tested, smectites and illite rehydroxylate most intensively during isothermal heating at 400 °C (H 2 O content = 0.28 mol H 2 O/m 3 ), which is the onset temperature of dehydroxylation of their rehydroxylated forms. The smectites capability to rehydroxylate comes from an interplay of the counterions' size, the tetrahedral substitution in the 2:1 layers, and the negative charge generated in the former-octahedral sheet that controls the ability of cations to move from the surface of the tetrahedral sheet or from the octahedral sheet back to the interlayer. The conventional (time) 1/4 power law and the logarithmic kinetic models of rehydroxylation produced good fits of the experimental data. Calculation of activation energies (E a ) for the rehydroxylation reaction produced values from 30 kJ/mol to 173 kJ/mol, highly dependent on the interlayer cation, 2:1 layer structure, and the experimental protocol. The kinetic models developed for rehydroxylation at temperature near ambient remain valid for the experiments up to 400 °C. At 500 °C the mechanism of rehydroxylation is influenced by other factors. [ABSTRACT FROM AUTHOR]

A multi-cycle heating and cooling thermogravimetric (TG) method was used to study the kinetic behavior of partially dehydroxylated illite, aluminoceladonite, and dioctahedral smectite samples. The method consists of consecutive heating cycles separated by intervals of cooling to room temperature, with the maximum cycle temperatures (MCTs) set incrementally higher in each consecutive cycle. In the studied samples, dehydroxylation of each portion of the initial OH groups follows the kinetics of a homogeneous zero-order reaction in each heating cycle. The activation energies (Ea) were calculated in terms of this model for separate heating cycles of each sample with regression coefficients R² ≥ 0.999. A zero-order reaction determined at each heating cycle indicates that at each stage of partial dehydroxylation, there is no formation of an intermediate phase and the reaction is probably the direct transformation of the original layers into completely dehydroxylated layers. The Wyoming montmorillonite and illite consisting of cis-vacant (cv) layers had the highest Ea values (53-55 kcal/mol). In the samples consisting of trans-vacant (tv) layers and having almost the same octahedral cation composition the maximum Ea values varied from 45 to 30 kcal/mol and the Ea of each sample in this group are similar over a wide range of the DT. For the samples consisting of a mixture of cv and tv illite structures, a bimodal distribution of the Ea values exists with increasing MCT and DT. The maximum Ea values for dehydroxylation of the tv and cv illite structures are different. The activation energies from the tv aluminoceladonite and Otay tv montmorillonite samples have similar maximum Ea values (39.4 to 41.8 kcal/mol), but the variation in Ea with DT has a skewed bell-like distribution that is probably related to a heterogeneous octahedral cation composition of the 2:1 layers. The Ea values calculated for the mineral structures in this study are compared with those published and the main factors controlling the Ea variation at different stages of the partial dehydroxylation are discussed. [ABSTRACT FROM AUTHOR]

A multi-cycle heating and cooling thermogravimetric (TG) method was used to study the kinetic behavior of two partially dehydroxylated pyrophyllite samples. In the original state, the S076 sample contains only trans-vacant (tv) layers, whereas in sample S037 tv and cis-vacant (cv) layers are randomly interstratified. The method consists of consecutive heating cycles (rate 2 °C/min) separated by intervals of cooling to room temperature, with the maximum cycle temperature set (MCT) incrementally higher than in each previous cycle. The activation energy (Ea) values were calculated for the S037 and S076 samples for all cycles in terms of a homogeneous zero-order reaction with the regression coefficients r² ≥ 0.9997. The S076 sample had Ea values that varied from 40 to 42 kcal/mol within a partial dehydroxylation (DT) range from 19 to 63%. However, at DT values <19% and >63% the Ea values are slightly lower at 38-39 kcal/mol and drop below 30 kcal/mol at DT = 90%. A bimodal distribution of the S037 sample Ea values exists with increasing MCT and DT. In the first cycles of intense dehydroxylation from tv layers the Ea values increase sharply from 34 kcal/mol at DT = 7% to 45 kcal/mol within the MCT interval from 525 to 575 °C and then decrease to 36-38 kcal/mol at the cycles with MCT = 650-700 °C. In the second portion of cycles corresponding to the dehydroxylation of cv layers, the Ea values increase from 36-37 kcal/mol at MCT = 700 °C to 44-46 kcal/mol at MCT = 775 °C. These results show that both tv and cv layers require exactly the same energy for dehydroxylation and have similar variation of the Ea values with MCT and DT. The pyrophyllite particle size distribution is a major factor that is likely responsible for a broad interval of dehydroxylation temperature. This implies that the lower the temperature, the smaller the particles that can be dehydroxylated. Therefore, the activation energy values at the beginning of the reaction are lower than those at higher degrees of dehydroxylation because of the combination of a slow experimental heating rate and thin crystallites, which both contribute to lower temperatures at which the reaction starts. The decrease in activation energy observed at the end of the dehydroxylation reaction of sample S076, and of the tv layer portion of sample S037 is accompanied by an increase in the "induction" temperature interval during which an accumulation of additional thermal energy decreases the activation energy. The kinetic parameters determined for the samples in this study correspond to a homogeneous reaction and are used to predict a general pattern of the structural transformations of pyrophyllite at different stages during dehydroxylation. [ABSTRACT FROM AUTHOR]

Fired-clay ceramics gain mass with time as a result of rehydroxylation (RHX) of the dehydroxylated structures of clay minerals. The mass gain is usually described using the time to the quarter (TTTQ) model, which is fundamental to the archeometric dating technique called RHX dating. In RHX dating practice, experimental mass gain fits can be improved by adding a time-offset correction (t 0), apparently corresponding to changes in diffusional parameters. Using theoretical mass gain data generated by diffusional, geometric contraction, and reaction-order kinetic models, we showed that TTTQ model can successfully fit data produced by any deceleratory kinetic model, especially if the t 0 is applied. The quality of the fits and linearity of Arrhenius plots of theoretical data likely would cause misidentification of the reaction model in favor of TTTQ model in RHX dating practice. The model misidentification results in incorrect values of activation energy (E a) despite ideal mass gain fits and linearity of Arrhenius plots. In this paper we question the validity of t 0 and propose a calculation of correct E a from any RHX mass gain data using a model-independent approach. • Time to the quarter (TTTQ) model can describe kinetic data generated with other kinetic models. • Application of the time-offset correction to TTTQ model cannot be justified solely by the improvement of the fit. • Application of time-offset correction may result in erroneous activation energy (E a) values. • Model-free E a can be obtained if data with the same fractional mass gain at each temperature are used. [ABSTRACT FROM AUTHOR]

The selective self-assembly of small molecules in the interlayer of kaolinite is a fundamentally important and technologically relevant process, typically studied ex situ by X-ray diffraction (XRD). Near-infrared (NIR) spectroscopy is now introduced to provide a complementary local structural description of intercalation with improved control of experimental conditions. New NIR- and XRD-based proxies were developed and applied to the real-time monitoring of N -methylformamide (NMF) intercalation in two reference kaolinites differing in stacking order. The commonly employed XRD-based formalism was found to overestimate reaction progress. The bonding of NMF in the interlayer was independent of reaction progress and kaolinite type. Both NIR and XRD recorded identical sigmoidal kinetics. Isothermal NIR monitoring (25–80 °C) yielded time-temperature superimposable sigmoidals with an apparent activation energy of ~60 kJ/mol, common to both samples. All NIR and XRD data series could be described as linear combinations of empty and fully intercalated kaolinite. The filling of the interlayer was too fast to be observed. The sigmoidal curves were instead modeled as the log-normally distributed response of an ensemble of intercalating entities, presumably crystallites. The multiplicative standard deviation of the distribution, which determines its steepness, is a sample-specific, temperature-independent property of kaolinite. [Display omitted] • First real-time in situ combined NIR and XRD study of N -methylformamide intercalation in kaolinite. • Same sigmoidal kinetics recorded by both techniques. Steepness depends on kaolinite. • Two kaolinites differing in structure and texture display the same activation energy. • Intercalation events are instantaneous on an interlayer-crystallite basis. • Sigmoidals represent the log-normal distribution of individual intercalation times. [ABSTRACT FROM AUTHOR]