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Thermally induced dehydroxylation and oxidative dehydrogenation drive the thermal decomposition of all Fe2+-containing phyllosilicates. Whereas the former produces H2O gas, the latter results in H2 evolution. Six chlorites representing the Mg-Fe2+ series from clinochlore to chamosite and biotite (as an analog of the 2:1 layer in chlorite) were investigated using thermogravimetry coupled to quadrupole mass spectrometry (TG-MS). A fast-ramp heating protocol was applied to identify if and how hydrogen gas was released from the crystal structure and whether it was quantitatively related to structural Fe2+ content. Dehydroxylation and oxidative dehydrogenation were tested under inert and oxidizing conditions. H2 liberation confirmed the H2 gas production by oxidative dehydrogenation, as shown by an evolution of the m/z = 2 signal for chamosites, Fe-rich clinochlores, and biotite heated under nitrogen gas atmosphere. Along with H2 evolution, H2O (m/z = 18) was released, suggesting that dehydroxylation is a trigger for dehydrogenation. The higher the Fe2+ content in the studied chlorites, the more intense the H2 evolution, thus the higher the H2/H2O ratios. The products of ramp-heating to 1000 °C resulted in varying amounts of newly formed Fe3+ (from 7 to 22%), however, biotite that converted into oxybiotite underwent almost complete oxidation, indicating a stronger tendency of 2:1 layer to dehydrogenation. The observed concurrent, but independent mechanisms of H2 and H2O evolution produced a feasible model of the thermal decomposition of chlorites. Despite O2 availability under oxidizing condition, the Fe2+ oxidation was not driven by attaching oxygen anions to the phyllosilicate structure, but also by dehydrogenation. Hydrogen was not detected using MS for any tested sample heated in synthetic air because any H2 if released was instantaneously combined with external O2, which resulted in an excess H2O MS signal not matched by mass loss on the TG profiles of chamosite and biotite. Without coupling of the evolved gas analysis with TG, the excess H2O produced by dehydrogenation in the O2-bearing carrier gas would result in misleading interpretations of dehydroxylation. Methodological and geological implications of the TG-MS experiments are discussed. The oxidation of Fe2+ in all Fe2+-containing phyllosilicates proceeds with simultaneous H2 gas release that is not dependent on oxygen fugacity nor temperature during the mineral formation. Therefore, the correlation between Fe3+/Fe2+ and remaining hydrogen in the structure must be considered during modeling the conditions that involve chlorite as geothermobarometer. H2 release during heating is proposed as an indicator of oxidative dehydrogenation of Fe2+-bearing minerals on Mars. [ABSTRACT FROM AUTHOR]

The primary aim of this study was the accurate determination of unit-cell parameters and description of disorder in chlorites with semi-random stacking using common X-ray diffraction (XRD) data for bulk powder samples. In the case of ordered chlorite structures, comprehensive crystallographic information can be obtained based on powder XRD data. Problems arise for samples with semi-random stacking, where due to strong broadening of hkl peaks with k ≠ 3n, the determination of unit-cell parameters is demanding. In this study a complete set of information about the stacking sequences in chlorite structures was determined based on XRD pattern simulation, which included determining a fraction of layers shifted by ±1/3b, interstratification with different polytypes and 2:1 layer rotations. A carefully selected series of pure Mg-Fe tri-trioctahedral chlorites with iron content in the range from 0.1 to 3.9 atoms per half formula unit cell was used in the study. In addition, powder XRD patterns were carefully investigated for the broadening of the odd-number basal reflections to determine interstratification of 14 and 7 Å layers. These type of interstratifications were finally not found in any of the samples. This result was also confirmed by the XRD pattern simulations, assuming interstratification with R0 ordering. Based on h0l XRD reflections, all the studied chlorites were found to be the IIbb polytype with a monoclinic-shaped unit cell (β ≈ 97°). For three samples, the hkl reflections with k ≠ 3n were partially resolvable; therefore, a conventional indexing procedure was applied. Two of the chlorites were found to have a monoclinic cell (with α, γ = 90°). Nevertheless, among all the samples, the more general triclinic (pseudomonoclinic) crystal system with symmetry C1 was assumed, to calculate unit-cell parameters using Le Bail fitting. A detailed study of semi-random stacking sequences shows that simple consideration of the proportion of IIb-2 and IIb-4/6 polytypes, assuming equal content of IIb-4 and IIb-6, is not sufficient to fully model the stacking structure in chlorites. Several, more general, possible models were therefore considered. In the first approach, a parameter describing a shift into one of the ±1/3b directions (thus, the proportion of IIb-4 and IIb-6 polytypes) was refined. In the second approach, for samples with slightly distinguishable hkl reflections with k ≠ 3n, some kind of segregation of individual polytypes (IIb-2/4/6) was considered. In the third approach, a model with rotations of 2:1 layers about 0°, 120°, 240° was shown to have the lowest number of parameters to be optimized and therefore give the most reliable fits. In all of the studied samples, interstratification of different polytypes was revealed with the fraction of polytypes being different than IIbb ranging from 5 to 19%, as confirmed by fitting of h0l XRD reflections. [ABSTRACT FROM AUTHOR]

In addition to dehydroxylation, thermal decomposition of Fe(II)-bearing chlorites also involves dehydrogenation. Dehydrogenation does not require the presence of oxygen and readily occurs in an inert gas atmosphere via electron transfer between the OH group and octahedral Fe(II). The reaction results in releasing one hydrogen atom that forms an H2 gas upon diffusing out of the crystallite, and leaves one structural Fe(II) oxidized, to keep the charge balance. Dehydrogenation removes structural hydrogen reducing the amount of OH groups available for dehydroxylation thus H2O released during heating. In the present study, the dehydrogenation was tracked thermogravimetrically (TG) for pure chlorites. Clinochlore, Fe-clinochlore, and Mg-chamosite were investigated under various isothermal and ramp-heating conditions under pure nitrogen flow. Thermally altered Mg-chamosite was analyzed ex-situ using Mössbauer spectroscopy, X‑ray diffraction (XRD), and infrared spectroscopy. Dehydrogenation and dehydroxylation were found to occur simultaneously, but by independent mechanisms during the heating of all Fe(II)-containing chlorites. The extent of these reactions was tracked using a combination of total mass loss and degree of Fe(II) oxidation. The dehydrogenation/dehydroxylation ratio varied with heating conditions. The slower the ramp-heating rate, thus the longer time at elevated temperatures before dehydroxylation starts, the greater the dehydrogenation that precedes dehydroxylation. Each studied chlorite had its unique range of isothermal temperatures where dehydrogenation can be enhanced with only minor dehydroxylation. For Mg-chamosite, a narrow range of isothermal temperatures, 390–410 °C, caused—after 48 h of heating—the oxidation of almost 70% of Fe(II), with respect to a maximum ~20% of oxidized Fe(II) after dehydroxylation-dominated ramp heating. Any lower or higher isothermal temperatures than the optimum resulted in a lower amount of Fe(III) and greater total mass losses. Enhanced dehydrogenation led to the formation of a discrete population of a ferric (oxy) chamosite phase, observed in XRD patterns with a d-space of 13.91 Å, coexisting with 14.17 Å of the original chamosite. The dehydroxylation and dehydrogenation of chamosite at 450 °C resulted in the broadening of 00l XRD reflections interpreted as a mixed-layer phase that consisted of original, dehydroxylated, and dehydrogenated layers. Each particular heating protocol enhancing either dehydroxylation or dehydrogenation resulted in different compositions of the product formed after chlorite structure breakdown at 1000 °C. Even with ramp heating, dehydrogenation can occur, especially with Fe(II)-rich chlorites, decreasing the total mass loss. The procedure for the determination of "structural water" content and "loss of ignition" and "total mass loss" commonly measured in rocks and minerals by thermogravimetric methods can be questioned for Fe(II)-bearing chlorites and hence in the case of all Fe(II)-containing phyllosilicates. In geological conditions, if dehydrogenation occurs prior to chlorite dehydroxylation, the quantity of "structural water" transported within chlorites to the metamorphic environment in subduction zones can be reduced even by 50%. If occurring in natural conditions, Fe(II)-oxidizing dehydrogenation reaction questions the applicability of chlorite in geothermometry. Increased Fe(III)/total Fe ratio results in the miscalculation of chlorite formation temperature as much as by hundreds of degrees Celsius. The presence of Fe(III) as a result of dehydrogenation should be considered for all Fe(II)-bearing phyllosilicates. [ABSTRACT FROM AUTHOR]

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]

The phase composition of partially dehydroxylated specimens of two Clay Mineral Society standards, named KGa-1 and KGa-2 samples, were studied by powder X-ray diffraction patterns (XRD), infrared (IR) spectroscopy, and thermogravimetric (TG) data. Each specimen was preheated for 6 h in isothermal conditions, and the heating temperatures were selected to cover the entire range of dehydroxylation for both kaolinites (380-550 °C for KGa-2 and 400-600 °C for KGa-1). Products preheated at 550 and 600 °C are named metakaolinite, as their XRD patterns showed no traces of kaolinite reflections. The contribution of metakaolinite to the experimental XRD pattern and IR spectrum of each preheated specimen (including 3550-3750 and 500-1300 cm-1 ranges) was determined by the simulation of the experimental pattern with a sum of the XRD patterns or IR spectra corresponding to the untreated sample and metakaolinite. This procedure showed that the contents of metakaolinite determined independently by diffraction and spectroscopic methods for each of the preheated specimen have almost identical values. Although the modeled and experimental XRD and IR spectra generally match, they contain particular ranges where the intensity differences are particularly significant. The misfits suggest the presence of a third phase forming in partially dehydroxylated kaolinite. To reveal the diffraction and spectroscopic features of the kaolinite and possible intermediate phase (or phases), the contribution of metakaolinite was subtracted from the experimental XRD pattern and diffuse reflectance infrared Fourier transform (DRIFT) spectrum of each preheated specimen. Analysis of the metakaolinite-free XRD patterns and DRIFT spectra supports the idea that each preheated specimen, along with metakaolinite and initial kaolinite, contains a kaolinite-like component termed the intermediate phase. To determine the contents and specific diffraction features of the intermediate phases, the contribution of initial, untreated kaolinite was subtracted from the metakaolinite-free XRD patterns for each preheated specimen. In both samples, the content of the intermediate phase, Cinter, decreases with the growth of metakaolinite from 23-25% at the beginning of dehydroxylation to 2% when the XRD-determined content of metakaolinite is 98%. The analysis of the diffraction features of the intermediate phases formed during partial dehydroxylation of the KGa-1 and KGa-2 samples indicates that they represent a new type of defective kaolinite-like structure, in which, along with the one-dimensional periodicity along the c* axis and the layer displacement vectors typical for natural kaolinite samples, the octahedral sheets of the individual layers of the structure are partially dehydroxylated. [ABSTRACT FROM AUTHOR]

Surface characteristics of sedimentary opal-A and -CT were investigated for a large collection of samples from the Monterey Formation, California, based on the bulk rock mineral and chemical analysis, cation exchange capacity (CEC), and water content. Two approaches were used: (1) modeling bulk CEC and adsorbed water content for the entire data set using the contents of opal and clay minerals measured by XRD, and (2) correcting the chemical composition and CEC of the most pure opal samples for the mineral impurities quantified by XRD. Modeling indicates that the bulk rock data can be best explained by mixing an illite-smectite (CEC = 59 meq/100 g, 7-8% H2O), consistent with the XRD characteristics of the clay fraction, with opal-A (8 meq/100 g, 3.4% H2O), and opal-CT (13 meq/100 g, 3.7%H2O). Correcting the chemical composition of the most pure opal samples leaves a large excess of cations (Al, Fe, Na, K, Ca, and Mg). Iron is suspect to form traces of separate (oxy-)hydroxide phases, not detected by XRD, while Al for Si substitution in the opal structure produced local negative charge, which was compensated by Na, K, Ca, and Mg exchange cations. A perfect balance of positive and negative charges is observed if the clay admixture in pure opals has the composition of montmorillonite. The concentration of heterogeneous impurities in silica network in opal leads to smectite formation on or within the diatom frustules. These dispersed smectite particles, perhaps monolayers, can be missed during the bulk rock mineral quantification. The recalculated CEC of the opal, assuming the occurrence of dispersed smectite particles, varies from 3 to 11 meq/100 g, which is slightly less than that evaluated by modeling all the rock samples in the set, and corresponds to ~10-50% of the total opal charge quantified by the degree of Al for Si substitution. The remaining charge of the opal structure represents non-exchangeable cations. As opposed to smectite, opal CEC may depend on the size of cation used for the CEC measurement. For opals in the Monterey Formation the content of water removable at 200 °C can be modeled as a sum of a constant value and a variable value dependent on CEC; the latter component is similar to the H2O-CEC relationship that is typical for smectite. The combined system of a constant H2O + variable H2O in opal can potentially be applied for mineral modeling programs in wireline log formation evaluation in diatomaceous hydrocarbon reservoirs. [ABSTRACT FROM AUTHOR]

The multi-cycle heating and cooling thermogravimetric (TG) method was used to study the kinetic behavior of three kaolinite samples: defect-free Keokuk kaolinite, KGa-2 with a very low degree of structural order, and KGa-1 having intermediate structural order. In each cycle, the maximum cycle temperature (MCT) was set to 25 °C higher than the preceding cycle. The TG patterns consist of a set of subsequent DTG maxima representing the portions of OH groups that did not dehydroxylate in previous cycles. Each stage of partial dehydroxylation consists of two kinetic mechanisms and for each of them the experimental dα/dt values that characterize the reaction rate of the dehydroxylated fraction, α, within a period of the reaction time, t, were computed. One mechanism corresponds to a zero-order reaction that occurs in each cycle and indicates that the reaction is homogeneous and each non-dehydoxylated layer is transformed into metakaolinite layer without formation of intermediate derivatives. For this step of the cycles activation energy, Ea, was calculated from the linear relationship between ln(dα/ dt) and reciprocal temperature, T; for KGa-2 kaolinite, the Ea varies from 32.0 to 38.1 kcal/mol; in KGa-1, Ea varies from 37.1 to 40.4 kcal/mol, whereas in Keokuk, Ea varies from 42.7 to 47.5 kcal/ mol. The particular variation of the Ea is discussed in terms of structural and morphological features of the samples. The kinetic mechanism of the second step of reaction corresponds to the temperature range higher than the first step of the same heating cycle. The second step starts from the point where α = αP that was found to vary between 0.25 and 0.45. The acceleration of the reaction rate of dehydroxylation within this interval decreases with increasing α and T, and the mechanism observed for each of the studied samples is independent of its stacking order, average particle size, and particle size distribution. The f(α) is a function of the reaction mechanism in the second step and has the form f(α) = (1 - α)n/(1 - n) where n is an empirical parameter and its value was found from <0.01 to 0.06-0.08 among cycles and samples. The value of n controls the reaction rate slowing or the deviation from the zero-order reaction and increases with increasing metakaolinite content. Using parameters n, α, and T determined for the second step, Ea values were calculated for the second step of reaction in each heating cycle. For the Keokuk kaolinite, Ea value varies from 31.6 to 37.5 kcal/mol, in KGa-1 Ea is 27.0-35.6 kcal/ mol, and in KGa-2 the Ea value varies from 26.3 to 34.9 kcal/mol. A structural model explaining the acceleration rate slowing is discussed. [ABSTRACT FROM AUTHOR]

The <1 µm fraction of a trans-vacant 1M illite (RM30) was studied by conventional and synchrotron X-ray diffraction (XRD) techniques, combined with thermogravimetric (TG, DTG) methods to investigate the structural transformation of illite at different temperatures and degrees of dehydroxylation (DT). The oriented specimens preheated at 300 and 680 °C correspond to the non-dehydroxylated (DT = 0) and completely dehydroxylated (DT = 100%) 1 M illite structures. Deviation of the basal reflection positions from rationality, expressed by the coefficient of variation of d(00l) values, progressively increase from 0.05 at DT = 4%, to 0.14 at DT = 51%, and then decrease to 0.06 at DT = 95%. Similarly, for each 00l reflection the full width at half-maximum (FWHM) shows a bell-like evolution with increasing preheating temperature. Both of these features are characteristic for mixed-layered structures. The experimental profiles of 00l reflections from the oriented partially dehydroxylated specimens perfectly matched the profiles from XRD pattern simulations calculated in terms of a mixed-layered structure in which the non-dehydroxylated (ND) and completely dehydroxylated (CD) illite layers are interstratified with a strong tendency to segregation. The content of the CD layers in the modeled mixed-layered structures of the preheated specimens show a significant linear correlation with the corresponding DT values (R² = 0.99). In random powder XRD patterns collected with synchrotron radiation, the preheated specimens show a distinctive trend in the unit-cell parameters. However, the accuracy in determination of the unit-cell parameters at first decreases up to DT = 61% and then increases with a further increase in DT. The evolution of FWHM values of individual hkl reflections is also similar to that observed for each 00l peak from oriented sample preparations. The unexpected evolution of the unit-cell parameters during progressive dehydroxylation is explained by the interstratification of ND and CD layers in illite. The formation of the mixed-layered structures during Al-rich 1M illite dehydroxylation is in agreement with the prediction from the kinetic model of partially dehydroxylated dioctahedral 2:1 clay structures where dehydroxylation of each portion of the initial OH groups corresponds to a zero-order reaction that is independent of the structural and chemical composition. The reaction is homogeneous and during partial dehydroxylation of the illite structure, ND layers transform into the CD layers without formation of an intermediate phase. A layer-by-layer dehydroxylation mechanism is suggested for thermally induced illite structural transformation. [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]

Rehydroxylation of the previously dehydroxylated dioctahedral 2:1 layer clay mineral occurs preferentially in specific sites within the former octahedral sheet. The rehydroxylation of dehydroxylated A1-rich and A1,Mg-rich 2:1 layers occurs as trans-vacant (tv) structural arrangements, regardless of whether the initial structure was tv or cis-vacant (cv). In nontronite (Fe-rich 2:1 layer clay), the dehydroxylate pseudo-cv structure is probably directly reconstructed into the rehydroxylated cv structure without migration of octahedral cations. Rehydroxylation occurs preferentially in the R3+-Or-R3+ former octahedral structural arrangements (Or = residual oxygen) over R2+-Or-R (R = R3+ or R2+ = A13+, Fe3+ or Mg2+, Fe2+). In the case of the R2+ octahedral substitution, the interlayer cation is attracted to the electrostatically undersaturated residual oxygen of the R2+-Or-R arrangement, which blocks the ability of water molecules to pass through the ditrigonal cavity and rehydroxylate the previously dehydroxylated local arrangement. The pyrophyllite-like type of octahedral R3+-Or-R3+ arrangements, formed due to the lack of tetrahedral substitution and resulting in the absence of interlayer cations, is thus favored for rehydroxylation over the mica-like R3+-Or-R3+ arrangements where Al occurs in the tetrahedral sheet. The valence of the interlayer cation and the charge density of the 2:1 layer clay mineral, which controls the interlayer cation content, also affect the degree of rehydroxylation.Dehydroxylated 2:1 layer minerals with a high-rehydroxylation potential, including beidellite and illite, use all the adsorbed water molecules that persist above 200 °C for rehydroxylation; the water vapor from the ambient environment also becomes a source of H2O molecules for rehydroxylation. The high demand for water molecules to use for rehydroxyltion results in a noticeable gain of mass in the temperature interval between 200 and 350 °C even during heating. [ABSTRACT FROM AUTHOR]

Thermal analysis experiments in the environment of an extremely low water vapor concentration provide insight into the first steps of the rehydration mechanism in smectite when completely dehydrated and the interlayer region is collapsed. The relative structural and compositional controls on dehydration and rehydration reactions are compared from a well-characterized suite of samples that vary with respect to chemical composition, octahedral and tetrahedral substitution, octahedral cation site vacancy, and degree of dehydroxylation. Techniques including multi-cycle heating-cooling thermogravimetric analysis and nitrogen gas adsorption on various smectite samples preheated at different temperatures followed by rehydration at ambient conditions were used to characterize the interaction of water molecules with completely dehydrated montmorillonite, beidellite, and nontronite smectite types. Beidellite with high-Al3+ tetrahedral substitution results in electrostatically undersaturated basal oxygen atoms that exert strong repulsion between the tetrahedral sheets of adjacent 2:1 layers. The interlayer region of dehydrated or dehydroxylated beidellite is capable of being spontaneously rehydrated even in low water vapor environments. In completely dehydrated montmorillonite and nontronite, the external surface area of the crystallites is a primary control on water adsorption at low humidity when the molecules form a shell around the exchangeable cations present on external surfaces. The potential of montmorillonite and nontronite to reopen a collapsed interlayer is significantly lower than beidellite because of their crystal-chemical features that result in 2:1 layer and interlayer cation attraction. With increasing water vapor partial pressure, the hydration potential of interlayer cations provokes a reopening of the interlayer. In a dehydroxylated nontronite, the undersaturated residual oxygen atom strongly bonds the interlayer cation within the ditrigonal ring of the tetrahedral sheet, resulting in a permanent interlayer collapse. The specific surface area calculated from a conventional N2 gas adsorption measurement using the BET model represents the external surface area of a dehydrated smectite crystallite and can be converted into the mean crystallite thickness. The mean crystallite thickness of a completely dehydrated smectite increases with an increase in preheating temperature. [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]

Two pyrophyllite samples, one S037, from the Coromandel region of New Zealand and the other, S076, from Berosovska, Ural, Russia, were studied by thermogravimetric (TG and DTG), infrared (IR), and X-ray diffraction (XRD) methods to investigate structural transformations of these samples at different stages of their partial dehydroxylation. The samples were heated at different temperatures during 45 min and the degree of dehydroxylation was estimated as a ratio of mass loss of each particular heated specimen to the total mass loss of the sample during total dehydroxylation. Sample S076 consists only of trans-vacant (tv) layers because its DTG curve and IR spectrum contain a single dehydroxylation maximum at Tmax = 723 °C and an OH stretching band 3675 cm-1, respectively. In sample S037 tv and cis-vacant (cv) layers are interstratified at random and its DTG and IR spectrum contain, respectively, two dehydroxylation maxima at 595 and 760 °C and two stretching bands at 3675 and 3668 cm-1. The positions and intensities of the reflections in the experimental powder XRD patterns of sample S037, as well as the refined parameters of the unit cell, almost coincide with those determined for 1A pyrophyllite. However, the XRD patterns contain an "additional" peak with d = 4.454(6) Å, which is absent in normal 1A pyrophyllite. This peak can be considered as an indicator of a structure in which tv and cv layers are interstratified. The XRD patterns from the oriented specimens of the studied samples heated above 500 °C show splitting of the basal reflections. Simulation of the XRD patterns from oriented specimens of the S076 sample show that the phase composition of the specimens is the same independent of the heating temperature and represents a physical mixture of a non-treated original pyrophyllite and an almost completely dehydroxylated phase, which contains 5% of non-dehydroxylated layers. The higher the temperature, the higher the content of the dehydroxylated-rich phase in the heated sample. Simulation of the XRD patterns from the heated S037 samples show that the phase composition of each specimen is a physical mixture of low-and high-dehydroxylation phases, referred to as LD and HD phases, respectively. Each of these phases is a mixed-layered structure in which the original non-dehydroxylated (ND) and completely dehydroxylated (CD) layers are interstratified at random. The main difference between these phases is that in the LD phase the content of ND layers prevail, whereas in the HD phase CD layers dominate. The increase in temperature and degree of dehydroxylation change the relative content of the LD and HD phases as well as the relative amount of ND and CD layers in these phases. Interstratification of ND and CD layers in the LD and HD phases is not consistent with the commonly accepted model according to which during dehydroxylation of pyrophyllite and related layer silicates, non-dehydroxylated and dehydroxylated domains can coexist within one layer. As a result of this inconsistency, the model explaining the wide temperature interval of pyrophyllite dehydroxylation needs to be reconsidered. A similar inconsistency arises with the model predicting the formation of several intermediate semi-dehydroxylated structures during pyrophyllite dehydroxylation because evidence for such phases was not observed. A new model of pyrophyllite dehydroxylation is presented, which is consistent with the hypothesis, that the reaction is homogeneous and spontaneous nucleation and growth of completely dehydroxylated layers takes place during pyrophyllite dehydroxylation. In terms of this model, the large temperature interval of pyrophyllite dehydroxylation is related to particle size distribution as well as to structural disorder. A mechanism for the formation of the mixed-layered structures of the LD and HD phases is proposed. [ABSTRACT FROM AUTHOR]