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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 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]