The effect of γ-irradiation on the structure and subsequent thermal decomposition of brucite

The effect of γ-irradiation on the structure, phase composition and kinetics of isothermal decomposition of natural textural brucite Mg(OH)₂ has been investigated by Mn²⁺ electron paramagnetic resonance (EPR), proton magnetic resonance (PMR), X-ray diffraction (XRD) and weight loss methods. Starting from a 10⁶-Gy dose, γ-irradiation (⁶⁰Co, 13.8 Gys^?1) is found to stimulate the formation of a new phase in the brucite structure, namely basic magnesium carbonate. The carbonate phase is assumed to form in brucite under γ-irradiation accordingly to the scheme \(\) (in the brucite structure). There is also a possibility that γ-irradiation forms particles with high reaction ability, CO^?₂ radicals and/or CO molecules, which can react with the brucite structure. Preliminary γ-irradiation (9.75 × 10⁷ Gy) slows down the subsequent isothermal dehydroxylation of natural brucite, which can be explained by the formation of the new carbonate phase in the Mg(OH)₂ structure. Dehydroxylation kinetics of both original and irradiated samples are interpreted by a two-stage nucleation model at 623, 648, 673, 698 and 723 K. The reaction rate is limited by the first nucleation stage rate (proton transition from an OH group near the reaction interface on a freed vacant orbital of an oxygen ion of the OH group in the nearest elementary cell, i.e., formation of a structured water molecule). The second-stage rate (water molecule removal from the structure and proton migration from the residual hydroxyl inside the structure) is about 1 order of magnitude higher. The activation energy of the limiting stage is 194 and 163 kJ mol^?1 for the original and irradiated samples, respectively. Non-linear Arrhenius dependencies for the first-stage rate constants are related to the potential barrier reduction due to thermal fluctuations of large structural zones (with radii of about 20 and 81 Å in original and irradiated samples, respectively), whose ions form this barrier.     

Linear relationship between thermo-dehydroxylation and induced-strain by mechanical processing in vacuum: The case of industrial kaolinite, talc and montmorillonite

The thermal behaviour and the structural properties of three important industrial phyllosilicates, such as kaolinite, talc and Ca-rich montmorillonite, have been examined after mechanical treatment in a specifically built planetary ball mill working in vacuum (P = 0.13 Pa) at room temperature (25 °C). It is found that, on increasing the grinding time, the temperature of the dehydroxylation reaction decreases linearly as a result of a decrease of the crystallite size and structural order. To be noted that the mechanical treatment in our milling conditions did not induce significant amorphization. The temperature at which there is the maximum of dehydroxylation and the weight losses of the intralayer OH are linearly related to the increase of the FWHM of the 001 basal plane. These results are useful for predicting the thermal behaviour of layer silicates to be subjected to mechanical processing in industrial application.