Kinetics of partial dehydroxylation in dioctahedral 2:1 layer clay minerals

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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 ( E a ) were calculated in terms of this model for separate heating cycles of each sample with regression coefficients R 2 ≥ 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 E a values (53–55 kcal/mol). In the samples consisting of trans -vacant ( tv ) layers and having almost the same octahedral cation composition the maximum E a values varied from 45 to 30 kcal/mol and the E a of each sample in this group are similar over a wide range of the D T . For the samples consisting of a mixture of cv and tv illite structures, a bimodal distribution of the E a values exists with increasing MCT and D T . The maximum E a 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 E a values (39.4 to 41.8 kcal/mol), but the variation in E a with D T has a skewed bell-like distribution that is probably related to a heterogeneous octahedral cation composition of the 2:1 layers. The E a values calculated for the mineral structures in this study are compared with those published and the main factors controlling the E a variation at different stages of the partial dehydroxylation are discussed.