• journal_title：American Mineralogist
• Contributor：Victor A. Drits ; Arkadiusz Derkowski ; Douglas K. McCarty
• Publisher：Mineralogical Society of America
• Date：2012-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2012.3971
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：97
• issue：5-6
• firstpage：930
• section：Articles

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 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 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.