Thermal reactions of smectites—Relation of dehydroxylation temperature to octahedral structure

Montmorillonites are structural and chemical heterogenic with the octahedral sheet being the most distinguished structural feature. The occupancy of cis- and trans-positions of the dioctahedral smectites was investigated by simultaneous thermal analysis. Cis-Vacant (cv) phyllosilicates dehydroxylate between 650 and 700 °C and trans-vacant (tv) varieties between 500 and 550 °C. Calculation of peak areas from the peak fitted evolved water curves were used to estimate the ratio of trans- to cis-vacancies. Limits for the proportion of cis- and trans-vacancies were defined resulting in four classes (cv; cv/tv; tv/cv; tv) for montmorillonites with respect to the distribution of the octahedral cations. Most common are mixed cv/tv montmorillonites with dominating amounts of cv 2:1 layers (74–50%). Pure cv montmorillonites occur in nature. Some montmorillonites classified as cv contain up to 25% tv layers. Pure tv montmorillonites were not found. All tv montmorillonites contain cv 2:1 layers (up to 24%).