Abstract
The structures of octahedral sheets of dioctahedral phyllosilicates show cis-vacant (cv) and trans-vacant (tv) configurations due to the different distributions of the octahedral cations over cis- and trans-sites. On the basis of the different dehydroxylation temperatures, a thermal analysis is an effective method used to identify the cv and tv configurations of an octahedral sheet in dioctahedral smectites. The proportions of cv and tv configurations were determined by fitting the derivative thermogravimetry (DTG) curves. A wide range of cv and tv proportions were detected in the samples. The dehydroxylation temperatures of samples consisting of cv configuration are about 150 to 200 °C higher than those consisting of tv configurations. The samples were classified as tv varieties when octahedral Fe3+ > 0.46 mol/FU, and the pure tv dioctahedral smectites were found when Fe3+ > 1.8 mol/FU. A clear linear relationship was found between the content of octahedral Fe3+ and Al3+ and the proportion of cv and tv configurations. The substitution of Al3+ by Fe3+ in the octahedral sheets is the main factor for the formation of tv varieties. There was no relationship detected between the layer charge density, octahedral Mg2+ content, and the proportion of tv and cv. The present results indicate that the iron content has a significant effect on the structure of the octahedral sheet.
Highlights
Smectites are used widely in industrial and environmental applications due to their physical and chemical properties, for example, cation exchange capacity, swelling behavior, and adsorption capacity [1]
The present results indicate that the iron content has a significant effect on the structure of the octahedral sheet
The present results show that there is no relation between the proportion tv and cv configurations to the total layer charge density (Figure 12a)
Summary
Smectites are used widely in industrial and environmental applications due to their physical and chemical properties, for example, cation exchange capacity, swelling behavior, and adsorption capacity [1]. The properties of smectites are closely related to their structural features [2,3]. Smectites are typical 2:1 layers of clay minerals, composed of two tetrahedral silicate sheets (T). The octahedral sheet is sandwiched between the two tetrahedral sheets (TOT) (Figure 1). The dominant cation is Si4+ , but it can often be substituted by trivalent cation such as Al3+.
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