A comprehensive characterization of dioctahedral smectites
- chair: Wolters, F. / Lagaly, G. / Kahr, G. / Nueesch, R. / Emmerich, K. (2009)
- place: Clays and Clay Minerals, 57 (2009), 1, 115-133
- Date: 2009
Wolters, F. / Lagaly, G. / Kahr, G. / Nueesch, R. / Emmerich, K. (2009): „A comprehensive characterization of dioctahedral smectites“. In: Clays and Clay Minerals, 57 (2009), 1, 115-133
The term 'montmorillonite' encompasses a wide range of chemical compositions and structures. Comprehensive and reliable characterization is essential for unambiguous classification. Twenty eight purified, Na-exchanged smectites (< 0.2 mu m) were characterized by layer-charge measurement using the alkylammonium method, by cation exchange capacity (CEC) measurement with Cu-triethylenetetramine, by determination of the chemical composition using X-ray fluorescence analysis, by calculation of the structural formula following determination of the octahedral structure (trans-vacant vs. cis-vacant) by Simultaneous thermal analysis, and by X-ray diffraction analysis. Mossbauer spectroscopy was applied to determine the oxidation state and coordination of Fe and changes thereof during purification of the Source materials.
The charge derived from chemical composition was considerably greater (by up to 30%) than the measured layer charge. The independently measured layer charge was used to calculate the structural Formula. The measured CEC values, comprising the permanent charges and the pH-dependent edge charges, were consistent with measured layer charge but not with layer charge derived from the chemical composition. Therefore, the Structural formula of smectites should be calculated using the measured layer charge.
The dehydroxylation temperature, which conveys information about the structure of the octahedral sheet, was correlated to the amount of Mg and Fe(3+) and the location of charges. No relationship was found among the dehydroxylation temperature and the mean layer charge or the Mg content. In contrast, a clear relationship was observed between the Fe content and the dehydroxylation temperature. Montmorillonites with all Fe content < 0.3/f.u. are cis-vacant and those containing Fe(3+) > 0.3 mol/f.u. are trans-vacant, mostly with additional cis-vacancies. Tetrahedral substitution also appeared to be a function of the number of trans-vacancies.