Clays and Clay Minerals, Vol. 57, No. 1, 115–133, 2009.
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Authors
Wolters, Felicitas
Lagaly, Gerhard
Kahr, Guenter
Nueesch, Rolf
Emmerich, Katja
Issue Date
2009
Type
Article
Language
Keywords
Periodicals , Geology , CEC , Chemical Composition , Cis-vacant , Dehydroxylation , Fe content , Layer Charge , Montmorillonite , Octahedral Charge , Simultaneous Thermal Analysis , Structural Formula , Tetrahedral Charge , Trans-vacant
Alternative Title
A Comprehensive Characterization Of Dioctahedral Smectites
Abstract
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 mm) 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. Mo¨ ssbauer 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 permanentcharges and the pH-dependentedge charges, were consistentwi th measured layer charge butnotwit h 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 Fe3+ 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 an Fe content <0.3/f.u. are cis-vacant and those containing Fe3+ > 0.3mol/f.u. are trans-vacant, mostly with additional cis-vacancies. Tetrahedral substitution also appeared to be a function of the number of trans-vacancies. The parameters analyzed provide the basis for a new descriptive classification system.
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Citation
Clays and Clay Minerals, Vol. 57, No. 1, 115–133, 2009.
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