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Clays and Clay Minerals; April 2007; v. 55; no. 2; p. 220-232; DOI: 10.1346/CCMN.2007.0550211
© 2007 Clay Minerals Society
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AB INITIO CALCULATIONS OF RELATIVE STABILITIES OF DIFFERENT STRUCTURAL ARRANGEMENTS IN DIOCTAHEDRAL PHYLLOSILICATES

Daniel Tunega1,2, Bernard A. Goodman2,3,*, Georg Haberhauer2, Thomas G. Reichenauer2, Martin H. Gerzabek4 and Hans Lischka1

1 Institute for Theoretical Chemistry, University of Vienna, Währingerstrasse 17, A-1090 Vienna, Austria
2 Department of Environmental Research, Austrian Research Centers GmbH – ARC, A-2444 Seibersdorf, Austria
3 Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
4 Institute of Soil Research, University of Natural Resources and Applied Life Sciences, Peter-Jordan-Strasse 82b, A-1190 Vienna, Austria

* E-mail address of corresponding author: bernard_a_goodman{at}yahoo.com

An ab initio theoretical approach has been used to calculate optimized geometries and the relative energies of various compositional arrangements in structures of dioctahedral smectites based on models consisting of two unit-cells. These calculations indicate that the energy differences between structures having vacancies in sites with cis- or trans-OH coordination are small and that their relative energies vary with the chemical nature of the substitutions. For example, a cis-OH coordination for the vacancy was the most stable when the interlayer charge originated from substitution of Al for Si in the tetrahedral sheet, whereas the trans-coordination was the more stable for most cases of substitution in the octahedral sheet, an exception being Fe(II) for Al where the cis-OH coordination was favored. It seems likely, therefore, that long-range structural disorder will be a common phenomenon in natural phyllosilicate specimens.

Key Words: Ab Initio • Dioctahedral • Phyllosilicates • Smectite • Cis- and Trans-coordination






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