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Clays and Clay Minerals; February 2008; v. 56; no. 1; p. 10-22; DOI: 10.1346/CCMN.2008.0560102
© 2008 Clay Minerals Society
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CATION MASS-VALENCE SUM (CM-VS) APPROACH TO ASSIGNING OH-BENDING BANDS IN DIOCTAHEDRAL SMECTITES

Will P. Gates

Department of Civil Engineering, Monash University, Clayton VIC3800, Australia

* E-mail address of corresponding author: gateswp{at}smectech.com.au

The assignments of OH-bending bands in the infrared (IR) spectra of dioctahedral smectites (montmorillonites, ferruginous smectites, and nontronites) have been revisited using a cation mass-valence sum (CM-VS) approach to quantify octahedral cation occupancy. The CM-VS approach enabled prediction of OH-bending band positions (in wavenumbers) related to OH-sharing octahedral cation pairs that had valence sums of 4 and 5, and cation masses associated with Fe(II) and Mg. Application of rules for the relationship enabled determination of the location of OH-sharing octahedral cation pairs containing Mg and Fe(II) for which previous assignments have been considered controversial, e.g. Fe(III)Mg-OH and MgMg-OH, or for which assignments have been missing due to lack of spectroscopic evidence, e.g. AlFe(II)-OH, Fe(III)Fe(II)-OH, Fe(II)Mg-OH, and Fe(II)Fe(II)-OH. Examples of these bands from several natural ferruginous smectites and nontronites are discussed. Quantification of IR spectra was used to develop a better understanding of the octahedral cation occupancy of this important class of Fe(III)-enriched smectites. While Fe(II) contents may be somewhat overestimated by the IR technique, those for four of the six ferruginous smectites studied here agree well with data from Mössbauer spectroscopy.

Key Words: Cation Mass • Cation Pairs • Infrared Spectroscopy • Octahedral • Smectite Chemistry • Valence Sum






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