PNNL-13501 - Pacific Northwest National Laboratory

undoubtedly not be the case for fixed unit cell conditions
where the forward exchange reaction would be large and
unfavorable. The small exchange energy overall suggests
that the muscovite structure is essentially indifferent with
respect to interlayer cation when the interlayer spacing is
free to change. Between the various models, the results
indicate that the exchange energy systematically follows
changes in this parameter, which in turn depends on the
layer charge. Where the interlayer site distortion is small,
the forward exchange reaction was found to be favorable,
and more so with increasing layer charge. But a large
deleterious effect on the exchange energy arises from
slight distortion of the interlayer site structure.
Overall, the calculations suggest that the incorporation of
cesium into muscovite interlayers is energetically feasible
and nearly isoenergetic when the expansion of the
interlayers is not inhibited. The driving force for the
overall exchange reaction is very small and therefore the
forward exchange behavior is likely to be dominated by
diffusion kinetics. Likewise, any long-lived fixation of
cesium in the interlayers of muscovite is likely due to
kinetic barriers, rather than thermodynamics.
Future work in this area should incorporate more
exchangeable cations and a larger variety of substitutions
in the 2:1 layers. Kinetic aspects of the exchange process
must also be addressed if the models are ever to be used
to simulate experimentally observed behavior.
Publications and Presentations
Rosso KM and JR Rustad. “The structures and energies
of AlOOH and FeOOH polymorphs from plane wave
pseudopotential calculations.” American Mineralogist (in
press).
Rosso KM, JR Rustad, and EJ Bylaska. “The Cs/K
exchange in muscovite interlayers: An ab initio
treatment.” Clays and Clay Minerals (submitted).
Rustad JR and KM Rosso. “Equation of state of diaspore
(α-AlOOH) calculated from plane wave pseudopotential
methods.” Geophysical Review Letters (submitted).
Rosso KM, EJ Bylaska, and JR Rustad. December 1999.
“The energetics of cation exchange in muscovite mica.”
Presented at the National Meeting of the American
Geophysical Union, San Francisco, California.
Earth System Science 247