Potential protonation sites in the Al2SiO5 polymorphs based on polarized FTIR
spectroscopy and bond critical point properties

Jason B. Burt
Department of Geosciences, University of Arizona, Tucson, AZ  85721 USA
Department of Geosciences, Virginia Tech, Blacksburg, VA 24061 USA

Nancy L. Ross, Gerry V. Gibbs
Departments of Geosciences,
Materials Science and Engineering, and Mathematics, Virginia Tech, Blacksburg, VA 24061 USA

George R. Rossman
Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena, CA 91125-2500, USA

Kevin M. Rosso
W.R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory,
P.O. Box 999, KB-96, Richland, Washington 99352, USA


Potential protonation sites for, kyanite, sillimanite, and andalusite, located in a mapping of the (3,-3) critical points displayed by their L(r) = −∇2r(r) distributions, are examined in terms of polarized FTIR spectroscopic results for kyanite and sillimanite crystals determined earlier and those determined for andalusite in this study.  For andalusite, seven peaks were observed when the electric vector, E, is parallel to [100]: four intense ones at  3440 cm-1, 3460 cm-1, 3526 cm-1, and 3597 cm-1 and three weaker ones at 3480 cm-1, 3520 cm-1, and 3653 cm-1.  Six peaks, three intense ones at 3440 cm-1, 3460 cm‑1, and 3526 cm-1 and three weaker ones at 3480 cm-1, 3520 cm-1, and 3653 cm-1 when E parallels [010]. No peaks were observed when E is parallel to [001]. The concentration of water in andalusite varies between 110 and 168 ppm by weight % H2O.  Polarized FTIR spectra indicate that the OH vector is parallel to (001) in andalusite and sillimanite and (11-1) in kyanite. Examination of the L(r) (3,-3) critical points in comparison with the polarized FTIR indicates that H prefers to bond to the oxygen atoms O1 and O2 in andalusite and O2 and O4 in sillimanite which correspond to the underbonded oxygen atoms and those with the largest L(r) maxima. In kyanite, comparison of the FTIR spectrum and the critical points indicates that H will preferentially bond to the two 4-coordinated O2 and O6 atoms.

Physics and Chemistry of Minerals 34, 295-306