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.