Single-crystal IR spectroscopy of very strong hydrogen bonds in pectolite,
NaCa2[Si3O8(OH)], and serandite, NaMn4Si3O8(OH)]
Vera M.F. Hammer
Mineralogisch-Petrographische Abteilung, Naturhistorisches Museum Wien, Burgring
7, A-1014 Wien, Austria
George R. Rossman
Division of Geological and Planetary Sciences, California Institute of Technology,
170-25
Pasadena, California 91125-2500, U.S.A.
Eugen Libowitzky
Institut für Mineralogie und Kristallographie, Universität Wien-Geozentrum
Althanstrasse 14, A-1090 Wien, Austria
Abstract
Polarized infrared absorption spectra of thin, oriented single-crystal slabs
of pectolite and serandite were recorded between 4000 and 350 cm-1
at 298 and 83 K. The spectra of both minerals show a broad absorption region
parallel to the silicate chains (b direction) that is centered around 1000
cm-1, which is interrupted by a transmission window, and which
is superimposed by sharp silicate, lattice, and overtone modes. This band
is assigned to the OH stretching mode consistent with the alignment of the
O-H...O hydrogen bond parallel to b and the short O-O distance of 2.45-2.48
Å that was found in previous Xray structure refinements. At 1396 cm-1
(pectolite) and 1386 cm-1 (serandite) an OH bending mode is observed
in the IR spectra parallel to c. At low temperatures, this mode shifts up
to higher frequencies (1403 cm-1 at 83 K in pectolite), whereas
the down-shift of the OH stretching mode cannot be observed due to the extremely
broad band width. The slightly higher energy of the bending mode in pectolite
indicates a slightly stronger hydrogen bond with respect to serandite. However,
the bond length in serandite is slightly shorter than that in pectolite.
An asymmetric O-H.. . O bond is confirmed in pectolite and serandite through
comparison with different materials with similar, very strong hydrogen bonds
and low-energy OH stretching modes.
American Mineralogist, Volume 83, pages 569-576, 1998