SINGLE-CRYSTAL IR SPECTROSCOPY OF VERY STRONG HYDROGEN-BONDS IN PECTOLITE, NACA2[SI3O8(OH)], AND SERANDITE, NAMN2[SI3O8(OH)]

Polarized infrared absorption spectra of thin, oriented single-crystal slabs of pectolite and serandite were recorded between 4000 and 350 c m(-1) at 298 and 83 K. The spectra of both minerals show a broad absor ption region parallel to the silicate-chains (b direction) that is cen tered around 1000 cm(-1), which is interrupted by a transmission windo w, 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 shor t O...O distance of 2.45-2.48 Angstrom that was found in previous X-ra y structure refinements. At 1396 cm(-1) (pectolite) and 1386 cm(-1) (s erandite) an OH bending mode is observed in the IR spectra parallel to c. At low temperatures, this mode shifts up to higher frequencies (14 03 cm(-1) at 83 K in pectolite), whereas the down-shift of the OH stre tching 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 pectolit e. 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.