Eugen Libowitzky and George R. Rossman
Division of Geological and Planetary
Sciences,
California Institute of Technology
Pasadena, CA 91125-2500 U.S.A.
Abstract
Lawsonite single crystals were investigated by polarized FTIR spectroscopy at wavenumbers between 8000 and 1000 cm-1 and temperatures between 82 and 325 K. This temperature range contains three lawsonite phases - Cmcm >273 K, 273 K > Pmcn > 150 K, P21cn < 1150 K - which are characterized by different rotations of hydroxyl groups and molecules. Unlike previous studies of H2O in minerals, which assumed weakly bounded, symmetric H2O molecules, the highly asymmetric H2O molecule in lawsonite required a modified approach that uses the single, uncoupled O-H stretching frequencies and orientations of the individual O grups in the H2O molecule.
The formation of a strong hydrogen-bond system with decreasing temperature is characterized by a shift of O-H stretching bands from 2968 and 3252 cm-1 at 82 K. These frequencies are in good agreement with the corresponding hydrogen-bond lengths (H···O = 1.66 and 1.74 Å, O-H···O = 2.60 and 2.66 Å) at low temperatures. The orientations of the O-H vectors determined from polarized IR measurements also confirm the H-atom positions refined from previous X-ray structure determinations at low temperatures. However, the disagreement between spectroscopically determined distances (and orientations and those from X-ray refinements at ambient conditions indicates that the room-temperature Cmcm structure of lowsonites contains dynamically disordered hydroxyl groups and H2O molecules. The smooth changes of stretching and bending frequencies across the phase boundaries at 273 and 150 K also suggest that the lawsonite phase transitions are of a dynamic order-disorder type rather than a displacive type.
Deuteration experiments on differently oriented, single-crystal lawsonite slabs at 350°C and 1.2-2.5 kbar showed that lawsonite has a preferred H-diffusion direction oparallel to [001]. This is consistent with the crystal structure showing channels parallel to [001], which are solely occupied by H atoms. The spectra of isotpically diluted samples, which are almost identical to those of natural lawsonite, indicate that band-coupling effects are generally weak.
The FTIR powder spectra of the lawsonite-type mineral hennomartinite, SrMn2[Si2O7](OH)2·H2O, are similar to the lawsonite Z spectra and confirm the existence of both strong and weak hydrogen bonds in its structure.