FTIR spectroscopy of lawsonite between 82 and 325 K

Lawsonite single crystals were investigated by polarized FfIR spectroscopy at wave numbers 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, P2_1cn < 150 K-which are characterized by different rotations of hydroxyl groups and H20 molecules. Unlike previous studies of H_2O in minerals, which assumed weakly bonded, symmetric H_2O molecules, the highly asymmetric H_2O molecule in lawsonite required a modified approach that uses the single, uncoupled O-H stretching frequencies and orientations of the individual OH groups in the H_2O 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 325 K to 2817 and 3175 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 lawsonite contains dynamically disordered hydroxyl groups and H_2O 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 ºC and 1.2-2.5 kbar showed that lawsonite has a preferred H-diffusion direction parallel to [001]. This is consistent with the crystal structure showing channels parallel to [001], which are solely occupied by H atoms. The spectra of isotopically 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, Sr-Mn_2[Si_2O_7](OH)_2.H_2O, are similar to the lawsonite Z spectra and confirm the existence of both strong and weak hydrogen bonds in its structure