FTIR SPECTROSCOPY OF LAWSONITE BETWEEN 82 AND 325 K

Lawsonite single crystals were investigated,by polarized FTIR spectros copy at wavenumbers between 8000 and 1000 cm(-1) and temperatures betw een 82 and 325 K. This temperature range contains three lawsonite phas es-Cmcm > 273 K, 273 K > Pmcn > 150 K, P2(1)cn < 150 K-which are chara cterized by different relations of hydroxyl groups and H2O molecules. Unlike previous studies of H2O in minerals, which assumed weakly bonde d, symmetric H2O molecules, the highly asymmetric H2O molecule in laws onite required a modified approach that uses the single, uncoupled O-H stretching frequencies and orientations of the individual OH groups i n the H2O molecule. The formation of a strong hydrogen-bond system wit h decreasing temperature is characterized by a shift of O-H stretching bands From 2968 and 3252 cm(-1) at 325 H to 2817 and 3175 cm(-1) at 8 2 K. These frequencies are in good agreement with the corresponding hy drogen-bond lengths (H ... O = 1.66 and 1.74 Angstrom, O-H ... O = 2.6 0 and 2.66 Angstrom) at low temperatures. The orientations of the O-H vectors determined from polarized IR measurements also confirm the H-a tom positions refined from previous X-ray structure determinations al low temperatures. However, the disagreement between spectroscopically determined distances (and orientations) and those from X-ray refinemen ts at ambient conditions indicates that the room-temperature Cmcm stru cture of lawsonite contains dynamically disordered hydroxyl groups and H2O molecules. The smooth changes of stretching and bending frequenci es across the-phase boundaries at 273 and 150 K also suggest that the lawsonite phase transitions are of a dynamic order-disorder type rathe r than a displacive type. Deuteration experiments on differently orien ted, single-crystal lawsonite slabs at 350 degrees C and 1.2-2.5 kbar showed that lawsonite has a preferred H-diffusion direction parallel t o [001]. This is consistent with the crystal structure showing channel s 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 generall y weak. The FTIR powder spectra of the lawsonite-type mineral hennomar tinite, Sr-Mn-2[SiO2O7](OH)(2) . H2O, are similar to the lawsonite Z s pectra and confirm the existence of both strong and weak hydrogen bond s in its structure.