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 . This is consistent with the crystal structure showing channels parallel to , 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.
© 1996 Mineralogical Society of America. Manuscript received December 1, 1995. Manuscript accepted May 28, 1996. The authors thank P. Wyllie (California Institute of Technology) for the use of his cold-seal equipment, and J. Eiler (California Institute of Technology) for technical help with the cold-seal vessels. G.A. Lager (University of Louisville) provided information on the results of his neutron refinement of lawsonite at room temperature, and S.P. Gabuda and S.G. Kozlova (Novosibirsk) provided preliminary data on a single-crystal proton NMR study of lawsonite at room temperature. The comments of referees H. Cynn (UCLA) and C. Merzbacher (Naval Research Laboratories), as well as the remarks of Associate Editor A. Hofmeister helped to improve the quality of the manuscript. E.L. appreciates financial support from the Fonds zur Förderung der wissenschaftlichen Forschung, Austria, during an Erwin-Schrödinger fellowship, project J01098-GEO. G.R.R. acknowledges support from the National Science Foundation, grant EAR-9218980. This paper represents the Division of Geological and Planetary Sciences of the California Institute of Technology, contribution 5623.