An infrared and Raman spectroscopic study of gypsum at high pressures

Citation
E. Knittle et al., An infrared and Raman spectroscopic study of gypsum at high pressures, PHYS CHEM M, 28(9), 2001, pp. 630-640
Citations number
32
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
PHYSICS AND CHEMISTRY OF MINERALS
ISSN journal
0342-1791 → ACNP
Volume
28
Issue
9
Year of publication
2001
Pages
630 - 640
Database
ISI
SICI code
0342-1791(200110)28:9<630:AIARSS>2.0.ZU;2-U
Abstract
We present Raman and infrared spectra of gypsum to 21 GPa at 300 K. Our mea surements encompass the internal modes of the (SO4)(-4) group that lie betw een 400 and 1150 cm(-1), hydroxyl-stretching vibrations between 3200 and 36 00 cm(-1), and a libration and bending vibrations of the molecular H2O grou p. All vibrations of the sulfate group have positive pressure shifts, while the hydroxyl-stretching and -bending vibrations have a mixture of positive and negative pressure shifts: the effect of pressure on the hydrogen bondi ng of the water molecule thus appears to be complex. Near 5 GPa, the two in frared-active bending vibrations of the water molecule coalesce, and the mo rphology of the hydroxyl-stretching region of the spectrum shifts dramatica lly. This behavior is consistent with a pressure-induced phase transition i n gypsum in the vicinity of 5-6 GPa, which is observed to be reversible on decompression to zero pressure. The spectral observations are consistent wi th the onset of increased disorder in the position of the water molecule in gypsum: the sulfate vibrations are largely unaffected by this transition. The Raman-active symmetric stretch of the sulfate group undergoes an appare nt splitting near 4 GPa, which is interpreted to be produced by Fermi reson ance with an overtone of the symmetric bending vibration. The average mode Gruneisen parameter of the 20 vibrational modes we sample is less than 0.05 , in contrast to the bulk thermal Gruneisen parameter of 1.20. Accordingly, the vibrations of both water and sulfate units within gypsum are highly in sensitive to volumetric compaction. Therefore, in spite of the changes in t he bonding of the water unit near 5 GPa, metastably compressed gypsum maint ains strongly bound molecular-like units to over 20 GPa at 300 K.