HR: 1330h
AN: V42A-0337    [PDF]
TI: A High Pressure Infrared Spectroscopic Study of PbCO3-Cerussite: Constraints on the Structure of the Post-Aragonite Phase
AU: * Catalli, K
EM: krystle@ucsc.edu
AF: University of California, Santa Cruz, Department of Earth Sciences 1156 High St., Santa Cruz, CA 95064 United States
AU: Santillan, J
EM: jds@es.ucsc.edu
AF: University of California, Santa Cruz, Department of Earth Sciences 1156 High St., Santa Cruz, CA 95064 United States
AU: Williams, Q
EM: quentw@es.ucsc.edu
AF: University of California, Santa Cruz, Department of Earth Sciences 1156 High St., Santa Cruz, CA 95064 United States
AB: We have measured the infrared spectrum of aragonite-structured PbCO3-cerussite to 42 GPa at 300 K in the diamond anvil cell. The in-plane bending, symmetric stretching, and asymmetric stretching vibrations of the carbonate group each shift to higher frequencies to pressures of 17 GPa at rates between 1.1 and 4.0 cm-1/GPa, while the out-of-plane bending vibration shifts to lower frequency by -1.3 cm-1/GPa. This softening is likely due to pressure-induced strengthening of the Pb-O bond. A phase transition is observed beginning at 17 GPa: this is in accord with previous Raman spectroscopic and x-ray crystallographic work on PbCO3 under pressure. The phase transition is manifested by a splitting of the out-of-plane bending vibration, and a broadening and dramatic decrease in amplitude of the symmetric stretching vibration: the absolute positions of the remaining bands are essentially identical between the low- and high-pressure phases, implying that the bonding environments of the carbonate unit in the two phases are similar. However, the pressure shifts of the vibrational bands are lower in the high pressure phase, with shifts between 0 and 0.3 cm-1/GPa: these smaller rates suggest that the carbonate group in the high pressure phase is less compressible than within the aragonite-structured phase. No further phase transitions were observed to 42 GPa. The high pressure phase has been proposed to crystallize in the orthorhombic space group P2sub122 with 16 formula units in the unit cell by Lin and Liu (1997). Yet, the band splittings and disappearance of the symmetric stretching vibration in the spectra of the high pressure phase are instead consistent with a substantially smaller trigonal unit cell, with space group P-31c: this is the same as that of the high pressure phase of BaCO3 (Holl et al., 2000), and our spectra strongly resemble those of this phase. Our results thus indicate that the post-aragonite high pressure phase in carbonates may generally be a comparatively high symmetry trigonal structure. Such a structure thus represents the most likely candidate for the ultra-high pressure structure of CaCO3-aragonite.
DE: 1025 Composition of the mantle
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3900 MINERAL PHYSICS
DE: 3924 High-pressure behavior
DE: 3934 Optical, infrared, and Raman spectroscopy
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting