HR: 1330h
AN: V42A-0336    [PDF]
TI: A High Pressure X-Ray Diffraction Study of CaCO3-Aragonite: Implications for the Post-Aragonite Phase Transition
AU: * Santillan, J D
EM: jds@es.ucsc.edu
AF: University of California, Santa Cruz, Dept. of Earth Sciences 1156 High Street, Santa Cruz, CA 95064 United States
AU: Williams, Q
EM: jds@es.ucsc.edu
AF: University of California, Santa Cruz, Dept. of Earth Sciences 1156 High Street, Santa Cruz, CA 95064 United States
AB: The volume and lattice parameters of CaCO3-aragonite have been measured as a function of pressure to 50 GPa using angle-dispersive synchrotron x-ray diffraction at 300 K. The Birch-Murnaghan equation of state of aragonite yields a best-fit bulk modulus (K0) of 83 (+/- 3) GPa, assuming a value of dK/dP of 4. A progressive pressure-induced change takes place in the diffraction patterns, with the (021) and (111) diffraction lines converging on one another with compression. This change is accompanied by a significant (~85 percent) loss in relative intensity of the (202) and (041) peaks. This convergence and shift in intensity of diffraction lines implies that CaCO3-aragonite is approaching a transition to a trigonal structure, such as occurs within BaCO3 at pressures of 7 GPa. Indeed, the b/a ratio of aragonite at 50 GPa approaches the square root of 3: the critical value for producing a transition to a trigonal structure, and close to that of BaCO3 at low pressure. A short extrapolation of our results indicates that a phase transition, with minimal volume change, is expected in CaCO3-aragonite at pressures exceeding ~60 GPa. This result is in general accord with the systematics of phase transitions in aragonite-structured divalent cation carbonates. If free CaCO3 exists in the deep mantle, we anticipate that it will occur in a trigonal structure.
DE: 1025 Composition of the mantle
DE: 3900 MINERAL PHYSICS
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting