HR: 1330h
AN: V42A-0338    [PDF]
TI: Cation Order-Disorder of Dolomite at High Pressure and Temperature
AU: * Antao, S M
EM: sytle.antao@stonybrook.edu
AF: State University of New York at Stony Brook, Mineral Physics Institute & Department of Geosciences, Stony Brook, NY 11794-2100 United States
AU: Hassan, I
EM: ishmael.hassan@uwimona.edu.jm
AF: University of the West Indies, Department of Chemistry, Kingston, KIN 7 Jamaica
AU: Crichton, W
EM: crichton@esrf.fr
AF: European Synchrotron Radiation Facility, BP220, Grenoble, F 38043 France
AU: Parise, J B
EM: john.parise@stonybrook.edu
AF: State University of New York at Stony Brook, Mineral Physics Institute & Department of Geosciences, Stony Brook, NY 11794-2100 United States
AB: The structure of dolomite, CaMg(CO$_{3}$)$_{2}$, was determined at a constant pressure of about 4 GPa and from 25 to $1345\deg$C by {\it in-situ} synchrotron X-ray diffraction ($\lambda$ = 0.91997 $\AA$) at ID30 of the ESRF, France. The sample (packed into a Au capsule) was first compressed and data were then collected at different temperatures. Previous studies on the ordering in dolomite were done on quenched samples, but the kinetics of reordering upon quenching were too rapid to preserve the high-temperature disorder state. In fully ordered dolomite, the fraction of Ca cations on the 3{\it a} site is {\it x}$_{Ca}$ = 1, whereas in a disordered dolomite, {\it x}$_{Ca}$ = 0.5. Alternatively an order parameter {\it S} = 2{\it x}$_{Ca}$-1, varies from {\it S} = 1 (where {\it x}$_{Ca}$ = 1) for a completely ordered dolomite to {\it S} = 0 (where {\it x}$_{Ca}$ = 0.5) for a completely disordered dolomite. The ratio of intensities of reflections sensitive and insensitive (ratio = I$_{(105)}$/I$_{(006)}$) to ordering was also used to test the disorder in dolomite, and this ratio shows that dolomite disorders with temperature. These methods for characterizing the order-disorder reaction is compared to the results obtained from Rietveld refinement methods. Initially, {\it S} = 1.0 ideally for ordered dolomite. On heating, there is no change in cation ordering until the temperature is high enough to cause exchange of Ca and Mg between the two octahedral sites to occur. This activation barrier is overcome beyond about $1000\deg$C, where the sample starts to disorder. On further heating the sample continuously disorders along a smooth pathway to the maximum temperature studied ({\it T}$_{max.}$ = $1345\deg$C, {\it S} = 0.2(2)). Trace amounts of CaO and MgO were observed in diffraction patterns at and beyond $1135\deg$C. This would indicate either decomposition or partial melting, but dolomite was still the dominant phase. This reaction may cause a slight deviation from the ideal stoichiometry for the remaining dolomite.
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting