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GAUDEFROYITE, Ca₈Mn³⁺₆[(BO₃)₆(CO₃)₂O₆]: HIGH-TEMPERATURE CRYSTAL STRUCTURE

Sytle M. Antao^1, and Ishmael Hassan²

¹ Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA
² Department of Chemistry, University of the West Indies, Mona, Kingston 7, Jamaica

E-mail address: sytle.antao{at}anl.gov

The structural behavior of gaudefroyite, Ca₈Mn³⁺₆[(BO₃)₆(CO₃)₂O₆], was determined by using Rietveld refinements based on synchrotron powder X-ray-diffraction data from 25 to 486°C. The structure was also refined at 25°C using synchrotron high-resolution powder X-ray-diffraction (HRPXRD) data that gave the pseudohexagonal unit-cell parameters a 10.60791(2), c 5.88603(1) Å, and V 573.605(2) ų at room temperature. The expansion of the gaudefroyite structure is nearly isotropic and is caused mainly by an increase in the Ca1–O3 distance. The volume change from 25° to 486°C is 2.2(2)%. Beyond 486°C, significant changes were observed in the X-ray-diffraction traces; they indicate the formation of a minor second phase that exsolved from the host gaudefroyite.

Keywords: gaudefroyite, high-temperature structure, exsolution, Rietveld refinements, synchrotron radiation.