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High-pressure phase transformation in MnCO₃: a synchrotron XRD study

S. Ono^*,

Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japan

^* E-mail: shigeaki.ono{at}ucl.ac.uk

ABSTRACT

The high-pressure behaviour of manganese carbonate was investigated by in situ synchrotron X-ray powder diffraction up to 54 GPa with a laser-heated diamond anvil cell. A phase transition from rhodochrosite to a new structure form was observed at 50 GPa after laser heating. The diffraction pattern of the new high-pressure form was reasonably indexed with an orthorhombic unit-cell with a = 5.361 Å, b = 8.591 Å and c = 9.743 Å. The pressure-induced phase transition implies a unit-cell volume reduction of ~5%. This result does not support the direct formation of diamond by dissociation of solid state MnCO₃ reported in a previous study. Fitting the compression data of rhodochrosite to a second-order Birch-Murnaghan equation of state (K₀' = 4) gives K₀ = 126(±10) GPa. The c axis of the unit-cell parameter was more compressive than the a axis.

KEYWORDS: MnCO₃, rhodochrosite, phase transition, high pressure, diamond anvil cell