High-pressure phase transformation of carbonate malachite Cu₂(CO₃)(OH)₂ driven by CuO₆ regularization and CO₃ rotation

High-pressure synchrotron X-ray diffraction and infrared absorption spectroscopy have been employed to study the crystal chemistry and phase transitions in an OH-bearing carbonate, malachite Cu₂(CO₃)(OH)₂, to determine the effect of OH on the stability of carbonate. We found that the crystal structure of malachite is stabilized by a high degree of CuO₆-octahedron distortion, as is manifested by large variations in Cu-O bond lengths resulting from oxygen atoms that connect to hydrogen at crystallographically different sites. External pressure offsets the effect of hydrogen bond, promotes CuO₆ compression and regularization and accordingly CO₃ rotation. Rotation of CO₃-triangles, in turn, assists in a conversion in the crystal orientation of the CuO₆ structural unit. During compression to above ~6 GPa, malachite begins to turn into the rosasite lattice, accompanied with a jump in density of 3.3%. Rosasite is characterized with a hardened lattice and preserves to the maximum pressure (18.2 GPa) of the present study. Phase transformation mechanism of malachite to rosasite is different from that of carbonates, with the latter being driven by an almost uniform compression of MO₆-octahedron (M=Ca, Cd, Mn, Fe, Zn, Mg, etc.) and rotation/translation of CO₃-triangle under pressure.