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

Abstract

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.