| Summary: | Malachite Cu<sub>2</sub>(CO<sub>3</sub>)(OH)<sub>2</sub> is a common hydroxycarbonate that contains about 15.3 wt % H<sub>2</sub>O. Its structural chemistry sheds light on other hydroxyl minerals that play a role in the water recycling of our planet. Here using Raman and infrared spectroscopy measurements, we studied the vibrational characteristics and structural evolution of malachite in a diamond anvil cell at room temperature (25 °C) up to ~29 GPa. Three types of vibrations were analyzed including Cu−O vibrations (300−600 cm<sup>−1</sup>), [CO<sub>3</sub>]<sup>2−</sup> vibrations (700−1600 cm<sup>−1</sup>), and O−H stretches (3200−3500 cm<sup>−1</sup>). We present novel observations of mode discontinuities at pressures of ~7, ~15, and ~23 GPa, suggesting three phase transitions, respectively. First, pressure has a great effect on the degree of deformation of the [CuO<sub>6</sub>] octahedron, as is manifested by the various shifting slopes of the Cu−O modes. [CuO<sub>6</sub>] deformation results in a rotation of the structural unit and accordingly a phase transition at ~7 GPa. Upon compression to ~15 GPa, the O−H bands redshift progressively with significant broadness, indicative of an enhancement of the hydrogen bonding, a shortening of the O···O distance, and possibly somewhat of a desymmetrization of the O−H···O bond. O−H mode hardening is identified above ~15 GPa coupled with a growth in the amplitude of the lower-energy bands. These observations can be interpreted as some reorientation or reordering of the hydrogen bonding. A further increment of pressure leads to a change in the overall compression mechanism of the structure at ~23 GPa, which is characterized by the blueshift of the O−H stretches and the softening of the O−C−O in-plane bending bands. The hydrogen bonding weakens due to a substantial enhancement of the Cu−H repulsion effect, and the O···O bond length shows no further shortening. In addition, the change in the local geometry of hydrogen is also induced by the softening of the [CO<sub>3</sub>]<sup>2−</sup> units. In this regard we may expect malachite and other analogous hydroxyl minerals as capable of transporting water downward towards the Earth’s transition zone (~23 GPa). Our results furnish our knowledge on the chemistry of hydrogen bonding at mantle conditions and open a new window in understanding the synergistic relations of water and carbon recycling in the deep Earth.
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