| Summary: | We report on high-pressure angle-dispersive synchrotron X-ray diffraction data of a natural Zn<sub>3.78(2)</sub>Cu<sub>1.22(2)</sub>(CO<sub>3</sub>)<sub>2</sub>(OH)<sub>6</sub> aurichalcite mineral up to 7.6 GPa and ab initio total energy calculations of the aurichalcite structure with three different Zn-Cu stoichiometries (Zn:Cu ratios = 10:0, 8:2 and 6:4). A monoclinic-to-triclinic displacive second-order phase transition was found experimentally at 3 GPa. The experimental bulk modulus of the initial <i>P</i>2<sub>1</sub>/<i>m</i> aurichalcite is B<sub>0</sub> = 66(2) GPa, with a first-pressure derivative of B<sub>0</sub>′ = 9(2). A comparison with other basic copper and zinc carbonates shows that this B<sub>0</sub> value is considerably larger than those of malachite and azurite. This relative incompressibility occurs despite the fact that aurichalcite features a layered structure due to the number of directed hydrogen bonds between carbonate groups and the cation-centered oxygen polyhedra forming complex sheets. The existence of different bond types and polyhedral compressibilities entails a certain anisotropic compression, with axial compressibilities <i>κ<sub>a</sub></i><sub>0</sub> = 3.79(5)·10<sup>−3</sup> GPa<sup>−1</sup>, <i>κ<sub>b</sub></i><sub>0</sub> = 5.44(9)·10<sup>−3</sup> GPa<sup>−1</sup> and <i>κ<sub>c</sub></i><sub>0</sub> = 4.61(9)·10<sup>−3</sup> GPa<sup>−1</sup>. Additional density-functional theory calculations on the <i>C</i>2/<i>m</i> hydrozincite-type structure with different Zn:Cu compositional ratios shows that the aurichalcite structure is energetically more stable than the hydrozincite one for compositions of Zn:Cu = 10:0, 8:2 and 6:4 at room pressure. The pure Zn aurichalcite phase, however, was predicted to transform into hydrozincite at 18 GPa, which suggests that the experimentally observed hydrozincite structure is a metastable phase.
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