CaCO 3-III and CaCO 3-VI, high-pressure polymorphs of calcite: Possible host structures for carbon in the Earth's mantle

Calcite, CaCO 3, undergoes several high pressure phase transitions. We report here the crystal structure determination of the CaCO 3-III and CaCO 3-VI high-pressure polymorphs obtained by single-crystal synchrotron X-ray diffraction. This new technical development at synchrotron beamlines currently affords the possibility of collecting single-crystal data suitable for structure determination in-situ at non-ambient conditions, even after multiphase transitions. CaCO 3-III, observed in the pressure range 2.5-15GPa, is triclinic, and it presents two closely related structural modifications, one, CaCO 3-III, with 50 atoms in the unit cell [a=6.281(1)Å, b=7.507(2)Å, c=12.516(3)Å, α=93.76(2)°, Β=98.95(2)°, γ=106.49(2)°, V=555.26(20)Å 3 at 2.8GPa], the second, CaCO 3-IIIb, with 20 atoms [a=6.144(3)Å, b=6.3715(14)Å, c=6.3759(15)Å, α= 93.84(2)°, Β=107.34(3)°, γ=107.16(3)°, V=224.33(13)Å 3 at 3.1GPa]. Different pressure-time experimental paths can stabilise one or the other polymorph. Both structures are characterised by the presence of non-coplanar CO 3 groups. The densities of CaCO 3-III (2.99g/cm 3 at 2.8GPa) and CaCO 3-IIIb (2.96g/cm 3 at 3.1GPa) are lower than aragonite, in agreement with the currently accepted view of aragonite as the thermodynamically stable Ca-carbonate phase at these pressures. The presence of different cation sites, with variable volume and coordination number (7-9), suggests however that these structures have the potential to accommodate cations with different sizes without introducing major structural strain. Indeed, this structure can be adopted by natural Ca-rich carbonates, which often exhibit compositions deviating from pure calcite. Mg-calcites are found both in nature (Frezzotti et al., 2011) and in experimental syntheses at conditions corresponding to deep subduction environments (Poli et al., 2009). At these conditions, the low pressure rhombohedral calcite structure is most unlikely to be stable, and, at the same time, Mg and Fe solubility in aragonite is hindered energetically in the 9-fold coordination site. Above 15GPa, and up to the maximum pressure investigated (40GPa), we observe the high-pressure polymorph CaCO 3-VI, triclinic [a=3.3187(12)Å, b=4.8828(14)Å, c=5.5904(14)Å, α=103.30(2)°, Β=94.73(2)°, γ=89.21(2)°, V=87.86(20)Å 3 at 30.4GPa] with 10 atoms in the unit cell. It is characterised by coplanar CO 3 groups but the structure is no longer layered, as in the lower pressure polymorphs. The density of the CaCO 3-VI structure (3.78g/cm 3 at 30.4GPa) is higher than aragonite. For this reason it could be supposed that a region may exist where this polymorph replaces aragonite in the Earth's intermediate mantle. The lower coordination number for the Ca site [7+2] instead of [9] in aragonite suggests that this structure could be easily adopted by an extended solid-solution range from calcite towards the dolomite [CaMg(CO 3) 2]-ankerite [CaFe(CO 3) 2] compositional join. The transitions from calcite to CaCO 3-III, CaCO 3-IIIb and CaCO 3-VI are perfectly reversible and after pressure release we always observe the calcite structure, with the sample recovered as a single-crystal. Indeed, it is highly unlikely that these structures can be observed in samples recovered from high-pressure environments.