Compressibility of single-crystal zircon up to 19 GPa: implications for the partitioning coefficient of trace elements

Abstract. Zircon is a widely studied accessory mineral that helps us understand the evolution of the Earth. The unique nature of zircon is due to its chemical and physical strength and the characteristic crystallographic zirconium site, which is compatible with rare Earth elements and actinoids. According to the lattice strain model, trace element partitioning coefficients can be predicted by zircon's compressibility of the cation site. However, the crystal structure of zircon has been precisely determined at less than 5 GPa by single-crystal X-ray diffraction measurements. Here, we have precisely determined the crystal structure of zircon under high-pressure conditions of up to 19 GPa, covering the whole stability field of zircon using a diamond anvil cell. We have also theoretically calculated the unit cell parameters of zircon at high pressure by molecular dynamics simulations. The unit cell parameters and the bond length showed abnormal trends above the high-pressure stability limit of zircon. The previous discrepancy in the compressibility is likely due to the non-hydrostatic condition and instability of the sample. We determined the compressibility of the Zr–O bond and ZrO8 polyhedra based on the determined crystal structure. The averaged length of the Zr–O bond showed a consistent modulus, with the partitioning coefficients predicted by the lattice strain model. On the contrary, the model with a Poisson solid assumption is inconsistent with the partitioning coefficients. The averaged lengths of the cation–anion bonds under high pressure can help in understanding the partitioning coefficient between minerals and melt.