Mineral inclusions in natural diamonds: the high-pressure behaviour of walstromite-structured CaSiO3

Triclinic CaSiO3 with walstromite-like structure is the dominant Ca-bearing phase in super-deep diamonds [1]. Such mineral is crucial to our understanding of the physical and chemical characteristics of the very deep Earth as it is considered the product of back-transformation from CaSiO3-perovskite, which is stable at lower mantle pressure and temperature conditions. Nevertheless, at the present day there are no reliable literature data on the pressure at which CaSiO3 crystallizes within diamonds and therefore no valid evidence for CaSiO3-walstromite to derive from CaSiO3-perovskite. A method recently improved by Angel et al. [2] allows us to estimate the entrapment pressure of the inclusion within the diamond by knowing its residual pressure, measured at ambient conditions, and the thermoelastic parameters of the mineral inclusion and the diamond host. Here for the first time we have determined the in situ high-pressure behaviour of CaSiO3 with walstromite-like structure by single-crystal X-ray diffraction using synchrotron radiation at ID09A beamline (ESRF, Grenoble, France). The crystal structure and the unit-cell parameters were measured up to about 8 GPa. This allowed us not only to determine its static bulk modulus and its first pressure derivative, but also to evaluate the degree of anisotropy under compression. Finally, our results were compared with the high-pressure data on CaSiO3-wollastonite and CaSiO3-perovskite, providing a wide scenario about the high-pressure behaviour of CaSiO3-walstromite.