First Observation of Quenched Davemaoite to Ambient Conditions: Its Electron Diffraction Pattern

Abstract

Calcium‐rich silicate perovskite (davemaoite) has been for the first time investigated at ambient pressure by electron diffraction in transmission electron microscopy. The obtained diffraction pattern in comparison to that of the coexisting Mg‐rich silicate perovskite (bridgmanite) has revealed the cubic symmetry, Pm$$\overline{3}$$m, the lattice parameter a = 0.3561(4) nm and the volume V₀ = 45.16(10) Å³ (27.19 cm³/mol), which is still larger molar volume than that of the coexisting bridgmanite (25.49 cm³/mol). With large volume difference davemaoite and bridgmanite coexist at equilibrium under the conditions at 40 GPa and 2,000°C. The nanometer‐sized crystalline davemaoite (4.27 g/cm³) is mantled by the amorphous CaSiO₃ (2.82 g/cm³). This core‐mantle structure prevents amorphization of the unstable high‐pressure mineral at the core due to a static pressure, 1.2(10) GPa generated by volume expansion on the transition at the mantle. This mechanism is the same as that of high‐pressure minerals preserved in shocked meteorites.