High-Pressure γ-CaMgSi2O6: Does Penta-Coordinated Silicon Exist in the Earth's Mantle?

In situ X-ray diffraction experiments with natural Fe-and Al- bearing diopside single crystals and density functional theory (DFT) calculations on diopside end-member composition indicate the existence of a new high-pressure gamma-diopside polymorph with rare penta-coordinated silicon. On compression alpha-diopside transforms to the gamma-phase at similar to 50 GPa, which in turn, on decompression is observed to convert to the known beta-phase below 47 GPa. The new gamma-diopside polymorph constitutes another recent example of penta-coordinated silicon (Si-V) in overcompressed metastable crystalline silicates, suggesting that V Si may exist in the transition zone and the uppermost lower mantle in appreciable quantities, not only in silicate glass and melts but also in crystalline phases contained in the coldest parts of subducted stagnant slabs. V Si may have significant influences on buoyancy, wave velocity anomalies, deformation mechanisms, chemical reactivity of silicate rocks, and seismicity within the slab. Plain Language Summary Earth is a rocky planet and is dominated by silicates. Crust and upper mantle of our planet feature minerals with silicon almost exclusively in four-coordinated sites, whereas minerals with silicon in six-coordinated sites dominate the transition zone and the lower mantle. It is a long-standing question whether five-coordinated Si phases exist. Five-coordinated Si is observed in silicate glass and melt upon compression as intermediate states and can provide important insight into the transformation mechanism in silica densification. In this paper, we observed a high-pressure crystalline silicate phase with five-coordinated Si. This affects chemical reactivity, elastic and plastic deformation, density of the subducted slab, and its buoyancy relative to the surrounding mantle.