High-Pressure $γ$-CaMgSi 2 O 6 : Does Penta-Coordinated Silicon Exist in the Earth's Mantle?
We report 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 γ-diopside polymorph with rare penta-coordinated silicon. On compression $α$-diopside transforms to the $γ$-phase at ~50 GPa, which in turn, on decompression is observed to convert to the known $β$-phase below 47 GPa. The new $γ$-diopside polymorph constitutes another recent example of penta-coordinated silicon ( V Si) 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.