Pentacoordinate silicon in high-pressure crystalline and glassy phases of calcium disilicate (CaSi2O5)

We report Si-29 MAS NMR spectra for the monoclinic and triclinic crystallin e phases of CaSi2O5. The latter, which is now known from single crystal x-r ay diffraction to contain pentacoordinate silicon (SiO5 or Si-[5]) units, h as an NMR peak for this group near to -150 ppm, for the first time definiti vely confirming previous peak assignments in the spectra of ambient and hig h-pressure glasses. This result, and new data on a CaSi2O5 glass quenched f rom a liquid at 2300 degrees C, 10 GPa, demonstrate that Si-[5] groups, alo ng with SiO6 (Si-[6]) units, form at high pressures in silicate liquids wit h abundant non-bridging oxygens, and are thus likely to be important for th e properties such as viscosity and density of mafic to ultramafic magmas in nature.