We report 29SiMASNMR spectra for the monoclinic and triclinic crystalline phases of CaSi2O5. The latter, which is now known from single crystal x-ray diffraction to contain pentacoordinate silicon (SiO5 or [5>Si) units, has an NMR peak for this group near to -150 ppm, for the first time definitively confirming previous peak assignments in the spectra of ambient and high-pressure glasses. This result, and new data on a CaSi2O5 glass quenched from a liquid at 2300 ¿C, 10 GPa, demonstrate that [5>Si groups, along with SiO6([6>Si) units, form at high pressures in silicate liquids with abundant non-bridging oxygens, and are thus likely to be important for the properties such as viscosity and density of mafic to ultramafic magmas in nature. ¿ 1999 American Geophysical Union