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Pentacoordinate silicon in high‐pressure crystalline and glassy phases of calcium disilicate (CaSi 2 O 5 )
Author(s) -
Stebbins J. F.,
Poe B. T.
Publication year - 1999
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/1999gl008364
Subject(s) - monoclinic crystal system , triclinic crystal system , anorthite , silicate , nmr spectra database , silicon , materials science , crystallography , spectral line , mineralogy , diffraction , analytical chemistry (journal) , crystal structure , chemistry , optics , organic chemistry , metallurgy , physics , astronomy
We report 29 Si MAS NMR spectra for the monoclinic and triclinic crystalline phases of CaSi 2 O 5 . The latter, which is now known from single crystal x‐ray diffraction to contain pentacoordinate silicon (SiO 5 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 CaSi 2 O 5 glass quenched from a liquid at 2300°C, 10 GPa, demonstrate that [5] Si groups, along with SiO 6 ( [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.