Structural and spectroscopic characterization of keatite (SiO2)

Keatite, a polymorph of silica rare in nature, was synthesized by hydrothermal treatment of silicon and water at 100 MPa and 600 °C. The crystal structure of keatite at 24 °C was refined by the Rietveld method using synchrotron X-ray diffraction data. The obtained structure is consistent with the results of previous studies in which some constraints were imposed during refinements. The 29Si MAS NMR spectrum of keatite shows two peaks at −113.9 and −114.3 ppm, which can be assigned to Si at the Si1 and Si2 sites, respectively. The Raman spectrum of keatite shows a prominent peak at 473 cm−1, which is attributable to the Si–O–Si bending mode of the 5-membered ring. These spectra, reported for the first time, are expected to be valuable for the identification of keatite in synthetic and natural samples.