2026/07/22 by Xianyu Xue, Masami Kanzaki, Jianjun Jiang
Earth and Planetary Sciences · Materials Science · #High-pressure geophysics and materials #Magnesium Oxide Properties and Applications #Glass properties and applications
paper · doi:10.2138/am-2026-10286
Abstract In order to clarify the incorporation mechanisms of water in nominally anhydrous MgSiO3 clinoenstatite (CEn), we performed comprehensive one- and two-dimensional 1H and 29Si NMR, and vibrational (Raman and infrared) spectroscopic measurements on CEn samples containing nominally 0.05–0.3 wt% H2O synthesized at 7–14 GPa and 1200 °C. All the recovered samples were confirmed by Raman and 29Si NMR to be the low-pressure CEn (LCEn) phase that was reverted from the unquenchable high-pressure CEn (HCEn) phase during room-temperature decompression. The LCEn samples synthesized at 7 GPa are dominated by two 1H MAS NMR peaks near 7.8 and 5.8 ppm, which are revealed by 1H double-quantum (DQ) MAS NMR to form 2H pairs, and two infrared bands near 3378 and 3042 cm−1. All the observed spectral features (including 1H-29Si CP MAS NMR spectra) match well with those predicted from first-principles calculations for (2H)M2 defects (proton pairs in Mg (M2) vacancies) of LCEn, confirming previous assignment of these two infrared bands. On the other hand, a LCEn sample synthesized at 14 GPa is dominated by four broader 1H MAS NMR peaks near 12, 8, 3 and 0 ppm, which are revealed by 1H DQ and triple-quantum (TQ) MAS NMR to belong to 4H clusters. Like those reported previously for CEn samples synthesized at relatively high pressures, its infrared spectrum is dominated by three narrower bands near 3687, 3675 and 3602 cm−1, which must correspond to the 1H NMR peaks near 3 and 0 ppm for weakly hydrogen-bonded OH groups of the 4H clusters. Previously unreported weak broad bands in the 2500–3000 cm−1 region are also present, and could correspond to the 1H NMR peaks near 8 and 12 ppm for strongly hydrogen-bonded OH groups of the 4H clusters. All the observed spectral features of the sample match well with those predicted for (4H)SiA + (4H)SiB defects (4H clusters in both SiA and SiB vacancies) of LCEn (converted from a single type of (4H)Si defects in HCEn) from first-principles calculations, with the OH groups of weak hydrogen bonding associated with the two nonbridging oxygens, and those of strong hydrogen bonding associated with the two bridging oxygens on each Si vacancy. Failure to observe the latter would lead to underestimation of the water content. The samples synthesized at 10–12 GPa contain both types of OH defects. These results, as well as the previously reported IR data for LCEn, indicate the water incorporation mechanisms of HCEn change from dominantly (2H)Mg defects to (4H)Si defects with increasing pressure. This trend resembles those reported previously for MgSiO3 orthoenstatite and Mg2SiO4 forsterite, suggesting possible common underlying mechanisms.