Incorporation mechanisms and infrared absorption coefficients of water in MgSiO3 orthoenstatite clarified via comprehensive NMR and vibrational spectroscopic measurements, and first-principles calculations

Abstract

We performed 1H and 29Si NMR and infrared measurements, and first-principles calculations to clarify the nature of OH defects in MgSiO3 orthoenstatite. An orthoenstatite sample synthesized at 7 GPa and 1200 °C from a composition of MgSiO3 + 0.1 wt% H2O yielded two 1H MAS NMR peaks near 5.9 and 7.6 ppm that are correlated in 2D NMR spectra, and two infrared bands near 3361 and 3066 cm− 1 that correspond to the previously reported A3 and A4 bands. The first-principles calculations confirmed that they are due to a pair of protons in a Mg (M2) vacancy. The previously reported A1 and A2 infrared bands near 3687 and 3592 cm− 1 for orthoenstatite synthesized at low silica activities were confirmed to arise from four protons in a SiB vacancy. The latter is predicted to give two additional OH stretching bands associated with two strongly hydrogen-bonded O3b-H bonds with frequencies below the spectral range reported thus far. The previously reported infrared absorption coefficients were thus revised to account for the undetected bands. 1H NMR may be used to quantitatively detect all four protons (expected at 1–12 ppm). Other mantle minerals should also be examined for potentially overlooked OH defects with strong hydrogen bonding.