R2: Crystal structure, crystal chemistry, physical properties of minerals, crystal growth and applied mineralogy

[R2-03] The incorporation mechanisms of water in aluminous orthoenstatite clarified via comprehensive NMR measurements and first-principles calculations

*Xianyu XUE¹, Masami Kanzaki¹, Rongzhang Yin² (1. Okayama University, 2. Peking University)

Keywords:orthopyroxene, water, aluminum, NMR, first-principles calculation

Aluminum is an important minor element in natural orthopyroxene, which has been shown to significantly enhance its water incorporation capacity. However, the incorporation mechanisms remain speculative. We performed a comprehensive one- and two-dimensional ¹H, ²⁹Si and ²⁷Al NMR measurements on hydrous aluminous orthoenstatite (OEn) samples containing 4 to 8 wt% Al₂O₃ synthesized at 1.5 GPa and 900°C, and first-principles calculations on the energy, and NMR and polarized infrared spectra for anhydrous and hydrous aluminous OEn models to clarify the issue. The combined ¹H MAS and static NMR, ¹H double-quantum and triple-quantum MAS NMR, and ²⁷Al-¹H CP MAS NMR and HETCOR results, along with first-principles calculation results, unambiguously revealed that a large part of the incorporated water are present as proton pairs in Mg vacancies adjacent to Al, with one proton of each pair exhibiting significantly weaker hydrogen bonding, and accordingly smaller ¹H chemical shifts and higher OH stretching frequencies, than those in Al-free OEn, as a result of interaction with Al (see Figure). Proton pairs in Mg vacancies remote from Al are minor or absent. Coupled substitutions of Al + H for 1Si and 2Mg (both with weak hydrogen bonding) were also detected, but are less abundant than hitherto considered. Thus, the enhancement of water solubility by Al is achieved dominantly through the modification of the hydrogen bonding of protons in Mg vacancies. Such a mechanism may also be important in other nominally anhydrous mantle minerals. The obtained polarized IR spectral characteristics from our first-principles calculations also allow us to decipher the incorporation mechanisms of water in synthetic OEn of lower Al concentrations and natural orthopyroxenes of diverse origins from the polarized infrared data reported thus far.