Water dissolved in olivine: A single‐crystal infrared study

Abstract

Polarized infrared spectra of the hydrogen impurities in an olivine single crystal, approximately (Mg0.9Fe0.1)2SiO4, from the Zargabad Island, Red Sea, are reported in the range 4200–3000 cm−1. In the O‐H stretching frequency region, two groups of IR absorption bands occur which show the same pleochroism and the same temperature‐induced shift between room temperature and 80 K. Each of these IR groups is composed of three equidistant bands at 3614, 3591, 3567 m−1 and at 3576, 3530, 3485 cm−1 (80 K values). These distinct multiplets are assigned to the OH−‐compensated octahedral and tetrahedral vacancy defects [OH·V″oct]′ and [OH·V′tetr]*, respectively. It is suggested that the reduced effective charge of the tetrahedral, i.e., Si‐type, vacancy is due to Si entering a nearby nonideal site for partial charge compensation. The multiplet splitting, 23 and 45 cm−1, is due to the isomorphous substitution of Mg and Fe on the adjacent M1 and M2 positions to which the proton‐bearing oxygen is bound. This leads to the combinations (Mg Mg Si), (Mg Fe Si), (Fe Fe Si) and (Mg Mg Mg), (Mg Mg Fe), (Mg Fe Fe) at the octahedral M2 and at the tetrahedral vacancy sites, respectively. An IR signal which would correspond to the fully OH−‐compensated divalent cation vacancy defect [OH·V″octHO·]* appears to be absent. Instead a weak absorption band is observed in the range of the H‐H stretching frequency of molecular H2. By analogy to molecular H2 in MgO and CaO single crystals and in fused silica it is concluded that OH− pairs, associated with this type of defects, undergo a charge transfer conversion into (H2 + O22−). The H2 are loosely bound to the peroxy entities which probably form peroxy bridges between SiO4 tetrahedra, e.g., Si‐O‐O‐Si. Outgassing of H2 would leave excess oxygen behind, shifting the stoichiometry of the olivine in a direction which has never been discussed before.