ON THE MECHANISMS FOR H AND AL INCORPORATION IN STISHOVITE

Abstract

The solubility and incorporation mechanisms of hydrogen in synthetic stishovite as a function of Al2O3 content have been investigated. Mechanisms for H incorporation in stishovite are more complex than previously thought. Most H in stishovite is incorporated via the Smyth et al. (Am Mineral 80:454-456, 1995) model, where H docks close to one of the shared O-O edges, giving rise to an OH stretching band in infrared (IR) spectra at 3,111-3,117 cm-1. However, careful examination of IR spectra from Al-stishovite reveals the presence of an additional OH band at 3,157-3,170 cm-1. All H is present on one site, with interstitial H both coupled to Al3+ substitutional defects on adjacent octahedral (Si4+) sites, and decoupled from other defects, giving rise to two distinct absorption bands. Trends in IR data as a function of composition are consistent with a change in Al incorporation mechanism in stishovite, with Al3+ substitution for Si4+ charge-balanced by oxygen vacancies at low bulk Al2 O3 contents, and coupled substitution of Al3+ onto octahedral (Si4+) and interstitial sites at high bulk Al2O3 contents. Trends in OH stretching frequencies as a function of AlAl2O3 content suggest that any such change in Al incorporation mechanism could alter the effect that Al incorporation has on the compressibility of stishovite, as noted by Ono et al. © Springer-Verlag 2006.