title: Ultrahydrous stishovite from high-pressure hydrothermal treatment of SiO2.
abstract: Stishovite (SiO(2) with the rutile structure and octahedrally coordinated silicon) is an important high-pressure mineral. It has previously been considered to be essentially anhydrous. In this study, hydrothermal treatment of silica glass and coesite at 350-550 °C near 10 GPa produces stishovite with significant amounts of H(2)O in its structure. A combination of methodologies (X-ray diffraction, thermal analysis, oxide melt solution calorimetry, secondary ion mass spectrometry, infrared and nuclear magnetic resonance spectroscopy) indicate the presence of 1.3 ± 0.2 wt % H(2)O and NMR suggests that the primary mechanism for the H(2)O uptake is a direct hydrogarnet-like substitution of 4H(+) for Si(4+), with the protons clustered as hydroxyls around a silicon vacancy. This substitution is accompanied by a substantial volume decrease for the system (SiO(2) + H(2)O), although the stishovite expands slightly, and it is only slightly unfavorable in energy. Stishovite could thus be a host for H(2)O at convergent plate boundaries, and in other relatively cool high-pressure environments.
authors.affiliation: Department of Materials and Environmental Chemistry, Stockholm University, S-10691 Stockholm, Sweden.
authors.forename: Kristina
authors.initials: K
authors.lastname: Spektor
authors.forename: Johanna
authors.initials: J
authors.lastname: Nylen
authors.forename: Emil
authors.initials: E
authors.lastname: Stoyanov
authors.forename: Alexandra
authors.initials: A
authors.lastname: Navrotsky
authors.forename: Richard L
authors.initials: RL
authors.lastname: Hervig
authors.forename: Kurt
authors.initials: K
authors.lastname: Leinenweber
authors.forename: Gregory P
authors.initials: GP
authors.lastname: Holland
authors.forename: Ulrich
authors.initials: U
authors.lastname: Häussermann
mesh_terms: D002151:Calorimetry; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D008903:Minerals / Q000138:chemical synthesis* / Q000737:chemistry; D008968:Molecular Conformation; D011312:Pressure; D011791:Quartz / Q000138:chemical synthesis* / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013696:Temperature; D014867:Water / Q000737:chemistry; D014961:X-Ray Diffraction
chemical_list: D008903:Minerals; D014867:Water; D011791:Quartz; D012822:Silicon Dioxide