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Ultrahydrous stishovite from high-pressure hydrothermal treatment of SiO2
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry. Arizona State University USA.
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2011 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 108, no 52, 20918-20922 p.Article in journal (Refereed) Published
Abstract [en]

Stishovite (SiO2 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 H2O 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 % H2O and NMR suggests that the primary mechanism for the H2O uptake is a direct hydrogarnet-like substitution of 4H+ for Si4+, with the protons clustered as hydroxyls around a silicon vacancy. This substitution is accompanied by a substantial volume decrease for the system (SiO2 + H2O), although the stishovite expands slightly, and it is only slightly unfavorable in energy. Stishovite could thus be a host for H2O at convergent plate boundaries, and in other relatively cool high-pressure environments.

Place, publisher, year, edition, pages
2011. Vol. 108, no 52, 20918-20922 p.
Keyword [en]
low temperature, high-pressure synthesis, hydrothermal environments, multianvil technique
National Category
Inorganic Chemistry Geochemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-123989DOI: 10.1073/pnas.1117152108ISI: 000298479900018OAI: oai:DiVA.org:su-123989DiVA: diva2:881090
Funder
Swedish Research Council
Available from: 2015-12-09 Created: 2015-12-09 Last updated: 2015-12-18Bibliographically approved
In thesis
1. Extreme water catalyzed transformations of SiO2, TiO2 and LiAlSiO4
Open this publication in new window or tab >>Extreme water catalyzed transformations of SiO2, TiO2 and LiAlSiO4
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The dramatic change in properties of water near its critical point (i.e. T = 374 °C and p = 22.1 MPa, note: 100 MPa = 0.1 GPa = 1 kbar ≈ 1000 atm) has been a subject of numerous studies and also lead to the development of various applications (e.g. in waste destruction, biomass processing, and the synthesis of advanced ceramic materials). However, comparatively little is known about the behavior of water at gigapascal pressures. The present study attempts to explore catalytical properties and reactivity of extreme water with respect to several oxide systems: SiO2, TiO2 and LiAlSiO4. “Extreme water” here is defined as existing at p,T conditions of 0.25–10 GPa and 200–1000 °C, thus considering both supercritical fluid and hot compressed ice. The study shows that extreme water can make high pressure mineral phases accessible at relatively mild T conditions. At the same time, high pressure aqueous environments appear efficient in stabilizing novel metastable structures and may be considered as a general route for synthesizing new materials.

The hydrothermal treatment of SiO2 glass at 10 GPa and 300–550 °C yielded an unusual ultrahydrous form of stishovite with up to 3% of structural water. At the same time, the extreme water environment enhanced notably the kinetics of stishovite formation, making it accessible at unprecedentedly low temperatures. Thus, for the SiO2–H2O system water acts as both catalyst and reactant. For TiO2 a hydrothermal high pressure treatment proved to be of high importance for overcoming the kinetical hindrance of the rutile – TiO2-II transformation. 6 GPa and 650 °C were established as the mildest conditions for synthesizing pure TiO2-II phase in less than two hours. The crystallization of LiAlSiO4 glass in an extreme water environment yielded a number of different phases. In the low pressure region (0.25 – 2 GPa) mainly a zeolite (Li-ABW) and a dense anhydrous aluminosilicate (α-eucryptite) were obtained. At pressures above 5 GPa the formation of novel pyroxene-like structures with crystallographic amounts of structural water was observed.

The overall conclusion of this study is that extreme water environments show a great potential for catalyzing phase transitions in oxide systems and for stabilizing novel structures via structural water incorporation.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry, Stockholm University, 2015. 81 p.
Keyword
extreme water environments, high pressure polymorphism, hydrous stishovite, titania phase transitions, lithium aluminosilicates, zeolites from glass precursors, hydrous pyroxenes
National Category
Inorganic Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-124010 (URN)978-91-7649-313-7 (ISBN)
Public defence
2016-01-22, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2015-12-28 Created: 2015-12-09 Last updated: 2015-12-18Bibliographically approved

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