Curtin University Homepage
  • Library
  • Help
    • Admin

    espace - Curtin’s institutional repository

    JavaScript is disabled for your browser. Some features of this site may not work without it.
    View Item 
    • espace Home
    • espace
    • Curtin Research Publications
    • View Item
    • espace Home
    • espace
    • Curtin Research Publications
    • View Item

    Volcanic SiO2-cristobalite: A natural product of chemical vapor deposition

    Access Status
    Fulltext not available
    Authors
    Schipper, C.I.
    Rickard, William
    Reddy, Steven
    Saxey, David
    Castro, J.M.
    Fougerouse, Denis
    Quadir, Z.
    Conway, C.
    Prior, D.J.
    Lilly, K.
    Date
    2020
    Type
    Journal Article
    
    Metadata
    Show full item record
    Citation
    Schipper, C.I. and Rickard, W.D.A. and Reddy, S.M. and Saxey, D.W. and Castro, J.M. and Fougerouse, D. and Quadir, Z. et al. 2020. Volcanic SiO2-cristobalite: A natural product of chemical vapor deposition. American Mineralogist. 105 (4): pp. 510-524.
    Source Title
    American Mineralogist
    DOI
    10.2138/am-2020-7236
    ISSN
    0003-004X
    Faculty
    Faculty of Science and Engineering
    School
    John de Laeter Centre (JdLC)
    School of Earth and Planetary Sciences (EPS)
    URI
    http://hdl.handle.net/20.500.11937/79358
    Collection
    • Curtin Research Publications
    Abstract

    © 2020 Walter de Gruyter. Cristobalite is a low-pressure, high-temperature SiO2 polymorph that occurs as a metastable phase in many geologic settings, including as crystals deposited from vapor within the pores of volcanic rocks. Such vapor-phase cristobalite (VPC) has been inferred to result from silica redistribution by acidic volcanic gases but a precise mechanism for its formation has not been established. We address this by investigating the composition and structure of VPC deposited on plagioclase substrates within a rhyolite lava flow, at the micrometer to nanometer scale. The VPC contains impurities of the form [AlO4/Na+]0 - coupled substitution of Al3+ charge-balanced by interstitial Na+ - which are typical of cristobalite. However, new electron probe microanalysis (EPMA) element maps show individual crystals to have impurity concentrations that systematically decline from crystal cores-to-rims, and atom probe tomography reveals localized segregation of impurities to dislocations. Impurity concentrations are inversely correlated with degrees of crystallinity [observed by electron backscatter diffraction (EBSD), hyperspectral cathodoluminescence, laser Raman, and transmission electron microscopy (TEM)], such that crystal cores are poorly crystalline and rims are highly ordered tetragonal α-cristobalite. The VPC-plagioclase interfaces show evidence that dissolution-reprecipitation reactions between acidic gases and plagioclase crystals yield precursory amorphous SiO2 coatings that are suitable substrates for initial deposition of impure cristobalite. Successive layers of cubic β-cristobalite are deposited with impurity concentrations that decline as Al-bearing gases rapidly become unstable in the vapor cooling within pores. Final cooling to ambient temperature causes a displacive transformation from β→α cristobalite, but with locally expanded unit cells where impurities are abundant. We interpret this mechanism of VPC deposition to be a natural proxy for dopant-modulated Chemical Vapor Deposition, where halogen-rich acidic gases uptake silica, react with plagioclase surfaces to form suitable substrates and then deposit SiO2 as impure cristobalite. Our results have implications for volcanic hazards, as it has been established that the toxicity of crystalline silica is positively correlated with its purity. Furthermore, we note that VPC commonly goes unreported, but has been observed in silicic lavas of virtually all compositions and eruptive settings. We therefore suggest that despite being metastable at Earth's surface, cristobalite may be the most widely occurring SiO2 polymorph in extrusive volcanic rocks and a useful indicator of gas-solid reaction having occurred in cooling magma bodies.

    Related items

    Showing items related by title, author, creator and subject.

    • Initial breakup of supercontinent Rodinia as recorded by ca 860–840 Ma bimodal volcanism along the southeastern margin of the Yangtze Block, South China
      Lyu, P.; Li, W.; Wang, Xuan-Ce; Pang, C.; Cheng, J.; Li, X. (2017)
      Abstract It is considered that mantle plumes play an important role in the breakup of supercontinents, but continental rifting and associated bimodal volcanism often predate mantle-plume magmatism and the major stage of ...
    • Co-precipitation of ferrihydrite and silica from acidic hydrometallurgical solutions and its impact on the paragoethite process
      Dyer, Laurence G (2010)
      Ferrihydrite is a common iron oxyhydroxide, produced both naturally and industrially. It is often found in association with silica; an example of this is its occurrence in the Paragoethite process applied in zinc ...
    • Catalytic partial oxidation of propylene to acrolein: the catalyst structure, reaction mechanisms and kinetics
      Fansuri, Hamzah (2005)
      Bismuth molybdates have long been known as active catalysts for selective oxidation of olefins. There are several phases of bismuth molybdates but only three of them are known to be active for partial oxidation of propylene ...
    Advanced search

    Browse

    Communities & CollectionsIssue DateAuthorTitleSubjectDocument TypeThis CollectionIssue DateAuthorTitleSubjectDocument Type

    My Account

    Admin

    Statistics

    Most Popular ItemsStatistics by CountryMost Popular Authors

    Follow Curtin

    • 
    • 
    • 
    • 
    • 

    CRICOS Provider Code: 00301JABN: 99 143 842 569TEQSA: PRV12158

    Copyright | Disclaimer | Privacy statement | Accessibility

    Curtin would like to pay respect to the Aboriginal and Torres Strait Islander members of our community by acknowledging the traditional owners of the land on which the Perth campus is located, the Whadjuk people of the Nyungar Nation; and on our Kalgoorlie campus, the Wongutha people of the North-Eastern Goldfields.