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dc.contributor.authorSchipper, C.I.
dc.contributor.authorRickard, William
dc.contributor.authorReddy, Steven
dc.contributor.authorSaxey, David
dc.contributor.authorCastro, J.M.
dc.contributor.authorFougerouse, Denis
dc.contributor.authorQuadir, Z.
dc.contributor.authorConway, C.
dc.contributor.authorPrior, D.J.
dc.contributor.authorLilly, K.
dc.date.accessioned2020-05-21T03:20:43Z
dc.date.available2020-05-21T03:20:43Z
dc.date.issued2020
dc.identifier.citationSchipper, 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.
dc.identifier.urihttp://hdl.handle.net/20.500.11937/79358
dc.identifier.doi10.2138/am-2020-7236
dc.description.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.

dc.languageEnglish
dc.publisherMINERALOGICAL SOC AMER
dc.subjectScience & Technology
dc.subjectPhysical Sciences
dc.subjectGeochemistry & Geophysics
dc.subjectMineralogy
dc.subjectCristobalite
dc.subjectcrystalline SiO2
dc.subjectatom probe
dc.subjectgas solid reaction
dc.subjectvapor phase mineralization
dc.subjectchemical vapor deposition
dc.subjectglass corrosion
dc.subjectrhyolite
dc.subjectSOUFRIERE HILLS VOLCANO
dc.subjectBETA-PHASE-TRANSITION
dc.subjectATOM-PROBE
dc.subjectALPHA-CRISTOBALITE
dc.subjectMERAPI-VOLCANO
dc.subjectCHEMISTRY
dc.subjectSILICA
dc.subjectCATHODOLUMINESCENCE
dc.subjectMONTSERRAT
dc.subjectASH
dc.titleVolcanic SiO2-cristobalite: A natural product of chemical vapor deposition
dc.typeJournal Article
dcterms.source.volume105
dcterms.source.number4
dcterms.source.startPage510
dcterms.source.endPage524
dcterms.source.issn0003-004X
dcterms.source.titleAmerican Mineralogist
dc.date.updated2020-05-21T03:20:36Z
curtin.departmentJohn de Laeter Centre (JdLC)
curtin.departmentSchool of Earth and Planetary Sciences (EPS)
curtin.accessStatusFulltext not available
curtin.facultyFaculty of Science and Engineering
curtin.contributor.orcidSaxey, David [0000-0001-7433-946X]
curtin.contributor.orcidReddy, Steven [0000-0002-4726-5714]
curtin.contributor.orcidRickard, William [0000-0002-8118-730X]
curtin.contributor.researcheridSaxey, David [H-5782-2014]
curtin.contributor.researcheridReddy, Steven [A-9149-2008]
curtin.contributor.researcheridRickard, William [E-9963-2013]
dcterms.source.eissn1945-3027
curtin.contributor.scopusauthoridSaxey, David [15059256300]
curtin.contributor.scopusauthoridReddy, Steven [7402263354]
curtin.contributor.scopusauthoridFougerouse, Denis [56418452200]
curtin.contributor.scopusauthoridRickard, William [35171231700]


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