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

Abstract

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.

A new kind of invisible gold in pyrite hosted in deformation-related dislocations

“…2). The linear features are also consistent with dislocations imaged by APT in other minerals (Piazolo et al, 2016;Kirkland et al, 2018;Fougerouse et al, 2019;Schipper et al, 2020). Trace-element enrichment in the dislocation is up to 3.2 at% increase, including Ni, Cu, As, Pb, Sb, Bi, and Au (Table 1).…”

Experimental Constraints on the Crystallization of Silica Phases in Silicic Magmas

“…cations other than Si (Holmquist, 1961;Roy & Roy, 1964;Dapiaggi et al, 2015). This is consistent with Crs in volcanic products having > 1 wt% Al2O3 (up to ~3 wt%) and significant alkali (especially Na2O) contents (Pallister et al, 2008;Horwell et al, 2012;Schipper et al, 2015Schipper et al, , 2020Ivanova et al, 2018), whereas Qtz generally hosts < 1 wt% Al2O3 (Pallister et al, 2008;Horwell et al, 2012;Breiter et al, 2012). For Trd, the available data (Pallister et al, 2008) indicate the presence of Al2O3 and Na2O in smaller concentrations than in Crs but in greater concentrations compared to Qtz.…”

“…The analytical errors on major elements were calculated at ~1% (relative) for SiO2, Al2O3 and CaO, and 5% for Na2O. Schipper et al (2020) reported Na migration under the electron beam during cristobalite analyses, as such, we have tested the effect of defocusing the beam under the above conditions. We did not observe differences in Na content between the focused (spot size of 1 µm) and defocused (10x10 µm) beam.…”

“…In compositionally complex systems, the studies attempting to constrain the mutual relations between the different polymorphs agree that melt chemistry plays an important role in favouring one silica polymorph over the other. In general, the presence of Crs at 'low' T outside its stability domain is attributed to the stabilizing influence of Na, K and Al in its crystal structure (Roy & Roy, 1964;Chao & Lu, 2002;Dappiagi et al, 2015;Liang et al, 2017;Schipper et al, 2020). In-situ investigations show that Qtz-alkali oxide mixtures reacted in the Trd stability field first crystallize Crs in greater proportions for the sodic than the potassic system.…”

Atom Probe Tomography: Development and Application to the Geosciences

“…Nanoscale trace element variations in platinum‐group alloys have also been identified by APT and this has allowed the discrimination of magmatic versus weathering origins for such particles (Parman et al ). Similarly, fine‐scale compositional variations formed during mineral growth, and the subsequent modification of such growth profiles at the nanoscale by diffusion (Bloch et al ), interface‐coupled dissolution–reprecipitation (Peterman et al ) or exsolution (Genareau et al ), could be measured by APT and have the potential to place valuable constraints on magmatic, metamorphic and ore‐related petrogenetic processes (Cao et al , Gamal El Dien et al , Schipper et al ).…”