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Cristobalite Formation in Synthetic Glasses through the Role of Mineralizing Agents in Alteration Experiments

Cindy Mikaelian 1  , Lukas Baumgartner 2 , Arthur Adams 3 , Marie Violay 1 , Alexandra Kushnir 1

  • Affiliations: 1 Rock Physics and Geofluids Laboratory (RPGL), School of Architecture, Civil and Environmental Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland 2 Institut des Sciences de la Terre (ISTE), Université de Lausanne (UNIL), Lausanne, Switzerland 3 Laboratory for Biological Geochemistry (LGB), School of Architecture, Civil and Environmental Engineering, École Polytechnique Fédérale de Lausanne (EPFL), CH−1015 Lausanne, Switzerland

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 30

  • Programme No: 3.8.14

  • Theme 3 > Session 8

Abstract

Cristobalite, a silica (SiO2) polymorph often observed in volcanic systems, forms from vapor precipitation and glass devitrification at conditions outside of its stability field. Here we study the role of mineralizing agents (Al, Na, and K) in promoting metastable cristobalite formation at temperatures below its thermodynamically stable state. To do this, we synthesize four rhyolitic glasses, controlling for Al, Na, and K content. These glasses are then powdered to grain sizes of 60-90 microns and altered in externally heated, closed system, stainless steel pressure vessels at 200°C and water vapor pressures of 1.5 MPa over 1 to 8 weeks, using acidic and alkaline fluids. This allows us to simulate rock-fluid interactions under relevant volcanic dome conditions. Preliminary post-experiment solid (XRD) and fluid (ICP-OES) geochemistry indicate that tridymite nuclei present in the initial glass were partially or completely transformed into cristobalite after four weeks, under alkaline fluid conditions. These preliminary results suggest a possible relationship between tridymite presence and cristobalite formation in volcanic glasses and highlight the efficiency of alkaline solutions for the dissolution, mobility, and local redistribution of silica. Future work is needed to quantify 1) the concentration of mineralizing agents in these silica polymorphs and 2) how these mineralizing agents drive polymorph formation outside of their stability fields.