Vapour-phase crystallisation of silica from SiF4-bearing volcanic gases

J. C. M. De Hoog, M. J. Van Bergen, M. H. G. Jacobs

Abstract


Thermodynamic modelling of magmatic gases shows that SiF4 may be an important F-bearing species at the high
pressures typical of magma reservoirs. Upon decompression during degassing, SiF4 will react with water vapour
to form HF and silica. Common magmatic gases of high-T fumaroles seem to contain too little SiF4 to be a significant
source of silica, except if extremely large amounts of gas percolate through a small volume of rock, as
is the case in lava domes. Only if fluorine contents of the gases exceed 1 mol% detectable amounts of silica may
be formed, but such high fluorine contents have not yet been observed in natural gases. Alternatively, silica may
be formed by heating of cool SiF4-rich gases circulating in cooling lava bodies. We suggest that these mechanisms
may be responsible for the deposition of crystalline silica, most probably cristobalite, observed in vesicles
in lavas from Lewotolo volcano (Eastern Sunda Arc, Indonesia). Silica occurs as vapour-crystallised patches in
vesicles, and is sometimes associated with F-phlogopite, which further supports F-rich conditions during deposition.
Because of the connection between F-rich conditions and high-K volcanism, we propose that late-stage
gaseous transport and deposition of silica may be more widespread in K-rich volcanoes than elsewhere, and
long-term exposure to ash from eruptions of such volcanoes could therefore carry an increased risk for respiratory
diseases. The dependence of SiF4/HF on temperature reported here differs from the current calibration used
for temperature measurements of fumarolic gases by remote sensing techniques, and we suggest an updated calibration.

Keywords


SiF4;vapour crystallisation;silica;degassing

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References


DOI: https://doi.org/10.4401/ag-3233
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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X