Vapour-phase crystallisation of silica from SiF4-bearing volcanic gases
Main Article Content
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.
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.
Article Details
How to Cite
De Hoog, J. C. M., Van Bergen, M. J. and Jacobs, M. H. G. (2005) “Vapour-phase crystallisation of silica from SiF4-bearing volcanic gases”, Annals of Geophysics, 48(4-5). doi: 10.4401/ag-3233.
Issue
Section
OLD
Open-Access License
No Permission Required
Istituto Nazionale di Geofisica e Vulcanologia applies the Creative Commons Attribution License (CCAL) to all works we publish.
Under the CCAL, authors retain ownership of the copyright for their article, but authors allow anyone to download, reuse, reprint, modify, distribute, so long as the original authors and source are cited. No permission is required from the authors or the publishers.
In most cases, appropriate attribution can be provided by simply citing the original article.
If the item you plan to reuse is not part of a published article (e.g., a featured issue image), then please indicate the originator of the work, and the volume, issue, and date of the journal in which the item appeared. For any reuse or redistribution of a work, you must also make clear the license terms under which the work was published.
This broad license was developed to facilitate open access to, and free use of, original works of all types. Applying this standard license to your own work will ensure your right to make your work freely and openly available. For queries about the license, please contact ann.geophys@ingv.it.