The influence of crystallinity on high--temperature syn--eruptive gas
uptake by volcanic ash
Abstract
Formation of surficial sulfate– and halide–bearing salts by
syn–eruptive ash–gas interactions is known to occur during volcanic
eruptions. For reactions between aluminosilicates and the gas SO2, at
high temperature regimes (T≥ 600 °C), the controlling mechanism is the
outward chemical diffusion of alkalis and alkaline earth metals,
predominantly Ca2+, that result in sulfate salt formation, mostly CaSO4,
on glass surfaces. However, most of the experimental research has been
conducted for SO2–reactions with pure crystal–free, aluminosilicate
glass, to simplify the complexities of crystal–bearing systems. Here,
we tested high temperature SO2–reactions using particles of a
rhyolitic, crystal–bearing dome material from a 2013 eruption of
Santiaguito volcano (Guatemala), by exposing 2 g of particles to 25 sccm
of SO2; at 600–800 °C, for 5–60 min each time. We then compare our
results with those of previous studies using pure glass particles,
aiming to determine the influence of crystal fraction and type on the
occurrence and efficiency of gas–ash reactions. We conducted chemical
and microscopic analysis of pre– and post–treated samples and observed
that diffusion of Ca2+ is reduced in crystal–bearing samples relative
to crystal–free samples at the same conditions. The rate of slow–down
of the diffusion process appears to be dependent on the crystal volume
fraction, providing a mechanism to account for this effect a priori. SEM
images also showed that surface componentry strongly affects presence of
CaSO4, as salts appear to be absent on specific surface spots
corresponding to crystal phases. Our results illustrate the need for
ash-gas reaction studies to further consider both the effect of bulk–
and surface–componentry, in order to more accurately assess
syn-eruptive gas uptake by ash.