Fjölrit RALA - 15.06.2004, Page 139

Proton/AI3+ exchange reaction as a precursor of the hydrolysis of volcanic
glasses
D. Wolff-Boenisch1, S. R. Gíslason1 and E. H. Oelkers2
1 Science Institute, University oflceland, Reykjavik, Iceland 2 Géochimie: Transferts et Mécanismes, CNRS/URM
5563—UniversitéPaulSabatier, 31400 Toulouse, France
Oelkers and Gislason (2001) and Gislason and Oelkers (2003) have proposed a mechanism
and rate equation for basaltic glass dissolution. This equation has two straightforward
implications:
1) The logarithm of the forward dissolution rate (r+) is proportional to the logarithm of the
quotient (a3H+/aAi3+). This relationship has been validated now not only for basaltic but
also for more acid volcanic glasses up to rhyolites. It follows that, under acid
conditions, natural glasses obey the same dissolution mechanism regardless of their
chemical composition. Caution must be applied as experimental results from a previous
study (Wolff-Boenisch et al., 2004a) indicate that glass dissolution rates are only
proportional to geometric than BET derived surface areas.
2) A reduction in the activity of Al3+ at fixed pH will inevitably lead to an increase in r+.
This assumption has been tested by mixed flow reactor experiments with volcanic
glasses where far from equilibrium steady state dissolution rates were measured at 25° C
and pH = 4 as a function of aqueous fluoride concentration. The additional fluoride is
expected to lower the aqueous Al3+ activity due to aqueous Al-F complex formation.
Indeed, natural glass dissolution rates increase dramatically with increasing aqueous
fluoride concentration by more than one order of magnitude in response to a rise in
aqueous fluoride concentration from 0 to 3.8 ppm.
The observed enhancement of volcanic glass dissolution rates with aqueous fluoride
concentration has numerous implications for major and trace element mobility in volcanic
terrains. For example, aqueous fluoride concentration of melted snow in contact with pristine
volcanic ash can vary from less than one ppm to more than 1200 ppm. Glass dissolution rates
in these natural systems may be far faster than previously estimated (Wolff-Boenisch et al.
2004b).
References
Gislason S.R. and Oelkers E.H. (2003) The mechanism, rates, and consequences of basaltic
glass dissolution: II. An experimental study of the dissolution rates of basaltic glass as a
function of pH at temperatures from 6°C to 150°C. Geochim. Cosmochim. Acta 67, 3817-
3832.
Oelkers E.H. and Gislason S.R. (2001) The mechanism, rates, and consequences of basaltic
glass dissolution: I. An experimental study of the dissolution rates of basaltic glass as a
function of aqueous Al, Si, and oxahc acid concentration at 25° C and pH = 3 and 11.
Geochim. Cosmochim. Acta 65, 3671-3681.
Wolff-Boenisch D., Gislason S.R., Oelkers E.H., and Putnis C.V. (2004a) The dissolution
rates of natural glasses as a function of their composition at pH 4 and 10.6, and temperatures
from 25 to 74°C. Geochim. Cosmochim. Acta, in press.
Wolff-Boenisch D., Gislason S.R., and Oelkers E.H. (2004b) The effect of fluoride on the
geometric surface area normalized dissolution rates of natural volcanic glasses at pH 4 and
25°C. Geochim. Cosmochim. Acta, inpress.
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