UC Merced

Far-from-equilibrium, steady-state dissolution rates at pH 4 of a

suite of natural glasses, ranging from basaltic to rhyolitic in composition,

have been determined as a function of aqueous fluoride concentrations up to 1.8

X 10(-4) mol/kg in mixed-flow reactors. Dissolution rates of each of these

glasses increase monotonically with increasing aqueous fluoride concentration.

Measured dissolution rates are found to be consistent with both the Furrer and

Stumm (1986) surface coordination model and the Oelkers (2001) multi-oxide

dissolution model. Application of the latter model yields the following

equation that can describe all measured rates as a function of both glass and

aqueous solution composition: log (r(+.geo)/(mol/m(2)/s)) = [-0.086 (.)

SiO2(wt%) - 2.23] + [0.0067 (.) SiO2(wt%) + 0.683] (.)

log(alpha(H+)(3)/alpha(Al3+)) where r(+),(geo) represents the

far-from-equilibrium dissolution rate, normalized to geometric surface area,

SiO2(wt.%) refers to weight percent of SiO2 in the glass, and alpha(i) denotes

the activity of the subscripted aqueous species. Computed glass dissolution

rates increase with increasing aqueous fluoride concentration due to the

formation of aqueous Al-fluoride complexes, which decrease alpha(Al)(3+). This

rate expression can be used to predict far-from-equilibrium dissolution rates

of natural glasses in a variety of natural environments. Comparison of rate

predictions with the composition of natural fluids suggests that the presence

of aqueous fluoride can enhance natural glass dissolution rates by an order of

magnitude or more in a variety of geochemical systems. Copyright (C) 2004

Elsevier Ltd.