Fjölrit RALA - 15.06.2004, Síða 99

Dissolution rate of basaltic glass.
S. R. Gislason1, and E. H. Oelkers2
'Science Institute, University of Iceland; 2 CNRS/URM 5563, Université Paul Sabatier, Toulouse, France
Rapid cooling of magma on the Earth produces approximately one billion cubic meters (1
km3) of glass each year, mainly along the 70,000 km oceanic ridge system (Morgan and
Spera, 2001). Most of this glass is of basaltic composition. The overall objective of the
studies reported here is to define and understand the dissolution rate of volcanic glasses in the
surface environment of the Earth.
Basaltic glass dissolution rates were measured far from equilibrium as a function of aqueous
aluminium, silica, and oxalic acid concentration at 25° C and pH 3 and 11, and as a function
of pH from 2 to 11 at temperatures from 6° to 50° C, and at near neutral conditions to 150° C
(Oelkers and Gislason, 2001; Gislason and Oelkers, 2003). Dissolution rates decrease
dramatically with increasing pH at acid conditions, minimize at near neutral pH, and increase
slowly with increasing pH at basic conditions. The pH, at which basaltic glass dissolution
minimizes, decreases with increasing temperature. Rates are most likely controlled by
partially detached Si at the basaltic glass surface, which is linked to the Al/proton exchange
on the glass surface. The exchange of aluminium for three protons at the surface leads to the
formation of three partially detached Si atoms on the glass surface. Regression of far-from-
equilibrium dissolution rates obtained in our studies and reported in the literature for basaltic
glass indicate that all data over the temperature and pH range 6° < T < 300° C and 1 < pH <
11 can be described within uncertainty using
,1/3
r+,geo = AA eXP
-\E, irt
Cl ,
V Al3
where r+,geo signifies the geometric surface area normalized steady-state basaltic glass
dissolution rate at far-from-equilibrium conditions, Aa refers to a constant equal tolO'5 6 (mol
of Si/cm2/s), E\, designates a pH independent activation energy equal to 25.5 kJ/mol, R
stands for the gas constant, T signifies temperature in K, and a\ represents the activity of the
subscripted aqueous species.
References
Gislason, S.R., and H.E. Oelkers. 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 and temperature. Geochim. Cosmochim. Acta, 67: 3817-3832.
Morgan, N.A., and F.J. Spera. 2001. Glass transition, stractural relaxation, and theories of
viscosity: A molecular dynamics study of amorphous CaAl2Si208. Geochim. Cosmochim.
Acta 65: 4019-4041.
Oelkers, E.H., and S.R. Gislason. 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 oxalic acid concentration at 25° C and pH = 3 and 11.
Geochim. Cosmochim. Acta 65: 3671 - 3681.
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