Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 443

crystalline phases. The nucleation of solid phases or growth sites, rather
than the binding of solutes at existing growth sites, is increasingly recog-
nized as a key factor that controls mineral precipitation rates. This is illus-
trated in Figure 9.8.6 , which shows that kaolinite dissolution at pH 4 and
22

C is well described by the rate equations, but kaolinite growth at the
same conditions is much better described by a model that accounts for
the rate of nucleation of growth sites on the kaolinite surface [9.39].

°

Dissolution at in-situ conditions

The long-term weathering rates of minerals at geological conditions can
be reconstructed from a range of sources. For example, silicate glass
weathering experiments with durations of tens of years have been carried

4 x 10 -13

Precipation

3 x 10 -13

Short term

2 x 10 -13

Long term

1 x 10 -13

0 x 10 -13

-1 x 10 -13

-2 x 10 -13

-3 x 10 -13

-4 x 10 -13

Equilibrium

Dissolution

-5 x 10 -13

-30 -25 -20 -15 -10 -5 0 5 10 15 20

∆ G (kJ/mol)

Figure 9.8.6 Dissolution and precipitation rates of kaolinite
Dissolution and precipitation rates of kaolinite as a function of the Gibbs free energy of reac-
tion at pH 4 and 22 C. The symbols represent measured values of the steady-state reaction

rate. The dashed blue curve was obtained by fi tting the rate equation described in this sec-
tion (with n

°

1) to the dissolution data. The dashed red curve was obtained by
fi tting a two-dimensional nucleation model to the precipitation data. At

= 0.5 and m =

5 kJ/mol two
experimental values are reported on the graph: the faster growth rate was observed on a
time scale of hours for a kaolinite that had previously been subjected to steady-state dis-
solution for fi ve months and the slower growth rate was observed on longer time scales.
Figure redrawn from Yang and Steefel [9.39] .

∆ G r ≈