Geology Reference
In-Depth Information
For a selection of papers concerning kinetics of solid-state processes in
geological systems, see Hofmann et al. (
1974
) and Lasaga and Kirkpatrick (
1981
).
3.3.5 Dissolution and Crystallization
Many processes in rocks proceed more rapidly when a fluid phase is present
because of the faster transport of material either by diffusion or by bulk transport
in the fluid, and some are only defined in the presence of a fluid. An example of
fluid involvement is recrystallizing of dissolved material as new crystals or as
additional growth on existing crystals.
The kinetics of dissolution are simplified by the absence of a nucleation barrier
but, as with growth, the kinetics can still be either interface controlled or transport
controlled (Berner
1981
). The kinetics of the interface processes may depend sen-
sitively on the presence of adsorbed impurities, occupying sites that might otherwise
be occupied by reactants, or by crystal imperfections that intersect the interface to
provide sites where bonding energies will be different; the latter effect is revealed in
etch pits located where dislocations or planar defects intersect the interface.
Crystallization is, in many aspects, the converse of dissolution although its overall
kinetics can be quite different because of the requirement of nucleation. Again,
interfacial imperfections can play an important part, as seen in the role of screw
dislocations in providing a persistent step at which growth occurs, revealed in spiral
growth patterns, and which obviates some of the nucleation difficulties (Burton et al.
1951
). A review for igneous systems has been given by Cashman (
1990
)
The driving force for dissolution or crystallization derives from the difference
between the saturation concentration and the actual concentration of the crystal
species in the fluid, although the resulting rates sometimes vary as a high power of
this difference rather than linearly (Berner
1981
). However, the saturation
concentration, or solubility, depends on the curvature of the solid-fluid interface. It
is determined thermodynamically through an equilibrium constant or solubility
product, K, based on the activities of the relevant components in fluid and solid
where K
¼
exp
DG
=
R
ð Þ
depends on the change in Gibbs energy DG per mole
crystallizing or dissolving under standard conditions of temperature and pressure.
There is also a contribution to G from the interfacial energy if the curvature of the
interface is changed. Thus, for a given distribution of solid particle sizes, the fluid
may be supersaturated with respect to some of the particles and undersaturated
with respect to others; the result is a trend for the smaller particles to dissolve and
the larger ones to grow, leading to a coarsening of particle or grain size, an
example of a so-called Ostwald ripening process (for example, Chai
1974
).
The solubility can also be affected by the state of stress or the defect content in
the solid. It is especially affected by a stress component normal to the interface,
giving rise to the ''pressure solution'' effect (Lehner
1990
; Paterson
1973
; and
Thompson
1962
). A number of natural structural features have been attributed to
pressure solution, although in some cases the influence of heterogeneity on the
