Box 49 Mineral reaction kinetics and solution saturation

The discussion in Section 4.6.2 suggests that weathering rates of minerals are proportional to the water flow rate. This is only true if the waters are close to saturation (Box 4.12) with respect to the weathering mineral. If water flow is continuous and sufficiently high, a limit is reached beyond which further flushing is no longer a rate-controlling factor.

With insoluble minerals (solubility <10-4mol l-1 (Box 4.12)), including all silicates and carbonates, ion detachment from mineral surfaces is very slow, such that ions never build up in solution close to the crystal surface. The weathering rate of these minerals thus depends mainly on the rate of ion detachment from the crystal surface, rather than the efficiency of flushing (water flow rate). Only in very soluble minerals (solubility > 2 x 10-3 mol l-1), for example evaporite minerals, can ions detach rapidly from the mineral surface to form a microenvironment close to the crystal surface which is saturated with respect to the dissolving mineral. The rate of dissolution is then controlled by the efficiency of dispersal of these ions and water flushing effects then become important.

microclimate too; these factors in turn affect the potential for erosion and dissolution.

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