Can accurate kinetic laws be created to describe chemical weathering ?

Knowledge of the mechanisms and rates of mineral dissolution and growth, especially
close to equilibrium, is essential for describing the temporal and spatial evolution of
natural processes like weathering and its impact on CO2 budget and climate. The Surface
Complexation approach (SC) combined with Transition State Theory (TST) provides an
efficient framework for describing mineral dissolution over wide ranges of solution
composition, chemical affinity, and temperature. There has been a large debate for several
years, however, about the comparative merits of SC/TS versus classical growth theories for
describing mineral dissolution and growth at near-to-equilibrium conditions. This study
considers recent results obtained in our laboratory on oxides, hydroxides, silicates, and
carbonates on near-equilibrium dissolution and growth via the combination of
complementary microscopic and macroscopic techniques including hydrothermal atomic
force microscopy, hydrogen-electrode concentration cell, mixed flow and batch reactors.
Results show that the dissolution and precipitation of hydroxides, kaolinite, and
hydromagnesite powders of relatively high BET surface area closely follow SC/TST rate
laws with a linear dependence of both dissolution and growth rates on fluid saturation
state (V) even at very close to equilibrium conditions (jDGj < 500 J/mol). This occurs
because sufficient reactive sites (e.g. at kink, steps, and edges) are available at the exposed
faces for dissolution and/or growth, allowing reactions to proceed via the direct and
reversible detachment/attachment of reactants at the surface. In contrast, for magnesite
and quartz, which have low surface areas, fewer active sites are available for growth and
dissolution. Such minerals exhibit rates dependencies on V at near equilibrium conditions
ranging from linear to highly non-linear functions of V, depending on the treatment of the
crystals before the reaction. It follows that the form of the f(DG) function describing the
growth and dissolution of minerals with low surface areas depends on the availability of
reactive sites at the exposed faces and thus on the history of the mineral-fluid interaction
and the hydrodynamic conditions under which the crystals are reacted. It is advocated that
the crystal surface roughness could serve as a proxy of the density of reactive sites. The
consequences of the different rate laws on the quantification of loess weathering along the
Mississippi valley for the next one hundred years are examined.