Chemical kinetics of water‐rock interactions

Abstract

The recent literature on the kinetics of water‐rock interactions is reviewed. The data are then extended to provide a quantitative framework for the description of weathering and alteration. The available experimental data on dissolution of silicates verifies quantitatively the usual mineral stability series in sedimentary petrology. The rate of hydration of carbonic acid is shown to be a possible limiting factor in water‐rock interactions. The framework is developed to enable use of laboratory dissolution experimental results and thermodynamics to arrive at a rate law applicable up to equilibrium and therefore applicable to natural systems. The kinetic justification for the significance of a water‐rock ratio is discussed. With a proper treatment of fluid flow, the equations are applied to the weathering profile leading to the development of bauxites from nepheline syenites.

Mathematical Formulation Requirements and Specifications for the Process Models

“…The temperature dependence of the reaction rate constant can be expressed reasonably well via an Arrhenius equation (Lasaga, 1984). Since many rate constants are reported at 25 • C, it is more convenient to write the rate constant at some temperature as…”

“…Another option to be implemented involves a simple geometric method for calculating surface area (Lasaga, 1984). If a simple cubic packing of spherical grains of radius r, is considered, then the cubic arrangement of spheres yields, in a cube of side 4r and volume (4r) 3 , a total of 8 spheres, 85 ascemdoe.org November 9, 2010 each of radius r, volume 4πr 3 3 , and area 4πr 2 .…”

“…If the aqueous phase reactions are not sufficiently fast for a given time scale of interest that they reach equilibrium, then they must be treated kinetically by solving an ordinary differential equation. A convenient way to represent the reactions is with a Transition State Theory (TST) type of rate law (Aagaard and Helgeson, 1982;Lasaga, 1981Lasaga, , 1984)…”

“…It does not mean, however, that one cannot obtain overall transport control on the mineral dissolution or precipitation rate since this depends on the magnitude of the reaction rate relative to the macroscopic transport rates. The rate laws used for mineral precipitation and dissolution are based loosely on transition state theory (Aagaard and Helgeson, 1982;Lasaga, 1981Lasaga, , 1984). …”

Precipitation sealing and diagenesis: 2. Theoretical analysis

“…Ro may be a function of the temperature and pH of solution, and possibly of the stress state in the system [Lasaga, 1984]. V is volume of fluid in contact with mineral 0, Ko is the equilibrium constant, and Qo is the ion activity product,…”