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Accueil du site → Doctorat → États-Unis → 1994 → A physico-chemical model for simulating neogenesis of clay minerals in arid soils

Indiana University (1994)

A physico-chemical model for simulating neogenesis of clay minerals in arid soils

Alsaaran, Nasser Abdalaziz

Titre : A physico-chemical model for simulating neogenesis of clay minerals in arid soils

Auteur : Alsaaran, Nasser Abdalaziz

Université de soutenance : Indiana University

Grade : Doctor of Philosophy (PhD) 1994

The aim of this study was to develop a physico-chemically based model for simulating clay mineral neogenesis. The model employs thermodynamic potential and kinetic laws to simulate mineral dissolution-precipitation. Homogeneous reactions are considered reversible while heterogeneous reactions are irreversible. Dissolution-precipitation rate laws are based on transition state theory, wherein the net rate of the isothermal-isobaric irreversible reaction is a function of the activity of the species forming the activated complex, surface area of the reacting mineral, and the chemical affinity of the irreversible reaction. The masses of the chemical elements in the aqueous phase, obtained by solving continuity equations for mineral dissolution-precipitation and solute transport, provide input for calculating the activities of the aqueous species. The numerical simulation, based on data from a semiarid climate station, is interpreted to indicate a set of feedbacks controlled by the dissociation equilibrium constants of the secondary aqueous species as well as the identity of the secondary minerals being precipitated (including their rates of precipitation). In the simulation, concentrations of H$\sb4$SiO$\sb4$, Al$\sp3+$, Ca$\sp2+$, and total concentrations of Si and Ca increase whereas Mg$\sp2+$, Al(OH)$\sb4\sp-$, and total concentrations of Al and Mg decrease in response to rain infiltration. Permanent undersaturation of the soil solution with respect to palygorskite and sepiolite during the simulation period (1 yr) supports a suggestion (Monger and Daugherty 1991a,b) that induration of the calcic horizon to the extent that calcite begins replacing silicate grains is a prerequisite for palygorskite neoformation. The amount of kaolinite precipitated during the simulation decreased with depth while the amount of Ca-Montmorillonite increased, which is in accordance with field observations. Although it has been generally assumed in the past that calcite in semiarid soils (with noncalcareous parent material) is of an eolian origin, the results of this study indicate that calcite can precipitate in the lower part of a soil profile throughout the year even when the parent material is noncalcareous and the rain water is calcium free.

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