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Thermodynamics. In the long run, the soil chemical system is headed towards its equilibrium state. Thermodynamics tells us what the equilibrium state will be and how far the system is from equilibrium at present. It tells us nothing about how long it will take to reach equilibrium. Many common silicate minerals are inherently unstable at earth surface conditions and they will no longer be present when (or if) the system reaches equilibrium. Nevertheless, these minerals persist in soils for millions of years. Equilibrium between carbonate minerals and water is reached much more quickly, however. Kinetics is concerned with the rate of chemical reactions, including mineral weathering. In congruent dissolution, all weathering products are in solution in soil water. Calcite and dolomite dissolve congruently. Incongruent dissolution results in solid as well as dissolved products, for example, when weathering of a silicate mineral produces both dissolved ions and a clay mineral. Carbonate Mineral Weathering. Depending on the particular problem, carbonate mineral weathering can be interpreted using either thermodynamics or kinetics. From a thermodynamic point of view, calcite solubility in soils is mainly influenced by:
Dolomite solubility is similar except that Mg++ in soil water is also important. The kinetics of carbonate mineral weathering may be important in some cases. The rate of weathering increases with increasing temperature and with increasing distance from equilibrium. That is, if the soil water is nearly saturated with respect to calcite, the rate of dissolution of calcite should be very slow. Silicate Mineral Weathering. The rates of weathering for most silicate minerals are very slow, and have to be interpreted using kinetics. One key control on silicate mineral weathering rates is temperature: weathering rates increase with increasing temperature. Other factors include:
the mineral surface, the local system is closer to equilibrium and reaction rates will slow down).
and above 8 for many minerals). natural organic acids and other organic compounds such as chelates can increase the weathering rate. Distinctive Weathering Processes Biotite weathering in granite apparently results from oxidation of the iron in the biotite structure. This decreases the electrical charges that locks the layers in the biotite structure together, and ultimately alters the biotite to vermiculite or smectite. The resulting increase in mineral volume breaks apart the surrounding granite. This can reduce granite to rubble (or grus) even in dry climates. Acid sulfate weathering occurs when sulfide minerals are oxidized, producing very low pH in the surrounding soil water. This can enhance silicate mineral weathering. Sulfides are oxidized when unweathered shale, glacial till, or marine sediments are exposed by erosion, sea level change, or artificial drainage. Weathering Sequence Least Rapidly Weathered, Most Resistant |
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