Weathering general reactions

Mineral weathering generally occurs whenever water comes into contact with primary mineral particles. The overall processes of weathering are not reversible even though individual reaction steps are often treated as reversible equilibria. The main chemical mechanisms of weathering are ... 

Following these general considerations of the source of protons and the role of oxidation-reduction reactions in chemical weathering, general weathering reactions can be written for silicate and carbonate minerals. Four situations are discussed ... 

These reactions, however, are complex and generally proceed through a series of reaction steps. The rate of weathering of siUcates may vary considerably, depending on the arrangement of the siUcon tetrahedra in the mineral and on the nature of the cations. 

Dipped Goods. Gloves with and without cloth support are a typical example. Unsupported appUcations involve specialty medical gloves and weather balloons. Eor CR to compete with less expensive natural mbber, it must provide added value. Natural mbber medical examination gloves contain proteins that can cause an allergic reaction in both patients and medical personnel. Polychloroprene gloves avoid the problem for those individuals, but are too expensive for general use. Natural mbber weather balloons have less lift than CR balloons and are rapidly attacked by ozone in the upper atmosphere. 

Overall, weathering controls the chemistry of material that is transported into the sediment and that which stays behind in the soil. As an example, consider a general weathering reaction for an aluminosilicate (Stumm and Morgan, 1995) ... 

The weathering of silicates has been investigated extensively in recent decades. It is more difficult to characterize the surface chemistry of crystalline mixed oxides. Furthermore, in many instances the dissolution of a silicate mineral is incipiently incongruent. This initial incongruent dissolution step is often followed by a congruent dissolution controlled surface reaction. The rate dependence of albite and olivine illustrates the typical enhancement of the dissolution rate by surface protonation and surface deprotonation. A zero order dependence on [H+] has often been reported near the pHpzc this is generally interpreted in terms of a hydration reaction of the surface (last term in Eq. 5.16). 

Over longer time scales, clay minerals can undergo more extensive reactions. For example, fossilization of fecal pellets in contact with a mixture of clay minerals and iron oxides produces an iron- and potassium-rich, mixed-layer clay called glauconite. This mineral is a common component of continental shelf sediments. Another example of an authigenic reaction is called reverse weathering. In this process, clay minerals react with seawater or porewater via the following general scheme ... 

Reverse weathering is thought to occur during halmyrolysis, diagenesis, and catagenesis as clay minerals react with the major ions, bicarbonate, and DSi in seawater. The nature of these reactions is not well known. Their general form is thought to be... 

---