Congruent weathering

P in soils is 600 years (Fig. 1). As noted above, the only significant weathering source for phosphorus in soils is apatite minerals. These minerals can be congruently weathered as a result of reaction with dissolved carbon dioxide ... 

Dissolution of CaCOs is a congruent reaction the entire mineral is weathered and results completely in soluble products. The above reaction is driven to the right by an increase of CO2 partial pressure and by the removal of the Ca and/or bicarbonate. Any impurities present in the calcareous rock, such as silicates, oxides, organic compounds, and others, are left as residue. As the calcium and bicarbonate leach... 

Minerals can weather congruently to produce only dissolved weathering products or incon-gruently to produce both dissolved cations... 

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). 

The term incongruent is generally used, if a mineral upon dissolution reacts to form a new solid or if the reversal of a dissolution process leads to a different composition. In natural environments incongruent solubility is probably more prevalent, e.g., in weathering of many clays, than congruent dissolution. 

Chemical weathering of minerals results not only in the introduction of solutes to the aqueous phase but often in the formation of new solid phases. Dissolution is described as congruent, where aqueous phase solutes are the only products, or incongruent, where new solid phase(s) in addition to aqueous phase solutes are the products. These reactions... 

At lower temperatures, as discussed in Chapter 5.16, congruent and incongruent weathering reactions and neoformation of clays and zeolite minerals, which are common secondary mineral phases in crystalline rocks, are postulated to alter fluid and isotopic chemistry (Fritz and Frape, 1982) and increase salinity during hydration reactions (Bucher and Stober, 2000). As an example, reaction (5) represents the formation of zeolites. These well-documented fracture-filling minerals have been found by several authors at Fennoscandian sites, and at research sites in Europe (Bucher and Stober, 2000) ... 

Another useful concept is that of congruent and incongruent reactions. These terms describe reactions involving the dissolution of minerals. If all the products of a dissolution reaction are soluble, the reaction is called congruent, as in the case of the quartz dissolution reaction (1.6) described above. Because, as written, the olivine weathering reaction leads to quartz precipitation it is an incongruent reaction. 

Note that because olivine contains no Al, no clays result from reactions (7.1) or (7.2). Also, reaction (7.1) is congruent, whereas reactions (7.2) and (7.3) are incongruent. In other words, in the first reaction all the weathering products are soluble, whereas in the second and third reactions, weathering results in precipitation of new solid phases. Reaction (7.2) involves oxidation of Fe(II) to Fe(lII), with precipitation of the ferric iron in a mineral such as goethite. Among the three examples, only the feldspar contains Al and so can result in a clay. In reaction (7,3) we assume that all the Al moves directly from the feldspar to the kaolinite. However, if the pH is below about 4 or 5, the kaolinite itself becomes soluble and appreciable Al also goes into solution. 

---