Weathering, minerals

Acidolysis is a similar weathering reaction to hydrolysis in that is used to weather minerals, but in this case the source of is not water but organic or inorganic acids. Humic and fulvic acids (discussed in Section 8.3.2), carbonic acid, nitric or sulfuric acid, and low-molecular-weight organic acids such as oxalic acid can all provide H to weather minerals. All of these acids occur naturally in soils in addition nitric and sulfuric acid can be added to soil by acid pollution. The organic acids are prevalent in the... 

The carbonate weathered mineral is calcium/magnesium and the reaction consumes one CO2 from the atmosphere and liberates one CO2 coming from the matrix. The main weathering equation is ... 

Class 5 (the least sensitive) soil include Kashtanozem, Brown soil and Sierozem soil zones in the Plateau of Inner Mongolia and the Loess Plateau, Desert soil zones in He-xi-zou-lang and the Talimu River Basin, Subalpine Steppe soil, Alpine Steppe soil and Alpine Desert soil in the Plateau of Tibet. These kinds of soils, belonging to the soil class of Xerosol or Alpine soil, consist of easy weathering minerals such as carbonate. They show alkaline reactions, with weak leaching and sparse vegetation. Those kinds of soils are insensitive to acid deposition. 

The weathering rate in d) corresponds to a "smoothed" line reflecting the actual data. The change with altitude is primarily caused probably by an increase in effective surface area of weathered mineral grains. With decreasing elevation more soil is produced partially also due to increased biological activity. 

In Table 3, susceptibility to weathering increases down the list as fewer silicon-oxygen bonds need to be broken to release silicate. Consequently, quartz and feldspars especially, but also mica in temperate soils, are common inherited minerals in the coarse particle size fractions of soil (the silt and sand fractions, 0.002-2 mm). The amphiboles, pyroxenes, and olivine are much more easily weathered. Thus, soils derived from parent material with rock containing a predominance of framework silicates e.g. granite, sandstone) tend to be more sandy, while those derived from rocks containing the more easily weathered minerals tend to be more clayey. 

As Figure 15.3 illustrates, aggrading vegetation (forests and intensive crop production) produces acidity because as more cations are taken up by the plants (trees) H" is released through the roots. The protons released react with the weatherable minerals to produce some of the cations needed by the plants. The H" balance in soils (production through the roots versus consumption by weathering) is delicate and can be disturbed by acid deposition. 

The formula of the solid product (Al2Si205(0H)4) is that for kaolinite, an important member of the serpentine-kaolin group of clay minerals (Section 4.5.1). This reaction demonstrates incongruent dissolution of feldspar, i.e. dissolution with in situ reprecipitation of some compounds from the weathered mineral. 

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