5.2 chemistry Weathering

Chemical weathering outcomes from chemical changes to minerals that end up being unstable once they are exposed to surface ar conditions. The kinds of transforms that take location are highly certain to the mineral and the environmental conditions. Part minerals, choose quartz, are virtually unaffected by chemical weathering, when others, like feldspar, are quickly altered. In general, the level of chemical weathering is best in warm and also wet climates, and least in cold and also dry climates. The important features of surface problems that result in chemical weathering room the presence of water (in the air and on the ground surface), the variety of oxygen, and the presence of carbon dioxide, i m sorry produces weak carbonic acid when merged with water. That process, which is fundamental to most chemical weathering, deserve to be presented as follows:

H2O + CO2 —->H2CO3 climate H2CO3 —-> H+ + HCO3–,

water + carbon dioxide —-> carbonic acid then carbonic mountain —-> hydrgen ion + lead carbonate ion

Here we have actually water (e.g., together rain) to add carbon dioxide in the atmosphere, combining to produce carbonic acid. Then carbonic mountain dissociates (comes apart) to form hydrogen and carbonate ions. The quantity of CO2 in the air is sufficient to do only very weak carbonic acid, yet there is frequently much more CO2 in the soil, so water the percolates with the soil can end up being significantly much more acidic.

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There are two main varieties of chemistry weathering. ~ above the one hand, part minerals come to be altered to other minerals. For example, feldspar is altered — through hydrolysis — come clay minerals. Top top the various other hand, some minerals dissolve completely, and their contents go into solution. Because that example, calcite (CaCO3) is dissolve in acidic solutions.

The hydrolysis that feldspar deserve to be written prefer this:

CaAl2Si2O8 + H2CO3 + ½O2 —-> Al2Si2O5(OH)4 + Ca2+ + CO32-

plagioclase + carbonic acid —-> kaolinite + dissolved calcium + lead carbonate ions

This reaction reflects calcium plagioclase feldspar, but comparable reactions could likewise be written for salt or potassium feldspars. In this case, we finish up with the mineral kaolinite, along with calcium and carbonate ions in solution. Those ions have the right to eventually integrate (probably in the ocean) to type the mineral calcite. The hydrolysis the feldspar to clay is illustrated in number 5.9, which reflects two pictures of the exact same granitic rock, a recently broken fresh surface on the left and a clay-altered weathered surface on the right. Various other silicate mineral can additionally go through hydrolysis, although the end results will certainly be a tiny different. Because that example, pyroxene deserve to be converted to the clay mineral chlorite or smectite, and also olivine deserve to be converted to the clay mineral serpentine.

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Figure 5.9 Unweathered (left) and weathered (right) surfaces of the same piece of granitic rock. ~ above the unweathered surface the feldspars are still fresh and glassy-looking. ~ above the weathered surface the feldspar has been transformed to the chalky-looking clay mineral kaolinite.

Oxidation is another very important chemistry weathering process. The oxidation that the stole in a ferromagnesian silicate starts v the dissolution of the iron. For olivine, the process looks choose this, whereby olivine in the visibility of carbonic acid is convert to liquified iron, carbonate, and silicic acid:

Fe2SiO4+ 4H2CO3 —> 2Fe2+ + 4HCO3– + H4SiO4

olivine + (carbonic acid) —> liquified iron + liquified carbonate + liquified silicic acid

In the presence of oxygen, the dissolved iron is then quickly converted come hematite:

2Fe2+ + 4HCO3– + ½ O2 + 2H2O —->Fe2O3 + 4H2CO3

dissolved iron + bicarbonate + oxygen + water—->hematite + carbonic acid

The equation presented here is for olivine, but it could apply to virtually any other ferromagnesian silicate, including pyroxene, amphibole, or biotite. Stole in the sulphide minerals (e.g., pyrite) can also be oxidized in this way. And the mineral hematite is not the only feasible end result, together there is a wide variety of iron oxide minerals that can kind in this way. The results of this process are depicted in figure 5.10, which reflects a granitic absent in which some of the biotite and amphibole have actually been changed to type the steel oxide mineral limonite.

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Figure 5.10 A granitic absent containing biotite and also amphibole which have been changed near to the rock’s surface to limonite, i beg your pardon is a mixture of iron oxide minerals.

A special form of oxidation takes place in areas where the rocks have actually elevated levels of sulphide minerals, especially pyrite (FeS2). Pyrite reacts with water and oxygen to kind sulphuric acid, as follows:

2FeS2 + 7O2 +2H2O —–> 2Fe2+ H2SO4+ 2H+

pyrite + oxygen + water —–> iron ions + sulphuric acid + hydrogen ions

The runoff from locations where this process is ensuing is known as acid rock drainage (ARD), and also even a rock v 1% or 2% pyrite have the right to produce far-reaching ARD. Several of the worst examples of ARD room at metal mine sites, especially where pyrite-bearing rock and waste material have been mined native deep underground and also then piled up and also left exposed to water and also oxygen. One instance of that is the Mt. Washington Mine close to Courtenay ~ above Vancouver Island (Figure 5.11), yet there space many comparable sites throughout Canada and also around the world.

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Figure 5.11 Exposed oxidizing and also acid generating rocks and also mine waste in ~ the exit Mt. Washington Mine, B.C. (left), and an example of acid drainage downstream native the mine website (right).

At many ARD sites, the pH that the runoff water is less than 4 (very acidic). Under this conditions, metals such as copper, zinc, and also lead are fairly soluble, which have the right to lead to toxicity because that aquatic and other organisms. For numerous years, the flow downstream from the Mt. Washington Mine had so much liquified copper in it that it was toxic to salmon. Remediation job-related has since been lugged out in ~ the mine and the instance has improved.

The hydrolysis of feldspar and also other silicate minerals and the oxidation of iron in ferromagnesian silicates all serve to create rocks that space softer and also weaker than they to be to begin with, and also thus much more susceptible to mechanical weathering.

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The weathering reactions the we’ve discussed so far associated the revolution of one mineral to an additional mineral (e.g., feldspar to clay), and the relax of part ions in systems (e.g., Ca2+). Some weathering procedures involve the finish dissolution that a mineral. Calcite, because that example, will certainly dissolve in weak acid, to produce calcium and also bicarbonate ions. The equation is together follows:

CaCO3 + H+ + HCO3– —–> Ca2+ + 2HCO3–

calcite + hydrogen ions + bicarbonate —–> calcium ion + bicarbonate

Calcite is the significant component the limestone (typically much more than 95%), and under surface conditions, limestone will dissolve come varying degrees (depending top top which minerals it contains, various other than calcite), as presented in number 5.12. Limestone likewise dissolves at fairly shallow depths underground, creating limestone caves. This is discussed in more detail in thing 14, whereby we look at groundwater.

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Figure 5.12 A limestone outcrop on Quadra Island, B.C. The limestone, which is primarily comprised of the mineral calcite, has been liquified to different levels in different locations because that compositional differences. The buff-coloured bands space volcanic rock, which is not soluble.