5.2 chemistry Weathering

Chemical weathering outcomes from chemical alters to mineral that become unstable when they room exposed to surface conditions. The kinds of transforms that take location are highly particular to the mineral and also the environmental conditions. Part minerals, like quartz, space virtually unaffected by chemical weathering, if others, favor feldspar, are quickly altered. In general, the degree of chemical weathering is biggest in warm and wet climates, and also least in cold and also dry climates. The important features of surface conditions that result in chemical weathering are the presence of water (in the air and also on the ground surface), the diversity of oxygen, and the visibility of carbon dioxide, which produces weak carbonic mountain when an unified with water. That process, which is fundamental to many chemical weathering, can be displayed as follows:

H2O + CO2 —->H2CO3 then 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) add to carbon dioxide in the atmosphere, combining to create carbonic acid. Climate carbonic mountain dissociates (comes apart) to kind hydrogen and also carbonate ions. The amount of CO2 in the wait is sufficient to do only really weak carbonic acid, however there is typically much an ext CO2 in the soil, so water the percolates through the soil can end up being significantly much more acidic.

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There are two main varieties of chemical weathering. ~ above the one hand, some minerals come to be altered to other minerals. Because that example, feldspar is changed — by hydrolysis — to 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 can be written like this:

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

plagioclase + carbonic mountain —-> kaolinite + liquified calcium + lead carbonate ions

This reaction shows calcium plagioclase feldspar, but comparable reactions could likewise be created for salt or potassium feldspars. In this case, we finish up with the mineral kaolinite, together with calcium and carbonate ion in solution. Those ions can eventually incorporate (probably in the ocean) to kind the mineral calcite. The hydrolysis of feldspar to clay is shown in number 5.9, which reflects two photos of the exact same granitic rock, a recently broken fresh surface ar on the left and a clay-altered weathered surface on the right. Various other silicate minerals can likewise go with hydrolysis, return the end results will certainly be a little different. Because that example, pyroxene can be convert to the clay minerals chlorite or smectite, and also olivine can be convert to the clay mineral serpentine.

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

Oxidation is another really important chemistry weathering process. The oxidation the the stole in a ferromagnesian silicate starts through the resolution of the iron. Because that olivine, the procedure looks like this, wherein olivine in the visibility of carbonic mountain is convert to dissolved iron, carbonate, and also silicic acid:

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

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

In the visibility of oxygen, the dissolved iron is then conveniently converted to hematite:

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

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

The equation shown here is for olivine, however it could use to virtually any other ferromagnesian silicate, including pyroxene, amphibole, or biotite. Iron in the sulphide minerals (e.g., pyrite) can additionally be oxidized in this way. And the mineral hematite is not the only possible end result, together there is a wide range of stole oxide minerals the can form in this way. The outcomes of this procedure are illustrated in figure 5.10, which mirrors a granitic rock in which few of the biotite and also amphibole have actually been changed to form the iron oxide mineral limonite.

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Figure 5.10 A granitic absent containing biotite and amphibole which have been transformed near come the rock’s surface ar to limonite, i m sorry is a mixture of stole oxide minerals.

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

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

pyrite + oxygen + water —–> iron ion + sulphuric mountain + hydrogen ions

The runoff from locations where this procedure is taking place is well-known as acid absent drainage (ARD), and even a rock through 1% or 2% pyrite have the right to produce far-reaching ARD. Several of the worst instances of ARD room at metal mine sites, particularly where pyrite-bearing rock and waste material have actually been mined from deep underground and then piled up and also left exposed come water and oxygen. One instance of the is the Mt. Washington Mine near Courtenay top top Vancouver Island (Figure 5.11), however there are many comparable sites throughout Canada and around the world.

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

At many ARD sites, the pH that the runoff water is less than 4 (very acidic). Under these conditions, steels such as copper, zinc, and also lead are rather soluble, which can lead come toxicity because that aquatic and other organisms. For countless years, the flow downstream native the Mt. Washington Mine had so much liquified copper in it the it was toxicity to salmon. Remediation job-related has because been brought out in ~ the mine and the case has improved.

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

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

CaCO3 + H+ + HCO3– —–> Ca2+ + 2HCO3–

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

Calcite is the significant component the limestone (typically an ext than 95%), and under surface ar conditions, limestone will certainly dissolve to varying degrees (depending top top which mineral it contains, other than calcite), as presented in figure 5.12. Limestone additionally dissolves at fairly shallow depth underground, forming limestone caves. This is debated in much more detail in chapter 14, where we look in ~ groundwater.

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Figure 5.12 A limestone outcrop on Quadra Island, B.C. The limestone, i m sorry is primarily consisted of of the mineral calcite, has been dissolved to different levels in different locations because the compositional differences. The buff-coloured bands are volcanic rock, i m sorry is not soluble.