5.2 chemical Weathering

Chemical weathering outcomes from chemical transforms to minerals that end up being unstable as soon as they room exposed to surface conditions. The kinds of changes that take location are highly details to the mineral and the ecological conditions. Part minerals, choose quartz, are virtually unaffected by chemistry weathering, when others, choose feldspar, are quickly altered. In general, the degree of chemical weathering is best in warm and wet climates, and also least in cold and also dry climates. The important qualities of surface conditions that cause chemical weathering room the existence of water (in the air and on the ground surface), the variety of oxygen, and also the existence of carbon dioxide, i beg your pardon produces weak carbonic acid when linked with water. The process, which is basic to many chemical weathering, can be presented as follows:

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

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

Here we have actually water (e.g., together rain) to add carbon dioxide in the atmosphere, combine to develop carbonic acid. Then carbonic acid dissociates (comes apart) to form hydrogen and carbonate ions. The amount of CO2 in the air is enough to make only an extremely weak carbonic acid, but there is typically much much more CO2 in the soil, so water the percolates v the soil can become significantly more acidic.

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There room two main varieties of chemistry weathering. On the one hand, part minerals end up being altered to other minerals. Because that example, feldspar is transformed — through hydrolysis — to clay minerals. On the other hand, part minerals dissolve completely, and their materials go right into solution. For example, calcite (CaCO3) is soluble in acidic solutions.

The hydrolysis of feldspar can be written choose this:

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

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

This reaction shows calcium plagioclase feldspar, but similar reactions could additionally be written for sodium or potassium feldspars. In this case, we end up with the mineral kaolinite, along with calcium and also carbonate ions in solution. Those ions can eventually integrate (probably in the ocean) to kind the mineral calcite. The hydrolysis of feldspar come clay is depicted in figure 5.9, which shows two pictures of the exact same granitic rock, a recently damaged fresh surface ar on the left and a clay-altered weathered surface on the right. Other silicate mineral can likewise go v hydrolysis, although the finish results will be a little different. For example, pyroxene deserve to be convert to the clay minerals chlorite or smectite, and olivine can be convert to the clay mineral serpentine.

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

Oxidation is another really important chemical weathering process. The oxidation of the stole in a ferromagnesian silicate starts v the dissolved of the iron. For olivine, the process looks choose this, wherein olivine in the presence of carbonic acid is converted to dissolved 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 easily converted come hematite:

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

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

The equation displayed here is because that olivine, however it could use to almost any other ferromagnesian silicate, consisting of pyroxene, amphibole, or biotite. Stole in the sulphide minerals (e.g., pyrite) can also be oxidized in this way. And the mineral hematite is no the only feasible end result, as there is a wide range of stole oxide minerals that can form in this way. The outcomes of this process are shown in figure 5.10, which shows a granitic absent in which few of the biotite and amphibole have actually been transformed to form the stole oxide mineral limonite.

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

A special kind of oxidation takes location in areas where the rocks have elevated level of sulphide minerals, particularly pyrite (FeS2). Pyrite reacts through water and oxygen to type sulphuric acid, together follows:

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

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

The runoff from areas where this process is taking place is recognized as acid absent drainage (ARD), and also even a rock through 1% or 2% pyrite can produce far-reaching ARD. Some of the worst examples of ARD are at steel mine sites, especially where pyrite-bearing rock and also waste material have been mined indigenous deep underground and then piled up and also left exposed come water and also oxygen. One example of that is the Mt. Washington Mine near Courtenay on Vancouver Island (Figure 5.11), yet there space many similar sites across Canada and around the world.

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

At countless ARD sites, the pH the the runoff water is much less than 4 (very acidic). Under this conditions, steels such together copper, zinc, and also lead are fairly soluble, which deserve to lead come toxicity because that aquatic and also other organisms. For countless years, the river downstream indigenous the Mt. Washington Mine had actually so much liquified copper in it that it was toxicity to salmon. Remediation occupational has since been lugged out in ~ the mine and also the instance has improved.

The hydrolysis of feldspar and also other silicate minerals and also the oxidation of stole in ferromagnesian silicates all serve to produce rocks that space softer and also weaker than they were to start with, and thus much more susceptible to mechanically weathering.

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

CaCO3 + H+ + HCO3– —–> Ca2+ + 2HCO3–

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

Calcite is the major component of limestone (typically more than 95%), and under surface ar conditions, limestone will certainly dissolve to varying levels (depending ~ above which mineral it contains, other than calcite), as displayed in number 5.12. Limestone likewise dissolves at reasonably shallow depth underground, creating limestone caves. This is questioned in more detail in thing 14, where we look at groundwater.

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Figure 5.12 A limestone outcrop ~ above Quadra Island, B.C. The limestone, which is primarily comprised of the mineral calcite, has actually been dissolved to different levels in different areas because that compositional differences. The buff-coloured bands are volcanic rock, which is not soluble.