5.2 chemistry Weathering

Chemical weathering results from chemical transforms to mineral that end up being unstable once they are exposed to surface conditions. The type of alters that take place are highly details to the mineral and also the eco-friendly conditions. Some minerals, favor quartz, are virtually unaffected by chemistry weathering, while others, favor feldspar, are easily altered. In general, the degree of chemistry weathering is greatest in warm and also wet climates, and least in cold and also dry climates. The important qualities of surface problems that cause chemical weathering space the visibility of water (in the air and also on the ground surface), the diversity of oxygen, and also the existence of carbon dioxide, i beg your pardon produces weak carbonic mountain when linked with water. That process, i beg your pardon is basic to most chemical weathering, deserve to be shown as follows:

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

water + carbon dioxide —-> carbonic acid then carbonic mountain —-> hydrgen ion + 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 acid dissociates (comes apart) to type hydrogen and carbonate ions. The quantity of CO2 in the waiting is sufficient to make only very weak carbonic acid, however there is typically much much more CO2 in the soil, so water the percolates v the soil can come to be significantly more acidic.

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

The hydrolysis the feldspar have the right to be written favor 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 end up v the mineral kaolinite, along with calcium and also carbonate ions in solution. Those ions have the right to eventually incorporate (probably in the ocean) to form the mineral calcite. The hydrolysis the feldspar come clay is illustrated in number 5.9, which shows two photos of the same granitic rock, a recently broken fresh surface ar on the left and also a clay-altered weathered surface on the right. Various other silicate mineral can also go through hydrolysis, back the finish results will be a small different. For example, pyroxene have the right to be converted to the clay minerals chlorite or smectite, and olivine deserve to 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. Top top the weathered surface the feldspar has actually been changed to the chalky-looking clay mineral kaolinite.

Oxidation is another very important chemical weathering process. The oxidation of the iron in a ferromagnesian silicate starts v the resolution of the iron. Because that olivine, the procedure looks prefer this, whereby olivine in the presence of carbonic acid is converted to liquified iron, carbonate, and also silicic acid:

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

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

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

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

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

The equation shown here is for olivine, yet it could apply to virtually any other ferromagnesian silicate, consisting of pyroxene, amphibole, or biotite. Iron 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, as there is a wide range of iron oxide minerals that can type in this way. The results of this process are portrayed in figure 5.10, which shows a granitic absent in which few of the biotite and amphibole have been transformed to type the iron oxide mineral limonite.

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

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

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

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

The runoff from areas where this process is occurring is well-known as acid rock drainage (ARD), and also even a rock through 1% or 2% pyrite can produce far-ranging ARD. Few of the worst instances of ARD space at metal mine sites, particularly where pyrite-bearing rock and also waste material have been mined from deep underground and then piled up and also left exposed come water and also oxygen. One example of the is the Mt. Washington Mine near Courtenay ~ above Vancouver Island (Figure 5.11), but there space many similar sites throughout Canada and also around the world.

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

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

The hydrolysis of feldspar and also other silicate minerals and also the oxidation of steel in ferromagnesian silicates all serve to develop rocks that room softer and also weaker 보다 they were to start with, and also thus more susceptible to mechanical weathering.

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The weathering reactions the we’ve debated so far affiliated the change of one mineral to one more mineral (e.g., feldspar come clay), and also the release of part ions in solution (e.g., Ca2+). Part weathering processes involve the complete dissolution of a mineral. Calcite, because that example, will certainly dissolve in weak acid, to create calcium and also bicarbonate ions. The equation is together follows:

CaCO3 + H+ + HCO3– —–> Ca2+ + 2HCO3–

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

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

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