5.2 chemistry Weathering

Chemical weathering outcomes from chemical transforms to minerals that come to be unstable when they are exposed to surface ar conditions. The kinds of alters that take location are highly details to the mineral and the eco-friendly conditions. Some minerals, favor quartz, space virtually unaffected by chemistry weathering, when others, choose feldspar, are quickly altered. In general, the level of chemistry weathering is biggest in warm and also wet climates, and also least in cold and dry climates. The important characteristics of surface conditions that lead to chemical weathering room the existence of water (in the air and on the soil surface), the diversity of oxygen, and the visibility of carbon dioxide, i m sorry produces weak carbonic mountain when merged with water. That process, i beg your pardon is fundamental to most chemical weathering, have the right to 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 water (e.g., together rain) add to carbon dioxide in the atmosphere, combining to produce carbonic acid. Climate carbonic mountain dissociates (comes apart) to type hydrogen and carbonate ions. The lot of CO2 in the air is enough to do only really weak carbonic acid, yet there is commonly much much more CO2 in the soil, therefore water that percolates through the floor can come to be significantly an ext acidic.

You are watching: Feldspars commonly decompose during weathering to clay minerals, silica, and soluble constituents.

There space two main species of chemistry weathering. Top top the one hand, some minerals become altered to various other minerals. Because that example, feldspar is altered — by hydrolysis — come clay minerals. ~ above the various other hand, some minerals dissolve completely, and their contents go right into solution. For 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 shows calcium plagioclase feldspar, but similar reactions could additionally be written for salt or potassium feldspars. In this case, we end up with the mineral kaolinite, together with calcium and also carbonate ion in solution. Those ions have the right to eventually incorporate (probably in the ocean) to form the mineral calcite. The hydrolysis of feldspar to clay is illustrated in number 5.9, which reflects two photos of the exact same granitic rock, a recently damaged fresh surface on the left and a clay-altered weathered surface on the right. Various other silicate mineral can likewise go with hydrolysis, although the end results will certainly be a small different. Because that example, pyroxene deserve to be converted to the clay mineral chlorite or smectite, and 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 item of granitic rock. On the unweathered surfaces the feldspars space still fresh and glassy-looking. On the weathered surface ar the feldspar has been altered to the chalky-looking clay mineral kaolinite.

Oxidation is another really important chemical weathering process. The oxidation the the iron in a ferromagnesian silicate starts with the dissolved of the iron. For olivine, the procedure looks favor this, whereby olivine in the presence of carbonic acid is converted 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 easily converted come hematite:

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

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

The equation shown here is because that olivine, yet it could use to practically any other ferromagnesian silicate, consisting of pyroxene, amphibole, or biotite. Steel in the sulphide mineral (e.g., pyrite) can likewise be oxidized in this way. And the mineral hematite is no the only possible end result, together there is a wide selection of steel oxide minerals the can form in this way. The outcomes of this process are portrayed in number 5.10, which mirrors a granitic absent in which some of the biotite and amphibole have actually been changed to kind the iron oxide mineral limonite.

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

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

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

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

The runoff from areas where this procedure is occurring is known as acid rock drainage (ARD), and also even a rock v 1% or 2% pyrite deserve to produce far-ranging ARD. Few of the worst examples of ARD room at metal mine sites, particularly where pyrite-bearing rock and waste material have been mined from deep underground and also then piled up and left exposed come water and oxygen. One instance of the is the Mt. Washington Mine near Courtenay on Vancouver Island (Figure 5.11), but there room many comparable sites throughout Canada and around the world.

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

At numerous ARD sites, the pH that the runoff water is much less than 4 (very acidic). Under these conditions, steels such together copper, zinc, and lead are quite soluble, which have the right to lead to toxicity for aquatic and 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 work has because been lugged out at the mine and also the case 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 begin with, and thus much more susceptible to mechanical weathering.

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The weathering reactions the we’ve questioned so far involved the transformation of one mineral to another mineral (e.g., feldspar come clay), and also the release of part ions in systems (e.g., Ca2+). Part weathering processes involve the complete dissolution of a mineral. Calcite, because that example, will certainly dissolve in weak acid, to develop 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 an ext than 95%), and under surface ar conditions, limestone will dissolve come varying degrees (depending ~ above which minerals it contains, other than calcite), as displayed in number 5.12. Limestone additionally dissolves at relatively shallow depths underground, forming limestone caves. This is discussed in much more detail in chapter 14, whereby we look in ~ groundwater.

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Figure 5.12 A limestone outcrop ~ above Quadra Island, B.C. The limestone, i beg your pardon is primarily comprised of the mineral calcite, has actually been dissolved to different degrees in different locations because of compositional differences. The buff-coloured bands space volcanic rock, i m sorry is no soluble.