Which of the Following is an Example of Chemical Weathering

Affiliate 5 Weathering and Soil

five.2 Chemical Weathering

Chemical weathering results from chemical changes to minerals that become unstable when they are exposed to surface conditions. The kinds of changes that take place are highly specific to the mineral and the environmental conditions. Some minerals, similar quartz, are virtually unaffected by chemical weathering, while others, like feldspar, are easily altered. In general, the degree of chemic weathering is greatest in warm and wet climates, and least in common cold and dry climates. The of import characteristics of surface conditions that lead to chemic weathering are the presence of water (in the air and on the ground surface), the abundance of oxygen, and the presence of carbon dioxide, which produces weak carbonic acid when combined with water. That procedure, which is fundamental to most chemical weathering, can be shown as follows:

H₂O + CO_ii
—->H_iiCO_three

then   H₂CO₃
—-> H⁺     + HCO_three
^–,

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

Hither we have water (east.k., every bit rain) plus carbon dioxide in the atmosphere, combining to create carbonic acid. And so carbonic acid dissociates (comes apart) to form hydrogen and carbonate ions. The amount of CO₂
in the air is enough to make merely very weak carbonic acid, but at that place is typically much more CO₂
in the soil, so water that percolates through the soil can become significantly more than acidic.

There are ii chief types of chemical weathering. On the 1 hand, some minerals become altered to other minerals. For example, feldspar is altered — by
hydrolysis
— to
clay minerals. On the other hand, some minerals dissolve completely, and their components go into solution. For example, calcite (CaCO₃) is soluble in acidic solutions.

The hydrolysis of feldspar can exist written like this:

CaAl₂Si_twoO₈
+ H₂CO₃  + ½O₂
—-> Al_twoSi₂O₅(OH)₄
+
Ca²⁺
+  CO_three
^2-

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

This reaction shows calcium plagioclase feldspar, simply like reactions could also be written for sodium or potassium feldspars. In this example, we cease upwards with the mineral kaolinite, along with calcium and carbonate ions in solution. Those ions can eventually combine (probably in the ocean) to form the mineral calcite. The hydrolysis of feldspar to clay is illustrated in Figure five.nine, which shows two images of the same granitic rock, a recently broken fresh surface on the left and a dirt-altered weathered surface on the right. Other silicate minerals tin also go through hydrolysis, although the finish results volition exist a petty different. For instance, pyroxene tin can exist converted to the clay minerals chlorite or smectite, and olivine tin be converted to the clay mineral serpentine.

Oxidation
is another very important chemical weathering process. The oxidation of the atomic number 26 in a ferromagnesian silicate starts with the dissolution of the atomic number 26. For olivine, the procedure looks like this, where olivine in the presence of carbonic acid is converted to dissolved iron, carbonate, and silicic acrid:

Fe_twoSiO₄+ 4H₂CO_three—> 2Fe
^₂+   +  4HCO_iii
^–       +  H₄SiO₄

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

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

2Fe
^₂+
+ 4HCO₃– + ½ O_two     +  2H₂O —->Fe₂O₃
+ 4H_iiCO_iii

dissolved fe + bicarbonate + oxygen + h2o—->hematite + carbonic acid

The equation shown here is for olivine, but it could use to well-nigh any other ferromagnesian silicate, including pyroxene, amphibole, or biotite. Fe in the sulphide minerals (eastward.m., pyrite) can also be oxidized in this way. And the mineral hematite is not the but possible end issue, equally there is a wide range of iron oxide minerals that can form in this way. The results of this process are illustrated in Figure five.x, which shows a granitic stone in which some of the biotite and amphibole have been contradistinct to grade the iron oxide mineral limonite.

A special type of oxidation takes place in areas where the rocks take elevated levels of sulphide minerals, specially pyrite (FeS_ii). Pyrite reacts with water and oxygen to class sulphuric acrid, every bit follows:

2FeS_two+ 7O_two +2H₂O —–> 2Fe²⁺   H₂And then_iv+ 2H⁺

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

The runoff from areas where this procedure is taking place is known as
acrid stone drainage
(ARD), and fifty-fifty a rock with 1% or ii% pyrite can produce significant ARD. Some of the worst examples of ARD are at metal mine sites, specially where pyrite-bearing rock and waste fabric take been mined from deep cloak-and-dagger and and then piled upward and left exposed to h2o and oxygen. Ane instance of that is the Mt. Washington Mine almost Courtenay on Vancouver Island (Effigy 5.xi), but in that location are many similar sites across Canada and around the globe.

At many ARD sites, the pH of the runoff water is less than iv (very acidic). Under these conditions, metals such as copper, zinc, and lead are quite soluble, which can lead to toxicity for aquatic and other organisms. For many years, the river downstream from the Mt. Washington Mine had so much dissolved copper in it that it was toxic to salmon. Remediation work has since been carried out at the mine and the state of affairs has improved.

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

The weathering reactions that we’ve discussed then far involved the transformation of ane mineral to another mineral (e.g., feldspar to dirt), and the release of some ions in solution (e.g., Ca^two+). Some weathering processes involve the complete dissolution of a mineral. Calcite, for example, will dissolve in weak acid, to produce calcium and bicarbonate ions. The equation is as follows:

CaCO₃
+ H⁺   + HCO₃
^–  —–>   Ca²⁺  + 2HCO₃
^–

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

Calcite is the major component of limestone (typically more 95%), and under surface conditions, limestone will dissolve to varying degrees (depending on which minerals information technology contains, other than calcite), as shown in Effigy 5.12. Limestone also dissolves at relatively shallow depths cloak-and-dagger, forming limestone caves. This is discussed in more detail in Chapter fourteen, where we expect at
groundwater.