Unit 1 L3 Chemical weathering

Chemical Weathering
Most common and important than physical weathering processes, even in arid climates.
Due to the low temperatures of the weathering environment, chemical weathering occurs very slowly.
Involves changes that can alter both the chemical and the mineralogical composition of rocks.
Minerals in the rocks are attacked by water and dissolved atmospheric gases (oxygen, carbon dioxide), causing some components of the minerals to dissolve and be removed in solution.
Some mineral constituents recombine in situ and crystallize to form new mineral phases.
The chemical changes, along with physical weathering, disrupt the fabric of the weathered rock, producing residual blocks and a loose residue of resistant grains and secondary minerals.
Simple Solution: (congruent dissolution) occurs when a mineral goes into solution completely without precipitation of other substances.
E.g., Highly soluble minerals like, calcite, dolomite, gypsum, and halite, and even less soluble minerals such as quartz, undergo simple solution during exposure to meteoric water (rainwater).
Chemical bonds between ions in the minerals are broken, destroying the minerals and releasing constituent ions into solution in surface and ground waters.
If carbon dioxide is dissolved in the rainwater through interaction with atmospheric or soil CO2, the solubilizing ability of water is enhanced. Dissolution of CO2 in water forms carbonic acid (H2CO3-), which subsequently dissociates to produce hydrogen ions and carbonate ions
(CO2 + H2O ↔ H2CO3-↔ H+ + HCO3-).
Increase in H+ ions, relative to OH- ions, makes meteoric waters more acidic and thus more aggressive dissolution agents, particularly for carbonate minerals.
Hydrolysis is an important chemical reaction between silicate minerals and acids that leads to breakdown of the silicate minerals and release of metal cations and silica.
The reaction does not lead to complete dissolution of the minerals.
The amount of ions from the mineral that are taken into solution during weathering does not correspond to the formula of the weathering mineral. This kind of incomplete dissolution is called incongruent dissolution.
If Al is present in the minerals undergoing incongruent dissolution, clay minerals such as kaolinite, illite, and smectite may form as a by-product of hydrolysis.
E.g., orthoclase feldspar can break down to yield kaolinite or illite, albite (plagioclase feldspar) can decompose to kaolinite or smectite, and so on.
Hydrolysis
The H+ ions are commonly supplied by the dissociation of CO2 in water- the more CO2 that is dissolved in water, the more aggressive the hydrolysis reaction.
Hydrolysis can also take place in water containing no dissolved CO2, with H+ ions being supplied either by clay minerals that have a high proportion of H+ ions in cation exchange sites or by living plants, which create an acid environment.
Silica set free during hydrolysis goes into solution as silicic acid (H4SiO4); some of the silica may separate as colloidal or amorphous SiO2 and be left behind during weathering to combine with aluminum to form clay minerals.
Hydrolysis is the primary process by which silicate minerals decompose during weathering.
pH
The acidity or alkalinity of a solution is expressed by its pH. The pH is defined as the negative logarithm to the base 10 of the approximate hydrogen-ion concentration in moles per liter.
The pH scale extends from 0 to 14, corresponding to H+ concentrations ranging from 100 to 10-14.
E.g., a solution containing a H+ concentration of 10-1 moles per liter has a pH of 1, an H+ concentration of 10-7 yields a pH of 7, and so forth.
Solutions with a pH of 7 are considered neutral.
Acids have pH values lower than 7 and bases have values greater than 7.
Oxidation and Reduction: Chemical alteration of Fe and Mn in silicate Another element that oxidizes during weathering is sulfur.
E.g., pyrite (FeS2) is oxidized to form hematite (Fe2O3), with release of soluble sulfate ions.
Under some conditions where material undergoing weathering is water saturated, oxygen supply may be low and oxygen demand by organisms high. These conditions can bring about reduction of iron (gain of an electron) from Fe3+ to Fe2+.
Ferrous iron (Fe2+) is more soluble, and thus more mobile, than ferric iron (Fe3+) and may be lost from the weathering system in solution.
minerals such as biotite and pyroxenes, caused by oxygen dissolved in water, is an important weathering process because of the abundance of Fe in the common rock-forming silicate minerals.
An electron is lost from iron during oxidation (Fe2+ to Fe3+ + e-, where e- = electron transfer), which causes loss of other cations such as Si4+ from crystal lattices to maintain electrical neutrality.
Cation loss leaves vacancies in the crystal lattice that make the mineral more susceptible to attack by other weathering processes.