is a process of alteration of rocks of the Earth’s crust.
is a chemical decomposition process.
is brought out by atmospheric gases and moisture.
End product has a different chemical composition and poorer physical constitution.
Chemical change in the nature of rock takes place in the presence of moisture containing many active gases such as CO2, N2, H2, and O2.
Rocks are made up of minerals.
All the minerals in a rock are not in chemical equilibrium with the atmosphere around them. Chemical weathering is essentially a process of chemical reactions between the surfaces of rocks and the atmospheric gases. This process takes place so as to establish chemical equilibrium.
Chemical weathering eats up the rocks in a number of ways.
The process depends upon
the rocks’ mineralogical composition, and
the nature of chemical environment surrounding them.
Some of the main processes of chemical weathering are:
2. hydration and hydrolysis,
3. oxidation and reduction,
5. base exchange, and
6. formation of colloids.
Some rocks contain one or more minerals that are soluble in water to some extent. Rock salt (NaCl), gypsum (CaSO4. 2H2O) and calcite (CaCO3)
As it is, pure water is not a good solvent of minerals in most cases. But when pure water is carbonated, its power of dissolving (solvent action) is enhanced. Limestone is not soluble in pure water.
But carbonated water dissolves limestone effectively.
Limestone gets pitted and porous due to chemical weathering. H2O + CO2 → H2CO3 → H+ + HCO3-
CaCO3 + H2CO3 → Ca+2 + 2HCO3
2. HYDRATION and HYDROLYSIS
In these two processes of chemical weathering, atmospheric moisture directly attacks on the individual minerals of a rock. It ultimately affects the mineral’s structural make-up.
The interior of many minerals is in electric equilibrium.
But the surfaces of many crystals are not.
These may have partially unsatisfied valences.
When polarised water molecules come in contact with such crystals, it may cause any one of the following two reactions: Hydration: addition of water molecules
Hydrolysis: exchange of ions
The ions tend to hold the polarised side of the water molecule and form a hydrate. Hydration is process of addition of water molecule.
Examples are provided by hydration of iron oxides and calcium sulphate crystals. In some minerals with ferrous iron, the ferrous iron ion (Fe++ ion) holds the water molecule and forms water-iron complex or a hydroxide. Anhydrite (CaSO4) gets slowly converted to gypsum by hydration: CaSO4 + 2H2O → CaSO4.2H2O
Ions may be exchanged whereby some ions from water may enter into the crystal lattice of the mineral. This process of exchange of ions is called hydrolysis.
Silicate minerals are quite abundant in rocks.
Hydrolysis is a very common process of weathering of silicate minerals.
Hydrolysis of mineral orthoclase:
(Orthoclase is a feldspar – potassium aluminium silicate.) KAlSi3O8 + H+ → HAlSi3O8 + K+
Chemical weathering process of hydrolysis is a replacement (leaching) of the cations within a mineral lattice by hydrogen ions. The most soluble ions remain in solution and ultimately are carried by streams and rivers to the oceans. Less soluble ions may be precipitated along the route forming new rocks and minerals.
3. OXIDATION and REDUCTION
Iron is a chief constituent of many minerals and rocks.
The iron-bearing minerals, and hence rocks, are especially prone to chemical weathering through the process of oxidation and reduction.
Ferrous iron (Fe++) of the minerals is oxidised to ferric iron (Fe+++) on exposure to air rich in moisture. Ferric iron is not stable.
It is further oxidised to a stable ferric hydroxide.
4Fe + 3O2 → 2Fe2O3
Fe2O3.H2O + H2O → Fe2O3.H2O
Pyrite (iron pyrite) – FeS2 – is a natural and common mineral present in many rocks in small...
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