Spheroid weathering of granite. Spheroidal weathering is a form of chemical weathering. This occurs when concentric shells of decayed rock are successively loosened and separated from a block of rock by water. This is the decomposition of rocks due to chemical reactions occurring between the minerals in rocks and the environment. The examples below illustrate chemical weathering. Water Water, and many chemical compounds found in water, is the main agent of chemical weathering. Feldspar, one of the most abundant rock-forming minerals, chemically reacts with water and water-soluble compounds to form clay. Acids Iron precipitation on Fisher Creek stream channel. Water contains many weak acids such as carbonic acid. This weak, but abundant, acid is formed when carbon dioxide gas from the atmosphere mixes with rainwater. Sulfur dioxide and nitrogen gases create other types of acid rain that act as chemical weathering agents. Some sources of sulfur dioxide are power plants that burn coal; as well as volcanoes and coastal marshes. Sulfur gases react with oxygen and rainwater to form sulfuric acid. Although relatively weak, acid’s abundance and long term effects produce noticeable damage to vegetation, fabrics, paints and rocks. Oxidation Oxidation is another kind of chemical weathering that occurs when oxygen combines with another substance and creates compounds called oxides. Rust, for example, is iron oxide. When rocks, particularly those with iron in them, are exposed to air and water, the iron undergoes oxidation, which can weaken the rocks and make them crumble.

1. CHEMICAL
WEATHERING OF SOIL
FORMATION.
BY :
AREPALLI NAVEEN KUMAR .

2. WEATHERING
 DEFINITION:
A process of disintegration and decomposition of rocks and minerals
which are brought about by physical agents and chemical processes, leading to
the formation of Regolith (unconsolidated residues of the weathering rock on the
earth’s surface or above the solid rocks).
Two basic processes
Physical /mechanical Chemical
(disintegration) (decomposition)
In addition, another process: Biological and all these processes are work hand in
hand.
Depending up on the agents taking part in weathering processes, it is classified
into three types

3. CHEMICAL WEATHERING.
(DECOMPOSITION)
Decomposition of rocks and minerals by various
chemical processes is called chemical
weathering. It is the most important process for
soil formation.
Chemical weathering takes place mainly at the
surface of rocks and minerals with
disappearance of certain minerals and the
formation of secondary products (new
materials).
This is called chemical transformation.

4. PROCESS INVOLVED IN CHEMICAL
WEATHERING
Totally there are six steps involved In the chemical weathering
process.
HYDRATION
HYDROLYSIS
SOLUTION
CARBONATION
OXIDATION
REDUCTION

5. 1. HYDRATION
 Chemical combination of water molecules with a particular substance or mineral
leading to a change in structure.
 Occurs only when rocks are exposed to humid conditions
 The minerals loose their luster and become soft.
 It is one of the most common processes in nature and works with secondary
minerals, such as aluminium oxide and iron oxide minerals and gypsum.
 EXAMPLE:
CaSO4 + 2H2O CaSO4 .2H2O
(Anhydrite) (Gypsum)
2Fe2O3 + 3HOH 2Fe2O3 .3H2O
(Haematite) (red) (Limonite) (yellow)

6. 2.HYDROLYSIS
Most important process in chemical weathering.
It is due to the dissociation of H2O into H+ and OH-ions
which chemically combine with minerals and bring about
changes, such as exchange, decomposition of crystalline
structure and formation of new compounds.
Water acts as a weak acid on silicate minerals.
EXAMPLE:
KAlSi3O8 + H2O HAlSi3O8 + KOH
(Orthoclase) (Acid silt clay)
HAlSi3O8 + 8 HOH Al2O3 .3H2O + 6 H2SiO3
(recombination) (Hyd. Alum.oxide) (Silicicacid)

7. This reaction is important because
of two reasons
clay, bases and silicic acid - the substances formed
in these reactions - are available to plants
water often containing CO2 (absorbed from
atmosphere), reacts with the minerals directly to
produce insoluble clay minerals, positively charged
metal ions (Ca++, Mg++, Na+, K+ ) and
negatively charged ions (OH-, HCO3-) and some
soluble silica – all these ions are made available for
plant growth.

8. Solution
Some substances present in the rocks are directly soluble in water.
 The soluble substances are removed by the continuous action of
water and the rock no longer remains solid and form holes, rills or
rough surface and ultimately falls into pieces or decomposes.
 The action is considerably increased when the water is acidified by
the dissolution of organic and inorganic acids. (e.g) halites,Nacl
NaCl + H2O. Na+, Cl-, H2O.
(dissolved ions with water)

9. Carbonation:
 Carbon di oxide when dissolved in water it forms carbonic
acid.
2H2O + CO2 H2CO3
 This carbonic acid attacks many rocks and minerals and
brings them into solution. The carbonated water has an
etching effect up on some rocks, especially lime stone.
 The removal of cement that holds sand particles together
leads to their disintegration.

10. Oxidation
The process of addition and combination of oxygen to minerals. The
absorption is usually from O2 dissolved in soil water and that present
in atmosphere.
 The oxidation is more active in the presence of moisture and results
in hydrated oxides. (e.g) minerals containing Fe and Mg.
 4FeO+ O2 2Fe2O3
(Ferrous oxide) (Ferric oxide)
4Fe3O4 + O2 6Fe2O3
(Magnetite) (Haematite)

11. Reduction
The process of removal of oxygen and is the reverse of oxidation and
is equally important in changing soil colour to grey, blue or green as
ferric iron is converted to ferrous iron compounds.
Under the conditions of excess water or water logged condition (less
or no oxygen), reduction takes place.
EXAMPLE:
2Fe2O3 - O2 4FeO
(Haematite) (Ferrous oxide) reduced form

12. CONCLUSION
During chemical weathering igneous and
metamorphic rocks can be regarded as involving
destruction of primary minerals and the production
of secondary minerals.
In sedimentary rocks, which is made up of primary
and secondary minerals, weathering acts initially to
destroy any relatively weak bonding agents (FeO)
and the particles are freed and can be individually
subjected to weathering.

13. REFERENCE
 http://www.agriinfo.in/?page=topic&superid=4&topicid=249
 http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1
124303183&topicorder=4&maxto=7
 http://www.ux1.eiu.edu/~cfjps/1300/weathering.html
 https://www.worldofchemicals.com/443/chemistry-articles/mechanism-of-
chemical-weathering.html
 https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.tulan
e.edu/~sanelson/eens1110/weathering.pdf&ved=2ahUKEwjPruny8pztAhUy7X
MBHR9KDMQQFjAHegQIBhAB&usg=AOvVaw1tDmYIE-G1oanqktzGQgTK
 https://www.go_in_the_Garhwal_Lesser_Himalaya&ved=2ahUKEwjPruny8pztA
hUy7XMBHR9KDMQQFjAEegQICRAC&usg=AOvVaw2oOvQb