Igneous rocks are generally termed as fire rocks formed either underground or above ground there are two types Intrusive, Extrusive igneous rocks mainly containing high silica content

1. Igneous rock formation
through chemical weathering
Article written and published
By
www.worldofchemicals.com

2. Introduction
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Igneous rocks are generally termed as
fire rocks, which are formed either
underground or above ground. Igneous
rocks are also formed when volcanoes
erupt, causing the magma to rise above
the earth's surface and by solidification
of molten rock material.
Igneous rock is one of the three main
rock types, the others being sedimentary
and metamorphic rock. Igneous rocks
are commonly classified by their
composition and texture.

3. Types of Igneous rocks

There are two basic types

1) Intrusive igneous rocks such as diorite, gabbro, granite and
pegmatite that solidify below Earth's surface.
2) Extrusive igneous rocks such as andesite, basalt, obsidian,
pumice, rhyolite and scoria that solidify on or above Earth's
surface.


Igneous rocks can be classified according to chemical or
mineralogical parameters

Felsic igneous rocks containing high silica content
Intermediate igneous rocks containing between 52-63 per
cent SiO2
Mafic igneous rocks
Ultramafic rock igneous rocks
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4. Elements in Igneous rocks

Most of the igneous rocks are composed of
both major and minor important elements in
the Earth's crust.

Major elements found in igneous rocks
include

Silicon dioxide
Aluminum oxide
Sodium oxide
Potassium oxide
Phosphorus pentoxide
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5. Elements in Igneous rocks Cont
..
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Minor elements in igneous rocks include
Lithium
Beryllium
Scandium
Zinc
Copper
Lead
Fluorine
Chlorine
Sulfur
Barium

6. Facts and process of chemical
weathering

The primary agents in chemical weathering
are water, oxygen, and acids. These react
with surface rocks to form new minerals that
are stable in, or in equilibrium. Any excess
ions left over from the chemical reactions are
carried away in the acidic water.

When a rock is brought to the surface millions
or billions of years after it has formed, the
original minerals that were crystallized deep
in the crust under high pressures and
temperatures are unstable in the surface
environment and eventually break down.

7. Facts and process of chemical
weathering Cont ..
Rocks are not only changed by water, wind (oxygen),
acids and ice but can also be changed by chemicals.
Chemical weathering is most intense in areas that have
abundant water. Acids are chemical compounds that
decompose in water to release hydrogen atoms.
Hydrogen atoms frequently substitute for other
elements in mineral structures, breaking them down to
form new minerals that contain the hydrogen atoms.
 Rainwater commonly contains dissolved carbon dioxide
and is slightly acidic in nature. The burning of coal, oil,
and gasoline elements releases carbon dioxide,
nitrogen, and sulfur into the atmosphere, which react
with rainwater to form much stronger carbonic, nitric,
and sulfuric acids that damage the environment. This
process of releasing acids into the environment is well
known as acid rain.
 One of the more common and visible chemical
weathering reactions is the combination of iron and
oxygen to form iron oxide (rust).


8. Chemical weathering
definition
Chemical weathering is the process by which the
mineral compositions of rocks are changed. Chemical
weathering can cause minerals to decompose and even
dissolve.
 Chemical weathering is much more common in
locations where there is a lot of water. It is the most
important process for soil formation. Chemical
weathering becomes more effective as the surface area
of the rock increases. Since the chemical reactions
occur largely on the surface of the rocks, therefore the
smaller the fragments, the greater the surface area per
unit volume available for reaction.
 Chemical weathering is the weakening and subsequent
disintegration of rock by chemical reactions. These
reactions include hydration, hydrolysis, carbonation,
oxidation and reduction. These processes either form or
destroy minerals, thus altering the nature of the rock’s
mineral composition. Temperature and, especially,
moisture are critical for chemical weathering.


9. Following processes are involved
in chemical weathering
Hydration
 Hydrolysis
 Carbonation
 Oxidation
 Reduction
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10. Hydration

Hydration is one of the most common
processes in nature and works with
secondary minerals, such as aluminum oxide
and iron oxide minerals and gypsum. Soil
forming minerals in rocks do not contain any
water and they undergo hydration when
exposed to humid conditions.
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a) 2Fe2O3 + 3HOH →
(Hematite) (Red)
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b) Al2O3 + 3HOH
(Bauxite)
Oxide)
→
2Fe2O3 .3H2O
(Limonite) (Yellow)
Al2O3 .3H2O
(Hyd. aluminum

11. Hydrolysis

Hydrolysis is the 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.
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→
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HAlSi3O8 + 8 HOH → Al2O3 .3H2O
+
(Recombination)
(Hyd. Alum. oxide)
KAlSi3O8 + H2O
 (Orthoclase)

HAlSi3O8 + KOH
(Acid silt clay)
6 H2SiO3
(Silicic acid)
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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.

NaCl + H2O → Na+, Cl-

12. Carbonation

Carbon dioxide when dissolved in water it
forms carbonic acid.

2H2O + CO2
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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.
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CaCO3
+
(Calcite)
carbonate)
→ H2CO3
H2CO3 →
Ca(HCO3)2
(Ca bi

13. Oxidation

Oxidation is 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.

4FeO (Ferrous oxide) + O2 → 2Fe2O3 (Ferric
oxide)
 4Fe3O4 (Magnetite) +
O2
→ 6Fe2O3 (Hematite)
 2Fe2O3 (Hematite) + 3H2O
→ 2Fe2O3 .3H2O
(Limonite)
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14. Reduction

The process of removal of oxygen and is
the reverse of oxidation and is equally
important in changing soil color 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.

2Fe2O3 (Hematite) - O2 → 4FeO
(Ferrous oxide)

15. Reference
[1] © From,
http://www.agriinfo.in/?page=topic&super
id=4&topicid=249
 [2] © From,
http://passel.unl.edu/pages/informationm
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3183&topicorder=4&maxto=7
 [3] © From,
http://www.ux1.eiu.edu/~cfjps/1300/weat
hering.html
 http://www.worldofchemicals.com/wocart/
443/mechanism-of-chemicalweathering.html
