Lec 4 WEATHERING
OF ROCKS
AND MINERALS
• A process whereby rocks and
minerals
experience
disintegration and decomposition
when exposed to the interactions
of air, water and organic matter
• A basic process in soil formation
Types of weathering
Physical weathering
• Disintegration of rocks to small size and finally
to minerals that formed that rocks.
• No chemical changes
Chemical weathering
• Rocks and minerals have chemical changes and
formed new mineral that different from
primary mineral
• Also results in materials that dissolve
and and leach out together with the
water flow or form secondary minerals
• e.g. Calcite and silica
Physical Weathering versus Chemical
Weathering
Both types may occur at the same time
• In areas with a hot and wet climate,
chemical weathering is dominant.
•Pyhsical weathering
increases the surface area of
particles to the actions of air,
water and organic matter.
This intensifies chemical
weathering process.
Types of Physical Weathering
1. exfoliation
2. expansion and contraction
3. frost action
4. Actions by organism
Exfoliation (kelupasan)
Occurs because the inner part of the rock is hotter
or colder than the surface; causing the surface
layer to exfoliate. Normally occurs to massive
plutonic rocks.
Expansion and contraction
(Pengembagan dan Pengecutan)
Expansion and contraction
• When hot, minerals expand and contracts
when cold
• The process when repeated will cause the
rock to crack and and subsequently
disintegrate
• Not common in Malaysia as temperature
change is small (requires a difference
between
maximum
and
minimum
temperature of > 10 oC). May occur during
forest fires.
Frost Action (Tindakan ibun)
• Water expands when frozen < 0 oC
(Volume of ice is 9% more than volume of
water)
• Significant in cold climate countries
Abrasion
Abrasion of rocks and sand. Rock
fragments, sand and soil particles
when carried by water, wind and
graviti will cause abrasion (geseran)
between them and will lead to further
disintegration.
Organism action (tindakan organisma)
• Roots of plants, as they grow bigger in
cracks of rocks will cause the rock to
crack further
• Human hastens rock weathering by
cutting through rocks for highways, etc.,
exposing it to air and water.
CHEMICAL
WEATHERING
Weathering agent:
1. Water
2. Oxygen
3. Carbon dioxide
4. Organic acids
1. Dissolution
• A process whereby the solute dissolves in
water and forms ions
• The rate of dissolution on availability of
water and solubility of mineral/rock
Dissolution e.g.
SOLID
ION
+H2O
• NaCl
Na+ + Cl( Chloride) solubility : 360 g/l (20º C)
+H2O
Ca2+ + CO32• CaCO3
(Calcite) solubility : 14mg/l (20º C )
+ H 2O
• CaSO4
Ca2+ +
SO2( Gypsum) solubility : 2.6 g/l (20ºC)
• Soluble minerals under chemical
changes more easily
2) Hydration
• Combination of water molecules and
minerals or salts, forming a compound
that is bigger, softer and easily
disintegrate.
•
Examples:
• 2FeCO3 + 3H2O
( Hematite )
2Fe2O3.3H2O
(Limonite)
• CaSO4 + 2H2O
CaSO4.2H2O
(Gypsum)
• The reactions
above is
exothermic
(release of heat) that may cause change in
volume and subsequently exfoliation of
rocks
3. Hydrolsis
• A weathering process that is most important in
the disintegration of silicate minerals.
• A reaction which causes the exchange of cations
(Na, K, Ca, Mg) in the main silicate structure by H
or OH ions
• Examples:
2K +
2KAlSi2O8 + 2H+ + H2O
Al2Si2O5 (OH) 4 + 4SiO2
4. Ion exchange ( in chemical weathering of rocks )
e.g. equation
Chemical weathering
5. Oxidation and reduction
a) Oxidation
• Combination with O or losing electron. Usually
happen to mineral that contain Fe and Mn,
• Oxidation : e.g
e-
Fe2+
Fe3+
e-
Mn2+
Mn3+
Examples :
4FeO + O2
(Ferum oxcide)
2Fe2O3
(Hematite)
• Happened in good aeration
• Stones turned into yellowish/redish color
because of oxidation of the ferum.
b) Reduction
• Increasing of electron
• Happened when aeration is not good (flooding)
• Reduction : e.g
+e
Fe3+
Fe2+
• SO42(Sulfate)
S2(Sulfida)
• Fe2O3 + 2e + 6H+
2Fe2+ + 3H2O
6. Carbonation
• Important weathering other than hydrolysis.
• Reactions of carbonic acid into mineral.
Carbonic acid formed when CO2 (from air)
dissolve in water. Carbonic acid dissolve
mineral harder than water and formed
bicarbonate. Bicarbonate more soluble than
carbonate in water.
Carbonation : examples
• CO2 + H2O
H2CO3
• CaCO3 + H + HCO3
(carbonate)
H+HCO3
Ca (HCO3)2
(bicarbonate)
RATES OF WEATHERING
Depends on several factors :
i) type of rocks
• structure and mineral
- Structure : coarse
fine
foliated
layer/ not layers
Examples : sandstones weathering slower than
limestone.
ii) Climate
• Humidity
high moister/ humidity and temperature, rates
of weathering will increase. Moisture and
temperature influence solubility, hydration and
hydrolysis.
• Temperature influences chemical reactions (for
every 10° C, rates of reaction increased 2 times)
In tropical areas, chemical weathering is more
important than physical weathering.
Mineral weathering (examples)
E.g.
Mineral
1. Quartz
2. Feldspar
(ortoklas)
Product
SiO2 ( solution )
SiO3 ( solution )
Kaolinite (clay)
monmorilonite(clay)
K+ , Na+ , Ca+ , Mg+
3. Amfibol, Piroksin
Biotite
Limonite (Ferum
hydrocide)
Hematite (Ferum
oxcide)
Mg2+ , Al3+