• 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

Lec 4 WEATHERING OF ROCKS AND MINERALS

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)

• 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.

Expansion and contraction

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

SOLID ION +H2O • NaCl Na+ + Cl- ( Chloride) solubility : 360 g/l (20º C)

+H2O • CaCO3 Ca2+ + CO3

2- (Calcite) solubility : 14mg/l (20º C )

Dissolution e.g.

+ H2O • 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 2Fe2O3.3H2O ( Hematite ) (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: 2KAlSi2O8 + 2H+ + H2O 2K + 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 2Fe2O3 (Ferum oxcide) (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- S2-

(Sulfate) (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 H+HCO3

• CaCO3 + H + HCO3 Ca (HCO3)2 (carbonate) (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 Product 1. Quartz SiO2 ( solution ) SiO3 ( solution ) 2. Feldspar Kaolinite (clay) (ortoklas) monmorilonite(clay) K+ , Na+ , Ca+ , Mg+

3. Amfibol, Piroksin Limonite (Ferum Biotite hydrocide) Hematite (Ferum oxcide) Mg2+ , Al3+