B. Chemical Weathering Definition

Chemical weathering is the decomposition or decay of rocks in situ due to the chemical reactions between rock minerals and air, moisture, rainwater, or seawater.

B. Chemical Weathering Causes / conditions favouring chemical

weathering1. High relative humidity2. Abundant rainfall

rainwater absorbs carbon dioxide in the atmosphere and form a mild carbonic acid. 稀碳酸 This acid rain can dissolve soluble minerals in rocks and cause chemical changes.

3. High temperatureHigh temperatures speed up the rate of chemical reactions.

4. Weak / soluble mineralsRocks contain minerals of different chemical compositions. The weak minerals undergo chemical changes when in contact with rainwater, acids or gases.

5. Organic acidsThese are released by decaying organic matter, e.g. decaying plants or animals. When in contact with rocks, these acids act on rock minerals and lead to chemical changes.

6. Bare / exposed / unvegetated surface

When rock minerals come into contact with air or gases like oxygen, they are also weathered.

B. Chemical Weathering Processes in chemical weathering

Water is an effective agent in chemical weathering. It works in several ways.1. Oxidation 氧化作用

Oxygen dissolved in water reacts with certain rock minerals especially iron, to form oxides and hydroxides.

Fe + O2 FeO2

2. Hydration 水化作用Some minerals absorb water and

expand, thereby producing stress within rocks

iron oxide + water iron hydroxide

Calcium sulfate + water gypsum

3. Hydrolysis 水解作用Water combines with rock minerals and

form new products such as clay minerals.

Feldspar + water kaolin (clay)

高嶺土

4. Carbonation 碳化作用Rainwater combines with carbon dioxide

in the air and forms a mild carbonic acid that acts on minerals in rocks.

CO2 + H2O H2CO3 carbonic acid

CaCO3 + H2CO3 Ca(HCO3)2Calcium carbonate carbonic acid calcium bicarbonate

5. Solution 溶解作用Water dissolves soluble minerals in

rocks. As a result, cavities or holes are formed on the rock surface. This process is known as honeycomb weathering.