Why does chemical weathering occur

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, 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. 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. Table 6. Silicates fall within the middle range.

The most common silicates in clastic sedimentary rocks are quartz, K-, Na-feldspars and micas. Amphiboles, pyroxene, olivine and Ca-feldspars are almost never found in sedimentary rocks.

Rocks sometime expand when exhumed. Repeated expansion and contraction of the rock during heating and cooling. Sometimes these curved layers fall away like skin on an onion. Soils may also form from transported material derived from elsewhere and deposited in a lowland or basin. Residual soils develop on plains and lowlands with moderate to gentle slopes and consist of loose, heterogeneous material left behind from weathering.

This material may include particles of parent rock, clay minerals, metal oxides and organic matter. This loose material is collectively called regolith , whereas the term soil is reserved for the topmost layer which contains organic matter. The A-horizon is the topmost layer and is usually a meter or two thick. The upper portion of the A-horizon is often rich in organic matter, called humus , and may also contain inorganic material like insoluble clays and quartz.

The A-horizon may take thousands of years to develop depending on the climate and acitivity of plants and animals. This is the layer that supports crops and other types of vegetation. These precipitated minerals often accumulate in small pods, lenses and coatings. Organic matter is sparse in the B-horizon. The lowest layer constitutes the C-horizon and is comprised of cracked and variably weathered bedrock mixed with clays.

Soils can vary significantly in color and composition. The particular type of soil that is produced in a region depends on the available materials, climate and also time. The high temperatures, heavy rainfall and humidity of tropical regions have driven chemical weathering to the extreme.

As a result, feldspars and other silicates have been completely altered while silica and calcite is extensively leached from the soil. The upper zone of laterite consists of insoluble precipitated iron and other oxides along with some quartz. At best, only a very thin layer of organic matter resides at the top of the soil to support the jungle vegetation. Figure 7. The images above show the dehydration reaction of limonite Fe 2 O 3.

H 2 O on the left to hematite Fe 2 O 3 on the right. The water, which was a structural component of limonite, has been removed in the process of dehydration. Experiential Activity A small amount of ground calcium feldspar is placed in the mortar.

Experiential Activity A small amount of limonite, Fe 2 O 3. H 2 O, is placed into a test tube. The tube is held with a test-tube holder just above the blue flame of a lighted Bunsen burner. The sample is cooled. Note: The color change of limonite from yellow to brick red; Reaction Fe 2 O 3.

Dehydration of limonite to hematite may be observed in the following experiential activity. Previous Page Next Page.