Describe chemically formed sedimentary rocks under the following headings; (a) mode of formation (b) examples (c) importance.

Chemically formed sedimentary rocks

(a) Mode of formation: Chemically formed sedimentary rocks are produced by chemical processes rather than the mechanical deposition of fragments. When water containing dissolved mineral salts evaporates, or when the water becomes saturated, the dissolved minerals are precipitated (deposited) out of solution. These precipitated minerals accumulate, settle in layers and are compacted and cemented to form solid rock. Examples of the processes include the evaporation of sea or lake water leaving behind salts, and the precipitation of calcium carbonate from lime-rich water in caves.

(b) Examples:

• Rock salt (halite) - from evaporation of salty water.
• Gypsum - from evaporation of sulphate-rich water.
• Some limestones and dolomite - from precipitation of calcium/magnesium carbonate.
• Stalactites and stalagmites and tufa - from precipitation of lime in caves.

(c) Importance:

1. Rock salt (halite) is used as table salt and as a raw material in the chemical industry.
2. Gypsum is used to manufacture plaster of Paris and cement.
3. Limestone is an important raw material for cement, and for building and glass making.
4. Some deposits store underground water and minerals, and limestone landscapes provide caves of tourist interest.