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

Chemically formed sedimentary rocks

(a) Mode of formation

Chemically formed sedimentary rocks are produced from minerals that were dissolved in water. When water containing these dissolved minerals evaporates, or when the water becomes over-saturated, the minerals are precipitated (deposited) out of solution. The precipitated material accumulates and, over time, is compacted and cemented to form solid rock. This process commonly takes place in seas, salt lakes, lagoons and hot springs where evaporation is high, and in limestone caves where deposition forms stalactites and stalagmites.

(b) Three examples

1. Rock salt (halite).
2. Gypsum.
3. Limestone (chemically formed) and potash.

(c) Four importance (uses)

1. Rock salt is a source of common salt used for food, and in the chemical industry.
2. Gypsum is used in making plaster of Paris, cement and fertilizer.
3. Limestone is used for the manufacture of cement and as a building and road-making material.
4. These rocks provide valuable raw materials that support industries and generate employment and revenue; some (potash) are used as fertilizers to improve agriculture.