Chemical Classification

The presence of chemical constituents in the rocks is the basis for their type of classification. They are as follows:

1. Siliceous rocks
2. Argillaceous rocks
3. Calcareous rocks

(i) Siliceous Rocks

In these rocks silica is the main constituent. The presence of silica in the free state is called sand, and in the combined state is a silicate. Rocks containing silica in the free state are harder, and those found in the combined form are likely to be disintegrated. Examples of silicate rocks are sandstone, quartzite, etc.

(ii) Argillaceous Rocks

In these rocks, clay or alumina is the main constituent. Examples of argillaceous rocks are slate, laterite, etc.

(iii) Calcareous Rocks

Calcium carbonate or lime is the main constituent in these rocks. They are readily acted upon by dilute hydrochloric acid. Examples are limestone, marble, etc.