INTRODUCTION

The earth’s crust is composed of rocks. Rock is a mineral mass of a more or less uniform composition. It may consist of a single mineral (monomineralic) or of several minerals (polymineralic). A mineral is a natural body, homogeneous in chemical composition and physical properties. Further minerals are the products of physical and chemical processes occurring in the earth’s crust.

Monomineralic rocks are quartz sand, pure gypsum and magnesite, and the polymineralic ones are granite, basalt and porphyries.

Natural rock materials are widely used for building purposes owing to their universal occurrence and physical and mechanical properties. They are used as mined materials, e.g., gravel, sand, or mechanically processed materials, e.g., crushed stone, sawn, dressed and polished stones, and for various architectural and decorative purposes.

Rocks are the main source of material for the manufacture of mineral binding materials, such as gypsum, lime, cement and of artificial rock materials, e.g., brick, glass, items for concrete and mortar.

2.2 CLASSIFICATION OF ROCKS

The origin and conditions of rock formation predetermine their chemical and mineralogical composition, crystalline structure and texture. These characteristics, underlying the strength and decorative properties of rocks, are the starting point for the engineering and economic evaluation of rocks for various structures of buildings and installations.

The three major classifications are based on the following aspects:

1. Geological or genetic classification
2. Physical classification and
3. Chemical classification

2.2.1 Geological or Genetic Classification

According to the geological classification, rocks are subdivided into three large groups, viz., igneous rocks, sedimentary rocks and metamorphic rocks.

1. Igneous rocks: Cooling and hardening of molten magma result in the formation of igneous rocks. Depending on the rate of cooling the sizes of crystals are formed. Granites, syenites, diorites and gabbros have large crystals, while basalts, rhyolites and andesites have small crystals. Igneous rocks are generally very hard.
2. Sedimentary rocks: Transportation agents like wind, water and ice may move the loose weathered rock materials and deposit them in the form of layers called sediments. Such sediments when subjected to heavy pressure undergo compaction and cementation, resulting in sedimentary rocks. Limestone, dolomite, shale, sandstone, etc., are sedimentary rocks.
3. Metamorphic rocks: Sedimentary rocks and to some extent igneous rocks when subjected to changes brought about by the combination of heat, pressure and plastic flow (called metamorphism) undergo changes in the structure, texture and mineral composition, and this results in the formation of metamorphic rocks. Gneiss, schist, slate, marble, etc., are metamorphic rocks.

2.2.2 Physical Classification

The basis for such a classification is the physical properties of rocks, the manner and arrangement of different particles and mass forming a stone. They are classified as follows:

1. Stratified rocks
2. Unstratified rocks
3. Foliated rocks

(i) Stratified Rocks

Stratified structure is formed due to the splitting of a series of parallel layers of sedimentary rocks. They exhibit distinct layers which can be separated. The plane of separation is called a cleavage plane. Limestone, slate and sandstone are examples of stratified rocks.

(ii) Unstratified Rocks

They are of granular or crystalline structure and become solid on cooling. They do not show any sign of strata, e.g., igneous rocks like granite, basalt, etc.

(iii) Foliated Rocks

These rocks show definite parallel arrangement of minerals showing a tendency to split in a specific direction, e.g., metamorphic rocks like gneiss, schist, etc.

2.2.3 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.