Classification of Rocks | Building Materials

Rocks or so-called building stones are naturally occurring materials used to construct residential, commercial, and industrial buildings. Building stones are euhedral shape stones and they are shaped and dressed according to the requirements. 

In this section, we will be dealing with the classification of building stones or rocks, the tests on rocks, and the Quarrying of stones. 

Classification of Rocks

All the stones that are used in building construction are rocks formed by various means. Each type of rock is formed through different processes and possesses different geological, physical, and chemical properties. Rocks can be classified under three categories:

1. Physical Classification
2. Chemical Classification
3. Geological Classification

We will first understand the concept behind all the classifications, then understand the relationship between each type with each other in detail to avoid confusion. 

1. Physical Classification of Rocks

The physical classification of rocks is based on the general structures of rocks. Hence, based on the structure, rocks can be classified into:

a. Stratified Rocks

Stratified rocks are formed by the formation, accumulation, or deposition of materials in layers. This might be due to weathering, erosion, or deposition of the sediments. The plane between two layers is called a bedding plane. Rocks can get spilt or break from the bedding plane easily. 

Examples: Limestone, Sandstone, Slate

b. Un-Stratified Rocks

Unstratified rocks are formed from magma or lava that cools. They are characterized by a dense granular or crystalline texture and a similar structure throughout their entire body.

Examples: Granite, Marble, Basalt and Rhyolite

c. Foliated Rocks

Foliated rocks are formed when pressure squeezes the minerals within a rock, causing them to align in parallel layers. This process creates a sheet-like structure with a striped appearance that reflects the direction of the applied pressure.

Foliated rocks are formed when rocks are subjected to extreme heat and pressure deep within the Earth. This process can take millions of years.

Examples: Gneiss, Granite, Slate

2. Chemical Classification of Rocks

Based on the chemical composition of the rocks, they can be classified into:

a. Argillaceous Rocks

The major composition of argillaceous rocks are alumina or clay. These rocks are consolidated and have very low porosity and permeability. 

Examples: Shale, Slate, Laterite, Claystone, Mudstone

b. Calcareous Rocks

The chief constituent of calcareous rocks is Lime (CaO) or calcium carbonates. These are also called carbonate rocks. Used for the manufacture of lime and cement.

Example: Limestone, Marble, gypsum, dolomite

c. Silicious Rocks

The chief constituent of silicious rocks are silica (SiO2) i.e. Sand. Therefore, these stone are fine-grained and hard with good compressive strength. 

Examples: Granite, Quartzite, Basalt and Trap

3. Geological Classification of Rocks

Based on the mode of formation of rocks, they can be classified into:

a. Igneous Rocks

Igneous rocks are rocks formed by the crystallization and cooling of molten lava or magma. Based on where earth it is formed, we have volcanic rocks, hypabyssal rocks and plutonic rocks. Hence, based on the first classification, Igneous rocks are unstratified rocks. 

Volcanic rocks are igneous rocks formed by the crystallization and cooling of lava on the earth surface. It is called as extrusive rocks. Their mineral crystals are fine-grained. 

Example: Basalt, Trap, Aplite, Andesite

Hypabyssal Rocks are igneous rocks formed by the crystallization and cooling of magma at intermediate depth usually between 2 to 4 km below the earth surface. Also called intrusive rocks. 

Examples: Dolerite

Plutonic Rocks are igneous rocks formed by the crystallization and cooling of magma within the earth crust at a depth of usually below 7 km. Mineral crystals are coarse grained. It is also intrusive rocks.

Example: Granite, Syenite, Pegmatite

b. Sedimentary Rocks

These rocks are formed by the deposition of products of weathering on the pre-existing rock. The agents that transport the weathering products can be frost, rain or wind. Hence, different layers are formed for these rocks. Hence they come under the category of stratified or aqueous rocks. 

Examples: Limestone, Sandstone, Mudstone, Gypsum, Shale, Dolomite, Laterite, Murrum (decomposed laterite, kankar

c. Metamorphic Rocks

The rocks formed by the metamorphism of pre-existing rocks especially igneous and sedimentary rocks are called as metamorphic rocks. During the process, the pre-existing rocks undergo mineralogical and structural changes under high temperature and pressure conditions.

Examples:

Marble = Limestone and Dolomite [Sedimentary]

Gneiss = Granite and Syenite [Igneous]

Slate = Mudstone and Shale [Sedimentary]

Quartzite = Sand Stone [Sedimentary]

Important Building Stones and Applications

Quartzite	High weather resistance, hardness, and crushing strength. Used for retaining walls, road metal, and rubble masonry.
Gneiss	Street Paving, Rough stone masonry work
Murrum	Garden walls, Fancy paths
Slate	Roof, sills, and damp proofing course
Marble	Flooring, Ornamental works, Facing works and columns
Kankar	Road metal, manufacture of hydraulic lime
Laterite	Building stone, road metal, rough stone masonry work.

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