STONES | CLASSIFICATION OF STONES

STONES

Stone is a ‘naturally available building material’ which has been used from the early age of civilization. It is available in the form of rocks, which is cut to required size and shape and used as building block. It has been used to construct small residential buildings to large palaces and  temples all over the world.

Classification of Rocks:

Building stones are obtained from rocks occurring in nature and classified in three ways.

1.   Geological classification

2.   Physical classification

3.   Chemical classification

1.       Geological Classification:

According to this classification, the rocks are of the following types.

(i)   Igneous Rocks: The molten material present in the inside portion of the earth's surface is known as magma and this magma occasionally tries to come out to the earth's surface through cracks or weak portions. The rocks which are formed by the cooling of magma are called igneous rocks. The igneous rocks are classified into the following three types

(a)    Plutonic rocks: They are formed by the cooling of magma at a considerable depth from the earth's surface. The cooling is slow and the rocks possess coarsely grained crystalline structure. This rock is mostly used for construction purposes. Granite is the leading example of this type of rock.

(b)    Hypabyssal rocks: Such rocks are formed due to cooling of magma at a relatively shallow depth from the earth's surface. The cooling is quick and hence these rocks possess finely

grained crystalline structure. The dolerite is an example of this type of rock.

(c)     Volcanic rocks: Such rocks are formed due to pouring of magma at earth's surface. The cooling is very rapid as compared to the previous two cases. Hence these rocks are extremely fine grained in structure. The basalt is an example of this type of rock.

(ii)  Sedimentary rocks: These rocks are formed by the deposition of products of weathering on the pre-existing rocks. All the products of weathering are ultimately carried away from their place of origin by the agents of transportation. Such agents are frost, rain, wind, water, etc.

Following four types of deposits occur:

(a)    Residual deposits: Some portion of the products of weathering remains at the site of origin. Such a deposit is known as a residual deposit.

(b)    Sedimentary deposits: The insoluble products of weathering are carried away in suspension and when such products are deposited, they give rise to the sedimemary deposits.

(c)     Chemical deposits: Some material that is carried away in solution may be deposited by some physio-chemical process such as evaporation, precipitation etc. which is called chemical deposits.

(d)    Organic deposits: Some portion of product of weathering gets deposited through the agency of organisms which is called as organic deposits.

Examples: Gravel, sandstone, limestone, gypsum, lignite etc.

(iii)   Metamorphic rocks: These rocks are formed by the change in character of the pre-existing igneous and sedimentary rocks when they are subjected to a great heat and pressure. This phenomenon is known as metamorphism.

Following four types of metamorphism occur with various combinations of heat, uniform pressure and directed pressure:

(a)       Thermal metamorphism: The heat is the predominant factor in this type of metamorphism.

(b)      Cataclastic metamorphism: At the surface of earth, the temperatures are low and metamorphismoccurs by direct pressure only. Such metamorphism is known as the cataclastic metamorphism.

(c)       Dynamo-thermal metamorphism: There is a rise in increase in depth. Hence, the heat in combination with stress brings about the change in the rock. Such metamorphism is known as the dynamo-thermal metamorphism.

(d)      Plutonic metamorphism: The metamorphic changes at great depth are brought about by uniform pressure and heat. Such metamorphism is known as plutonic metamorphism.

The examples of metamorphic rocks are marble, Gneiss, slate etc.

2.       Physical classification

Based on the general structure, the rocks may be classified as follows:

(a)    Stratified Rocks: These rocks are having layered structure. They possess planes of stratification or cleavage. They can be easily split along these planes. The sedimentary rocks like sand stones, lime stones etc. are the examples of this class of stones.

(b)    Unstratified Rocks: These rocks are not stratified. They possess crystalline and compact granular structure. They cannot be split in to thin slab. The volcanic rocks and some sedimentary rocks are coming under this type of rocks.

(c)     Foliated Rocks: These rocks have a tendency to split along a definite direction only. The direction need not be parallel to each other as in case of stratified rocks. This type of structure is very common in case of metamorphic rocks.

3.       Chemical classification

On the basis of their chemical composition the rocks are classified in to following three types.

(a)    Silicious rocks: The main content of these rocks is silica. They are hard and durable. Examples of such rocks are granite, quartzite, trap, sand stones etc.

(b)    Argillaceous rocks: The main constituent of these rocks is argil i.e., clay. These stones are hard and durable but they are brittle. They cannot withstand shock. Slates and laterites are examples of this type of rocks.

(c)     Calcareous rocks: The main constituent of these rocks is calcium carbonate. The durability of these rocks will depend on the constituents present in the surrounding atmosphere. Limestone is a calcareous rock of sedimentary origin while marble is a calcareous rock of metamorphic origin.


** basic civil engineering notes for bput students.