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
I. Geological Classification:
According to this classification, the rocks are of the following types.
a. Igneous rocks: Rocks that are formed by cooling of Magana (molten or pasty rocky material) are known as igneous rocks. Eg: Granite, Basalt and Dolerite etc.
b. Sedimentary rocks: these rocks are formed by the deposition of production of weathering on the pre-existing rocks. Examples: gravel, sandstone, limestone, gypsum, lignite etc.
c. Metamorphic rocks: These rocks are formed by the change in character of the pre-existing rocks. Igneous as well as sedimentary rocks are changed in character when they are subject to great heat and pressure. Known as metamorphism. Examples: Quartzite, Schist, Slate, Marble and Gneisses.
II. Physical Classification:
This classification based on general structure of rocks.
According to this, the rocks are classified into three types
a. Stratified Rocks: These rocks posses planes of stratification or cleavage and such rocks can be easily split along these planes
Ex: sedimentary rocks
b. An stratified rocks: The structure may be crystalline granular or compact granular. Examples: Igneous rocks and Sedimentary rocks affected by movements of the earth.
c. Foliated Rocks: These rocks have a tendency to split up in a definite direction only. Ex: Metamorphic rocks.
III. Chemical Classification:
According to this classification rocks are classified into three types.
a. Siliceous rocks: In these rocks, silica is predominates. The rocks are hard; durable and not easily effected by weathering agencies. Ex: Granite, Quartzite, etc.
b. Argillaceous Rocks: In these rocks, clay predominates. The rocks may be dense and compact or may be soft.
Ex: slates, Laterites etc.
c. Calcareous rocks: In these rocks, calcium carbonate predominates. The durability to these rocks will depend upon the constituents present in surrounding atmosphere. Ex: Lime Stone, marble etc.