Stones
Stones are obtained from rocks. These rocks are in the shape of the crust in the earth and have no definite shape or chemical composition, but are a mixture of two or more types of minerals.
The rocks from which stones are obtained, classify those rocks into three groups of the following types:
1. Geological cassification of rock
(a) Igneous rocks
Igneous rocks are formed by the solidification of molten magma ejected from volcanoes at or below the Earth's surface. It has a shiny crystalline texture.
(b) Sedimentary rocks
Sedimentary rocks are formed due to the gradual deposition of sand, clay etc., usually sedimentary rocks are also formed by the freezing of water. These rocks are formed in layers so they are also called stratified. Sandstone and limestone belong to the same category of rocks.
(c) Metamorphic rocks
Metamorphic rocks are formed due to changes in the basic structure under conditions of heat and extreme pressure. Marble belongs to metamorphic rock.
2. Physical classification of rocks
Physically, the rocks are classified as follows :
(a) Stratified rocks
The stratified rocks are those which exhibit distinet layers which can be separated. Examples of stratified rocks are limestone, sandstone and slate.
(b) Unstratified rocks
The unstratified rocks are those which do not show any sign of strata and cannot be easily split into slabs. Marble and granite are examples of unstratified rocks.
3. Chemical classification of rocks
Chamically, the rocks are classified as follows :
(a) Silicious rocks
Siliceous rocks are those rocks in which silica is present as the main component. The granite, quartzite, gneiss are examples of silicious rocks.
(b) Argillaceous rocks
Argillaceous rocks contain clay or alumina as the main constituent. The slate, laterite, Kaoline are the examples of argillaceous rocks.
(c) Calcarious rocks
Calcareous rocks contain lime or calcium carbonate as the main constituent. Marble and limestone are examples of calcareous rocks.
Important Building Stones
Following are the important construction stones, their composition, properties and uses:
1. Granite
It is an igneous rock. It is mainly composed of quartz, mica and *felspar. Its specific gravity is 2.64 and compressive strength ranges from 70 to 130 MN/m².Its color depends on the feldspar which can be brown, green, grey and pink. A fine-grained granite provides high resistance to weathering. It can be easily polished and worked. It is used for the exterior face of buildings.
*Feldspar is a silicate of aluminum that contains varying amounts of soda, potash or lime.
2. Slate
It is an argillaceous rock. It is mainly composed of alumina mixed with lime carbonate or sand. Its specific gravity is 2.8 and when it comes to compressive strength, it varies from 60 to 70 MN/m². It has grey or dark blue colour. Agood slate is hard, tough and fine grained. It is suitable for use in cisterns. Slate in the form of tiles is used as a good roofing material.
3. Gneiss
It is a siliceous rock. It consists mainly of feldspar and quartz. It works more easily than granite. It is a good material for road paving.
4. Sandstone
It is a siliceous type of sedimentary rock. It is mainly composed of quartz, silica and lime. Its specific gravity is 2.65 to 2.95 and compressive strength varies from 35 to 40 MN/m². Its common colors are white, grey, brown, pink etc. Fine-grained stones are strong and durable. It is suitable for ashlar work, moulding, carving etc.
5. Limestone
It is a calcareous type of sedimentary rock. Its specific gravity is 2.6. It is available in brown, yellow and dark brown colours. It is widely used in blast furnaces. It can be used as stone masonry for walls.
6. Marble
It is a calcareous type of metamorphic rock. It has a specific gravity of 2.7 and is available in many colors. It is very tough and takes a good polish. It is used for decoration and carving work.
7. Kankar
It is an impure limestone with the presence of 30% alumina and silica. Hard kankar are used for the foundation of buildings.
8. Laterite
It is a sandy clay stone that contains a high percentage of iron oxide. It has a porous and cellular structure. The specific gravity of laterite ranges from 2 to 2.2. Laterite blocks are suitable for stone formation whereas nodular laterite proves to be a very good road metal.
9. Murum
It is a type of decomposed laterite and its color is dark brown or red. It is used for fancy walkways and garden walks.
10. Quartzite
It is a type of silicious sandstone that has been subject to metamorphic action. It is strong and durable. It is used in road metal or railway ballast or concrete.
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