STONE | COMPOSITION OF STONES

Composition

Stones are obtained from rock, rocks has no definite chemical composition or shape. It may contain one mineral or several minerals. A mineral indicates a substance haying definite chemical composition and molecular structure. Such minerals, when combine, form rocks of

various types and the properties of a rock are then governed by the properties of minerals. The minerals present in different rocks are given bellow .

1.    Minerals present in igneous rocks

(i)                                          Augite: It is octagonal crystals having density of 23 to 36 kN/m³ and hardness 5 to 6. It is  also called Pyroxene.

(ii)                                      Chlorite: It is a green colour mineral derived from the decomposition of augite, biotite and hornblende.

(iii)                                  Feldspar: It has many varieties like orthoclase and plagioclase series. The orthoclase is whitish, greyish or pinkish in colour. The rocks disintegrate easily, if orthoclase is in abundance in their structure.

(iv)                                 Hornblende: This mineral is heavy, strong and durable, but brittle. It is a hexagonal crystal of dark green and black colour. Its density is 29 to 30 kN/m³ and hardness is 5 to

6. It is an essential component of many igneous rocks.

(v)                                     Mica: It is silicate of aluminium with potassium and occurs in thin transparent laminae or plates. The layers of mica shine with metallic luster. The hardness varies from 2 to 3 and density varies from 27.60 to 31 kN/m³.

(vi)                                 Olivine: It is a black, olive-green or yellow colour mineral.

(vii)                             Plagioclase: It belongs to feldspar series and composed of sodium aluminium silicate and calcium aluminium silicate. It is an oblique-splitting mineral.

(viii)                         Quartz: It is the crystalline form of silica. It is usually colourless. But it may be sometimes brownish, pink or yellow in colour. It is weather -proof and  resists  the attack of acids with the exception of hydrofluoric acid. Its density is 26.50 kN/m³ and  its hardness is 7. The common sand is a variety of quartz.

2 .    Minerals in the sedimentary rocks:  

 

Following are the commonly found minerals in the sedimentary rocks:

(i)         Calcite: This mineral rarely occurs in igneous rocks. But it is the chief constituent of many sedimentary rocks. The calcite is calcium carbonate CaCO₃. It is generally colourless. It is poorly soluble in water and it reacts vigorously with acids. Its density   is 27 kN/m³ and its hardness is 3.

(ii)        Magnesite: This mineral occurs rarely in nature and its chemical composition is MgCO₃. It is harder and less soluble than calcite.

(iii)      Dolomite: Its chemical composition is  MgCO3.CaCO₃.  It consists of 46% of MgCO₃ and 54% of CaCO₃. It  is stronger and heavier than calcite. It  is  brittle and insoluble   in ordinary water.

(iv)      Glauconite: This mineral has no definite crystal form. Its colour varies from yellow to green. It is insoluble in acids as well as ordinary water.

(v)        Limonite: Its colour varies from yellow to reddish brown. It is insoluble in ordinary water, but it dissolves in acids.

(vi)      Gypsum: It is the hydrated sulphate of calcium with chemical composition is CaSO₄.2H₂0. It is a white crystalline substance with density 23 kN/m³ and its hardness 2.

(vii)     Anhydrate: This is an anhydrous variety of gypsum and its chemical composition is CaSO₄. Its density varies from 28 to 30 kN/m³ and its hardness varies from 3.00 to 3.50.




** basic civil engineering notes for bput students.