The composition of the original crust of igneous rock which covered the earth must have been quite uniform all over the surface of the Globe. The present composition of the earth what we are seeing is the result of later changes. Below few thousands of feet, the earth is composed mainly of a basalt layer, which is more than hundred miles thick. Below that we are uncertain what is the exact nature of the earth. Silica and alumina make up about 75% of the crust of the earth and these two oxides are the essential elements of the clay.
When the surface of the earth cooled into a solid, various minerals where formed. The following is the approximate percentage of various minerals which make up the crust of the earth.
Feldspar 59.5
Ferro-magnesium group 16.8
Quartz 12.0
Biotite 3.8
Titanium minerals 1.5
All others 6.4
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100.0
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Feldspar is the most common mineral and weathering of this mineral largely accounts for clay.
To be continued…………..
