Q. Show the Occurrence of Metals?

Earth's crust and sea are the two main sources of metals. In the earth's crust metals occur both in the combined state in the form of minerals as well as in the native or free state. Earth's crust is the outermost part of the earth, which has an average thickness of about 17 km. The crust is thinner under the oceans and thicker under the continents. The minerals from which the extraction of any metal is chemically feasible and economically competitive are known as ores of that metal. Metals occur in widely varying quantities in the earth's crust. You may note that about 75% of the earth's crust is composed of non metals, oxygen and silicon. The relative abundance of only three industrially important metals, i.e., aluminium, iron and magnesium is more than 2%. The abundance of most other useful metals in the earth's crust is very low. Therefore, if the metals had been uniformly distributed in earth's crust, it would have not been possible to extract them. But luckily, the metals generally in the form of their minerals are unevenly distributed and are accumulated at some locations, making their extraction easier. These accumulations of minerals are termed as mineral deposits. Usually, the mineral is covered with a layer of soil, known as overburden. The thickness of overburden may vary from a few metres as in case of iron ore to thousands of metres as in case of deposits of gold. The mineral deposit is brought to the surface by mining.