Describe the main types of formations in which minerals occur.
The minerals occur in various types of formations as given below : (1) Igneous and metamorphic rocks : See Textbook Exercise Question 2(3). (2) In sedimentary rocks : Minerals occur in beds or layers. They are formed, as a result of deposition, accumulation and concentration in horizontal strata e.g., coal and some forms of iron ore have been concentrated as a result of long periods under heat and pressure. Gypsum, potash, salt and sodium salt are formed as a result of evaporation, especially in arid regions. (3) Some minerals like bauxite are formed due to decomposition of surface rocks and the removal of soluble constituents, leaving a residual mass of weathered material containing ores, e.g., bauxite is formed this way. (4) Certain minerals like gold, silver, tin and platinum occur as alluvial deposits in sands of valley floors and the base of hills. These are called ‘placer deposits’ and contain minerals, which are not corroded by water. (5) Ocean waters : Some minerals like common salt, magnesium and bromine are largely derived from ocean waters. The ocean beds too are rich in manganese nodules. Thus, minerals are found in various types of formations.
