The different forms of occurrence of minerals are:
a. Minerals occur in the cracks or joints in igneous and metamorphic rocks. Smaller occurrences are called veins and the larger occurrences are called lodes. They are formed when minerals in liquid and gaseous forms are forced upwards through cavities towards the surface of the earth. They cool and solidify as they rise.
The minerals that are mainly obtained from veins and lodes are tin, copper, led and zinc.
b. In sedimentary rocks, a number of minerals occur in bed or layer. They form due to the deposition, accumulation, and concentration in horizontal Strata. For example coal and iron ore. Another group of sedimentary minerals includes gypsum, potash salt, and sodium salt. They are formed due to evaporation, especially in arid regions.
c. Minerals are also formed due to the decomposition of surface rocks and the removal of soluble substances leaving a residual of weathered material containing ores. For example bauxite.
d. Minerals may occur as Alluvial deposits in sands of valley floors and the base of Hills. They are called placer deposits and contain minerals which are not corroded by water. For example gold and silver.
e. Ocean water also contains a vast quantity of minerals but most of them are too diffuse to be of any economic use. Common salt and magnesium are derived from ocean water.
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