Answer:
Minerals
generally occur in these forms:
(i) Igneous and metamorphic rocks: Minerals may occur in the cracks,
crevices, faults or joints. The smaller occurrences are called veins or lodes.
In most cases, they are formed when minerals in liquid, molten and gaseous forms
are forced upward through cavities towards the earth's surface.
They cool and solidify as they rise.
Example: Metallic minerals like tin, copper, zinc and
lead, etc. are obtained from the veins and lodes.
(ii) In sedimentary rocks: A number of minerals occur in
beds or layers.
They have been formed as a result of deposition,
accumulation and concentration in horizontal strata. Coal and some forms of
iron ore and sedimentary minerals include gypsum, potash salt and sodium salt.
(iii) Another mode of formation involves the decomposition
of surface rocks, and the removal of soluble constituents, leaving a residual
mass of weathered material containing ores. Bauxite is formed in this way
(iv) Certain minerals may occur as alluvial deposits in
the sands of valley floors and the base of hills. These deposits are called
'placer deposits' and generally contain minerals which are not corroded by
water.
Example: Gold, silver, tin and platinum are most important
among such minerals.
(v) The ocean waters contain vast quantities of minerals,
but most of these are too widely diffused to be of economic significance.
Example: Common salt, magnesium and bromine are largely
derived from the ocean waters. The ocean beds, too, are rich in manganese
nodules.
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