Minerals generally occur in these forms: . 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. . 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. . 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. . 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. . 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.