Minerals occurs in various forms :-   In igneous and metamorphic rocks minerals may occur in the cracks, crevices, faults or joints.  Major metallic minerals like tin, copper, zinc and lead etc. are obtained in rocks.  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 this way. Certain minerals may occur as alluvial deposits in 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. Gold, silver, tin and platinum are most important among such minerals. The ocean waters contain vast quantities of minerals. common salt, magnesium and bromine are largely derived from ocean waters.