Minerals from the sea
The sea offers a variety of minerals, many of which are considered to be alternative sources of metals in the future. Among these are the placer deposits, which are mechanically concentrated minerals that originate from eroded onshore rocks. The near-shore waves separate the minerals brought by rivers and glaciers into heavy (sp. gravity >2.8) and light minerals, and concentrate heavy minerals on the beaches and estuaries (Fig. 37). Elements in native state (diamond, gold, and platinum) or minerals such as ilmenite, rutile, magnetite, zircon, monazite, garnet, and corundum are some examples of placer deposits. The ilmenite sands of Ratnagiri, monazite and zircon sands of Kerala, and garnet sands of Visakhapatnam are the better known placer deposits on the beaches of India.
Oolites are the inorganic chemical precipitates of calcium carbonate that form in hyper-saline, shallow marine environment (<10 m). Oolites are found on the continental shelf off Mumbai, Visakhapatnam, and Chennai. Phosphorites or sedimentary phosphate (>18% P2O5) deposits form on the continental shelf or upper continental slope by biogeochemical processes associated with organic-rich sediments (Fig. 38). Phosphorites occur both on the east and west coasts of India. The phosphorite deposits in Mussoorie and Udaipur in northern India were formed in marine conditions millions of years ago. Barite (BaSO4) deposits occur in deep-sea sediments in association with organic and biogenic remains and/or with volcanic activity. Zeolites are the alteration products of the submarine volcanic rocks in the deep ocean floor. Phillipsite, analcime, harmotome, and clinoptilolite are some of the zeolite minerals.
Ferro-manganese deposits form in the deep ocean basins (4-5 km depth) in areas away from the influence of terrigenous (from land) fluxes (Fig. 39). They occur as nodules (round objects upto 10 cm in size) and encrustations (as layers on rocks exposed on the seafloor). These are valuable deposits not because they have a high iron and manganese content, but because of their copper, nickel, and cobalt (total 2.5%) content. Manganese nodules grow at a rate of 1-3 mm per million years and occur on the ocean floor or a few centimetres below it. Crusts with high cobalt content (0.25% - 1%) usually occur on seamounts, elevated marginal areas, and mid-ocean ridges.
Hydrothermal deposits are formed by the interaction of seawater with submarine volcanic activity. They occur intermittently all along the midocean ridge system (especially Mid-Atlantic Ridge and Central Indian Ocean Ridge) and in the Red Sea. The hydrothermal fluids that gush out of the vents on the seafloor deposit metal sulphides (iron, copper, and zinc) and sulphate (barium and calcium) minerals. The sediments close to the vent are enriched with high content of iron, manganese, silver, chromium, lead, and zinc.
Gas hydrates are compounds where gas molecules are physically trapped inside an expanded lattice of water molecules. They can be present below the ocean floor on the continental slopes and deeper areas of high rate of deposition of sediments with moderate organic content (0.5%). Seabed gas hydrates could be an energy source of the future. A potential gas hydrate province covering an area of 1400 sq. km has been identified in the Krishna-Godavari offshore basin.
Seawater can be evaporated to produce sodium and potassium chlorides. In countries like Saudi Arabia where fresh water is scarce, salts are separated from seawater (desalination) to provide pure water for drinking. Treating seawater with calcium hydroxide precipitates magnesium hydroxide, which is used in the manufacture of magnesium metal. Bromine and iodine are obtained by treating seawater with chlorine gas.