(a) Give an account of the geology and the process of formation of aluminium mineral deposits of India. (10 marks) (b) What are…

(a) Aluminium mineral deposits. India’s bauxite deposits are chiefly residual products of lateritization, in which aluminous rocks are subjected to prolonged tropical chemical weathering on flat, well-drained plateau tops. The main parent rocks are khondalites (garnet–sillimanite gneisses) of the Eastern Ghats, Deccan basalts in Maharashtra, Gujarat and Madhya Pradesh, and Precambrian rocks of the Chhotanagpur plateau (Lohardaga, Jharkhand). A typical profile has a hard laterite or bauxite cap, a bauxite zone, a kaolinitic lithomarge and then the parent rock; gibbsite is the dominant mineral in these young Indian laterites. Leaching removes silica, alkalis and alkaline earths, while aluminium (with residual ferric iron) concentrates as gibbsite, boehmite and diaspore in laterite crusts. Uplift, erosion and repeated weathering cycles control thickness and grade. The Eastern Ghats belt in Odisha and Andhra Pradesh hosts the largest lateritic bauxite occurrences; Gujarat (Kutch) and Madhya Pradesh (Amarkantak) contain important lateritic deposits; the Panchpatmali deposit of Koraput (Odisha), worked by NALCO, is one of Asia's largest, and Odisha holds about half of India's bauxite resources. Karstic bauxite forms in limestone solution features where aluminous clays are enriched, especially in the Jammu region, where older diaspore-rich bauxite lies on the Sirban Limestone, whereas lateritic bauxite forms as a residual cap over crystalline rocks. Economic bauxite generally requires Al₂O₃ above 40 per cent and SiO₂ below 5 per cent, with low impurities, because silica raises alumina losses and caustic consumption in Bayer processing. These deposits underpin India’s aluminium industry (Bayer process followed by Hall–Héroult smelting) and export earnings.

(b) Iron-titanium oxides in igneous rocks. Iron-titanium oxides are common accessory or dominant minerals in mafic and ultramafic igneous rocks, especially layered gabbroic intrusions, gabbro–anorthosite complexes and basalts. The principal minerals are ilmenite (FeTiO₃), magnetite (Fe₃O₄), ulvöspinel (Fe₂TiO₄) and hematite (Fe₂O₃), with titanomagnetite forming a solid-solution series between ulvöspinel and magnetite; oxidation state, expressed as fO₂, controls the relative stability of magnetite, ilmenite and hematite. They commonly occur with plagioclase, pyroxene, olivine and apatite (nelsonite is an ilmenite/magnetite–apatite rock), and magnetite often carries vanadium; examples are the magnetitite layers of the Bushveld Complex and the ilmenite norite of Tellnes (Norway). Textures reveal their history: oxide cumulate layers, oikocrystic magnetite enclosing silicates, resorbed xenocrysts, brecciated oxide bands, and oxide-silicate reaction rims. On cooling, oxy-exsolution produces ilmenite lamellae in magnetite in trellis, sandwich and composite forms, ulvöspinel exsolves as fine cloth-like intergrowths, ilmenite may host hematite lamellae, and later oxidation converts magnetite to hematite (martitisation). Titanomagnetite is the principal natural remanent-magnetisation carrier in basalts, and magnetite–ilmenite pairs serve as oxygen barometers. In India, the vanadiferous titanomagnetite of the Mayurbhanj gabbro–anorthosite complex (Odisha) and of Nuggihalli–Masanikere (Karnataka) are the important igneous occurrences; the ilmenite-rich beach placers of Chavara (Kerala) are detrital concentrations derived from such sources. Such textures are used to infer magma evolution, oxidation conditions and ore-forming processes.

(c) Prospecting, exploration and sampling. Prospecting is the early, regional search for mineral targets using geological mapping, remote sensing, geophysics and reconnaissance geochemistry; it is broad, low-cost and risk-oriented. Exploration is the detailed, deposit-scale investigation that tests a target by trenching, drilling, geotechnical work and resource estimation, moving from inference to bankable confidence. In the UNFC classification, prospecting corresponds broadly to G4 reconnaissance and G3 prospecting, while exploration generally spans G2 to G1, with G1 representing detailed, mine-planning grade data. Sampling links field evidence to assay results. Chip samples are taken from exposed outcrops or trenches; channel samples follow a fixed width along a face; bulk samples represent larger volumes for metallurgical or grade tests; core samples from drill holes provide continuous stratigraphy and grade control. Sampling also includes pit and trench samples, grab samples for reconnaissance, and blasthole grade-control samples in operating mines. Good sampling requires systematic or random spacing, composite intervals, avoidance of bias, proper splitting (riffle or cone-and-quarter), crushing, grinding and preparation for chemical or instrumental assay, together with QA/QC using blanks, duplicates and standards, and geostatistical methods to estimate continuity and uncertainty.

(d) Cosmic abundance and Oddo-Harkins rule. The Universe is dominated by hydrogen and helium, from Big Bang nucleosynthesis, with heavier elements produced by stellar nucleosynthesis, supernovae and neutron-star mergers. By mass, hydrogen and helium exceed 98 per cent, while oxygen, carbon, neon, iron, nitrogen and silicon are the principal “metals”. Earth’s crust differs sharply: oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium dominate through silicate differentiation in continental crust. The Oddo-Harkins rule observes that elements with even atomic numbers are usually more abundant than their odd-numbered neighbours, because even-even nuclei, with paired protons and neutrons, generally have higher binding energy per nucleon and are more stable. It is not universal. Exceptions are important: hydrogen (odd Z) is the most abundant element, and lithium, beryllium and boron are anomalously depleted because they are destroyed inside stars. Typical examples include iron (Z=26) being more abundant than manganese (Z=25) and cobalt (Z=27), oxygen (Z=8) than nitrogen (Z=7) and fluorine (Z=9), and silicon (Z=14) than aluminium (Z=13) and phosphorus (Z=15); the rule also explains the saw-tooth pattern of rare-earth abundances that becomes a smooth curve after chondrite normalisation.

(e) Earthquake hazards and mitigation. Earthquakes generate direct and secondary hazards. Ground shaking damages buildings, bridges and lifelines; liquefaction can cause soft ground to flow, toppling structures; surface rupture breaks land, pipelines and roads; co-seismic landslides and rockfalls block valleys and rivers; undersea earthquakes may generate tsunamis that inundate coasts. Secondary effects include fires from gas leaks, collapse of unsafe structures, disruption of water and sanitation, and disease. Mitigation must be multi-layered. Seismic hazard zoning and land-use planning keep critical facilities away from active faults, liquefiable soils and steep slopes. Building codes such as IS 1893 (design), IS 4326 (earthquake-resistant construction) and IS 13920 (ductile detailing), with the National Building Code, local bylaws and competent design, ensure ductile, base-isolated or seismic-resistant construction; retrofitting of old schools, hospitals and industrial plants reduces vulnerability. Early warning — including the Indian Tsunami Early Warning Centre (INCOIS, Hyderabad), which issues advisories within minutes, and regional earthquake early-warning pilots such as IIT Roorkee's system in Uttarakhand — together with seismic microzonation, community preparedness, school and hospital safety programmes, mock drills, emergency planning and insurance, improves resilience. The National Disaster Management Authority and state authorities must enforce codes, monitor construction and integrate risk into planning, because mitigation is cheaper than reconstruction.