Major mineral groups, metallic and non-metallic minerals, and how ores occur

Detail

1. Basic terms

• Mineral: a natural, inorganic substance with a fixed chemical make-up and a regular inner (crystal) structure. Rocks are made of one or more minerals.
• Ore: a rock or mineral mass that holds enough of a useful metal (or mineral) to be mined at a profit.
• Silicate minerals: minerals built from units of silicon and oxygen (the silicon–oxygen tetrahedron: one silicon atom joined to four oxygen atoms), often with metals like Na, K, Ca, Al, Mg and Fe added.
• They make up over 90% of the Earth's crust.
• Oxygen and silicon are the two most common elements in the crust, so most rocks are silicate-rich.

2. The main rock-forming silicate groups

Group	What it is made of	Look and features	Where found / use
Feldspar	Si + O with Na, K, Ca, Al	Light-coloured (cream, pink, white); most common mineral group, about half the crust (NCERT figure)	Ceramics (pottery, tiles), glass
Quartz	Silica (SiO₂) only	Hard (7 on the Mohs scale); does not dissolve in water; white or colourless	Main grain of sand and sandstone; a part of granite; glass, electronics, quartz watches
Pyroxene	Ca, Al, Mg, Fe + silica	Green or black; about 10% of the crust	Common in meteorites
Amphibole	Al, Ca, Fe, Mg + silica	Green or black; about 7% of the crust	Asbestos is one form of amphibole
Mica	K, Al, Mg, Fe + silica	Splits into very thin, clear sheets	Electrical and electronic goods
Olivine	Mg, Fe + silica	Green	Found in basalt and the mantle; gem form is peridot

• Mohs scale: a 1–10 scale of mineral hardness (talc = 1, diamond = 10). A harder mineral scratches a softer one. Quartz at 7 is harder than steel, so sand grains survive long weathering.
• Light vs dark silicates:
• Feldspar and quartz are light-coloured. They are rich in silica and common in granite (an acidic, light igneous rock of the continents).
• Pyroxene, amphibole and olivine are dark. They are rich in iron and magnesium and common in basalt (a basic, dark igneous rock, as in the Deccan Traps) and in the mantle.

3. Mica in detail

• Structure: mica is a group of 34 phyllosilicate minerals (silicates arranged in flat layers). That is why it splits into very thin sheets [2].
• Main commercial types [2]:
• Muscovite (potash or white mica). It comes from granitic pegmatites (very coarse-grained granite veins).

• Phlogopite (magnesium or amber mica). It forms in metamorphosed sedimentary rocks where pegmatite-rich granite has pushed in.

• Why it suits electrical work: it resists heat and sudden temperature change, has high dielectric strength (it does not let electricity pass), does not absorb water and does not react with chemicals [2]. It is used in capacitors and "micanite" (built-up mica sheets) [2].

• Indian belts (NCERT): the Koderma–Gaya–Hazaribagh belt (Jharkhand–Bihar), Ajmer (Rajasthan) and Nellore (Andhra Pradesh).
• Resources (as on 1.4.2015): 6,35,302 tonnes in total [2]. State shares [2]:
• Andhra Pradesh 41%
• Rajasthan 28%
• Odisha 17%
• Maharashtra 13%
• Bihar 2%

• Small amounts in Jharkhand and Telangana

• Production: Andhra Pradesh 16,822 t and Rajasthan 16,000 t in 2019-20 [2].

• Legal status: mica was declared a minor mineral on 10 February 2015 (Notification S.O. 423(E)). Producers now report to the states, not to the Indian Bureau of Mines (IBM) [2].
• Trend: for over a hundred years India held a near-monopoly in sheet mica production and export. Output has fallen since, because reconstituted mica and mica substitutes have cut demand [2].

4. Metallic, non-metallic and fuel minerals

Class	Sub-type	Examples
Metallic	Precious	Gold, silver, platinum
	Ferrous (contain iron, or are alloyed with iron)	Iron ore, manganese (also nickel, cobalt)
	Non-ferrous (contain no iron)	Copper, lead, zinc, tin, aluminium (from bauxite)
Non-metallic	—	Mica, limestone, gypsum, sulphur, phosphates, nitrates
Energy (mineral fuels)	Organic (from buried plant and animal matter)	Coal, petroleum, natural gas

• Ferrous minerals feed the iron and steel industry. Manganese is mixed with iron to make steel harder.
• Non-metallic minerals yield no metal when melted. They are used as they are, or in chemicals, fertiliser and building.
• Cement is NOT a mineral. It is a factory-made mix, mostly limestone heated with clay and gypsum. This is a common exam trap.
• Mineral fuels are organic, so they are not true minerals in the strict sense. They are still grouped with minerals as "energy minerals".

5. How minerals occur — five modes (linked to rock type)

(i) Veins and lodes — in igneous and metamorphic rocks

• Cause: hot, mineral-rich fluids from cooling magma (molten rock below the surface) move up through cracks, faults and joints.
• Process: the fluid cools, and minerals settle out on the crack walls.
• Result:
• Vein = a small, thin filling.

• Lode = a large, thick filling.

• Examples: tin, copper, zinc, lead. In India, Khetri copper (Rajasthan) and Zawar lead–zinc (Rajasthan) are found in old metamorphic rocks.

(ii) Beds or layers — in sedimentary rocks

• Minerals build up slowly, layer by layer, with the sediments. Heat and pressure over a long time then harden them.
• Examples: coal (Gondwana coalfields of the Damodar, Mahanadi and Godavari valleys) and some iron ores.
• Evaporites form in dry (arid) regions: shallow, salty water dries up and leaves behind gypsum, potash and sodium salts (example: gypsum in Rajasthan).

(iii) Residual deposits — left after weathering

• Cause: heavy rain and high heat in the tropics.
• Process: chemical weathering (rocks broken down by rainwater and chemical reactions) washes away the soluble parts of surface rocks, such as silica and bases.
• Result: insoluble, metal-rich leftovers stay in place and form an ore. This is called ore enrichment through weathering (NCERT Class 11).
• Example: bauxite (Al₂O₃·nH₂O, hydrated aluminium oxide), the principal ore of aluminium [3].
• Bauxite resources (as on 1.4.2020): 4,958 million tonnes [3].
• About 79% of this is metallurgical grade [3].
• State shares of resources [3]:
  • Odisha 41%
  • Chhattisgarh 20%
  • Andhra Pradesh 12%
  • Gujarat 8%
  • Jharkhand 6%
  • Maharashtra 5%
  • Madhya Pradesh 4%
• Main concentration: the East Coast bauxite deposits of Odisha and Andhra Pradesh, on the Eastern Ghats [3].
• Production in 2021-22: 22.49 million tonnes. Odisha gave about 73% and NALCO was the top producer at 33% [3].

(iv) Placer deposits — in valley-floor sands

• Cause: rivers wear away rocks that hold heavy, hard-wearing minerals.
• Process: running water carries off the light particles. The heavy, chemically stable grains settle in the sands of valley floors and the bases of hills.
• Result: deposits of gold, silver, tin and platinum. (Example: alluvial gold in the Subarnarekha sands.)

(v) Ocean deposits

• Ocean water yields common salt, magnesium and bromine. Most of these minerals are spread too thinly to be worth extracting.
• Ocean beds hold manganese nodules (also called polymetallic nodules: potato-sized lumps on the deep sea floor rich in manganese, nickel, copper and cobalt).
• India's site is in the Central Indian Ocean Basin (CIOB), in international waters. It was allotted by the International Seabed Authority (ISA) [4].
• India signed a 15-year exploration contract with the ISA on 25 March 2002 [4].
• The site India kept covers 75,000 sq km. It holds an estimated 380 million tonnes of nodules, containing about [4]:
  • 92.59 MT manganese
  • 4.7 MT nickel
  • 4.29 MT copper
  • 0.55 MT cobalt

6. Summary — rock type and how ores form

Mode	Host rock / setting	Process	Examples
Veins and lodes	Igneous, metamorphic	Hot fluids fill cracks, faults and joints	Tin, copper, zinc, lead
Beds / layers	Sedimentary	Deposition and evaporation	Coal, some iron ore; gypsum, potash, salt
Residual	Weathered surface rock	Soluble parts washed away	Bauxite
Placer	Valley-floor sands	Heavy grains sorted by water	Gold, silver, tin, platinum
Oceanic	Seawater and ocean floor	Dissolved salts; slow growth on the sea floor	Salt, Mg, bromine; manganese nodules

Prelims Hooks

• Silicate minerals make up over 90% of the crust. Feldspar is the most common group (about half the crust, NCERT).
• Quartz has hardness 7 on the Mohs scale and does not dissolve in water. It is the main grain of sand and a part of granite.
• Asbestos is a form of amphibole. The gem peridot is olivine.
• Pyroxene (about 10% of the crust) is common in meteorites.
• Mica was declared a minor mineral in February 2015. Andhra Pradesh leads mica resources (41%), followed by Rajasthan (28%) (as on 1.4.2015) [2].
• Manganese is a ferrous mineral. Aluminium, copper, lead, zinc and tin are non-ferrous.
• Trap: cement is a manufactured product, not a mineral. Coal, petroleum and natural gas are organic.
• Mode–mineral pairs: veins and lodes – tin, copper, zinc, lead; residual – bauxite; placer – gold, platinum; evaporite beds – gypsum, potash.
• Bauxite resources: Odisha 41% > Chhattisgarh 20% > Andhra Pradesh 12% (as on 1.4.2020). Odisha gave about 73% of output in 2021-22 [3].
• India's polymetallic nodule site: Central Indian Ocean Basin, 75,000 sq km, under an ISA contract signed in 2002 [4].

Mains Points

• Rock type controls where ores are found (GS-I)
• Old igneous and metamorphic rocks of the Peninsula (Archaean and Dharwar) hold metallic ores in veins and lodes.
• Gondwana sedimentary basins hold coal in beds.
• Tropical laterite caps on the Eastern Ghats hold residual bauxite.

• So most of India's minerals lie in the Peninsular plateau, and few lie in the alluvial plains.

• Weathering adds value: heavy monsoon rain and tropical heat wash out silica and leave aluminium-rich bauxite behind. This explains the East Coast bauxite belt of Odisha and Andhra Pradesh and India's aluminium industry (NALCO) [3].

• Changing demand for mica (GS-III)
• India's old sheet-mica monopoly has declined because of substitutes [2].
• Making mica a minor mineral handed regulation to the states and weakened national data [2].

• Poorly controlled mica-scrap collection in the Koderma–Giridih belt is linked to illegal mining and child-labour concerns. Better regulation is needed.

• Deep-sea minerals for strategic security (GS-III)

• The polymetallic nodules of the CIOB hold nickel, cobalt and copper [4]. These are critical minerals for batteries and the energy transition.
• India's ISA contract [4] needs both mining technology and baseline environmental studies, so that deep-sea ecosystems are not harmed.