Unit 5: Industrial Minerals and Rocks of India

Introduction

Industrial minerals and rocks are geological materials that are mined for their commercial value, based on their unique physical and/or chemical properties. This is distinct from "ore minerals," which are mined for the metals they contain (though some, like bauxite, are both).

Gems and Precious Metals

Gold (Au)

• Properties: Bright yellow metal, high metallic lustre, very soft (H: 2.5-3), extremely high specific gravity (heavy), ductile and malleable.
• Occurrences: (1) Lode/Vein Deposits: Found in its native state within quartz veins in metamorphic/igneous rocks. (2) Placer Deposits: As "alluvial gold" (dust, flakes, nuggets) concentrated in river sands and gravels, weathered from lode deposits.
• Distribution in India: Hutti Gold Field (Karnataka) is the main producer. Historically, Kolar Gold Fields (Karnataka) was the most famous. Alluvial gold is found in rivers like the Subarnarekha.

Diamond (C)

• Properties: Hardest known natural substance (H: 10), adamantine (brilliant) lustre, high refractive index. It is a polymorph of Carbon (C).
• Occurrences: (1) Primary: Forms under extreme pressure deep in the mantle and is brought to the surface explosively in an igneous rock called Kimberlite (which forms "pipes"). (2) Secondary: In alluvial (placer) deposits, weathered from kimberlite pipes.
• Distribution in India: Panna (Madhya Pradesh) is the main primary source from kimberlite pipes. Historically, the Golconda region (Andhra Pradesh) was famous for alluvial diamonds.

Corundum (Al₂O₃)

• Properties: Extremely hard (H: 9) (second only to diamond), typically forms barrel-shaped hexagonal crystals, high specific gravity.
• Uses: Gem varieties are Ruby (red) and Sapphire (blue). Opaque varieties are used as abrasives (emery).
• Occurrences: In silica-poor igneous rocks (like syenites) and high-grade metamorphic rocks (schists, gneisses).
• Distribution in India: Karnataka, Andhra Pradesh, Tamil Nadu, and Odisha.

Metallic Ores

Chromite (FeCr₂O₄)

• Properties: Black, sub-metallic lustre, brownish-black streak, H: 5.5.
• Uses: The only ore of Chromium (Cr), used to make stainless steel.
• Occurrences: Forms as layers (magmatic segregation) in ultramafic igneous rocks (e.g., dunite, peridotite).
• Distribution in India: The Sukinda Valley (Odisha) holds over 90% of India's chromite reserves, one of the largest deposits in the world.

Bauxite (Rock of Al-hydroxides)

• Properties: A rock, not a single mineral. Earthy lustre, reddish-brown/grey, often has a pisolitic (small, "pea-like") texture.
• Uses: The primary ore of Aluminum (Al).
• Occurrences: A residual deposit. It is the product of intense chemical weathering (laterization) of aluminum-rich rocks (like granite or basalt) in hot, wet, tropical climates.
• Distribution in India: East Coast Bauxite (Odisha & Andhra Pradesh) is the largest deposit. Also found in Jharkhand, Gujarat, and Maharashtra.

Magnetite (Fe₃O₄)

• Properties: Black, metallic lustre, black streak, strongly magnetic (its key identifying feature). H: 5.5-6.5.
• Uses: An important ore of Iron (Fe).
• Occurrences: Common in igneous rocks (layers in gabbros) and metamorphic rocks (Banded Iron Formations - BIF).
• Distribution in India: Kudremukh (Karnataka), Salem (Tamil Nadu), Goa.

Hematite (Fe₂O₃)

• Properties: Steel-grey (specular) or earthy red (oolitic), cherry-red streak (its key identifying feature). H: 5-6.
• Uses: The most important ore of Iron (Fe).
• Occurrences: Mainly in sedimentary deposits, especially Banded Iron Formations (BIF).
• Distribution in India: This is India's main iron ore. Found extensively in the Barbil-Koida Valley (Odisha), Bailadila (Chhattisgarh), Singhbhum (Jharkhand), and Goa.

Sphalerite (ZnS)

• Properties: Variable color (yellow, brown, black), resinous lustre, pale yellow/brown streak (often smells of sulfur), H: 3.5-4.
• Uses: The primary ore of Zinc (Zn).
• Occurrences: In hydrothermal veins, often found with Galena (lead ore) and Pyrite.
• Distribution in India: Rampura-Agucha (Rajasthan) is one of the world's largest zinc mines.

Industrial Rocks and Minerals

Limestone (Rock of CaCO₃)

• Properties: A sedimentary rock (composed of Calcite). Usually white, grey, or buff. Fizzes vigorously in dilute HCl. Soft (H: 3).
• Uses: Essential for manufacturing cement, also used as building stone ("dimension stone") and as a "flux" in steelmaking.
• Occurrences: Forms in shallow, warm marine environments from the accumulation of shells, corals, and chemical precipitates.
• Distribution in India: Widespread. Major deposits in the Vindhyan Range (MP, Rajasthan), Cuddapah (Andhra Pradesh), Gujarat.

Gypsum (CaSO₄·2H₂O)

• Properties: Very soft (H: 2, can be scratched with a fingernail), white/transparent, 1 perfect cleavage.
• Uses: Manufacturing Plaster of Paris, drywall, and as an additive to cement to control setting time.
• Occurrences: An "evaporite" mineral. Forms from the evaporation of saline water (seawater) in coastal lagoons.
• Distribution in India: Rajasthan is the largest producer (Nagaur, Jodhpur). Also found in Tamil Nadu and Gujarat.

Graphite (C)

• Properties: Grey to black, metallic/greasy feel, writes on paper, very soft (H: 1-2). A polymorph of Carbon.
• Uses: "Lead" in pencils, lubricant, high-temperature crucibles, batteries.
• Occurrences: Forms from the high-grade metamorphism of organic-rich sedimentary rocks (like coal or shale).
• Distribution in India: Odisha is a major producer, followed by Jharkhand and Tamil Nadu.

Mica (Muscovite/Biotite Group)

• Properties: Perfect 1-direction cleavage, splitting into thin, flexible, elastic sheets. Pearly lustre. (Muscovite is white/silvery, Biotite is black/brown).
• Uses: Sheet mica (Muscovite) is an excellent electrical and thermal insulator. Used in electronics, paints, and cosmetics.
• Occurrences: Found in large "books" in pegmatites (a very coarse-grained igneous rock). Also common in schists and gneisses.
• Distribution in India: India is world-famous for its high-quality sheet mica. Kodarma (Jharkhand) is the "Mica Capital", along with the Nellore belt (Andhra Pradesh) and parts of Rajasthan.

Marble (Metamorphosed CaCO₃)

• Properties: A non-foliated metamorphic rock (composed of recrystallized Calcite). Fizzes in HCl. Crystalline, interlocking texture.
• Uses: Primarily as a building and ornamental stone (dimension stone).
• Occurrences: Formed by the contact or regional metamorphism of limestone or dolostone.
• Distribution in India: Makrana (Rajasthan), famous as the source of the marble for the Taj Mahal. Also found in Jabalpur (MP) and Gujarat.

Clay Minerals (Hydrous Al-Silicates)

• Properties: A group of minerals (e.g., Kaolinite, Smectite). Extremely fine-grained (microscopic), earthy lustre, soft, plastic when wet.
• Uses: Ceramics (pottery, bricks, tiles), paper (as a filler and coating), cosmetics, drilling mud.
• Occurrences: A "secondary" mineral, formed by the chemical weathering (hydrolysis) of feldspar-rich rocks like granite.
• Distribution in India: Widespread. Major deposits of Kaolin (China Clay) are found in Kerala, West Bengal, Rajasthan, and Jharkhand.