Classification of Rocks & Minerals – Comprehensive Bullet-Point Notes

Rock Basics and General Classification

• A rock is a naturally-occurring, solid aggregate of one or more minerals.

  • Commonest colour: grey, but virtually any hue is possible.

• Three fundamental rock families:

  • Igneous

  • Sedimentary

  • Metamorphic

• Colour, texture, hardness, layering and mineral content are the usual diagnostic clues.

Igneous Rocks

• Definition

  • Formed by cooling and solidification of molten material (magma or lava).

  • Root word “igneous” ≈ “from fire”.

• Chemical makeup

  • Dominantly silicates (rich in $\text{Si}$ and $\text{O}$).

  • Rarer carbon-based igneous rocks contain carbonates.

• Textural end-members are controlled by cooling rate.

  • Intrusive (plutonic)

  • Magma cools slowly beneath Earth’s surface → large, visible crystals.

  • Coarse-grained; often described as phaneritic.

  • Examples: granite, diorite, gabbro, pegmatite, peridotite.

  • Extrusive (volcanic)

  • Lava erupts, cools rapidly on or above surface.

  • Fine-grained (aphanitic), glassy, or vesicular.

  • Examples & special features:

    • Basalt, andesite, dacite, rhyolite, tuff (fine ash welded together).

    • Obsidian (glassy), pumice/scoria (vesicular due to trapped gases).

• Diagrammatic reminder

  • Surface ⇄ subsurface positions are linked: intrusive magma bodies may feed extrusive flows.

Sedimentary Rocks

• General process

  • Weathering → Erosion → Transport → Deposition → Burial → Compaction & Cementation.

  • Often accumulate in water bodies, forming visible layers (beds/strata).

• Three genetic sub-groups

  1. Clastic (detrital)

  • Produced from mechanical breakdown of pre-existing rocks.

  • Grain size spectrum: breccia (angular gravel), conglomerate (rounded gravel), sandstone (sand), siltstone (silt), shale (clay).

    • Arkose sandstone: notable feldspar content.

  • Diagnostic textures: gritty to powdery; may feel like sandpaper/fine flour.

  1. Chemical

  • Precipitation from supersaturated solutions.

  • Examples: rock salt (halite), gypsum, chert/flint, iron ore, travertine, some dolomites.

  • Often display crystalline textures or evaporite layering.

  1. Organic (biochemical)

  • Derived from accumulation of biological debris.

  • Examples: coal, chalk, fossiliferous limestone.

Metamorphic Rocks

• Metamorphism = “change in form.” Parent rock (protolith) may be igneous, sedimentary or older metamorphic.

  • Agents: heat, pressure, chemically active fluids.

  • Increasing temperature & pressure → increasing metamorphic grade.

• Textural categories

  1. Foliated

  • Directed pressure aligns platy/elongate minerals → banding/foliation.

  • Progressive grade series: slate → phyllite → schist → gneiss.

  1. Non-foliated

  • No obvious banding; dominated by equant minerals or contact-metamorphic growth.

  • Examples: marble (from limestone), quartzite (from sandstone), hornfels, novaculite, anthracite coal, greenstone.

The Rock Cycle (integrative framework)

• Conceptual steps (numbers match transcript diagram):

  1. Magma

  2. Igneous rock (cooling & crystallisation)

  3. Weathering → sediments → deposition, compaction, cementation

  4. Sedimentary rock

  5. Metamorphic rock (heat & pressure) → melting returns to magma

• Emphasises continual recycling; any rock can transition to any other given the right conditions.

Minerals: Definition & Fundamental Criteria

• "A mineral is a naturally occurring, inorganic, solid substance with a defined chemical composition and an ordered (crystalline) atomic structure."

• Essential characteristics

  • Occurs naturally

  • Inorganic origin

  • Solid state at surface conditions

  • Definite chemical formula

  • Crystal lattice/structure

Everyday & Industrial Importance of Minerals

• Household

  • Toothpaste contains fluoride from fluorite; table salt is halite.

  • Talc gives silky texture to face powder.

  • Kitchenware: aluminium pans, stainless steel cutlery (Fe–Cr–C alloy).

• Health & Dentistry

  • Titanium implants, gypsum dental casts, stainless-steel surgical tools.

  • Supplements rich in Ca, Mg, Zn support biological functions.

• Construction

  • Steel (Fe + C), concrete (limestone, lime, chalk), glass (quartz + silica), floor tiles of granite & marble, aluminium frames.

• Technology & Electronics

  • Silicon, silver, gold inside microchips; copper predominant in wiring because $\text{Cu}$ balances cost & conductivity.

• Agriculture

  • NPK fertilisers supply $\text{N}$, $\text{P}$, $\text{K}$; lime (CaCO$_3$) neutralises acidic soils.

• Energy Sector

  • Nuclear: uranium-235/238 heat source.

  • Coal-fired plants use coal.

  • Batteries (e-cars): nickel, copper, lithium.

• Note on skyscrapers: up to 50 storeys rely on mineral-derived materials for wind & quake resistance.

Ethical, Environmental & Health Implications

• Talc may contain asbestos fibers → carcinogenic risk.

• Radioactive waste from nuclear reactors.

• Coal combustion releases $\text{CO}_2$ and pollutants → global warming.

• Excess synthetic fertiliser → eutrophication of water bodies.

Physical Properties Used to Identify Minerals

• 1. Crystal Habit

  • Equant (≈ cube/sphere e.g., garnet), elongate/prismatic (e.g., indicolite), platy (e.g., wulfenite).

• 2. Luster

  • Metallic: opaque, highly reflective (gold, pyrite).

  • Non-metallic: dull, silky, greasy, pearly (quartz, silicates).

• 3. Cleavage vs. Fracture

  • Cleavage: tendency to split along flat, shiny planes.

  • Fracture: irregular break (conchoidal, jagged, splintery, etc.).

• 4. Hardness (Mohs Scale)

  • Relative resistance to scratching; ranked 1–10.

  • Table summary with common objects

  • $1$ Talc — scratched by fingernail ($2.5$)

  • $2$ Gypsum

  • $3$ Calcite — copper coin ($3.5$)

  • $4$ Fluorite

  • $5$ Apatite — steel knife ($5.5$)

  • $6$ Orthoclase (Feldspar)

  • $7$ Quartz — masonry drill bit ($8.5$ scratches $<8.5$)

  • $8$ Topaz

  • $9$ Corundum

  • $10$ Diamond

• 5. Color

  • Visually obvious but unreliable alone; quartz shows multiple colours due to trace impurities (ppb levels).

• 6. Streak

  • Powder colour on unglazed porcelain; more diagnostic than bulk colour (hematite always red-brown streak).

• 7. Reaction with Acid

  • Carbonates fizz with dilute $\text{HCl}$. Example reaction:
    $\text{CaCO}<em>3 + 2\,\text{HCl} \rightarrow \text{CaCl}</em>2 + \text{H}<em>2\text{O} + \text{CO}</em>2 \uparrow$

• 8. Magnetism

  • Magnetite is strongly magnetic.

• 9. Taste / Odor / Feel

  • Halite tastes salty; sphalerite powder smells like rotten eggs (H$_2$S); talc feels slippery.

• Specific Gravity (SG)

  • $\text{SG} = \dfrac{W<em>{\text{mineral}}}{W</em>{\text{water}}}$ (equal volumes).

  • Silicates: $2.6–3.4$; dense ores (barite, galena) $\approx 5–8$.

• Striations

  • Fine parallel grooves on cleavage faces (help to separate plagioclase from K-feldspar).

Chemical Classification of Minerals (Dana System – 8 Classes + Mineraloids)

• Native Elements

  • Pure metallic or semi-metallic substances (e.g., gold, silver, copper, sulfur).

• Silicates (largest class)

  • Fundamental $\text{SiO}_4^{4-}$ tetrahedra linked in various ways.

  • Examples: feldspar, quartz, olivine, mica.

• Oxides

  • Metal + oxygen; range from ores (bauxite) to gemstones (corundum → ruby/sapphire). Example: magnetite.

• Sulfides

  • Metal + sulfur; usually dense, metallic, opaque. Example: pyrite ("fool’s gold").

• Sulfates

  • Metal + sulfate group $\text{SO}_4^{2-}$. Example: gypsum.

• Halides

  • Metal + halogen (Cl, F, Br, I). Example: halite (NaCl); typically soft & water-soluble.

• Carbonates

  • Metal + carbonate group $\text{CO}_3^{2-}$. Example: calcite, dolomite.

• Phosphates

  • Metal + phosphate $\text{PO}_4^{3-}$, often weathering products; colourful (e.g., apatite).

• Mineraloids (non-crystalline, not strictly minerals)

  • Examples: amber, opal, obsidian.

Additional Numerical / Formulaic References

• Uranium atomic mass example: $238.02891$ (U-238 in nuclear fuel).

• Silicon–gold–silver utilised in semiconductor doping: ppm to ppb scale impurities drastically change conductivity.

Connections & Integrated Significance

• Geological context: rock type governs soil fertility, landscape, natural hazards.

• Engineering relevance: knowing texture & mineral content guides stone selection (e.g., granite vs. limestone façades).

• Environmental stewardship requires balancing mineral extraction benefits with pollution, waste, and health risks.

• Foundational principle: Plate tectonics drives pressure-temperature regimes that underlie the entire rock cycle and mineral genesis.