Rocks & Minerals

Minerals are naturally occurring crystalline compounds; rocks are aggregates of minerals. Learn the rock cycle and three rock types, how minerals are identified (hardness, specific gravity, lustre), the key ore minerals, and how to calculate metal grade from mineral content and bulk density of mixtures.

Every mining decision starts with the rock: its minerals determine value, its texture affects processing, and its strength controls how it is mined. Ore minerals are those containing valuable elements in recoverable form — chalcopyrite for copper, hematite for iron, sphalerite for zinc.

An analogy

Minerals are the ingredients, rocks are the dishes: granite is a 'recipe' of quartz, feldspar and mica.

Rock cycle

Igneous rocks crystallise from magma (granite, basalt); sedimentary rocks form from deposited particles or precipitates (sandstone, limestone, coal); metamorphic rocks are transformed by heat and pressure (schist, gneiss, marble). Weathering, burial and melting cycle rocks between types over millions of years.

Identifying minerals

Hardness (Mohs scale: talc 1 → diamond 10; quartz 7 scratches glass), specific gravity (quartz 2.65, pyrite 5.0, galena 7.5, gold 19.3), lustre (metallic, vitreous), colour and streak, cleavage and crystal form.

Key ore minerals

Copper: chalcopyrite CuFeS₂ (34.6 % Cu), bornite, chalcocite, malachite. Iron: hematite Fe₂O₃ (70 %), magnetite Fe₃O₄ (72 %). Zinc: sphalerite ZnS. Lead: galena PbS. Gold: native gold, often microscopic in pyrite/arsenopyrite. Lithium: spodumene, brines. Gangue minerals (quartz, feldspar, calcite) have no value.

Mineralogy drives processing

Grain size and liberation, mineral associations, and deleterious elements (arsenic, fluorine) determine recovery and penalties. Automated mineralogy (QEMSCAN, MLA) and geometallurgy map these properties through the orebody.

Formulas

• Metal in mineral: f_m = (n × A_metal) / M_mineral
• Rock grade: grade = w_mineral × f_m
• Mixture density: 1/ρ_mix = Σ w_i / ρ_i

Worked example: Grade from mineral content

Given: Rock with 3 % chalcopyrite (CuFeS₂, M = 183.5, Cu = 63.55)

Find: Cu grade.

• f_Cu = 63.55/183.5 = 0.346
• Grade = 0.03 × 0.346 = 1.04 % Cu

Answer: ≈ 1.04 % Cu

Worked example: Bulk density of ore

Given: 3 % chalcopyrite (SG 4.2) in quartz (SG 2.65)

Find: Density.

• 1/ρ = 0.03/4.2 + 0.97/2.65 = 0.3732
• ρ = 2.68 t/m³

Answer: ≈ 2.68 t/m³

Common mistakes

• Wrong: Confusing mineral content with metal grade. — Right: Multiply by the metal fraction of the mineral.
• Wrong: Using a single density for all rock types. — Right: Measure density per domain (oxide, sulphide, waste).

Applications

• Core logging: Geologists record rock type, minerals and alteration.
• Density for tonnage: Accurate densities prevent tonnage errors.
• Processing design: Mineralogy selects flotation, leaching or gravity.

Key takeaways

• Rocks: igneous, sedimentary, metamorphic.
• Identify minerals by hardness, SG, lustre.
• Grade = mineral % × metal fraction.
• Mixture density from 1/ρ = Σ w/ρ.

Going further

• Geometallurgy: Spatial models of hardness and recovery, not just grade.
• Automated mineralogy: SEM-based mapping of liberation and associations.
• Critical minerals: REEs, lithium, cobalt for the energy transition.