Rocks and Minerals – Quick Reference

Mineral definitions

  • Mineral: homogeneous, naturally occurring, inorganic substance with a definable chemical composition and an orderly internal arrangement of atoms (lattice).
  • Rocks vs minerals: rocks are aggregates of one or more minerals; minerals are the building blocks.

Rocks and rock types

  • Three broad rock classes: Igneous, Metamorphic, Sedimentary.
  • Metasomatic (altered) rocks involve fluid-driven chemical changes during metamorphism.
  • The rock cycle: tectonics and geodynamics drive transitions among igneous, sedimentary, and metamorphic rocks.

Ore vs gangue

  • Ore mineral: naturally occurring mineral containing a valuable constituent (e.g., metal) that is mined.
  • Gangue: valueless minerals in an ore; not economically desirable but accompany ore minerals.
  • Not the same as waste rock.

Ore grade and deposit economics

  • Ore grade formula: ext{Ore Grade} = rac{c imes M{ ext{metal}}}{M{ ext{mineral}}} imes 100 where:
    • $c$ = concentration (as a fraction, e.g., 0.02 for 2%) of the ore mineral in rock,
    • $M_{ ext{metal}}$ = molar mass of the economic metal,
    • $M_{ ext{mineral}}$ = molar mass of the ore mineral.
  • Ore deposit: tested and known to be of sufficient size, grade, and accessibility to be mined at a profit.
  • Cut-off grade: ore grade below which mining is not economic.

Cu grade example (illustrative)

  • Rock contains 2% chalcopyrite (CuFeS2) and 0.5% bornite (Cu5FeS4).
  • Example results (Cu grade):
    • Chalcopyrite contribution: 0.32% Cu
    • Bornite contribution: 0.69% Cu
    • Total Cu grade: 1.01% Cu

Crystal structure and crystallography

  • Crystals have a lattice; unit cell is the smallest repeating unit with the crystal’s symmetry.
  • Seven lattice systems classified by unit cell shape and translational symmetry.
  • 14 Bravais lattices derived from the 7 lattice systems.
  • Seven lattice systems:
    • Cubic, Tetragonal, Orthorhombic, Hexagonal, Trigonal, Monoclinic, Triclinic.

Mineral classes by anions (Berzelius system)

  • Major classes: Silicates, Non-Silicates, Oxides, Sulphides, Sulphates, Halides, Carbonates, Native Metals.
  • Note: this is non-exhaustive (tellurides, arsenides, etc. are important ore minerals too).
  • Silicates sub-classes (based on silica tetrahedra links):
    • Olivines; Pyroxenes; Amphiboles; Micas; Clays; Quartz; Feldspars.

Ore commodities and minerals

  • Major groups include:
    • Non-metallic industrial minerals (e.g., feldspars, clays, phosphates, quartz).
    • Construction aggregates (limestone, dolostone, sandstone, shale, etc.).
    • Precious stones and gems (e.g., diamond, garnet).
    • Metallic minerals: ferrous and non-ferrous metals, PGE, REE, etc.
  • Classification supports mining and processing planning.

Mineral processing fundamentals

  • Processing aims to separate ore minerals from gangue by exploiting differences in properties:
    • Size (sieving, screening)
    • Magnetism (magnetic separation)
    • Density (cyclones, spirals, jigs)
    • Surface chemistry (flotation)
    • Optical properties (sorting)
    • Solubility and dissolution kinetics (hydrometallurgy)
    • Melting points (pyrometallurgy)
    • Texture (grinding, comminution, separation)

Sedimentary rocks

  • Sedimentary rocks form from loose sediments or chemical/biogenic precipitation near the surface.
  • Four main classes:
    • Clastic (detrital): cemented fragments; economic example includes Au-U bearing conglomerates (Witwatersrand).
    • Biochemical: shells of organisms (carbonates used in cement).
    • Chemical: direct mineral precipitation from solution (e.g., evaporites, carbonates, BIFs).
    • Organic: carbon-rich relicts of plants (coal).
  • Sediments cover >80% of Earth’s surface and constitute >1% of mass.

Igneous rocks

  • Igneous rocks crystallize from molten magma.
  • Magma generation mechanisms:
    • Decompression melting (rises with reduced pressure).
    • Melting due to volatiles lowering melting point.
    • Heat transfer from rising magma to surrounding rock.
  • Magma evolution factors (Bowen's concepts):
    • Source rock composition; Partial melting; Contamination/assimilation; Fractional crystallization.
  • Texture:
    • Intrusive (coarse-grained, phaneritic) vs. Extrusive (fine-grained, aphanitic).
  • Classification by composition and texture:
    • Felsic to ultramafic; felsic minerals typically include quartz, K-feldspar, plagioclase; mafic minerals include olivine, pyroxene, amphibole.
    • QAPF diagram for intrusive rocks; TAS diagram for volcanic rocks.
  • Common rock types:
    • Granites, diorites, gabbros (intrusive);
    • Rhyolites, andesites, basalts (extrusive);
    • Peridotite, dunite, gabbro (mafic to ultramafic derivatives).

Metamorphic rocks

  • Metamorphism: solid-state changes in minerals and texture due to pressure and/or temperature changes.
  • Observable changes:
    • Replacement of original minerals by metamorphic minerals (chemical reactions/phase changes).
    • Recrystallisation leading to textures (slaty cleavage, schistosity, gneissic banding).
  • Factors controlling metamorphism:
    • Temperature; Pressure; Fluids (metasomatism);
    • Fluid/rock ratio and rock composition; deformation (deviatoric stress).
  • Deformation and stress concepts:
    • Lithostatic pressure vs. deviatoric stress (drives deformation, not mineralogy).
  • Metasomatic rocks: hydrothermal alteration; fluids alter chemistry and mineralogy; important for ore deposits.

Metamorphic textures and classification

  • Foliated metamorphic rocks: slate, phyllite, schist, gneiss (alignment of platy minerals).
  • Non-foliated rocks: quartzite, marble, hornfels (granular, no preferred orientation).

Metasomatic and hydrothermal systems

  • Fluids mobilize through crust via grain boundaries or fractures.
  • Mineral assemblages depend on temperature, pressure, rock/fluid composition, and fluid/rock ratio.
  • Alteration zones reflect ore-forming hydrothermal processes.

Quick recall anchors

  • Key definitions: mineral vs rock; ore vs gangue; rock cycle; metamorphism vs metasomatism.
  • Major rock types and their formation controls: igneous (magma crystallization), sedimentary (weathering/deposition), metamorphic (pressure/temperature/fluids).
  • Processing-relevant properties: size, density, magnetism, surface chemistry, solubility, texture.
  • Crystal chemistry: unit cell, lattice systems (7 systems, 14 Bravais lattices).
  • Cu grade example: total Cu grade = 1.01% Cu from mixed ore minerals.

Summary formulas

  • Ore Grade formula (recap): ext{Ore Grade} = rac{c imes M{ ext{metal}}}{M{ ext{mineral}}} imes 100
  • Practical implications: higher grade or higher mass proportion of a valuable mineral increases economic viability.