Minerals form through various processes, shaping Earth's crust and providing clues about geological history. From magma crystallization to precipitation from solutions, these processes create diverse mineral structures and compositions, each telling a unique story about its formation environment.

Understanding mineral classification helps geologists identify and interpret Earth materials. Silicates dominate the crust, while other groups like oxides, sulfides, and carbonates play crucial roles in rock formation and economic resources. This knowledge is essential for unraveling Earth's complex geological narrative.

Mineral Formation Processes

Processes of mineral formation

• Crystallization from magma
  • Cooling and solidification of molten rock forms minerals as atoms arrange into orderly structures
  • Fractional crystallization separates minerals based on melting points (olivine crystallizes first)
  • Bowen's reaction series predicts mineral formation sequence in cooling magma (discontinuous and continuous branches)
• Precipitation from aqueous solutions
  • Evaporation of mineral-rich water concentrates dissolved ions forming evaporite minerals (halite, gypsum)
  • Chemical reactions in water produce new mineral phases (calcite precipitation in limestone caves)
  • Biological processes create biominerals through controlled crystallization (calcium carbonate shells, silica diatom frustules)
• Metamorphic reactions
  • Recrystallization under high pressure and temperature alters mineral structures and compositions (schists, gneisses)
  • Solid-state diffusion redistributes atoms within crystal lattices modifying mineral properties
  • Dehydration reactions release water from hydrous minerals forming new anhydrous phases (clay to mica transformation)
• Hydrothermal processes
  • Mineral deposition from hot, mineral-rich fluids circulating through rock fractures (quartz veins, ore deposits)
  • Formation of ore deposits concentrates valuable metals through precipitation and replacement reactions
• Weathering and secondary mineral formation
  • Chemical weathering of existing minerals breaks down primary minerals into new phases
  • Formation of clay minerals results from the alteration of feldspars and other silicates in surface environments

Mineral Classification and Identification

Classification of minerals

• Silicates

  • Most abundant mineral group comprising ~92% of Earth's crust
  • Silicon and oxygen tetrahedra serve as basic building blocks for diverse structures
  • Subgroups organized by silica tetrahedra arrangement:
    1. Nesosilicates: isolated tetrahedra (olivine, garnet)
    2. Sorosilicates: paired tetrahedra (epidote)
    3. Cyclosilicates: ring structures (tourmaline)
    4. Inosilicates: single or double chains (pyroxenes, amphiboles)
    5. Phyllosilicates: sheet structures (micas, clay minerals)
    6. Tectosilicates: three-dimensional frameworks (quartz, feldspars)

• Oxides

  • Metals combined with oxygen form compact, often hard minerals
  • Examples include hematite (iron ore), magnetite (magnetic iron oxide), corundum (aluminum oxide)

• Sulfides

  • Metals combined with sulfur often have metallic luster and economic importance
  • Examples include pyrite (fool's gold), galena (lead ore), sphalerite (zinc ore)

• Sulfates

  • Sulfate anion combined with metals creates softer minerals often found in evaporite deposits
  • Examples include gypsum (used in drywall), anhydrite (anhydrous calcium sulfate), barite (barium sulfate)

• Carbonates

  • Carbonate anion combined with metals forms minerals reactive to acid
  • Examples include calcite (limestone component), dolomite (sedimentary rock-former), aragonite (mollusk shells)

• Native elements

  • Occur in pure form without combining with other elements
  • Examples include gold, silver, copper, diamond (pure carbon)

Mineral formation environments

• Igneous environments
  • High-temperature minerals crystallize from cooling magma (olivine, pyroxene)
  • Mineral zonation in intrusive bodies reflects fractional crystallization (Bowen's reaction series)
• Sedimentary environments
  • Evaporite minerals form in arid basins through water evaporation (halite, gypsum)
  • Clay minerals develop in low-energy depositional settings from weathering products
• Metamorphic environments
  • Pressure-temperature indicator minerals help determine metamorphic conditions (garnet, staurolite)
  • Metamorphic facies characterize mineral assemblages formed under specific P-T conditions
• Hydrothermal environments
  • Ore mineral associations concentrate valuable metals (gold-quartz veins, porphyry copper deposits)
  • Alteration halos around mineral deposits indicate fluid-rock interactions
• Surface weathering environments
  • Formation of secondary minerals alters primary rock-forming minerals (feldspars to clay minerals)
  • Oxidation produces iron oxides and hydroxides (rust-colored soils)

Common rock-forming minerals

• Igneous rock-forming minerals
  • Quartz, feldspars, micas, amphiboles, pyroxenes, and olivine constitute major igneous rocks
  • Roles in magmatic differentiation and rock classification determine igneous rock types (granite, basalt)
• Sedimentary rock-forming minerals
  • Quartz, feldspars, clay minerals, calcite, and dolomite form clastic and chemical sedimentary rocks
  • Importance in determining sedimentary environments and diagenetic processes
• Metamorphic rock-forming minerals
  • Garnet, staurolite, kyanite, and sillimanite indicate metamorphic grade and pressure-temperature conditions
  • Help reconstruct metamorphic history and tectonic settings
• Accessory minerals
  • Zircon, apatite, and magnetite occur in small amounts but provide valuable information
  • Importance in geochronology (zircon dating) and paleomagnetism (magnetite orientation)
• Mineral stability
  • Influence of temperature, pressure, and chemical environment determines mineral persistence
  • Reaction series and mineral replacement reflect changing geological conditions over time