12.1 Principles of Mineral Associations

Mineral associations reveal the story of rock formation. These groups of minerals that occur together provide clues about geological environments and processes. By understanding these associations, geologists can decipher the conditions under which rocks formed.

Factors like magma composition, weathering, and pressure-temperature conditions control mineral associations in different rock types. These associations help identify rocks, determine their origins, and solve geological puzzles. They're key to understanding Earth's history and finding valuable resources.

Mineral Associations and Significance

Common Mineral Associations

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• Mineral associations form groups of minerals occurring together due to similar formation conditions or chemical affinities
• Paragenesis principle states certain minerals form together under specific geological conditions, providing insights into rock formation environment
• Quartz, feldspar, and mica association characterizes felsic igneous rocks, indicating silica-rich magmatic source (granites)
• Olivine, pyroxene, and plagioclase feldspar commonly occur in mafic igneous rocks, suggesting iron and magnesium-rich magma composition (basalts)
• Garnet, kyanite, and staurolite association indicates high-grade metamorphic rocks, particularly in pelitic protoliths (schists)
• Calcite, dolomite, and gypsum commonly occur in sedimentary environments, often indicating evaporitic or marine depositional settings (limestone, dolostone)
• Presence or absence of certain mineral associations provides valuable information about pressure, temperature, and chemical conditions during rock formation
  • Example: Presence of andalusite in a metamorphic rock suggests low-pressure, high-temperature conditions
  • Example: Absence of olivine in a basaltic rock may indicate extensive alteration or fractionation processes

Geological Significance

• Mineral associations serve as indicators of specific geological environments and processes
• Felsic mineral associations (quartz, feldspar, mica) suggest continental crust or evolved magmatic systems
• Mafic mineral associations (olivine, pyroxene, plagioclase) indicate oceanic crust or primitive magmatic sources
• Metamorphic mineral associations reveal information about burial depth, tectonic settings, and heat flow
  • Example: Blueschist facies minerals (glaucophane, lawsonite) indicate subduction zone environments
• Sedimentary mineral associations provide insights into depositional environments and diagenetic processes
  • Example: Evaporite mineral sequence (halite, gypsum, anhydrite) suggests arid climate and restricted basin conditions
• Hydrothermal mineral associations help identify ore deposits and geothermal systems
  • Example: Quartz-pyrite-chalcopyrite association indicates potential for porphyry copper deposits

Factors Controlling Mineral Associations

Igneous Rocks

• Magma composition serves as primary factor in igneous mineral associations
  • Felsic magmas produce quartz and alkali feldspar-rich assemblages (rhyolites, granites)
  • Mafic magmas yield olivine and calcium-rich plagioclase assemblages (basalts, gabbros)
• Bowen's reaction series explains sequence of mineral crystallization in cooling magmas, influencing resulting mineral associations
  • Discontinuous series: olivine → pyroxene → amphibole → biotite
  • Continuous series: Ca-rich plagioclase → Na-rich plagioclase
• Magma cooling rate affects crystal size and texture of mineral associations
  • Rapid cooling produces fine-grained or glassy textures (obsidian)
  • Slow cooling allows for larger crystal growth (pegmatites)
• Volatile content influences formation of hydrous minerals and late-stage crystallization products
  • High water content promotes formation of amphiboles and micas
  • Presence of fluorine or boron can lead to unique mineral associations (tourmaline in pegmatites)

Sedimentary Rocks

• Source rock composition controls initial mineral assemblage in sedimentary deposits
  • Quartz-rich source rocks produce quartz arenites
  • Feldspar-rich source rocks lead to arkosic sandstones
• Weathering processes alter original mineral compositions
  • Chemical weathering breaks down unstable minerals (feldspars) and produces clay minerals
  • Physical weathering affects grain size and sorting of sedimentary mineral assemblages
• Transportation mechanisms influence mineral associations through sorting and abrasion
  • Heavy minerals (zircon, magnetite) concentrate in placer deposits
  • Softer minerals may be preferentially removed during transport
• Depositional environment determines final mineral assemblage and diagenetic processes
  • Marine environments promote carbonate mineral formation (calcite, aragonite)
  • Evaporitic settings lead to precipitation of soluble minerals (halite, gypsum)
• Chemical precipitation in sedimentary environments creates distinct mineral associations
  • Evaporite sequences form through progressive concentration of seawater (halite, sylvite, carnallite)
  • Banded iron formations result from alternating precipitation of iron oxides and chert

Metamorphic Rocks

• Original rock composition (protolith) serves as starting point for metamorphic mineral associations
  • Pelitic rocks (shales) produce mineral assemblages rich in aluminosilicates
  • Carbonate rocks (limestones) yield calc-silicate mineral associations
• Pressure and temperature conditions control stability of metamorphic minerals
  • Low-grade metamorphism preserves some original sedimentary or igneous minerals
  • High-grade metamorphism produces new mineral assemblages stable at elevated P-T conditions
• Presence of fluids during metamorphism facilitates mineral reactions and element mobility
  • Hydrous fluids promote formation of hydrated minerals (micas, amphiboles)
  • CO2-rich fluids can lead to decarbonation reactions in carbonate rocks
• Concept of metamorphic facies describes specific mineral assemblages forming under particular P-T conditions
  • Greenschist facies: chlorite, muscovite, albite (low-grade metamorphism)
  • Granulite facies: garnet, pyroxene, K-feldspar (high-grade metamorphism)
• Metasomatism, or introduction of new chemical components during metamorphism, alters mineral associations
  • Skarn deposits form through interaction of magmatic fluids with carbonate rocks
  • Serpentinization of ultramafic rocks produces serpentine minerals through hydration reactions

Interpreting Mineral Assemblages

Formation Environment Indicators

• Index minerals in metamorphic rocks indicate specific pressure-temperature conditions
  • Andalusite forms at low pressure, high temperature
  • Kyanite indicates high pressure, moderate temperature
  • Sillimanite suggests high temperature, variable pressure
• Mineral stability fields represented in pressure-temperature diagrams allow interpretation of formation conditions
  • Aluminosilicate triple point (andalusite-kyanite-sillimanite) serves as important P-T reference
  • Garnet-in and staurolite-in reactions mark specific P-T conditions in metapelites
• Coexistence of certain minerals estimates oxygen fugacity during rock formation
  • Quartz-magnetite-fayalite assemblage indicates specific oxygen fugacity conditions
  • Presence of graphite suggests reducing conditions
• Fluid inclusions within minerals preserve information about composition and temperature of fluids during mineral growth
  • Saline fluid inclusions in quartz may indicate hydrothermal processes
  • CO2-rich fluid inclusions suggest deep crustal or mantle-derived fluids
• Texture and grain size of mineral assemblages provide information about cooling rates or recrystallization
  • Porphyritic textures in igneous rocks indicate two-stage cooling history
  • Granoblastic textures in metamorphic rocks suggest high-grade recrystallization

Environmental Condition Indicators

• Zoning patterns in minerals record changes in pressure, temperature, or chemical conditions during crystal growth
  • Metamorphic garnets may show prograde growth zoning (Mn-rich core to Fe-rich rim)
  • Igneous plagioclase often displays normal zoning (Ca-rich core to Na-rich rim)
• Presence of hydrous minerals versus anhydrous minerals indicates availability of water during rock formation
  • Amphiboles and micas suggest water-rich conditions
  • Pyroxenes and feldspars indicate relatively dry conditions
• Reaction textures and mineral pseudomorphs reveal changing environmental conditions
  • Coronas around olivine in gabbros indicate subsolidus reactions during cooling
  • Chlorite pseudomorphs after garnet suggest retrograde metamorphism
• Mineral assemblages in contact metamorphic aureoles record thermal gradients
  • Progression from low-grade to high-grade assemblages towards igneous intrusion
  • Width of aureole provides information on size and depth of intrusion
• Presence of high-pressure minerals indicates subduction or deep crustal environments
  • Coesite (high-pressure quartz polymorph) suggests ultra-high-pressure metamorphism
  • Eclogite facies assemblages (omphacite + garnet) indicate subduction of oceanic crust

Mineral Associations for Geological Problems

Rock Identification and Classification

• Mineral associations distinguish between igneous, sedimentary, and metamorphic rocks
  • Quartz + feldspar + biotite association suggests granite (igneous)
  • Calcite + dolomite + quartz association indicates limestone or dolostone (sedimentary)
  • Garnet + biotite + sillimanite association suggests high-grade metapelite (metamorphic)
• Presence of certain mineral assemblages indicates economic potential
  • Chalcopyrite + bornite + chalcocite association suggests porphyry copper deposits
  • Sphalerite + galena + pyrite association indicates potential for lead-zinc deposits
• Metamorphic mineral assemblages reconstruct pressure-temperature paths (P-T paths) of rocks
  • Clockwise P-T paths often indicate crustal thickening followed by exhumation
  • Counterclockwise P-T paths may suggest contact metamorphism or rifting environments
• Incompatible mineral associations reveal disequilibrium conditions or metastable preservation
  • Olivine + quartz in a single rock indicates disequilibrium or rapid quenching
  • Presence of high-pressure minerals in low-pressure rocks suggests rapid exhumation

Geological Problem-Solving Applications

• Mineral associations in detrital sediments determine provenance
  • Heavy mineral assemblages (zircon, rutile, tourmaline) help identify source rock types
  • Presence of glaucophane in sediments suggests erosion of high-pressure metamorphic terranes
• Recognition of characteristic mineral assemblages in hand specimens or thin sections aids rock classification
  • QAPF diagram for igneous rocks based on quartz, alkali feldspar, plagioclase, and feldspathoid content
  • ACF-AKF diagrams for metamorphic rocks help visualize mineral assemblages in different bulk compositions
• Understanding mineral associations predicts rock properties
  • Quartz-rich rocks tend to have high strength and low porosity
  • Clay-rich rocks often have low permeability and high plasticity
• Mineral associations help in unraveling complex geological histories
  • Overprinting of multiple metamorphic events recorded in porphyroblast growth zones
  • Hydrothermal alteration assemblages provide information on fluid-rock interactions
• Application of mineral associations in exploration geology
  • Alteration halos around ore deposits show characteristic mineral zonation
  • Indicator minerals in stream sediments guide exploration for diamond deposits (pyrope garnet, chrome diopside)