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Chapter 16

Chapter 16. Mineral genesis. Mineral genesis and genetic mineralogy. Genesis = origin Primary crystallization Subsequent history: transitions, exsolution , reaction Mineral properties explained by forming conditions Genetic mineralogy:

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Chapter 16

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  1. Chapter 16 Mineral genesis

  2. Mineral genesis and genetic mineralogy • Genesis = origin • Primary crystallization • Subsequent history: transitions, exsolution, reaction • Mineral properties explained by forming conditions • Genetic mineralogy: • Investigate principles controlling mineral formation • Quantitative: geothermometer, geobarometer, chemical studies • Modelling of crystal growth • Mineral deposit: • Geological body formed under specific conditions • Contains characteristic minerals: scattered, segregated, lenses, strata, veins / veinlets

  3. Mineral forming environments • Aqueous solutions: • 2 Types of solutions: • Hydrothermal solutions (endogenic) • Sources: Crystallizing magma; dehydrating sedimentary rocks; mantle degassing; migrated meteoric and seawater • Heated solution dissolve, transport and precipitate minerals from rocks along pathway - specific minerals can be concentrated and accumulated, often forming ore mineral deposits in this way • Quartz and calcite and sulphide ore deposits • Surface solutions or brines (exogenic) • Ground, karst and soil water – carbonates: calcite, aragonite • Lacustrine, oceanic, lagoon waters – evaporite minerals: halite, gypsum

  4. Mineral forming environments • Gas • Rare environment • Hematite, native sulfur, realgar from volcanic gases • Ice crystals from vapor: dendritic snowflakes • Fluids • Fluid mixtures of CO2 and H2O important during formation of skarns and metamorphism of limestone • Water can cause significant alteration and dissolution of minerals especially at high P and T • Eg: Quartz

  5. Mineral forming environments • Colloidal solutions • Typical in ocean floor silt rich in clay minerals, Al-, Fe-, Mn-hydroxides • Rarely in thermal springs with recent volcanic activity: amorphous opal • Magma • Not a simple pure melt: a mixture of substances and the compositions is not necessarily corresponding to the rocks that form from them • Liquid and solution properties • Anion groups in polyhedra ‘dissolved’ as clusters in the magma • Also large cations such as K+, Na+, Mg+, Ca+

  6. Mineral forming environments • Solid systems • Crystalline • Polymorphic transitions (no change in chemical composition) • Diamond to graphite; high quartz to low quartz; opal to quartz • Transform precursor mineral to new phases with different compositions • Pseudomorhps: pyrite replaced by limonite • Replacement processes • Porphyroblasts: garnet growing in a gneiss replacing (and including) pre-existing minerals • Usually associated with molecular water at grain boundaries

  7. Types of mineral crystallization • Why do minerals form? • More stable at new P, T or concentration than the melt, solution or pre-existing minerals from which they are forming • Types: • Free space crystallization • Metasomatism • Recrystallization

  8. Types of mineral crystallization • Free space crystallization: • Grow freely in gas, melt, solution • Examples: • Sulfur in volcanic gas • Porphyritic feldspars in magma • Amethyst in hydrothermal solution • Usually euhedral crystal habits

  9. Types of mineral crystallization • Metasomatism: • Definition: • a metamorphic process in which the chemical composition of a rock is changed significantly, usually as a result of fluid flow • a process of simultaneous capillary dissolution and crystallization by which a new mineral completely or partially replaces an initial mineral, often changing the chemical composition • Formation of compositionally diverse ores and rocks • Greisen forms from granite subject to hydrothermal solution: 3K(AlSi3O8) + 2H+  KAl2(ALSi3O10)(OH)2 + 6SiO2 + 2K+ Microcline Muscovite Quartz Simultaneously cassiterite (SnO2) forms when tin is added to the greisen

  10. Types of mineral crystallization • Recrystallization: • New crystals replace earlier ones • Increase or decrease in grain size • Compositional changes or not • Proceeds in solid state, driven by free chemical energy or deformation defects in crystals • NB factor in diagenesis and metamorphism • Recrystallization at low temperature and polymorphic transformations at higher temperature

  11. Types of mineral deposit

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