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:– 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

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

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

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+

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

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

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

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

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

Types of mineral depositTypes Genetic groups

Endogenic

Magmatic IgneousPegmatiteSkarnHydrothermalExhalational

Metamorphic Metamorphic

Exogenic

Supergene VadoseWeathering and oxidation zones

Sedimentary MechanicalChemicalBiogenic

Endogenic-exogenic

Hydrothermal sedimentary