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 deposit
Types
Genetic groups
Endogenic
Magmatic
Igneous
Pegmatite
Skarn
Hydrothermal
Exhalational
Metamorphic
Metamorphic
Exogenic
Supergene
Vadose
Weathering and oxidation zones
Sedimentary
Mechanical
Chemical
Biogenic
Endogenic-exogenic
Hydrothermal sedimentary