Mineral Formation Processes
Mineral Formation
Crystallization Processes
Crystal Habit: Describes the macroscopic appearance of minerals. The habit is influenced by external factors, such as environmental conditions during formation.
Elementary Cell: Characterized by a parallelepiped defined by the positions of atoms, ions, or molecules in space.
Node: The position of particles within the unit cell.
Formation Methods
- Solidification: By cooling molten material, as with volcanic lava.
- Sublimation: From the rapid cooling of a gas, such as sulfur from a fumarole.
- Weathering: Through the action of water, oxygen, or atmospheric CO2. For example, pyrite oxidation results in goethite.
- Precipitation: From a solution, such as the evaporation of seawater forming salt crystals. Can be induced by living organisms.
- Recrystallization: Occurs when crystal atoms are replaced by others; for example, the replacement of calcium by magnesium in calcite to form dolomite.
Crystal Structures
- Geode: Crystals growing within a rock hollow.
- Druze: Crystals growing in a dome-shaped rock.
- Lawn: Very small crystals arranged like a stain on the rock.
- Dendrites: Intergrown crystals.
Igneous Rock Formation
Intrusive Bodies
- Batholith: A large mass of plutonic rock extensively exposed at the surface.
- Lacolito: A lenticular mass of plutonic rock concordant with surrounding rocks, often formed by magma injection.
- Dike: An intrusive body that cuts across the surrounding rock structure.
- Sill: A tabular intrusive body parallel to the surrounding rock.
Bowen’s reaction series has two branches: the discontinuous series involves the formation of increasingly complex minerals, and the continuous series involves the reaction of anorthite with molten minerals, resulting in visually indistinguishable products.
Magmatic Processes
- Fractional Crystallization: Occurs when the reaction process is interrupted, separating solid and liquid phases, resulting in different rock types.
- Magmatic Assimilation: Magmatic fluid reacts with surrounding rock, altering the initial magma composition.
Volcanic Activity
Volcanoes are structures with magma chambers communicating with the surface. Eruptive style depends on magma characteristics. Fluid magma allows for slow gas release, while viscous magma traps gases, leading to explosive eruptions.
Volcanic Products
- Gases: Primarily CO2
- Liquids: Lava flows, forming mantles. Rapidly cooling lava can form blocks or exhibit columnar disjunction. Slow-cooling lava flows smoothly.
- Solids: Pyroclastics—volcanic ash, lapilli (2-64 mm), blocks, bombs (larger fragments).
Metamorphic Rock Formation
Metamorphism: The transformation of rocks under pressure and temperature conditions different from their formation. These changes occur without melting. Mineral crystal lattices are destroyed and reformed, adapting to new conditions. Metamorphism occurs in high-pressure zones, such as plate boundaries or contact zones with magmatic material. Rocks formed by thermal metamorphism include hornfels and corubianite.
Metamorphic Halo: Forms around a pluton due to retained heat, creating concentric zones of decreasing metamorphism away from the pluton.
Sedimentary Rock Formation
Sediments:
Debris: Transported solid particles from bedrock. Clasts are larger grains, matrix is finer material between clasts, and cement is the binding material.
Sedimentary Rock Types
- Chemical: Formed by chemical precipitation of dissolved ions.
- Biochemical: Precipitation aided by organisms, such as carbonate precipitation by algae.
- Organic: Formed by the accumulation of organic remains, leading to coal and oil formation.
Strata: Horizontal layers of sediments deposited over time. These layers often retain their original layering.
Sedimentary Structures
- Cross-stratification: Inclined layers within the strata.
- Granulo-classification: Particles sorted by size in aqueous environments.
- Tool marks: Created by dragged sediment particles.
- Organism marks: Footprints or other traces of organisms.
- Desiccation cracks: Formed by water loss in muddy areas.
Fossils: Abundant, geographically widespread, and rapidly changing organisms useful for dating.
Eon: The largest chronological unit of geologic time.
Soil Horizons
O Horizon: The uppermost layer, composed of decaying organic matter.
A Horizon: The leaching horizon, where soluble salts and clays are washed downward.
B Horizon: The accumulation horizon, where clays, oxides, and salts accumulate.
C Horizon: Weathered parent rock material.
Bedrock: Unweathered parent rock.