Earth 2404-L2

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Carleton University *

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MISC

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Geology

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Oct 30, 2023

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Lecture 2 - Minerals and Igneous Rocks What is a Mineral? - Naturally occurring solid - Inorganic - Homogeneous solid - Crystal structure - Specific fixed chemical composition - 25 minerals compose 99% of Earth’s crust Crystalline structure: ➔ Minerals have orderly internal structure ◆ Array of atoms in a regular repeating structure ◆ Dependent on the chemical bonds between atoms ◆ You can cut a mineral into pieces until you reach the atom level, and the chemical composition remains the same (unlike rocks) Crystal: the macroscopic expression of the crystalline structure. Internal structures expressed by external plane faces. What is a Rock? - Natural solid aggregate of one or more minerals o Minerals are the building block of rocks - Monomineralic rock o Composed of one mineral - Polymineralic rock o Composed of several minerals o Granite for example Mineral Classification - Chemical composition/properties - Internal structure

- Polymorphs o Minerals with same chemical composition but different crystal structures (therefore properties) o Diamond vs Graphite - Mineral growth o Solidification of a melt (magma cooling) o Precipitation from a solution (geode) o Solid state diffusion (metamorphic rocks) o Biomineralization (shells, coral) o Fumarolic mineralization (from gas near a volcano) Silicate Mineral Groups - Isolated tetrahedra o Four oxygen irons surrounding silicon ion - Two dimensional sheets o Crystal layers that can be peeled apart o Example: Mica o Clay minerals o Thin stacked sheets Non-silicate elements: Oxides: an important ore mineral Sulfide: important source of metallic ore. . Native Elements

- Minerals composed of one element o Copper, gold, silver… Laboratory Mineral Identification - Small samples - Assays and Geochemical (analyze powder) - Thin sections under microscope - X ray diffraction Field Mineral Identification - Luster - Color - Streak - Crystal habit (growth) - Hardness: Hardness is related to the strength of atomic bonds and is controlled by the weakest Bond. Scale of hardness is relative and non linear . - Cleavage (breaks on particular plane) - Fracture (breaks not on a particular plane) Introduction to Rocks - Igneous o Cooling and solidification of magma - Sedimentary o Erosion and deposition of rock fragments - Metamorphic o Modifying existing rocks through heat/pressure/fluids - Igneous + Metamorphic = 95% of Earth’s crust

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Igneous Rocks - Intrusive (plutonic) o Crystallizes within the crust o Uniform large grain size - Extrusive o Crystallizes on the surface o Big and small crystals o Grain size depends on cooling speed (fast = small) Crystallization: the process of mineral formation in cooling magma Volcanic Materials - Melt o Liquid portion - Pyroclastic (solids) o Ash, cinders, minerals crystallized from the melt - Gasses (volatiles) Igneous Minerals and Chemistry - Felsic o Rich in silica o SiO2 > 65% - Intermediate o 52% < SiO2 < 65% - Mafic

o Rich in ferromagnesian o 45% < SiO2 < 52% - Ultramafic o SiO2 < 45% Bowen reaction series: ➔ Bowen's Reaction Series describes the temperature at which minerals crystallize when cooled, or melt when heated The continuous branch describes the evolution of the plagioclase feldspars as they evolve from being calcium-rich to more sodium rich. The discontinuous branch describes the formation of the mafic minerals olivine, pyroxene, amphibole, and biotite mica. Magma and Silica - High % SiO2 o Viscous magma o Low temperature o Large plutonic bodies or explosive eruptions - Low % SiO2 o Fluid magma

o High temperature o Reaches surface easily, no explosions Cooling Based Textures - Phaneritic texture o Intrusive o Course, uniform grain § Slow cooling within crust - Aphanitic texture o Extrusive o Fine grain § Fast cooling on surface Volatile (Gas) Based Textures - Pumiceous o High in volatiles o Lots of holes o Considered a glass - Porphyritic o Poor in volatiles o Two stage cooling process (below and above ground) § Phenocrysts (Large crystals) § Matrix (Fine grains surrounding phenocrysts) Magma Rich in Volatiles - Vesicles o Small holes on top of lava flows where gas escaped

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- Amygdule o Infilling of vesicles with secondary material (quartz, calcite…) Intrusive Processes - Intrusion o Movement of magma from a magma chamber to a different subsurface location - Plutons o Bodies of rocks formed by the intrusion of magma into older rocks (“country rocks”) Types of Plutons - Batholith o Assemblage of plutons § Hundreds of square kilometers o Non tabular

o Discordant Extrusive Processes - Volcanism o Process by which magma rises into the crust and is extruded onto the surface - Lava Viscosity o § Increases with volatile content § Increases with % SiO2 o Basaltic lava § Flows quickly § Smooth surface § Pahoehoe (flow wrinkles) o Felsic lava § Flows slowly or not at all § Jagged surface § Can create domes - Tephra o Solid matter (ash, dust…) ejected into the air by volcano eruption o Blocks § Solid fragments with angular shapes o Bombs § Semi molten when airborne

Pyroclastic Rocks - Transitional between igneous and sedimentary o Igneous on the way up, sedimentary on the way down - Ex: Tuff - Very hot Tephra that fuses together -

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