Geo Mid PDF

An Introduction to Geology WHAT IS GEOLOGY? The science that examines Earth, its form and composition, and the changes that it has undergone and is undergoing. Two Major Fields of Geology Historical Geology - understands the origin of earth and its development through time. Physic...

An Introduction to Geology WHAT IS GEOLOGY? The science that examines Earth, its form and composition, and the changes that it has undergone and is undergoing. Two Major Fields of Geology Historical Geology - understands the origin of earth and its development through time. Physical Geology - examines the materials composing Earth and tries to understand processes occurring beneath and upon its surface Different Areas of Geologic Study Archeological geology Paleoclimatology Bio geosciences Paleontology (i.e., fossils) Engineering geology Petrology Geochemistry Planetary Geology Geomorphology (i.e., rocks) Sedimentary geology Geophysics Seismology Historical Geology Structural Geology Mineralogy Tectonics (Plate Tectonics) Ocean Sciences Volcanology CENTRAL IDEAS IN GEOLOGY Principle of Uniformitarianism 1795 – James Hutton published his ‘Theory of the Earth’ Most of the features on the surface of the Earth were formed by slow, ongoing geologic processes, not by sudden catastrophic events. The erosion of landforms, the deposition of sediments, the drifting of continents and the eruption of volcanoes - all of these were happening long ago, on roughly the same scale and at roughly the same rate as they are today. Geological processes that happened in the past are still operating today. Example 1  The Mount St. Helen’s volcanic eruption that took place on May 18th 1980 has Every layer of rock is formed by the uniform happened many times throughout Earth’s laying down of sediment that still occurs today history. Relative Dating and Geological Time Scale Events are placed in correct order (without knowing their age). The proper order of events can be determined without knowing their age in years Shown by applying the law of superposition 2  In layers of sedimentary rocks, or lava flows, the older beds are placed on the bottom and the younger at the top. A quarry site in Deborah, Iowa Principle of Fossil Succession: Fossils succeed (come after) each other in a definite order, therefore a time period (or sedimentary bed) can be recognized by its fossil content. 1. Fossils represent the remains of once-living organisms. 2. Most fossils are the remains of extinct organisms; that is, they belong to species that are no longer living anywhere on Earth. 3. The kinds of fossils found in rocks of different ages differ because life on Earth has changed through time. 3 THE EARTH The Earth is about 4600 million years old and made up of four spheres Geology is the science of the earth Four Spheres of the Earth Hydrosphere Oceans cover 71% of Earth’s surface 3% of Earth’s water consists of Fresh water ( streams, lakes, glaciers, underground) Atmosphere Consists of about 21% O2 (g), 78% N2 (g), other gases, such as CO2 , Ar, H2O, etc. Extends to about 100 km from the surface Protects Earth from the Sun’s heat and dangerous ultraviolet rays Allows life to exist Protects Earth from meteors, etc. Biosphere Includes all life on or near Earth’s surface Solid Earth (Geosphere / Lithosphere) Supports all the other spheres 4 Earth as a System Earth is really a system in which the different spheres interact continuously Two energy sources supplying the System are: o Sun’s heat which drives the external processes in the biosphere, atmosphere, hydrosphere and Earth’s surface o Earth’s internal heat that drives internal processes producing volcanoes, earthquakes and mountains; the internal heat comes from: § When the planet was first formed § Radioactive decay from elements such as U-238, K-40 Illustrative example of two energy sources when volcanoes erupt Ash enters the atmosphere Lava cooled and forms igneous rocks Sunlight is diminished Wind and water cause the weathering Lessened plant growth of rocks Animals get less food Sedimentary rocks are formed Heat and pressure turns the rocks into metamorphic 5 The Rock Cycle The rock cycle describes a process where the three main types of rocks (igneous, sedimentary and metamorphic) into one type or the other. Process of Type of Rock Type of Raw Material Examples Formation Solidification of Molten rock Igneous molten rock Granite Magma (below surface) Rock Cooling of molten Basalt Lava (above surface) rock Sediments from the Sedimentary Compaction and Limestone weathering, erosion and Rock Lithification Sandstone transportation of rocks Intense heat and Metamorphic Pre-existing, interior pressure in the Marble Rock rock under the surface Earth’s crust and Slate upper mantle 6 Schematic Diagram for the Rock Cycle 7 THE EARTH’S LAYERS Early Formation of Earth After the formation of the Sun, metals such as iron, and nickel together with rock forming elements such as Si, Ca, Na, and Mg, etc., formed metal lumps that orbited the Sun and growing into large bodies. Gravitational force attracted the masses together and high pressures built up deep inside Earth which caused the temperature to rise. Radioactive reactions also added to the heat produced causing the high density iron and nickel to melt and sink towards the center of the planet forming the Core Lighter elements, such as Si, Al, Na, K, etc. that could form rocks, rose to the surface to form a basic crust. The separation of elements allowed gases to escape upwards and a primitive atmosphere was formed. Earliest rocks solidified around 4000 Ma years ago on the crust. 8 The Earth’s Interior The Earth’s Interior can be divided based on two criteria: (1) Compositional Layers; and (2) Mechanical Layers. Compositional layers: Earth’s layers differentiated by chemical composition. Mechanical layers: Earth’s layers differentiated by physical properties (strength and rigidity). 9  Compositional (chemical) Layers Mechanical (physical) Layers Crust Lithosphere Earth’s relatively thin, rocky Outermost and most rigid layer of the Earth. outer skin that is generally divided into oceanic and Includes the crust (continental and oceanic) and continental crust upper mantle. o Oceanic crust: 7 km thick o Mohorovičić discontinuity (Moho): and composed of igneous boundary between crust and mantle defined rock called basalt. by characteristic change in velocity of o Continental crust: 35-40 seismic waves as they pass through rocks of km thick but may exceed different densities. 70 km; composed of Average thickness is 70 km. granite rocks Composed mostly of granite (continental crust), Low Density: 2.7 – 3.2 g/cm3. basalt (oceanic crust), and peridotite (upper mantle). Mantle Asthenosphere A solid rocky shell that extends Layer in the upper mantle that is weak, hotter and to a depth of 2900 km ductile. Contains over 82% of Earth’s Moves in a semiplastic manner. volume o Higher temperature (just below melting point Density = 3.3 to 5.5 g/cm3 of the rocks) allows the rocks to deform and Provides much of Earth’s heat flow. Upper mantle contains mainly Contains mainly peridotite rocks (rich in Fe, Mg) peridotite rocks (rich in dark Mg and Fe) Lower Mantle (Mesosphere) Lower mantle layer that is hard and more rigid due to high pressure. Outer Core Made of Fe and Ni High temperature liquid Core Generate Earth’s magnetic field An iron (Fe) – nickel (Ni) alloy Density = 11 g/mL with minor amounts of oxygen, silicon, and sulfur. Inner Core Made up of Fe and Ni Solid due to high pressure Density = 12 to 18 g/mL 10 11 THE EARTH’S CRUST Continents and Oceans Average height of the continent is about 850 m above sea level Average depth of the ocean is about 3800 m deep Conclusion: Most of Earth’s surface is below the sea. Continental Crust This type of crust forms from Earth’s continents and it continues into the sea as the continental shelves, continental slopes, and continental rise (collectively referred to as the continental margin). The upper part is called the granitic crust because of its composition is like granite and it is very old - up to 3700 Ma years old Continental crust is usually much thicker than the oceanic crust and can be up to 70 km thick The boundary between the continental crust and the oceanic crust is near the base of the continental rise Mountain belts are the main feature of the continental crust and are situated around the Pacific Rim and eastwards from the Alps to the Himalayas 12 Oceanic Crust Average thickness of the crust is about 6 km Oceanic ridges are important in the Oceanic crust. These are chains of huge mountains that circle Earth and are about 70000 km long and about 1000 km wide. One example is the Mid-Atlantic Ridge that rises 4 km from the ocean floor Near the coasts of active margins, oceans become quickly very deep forming trenches. One example is the Peru-Chile Trench, parallel to the Andes Mountains. Composition of the oceanic crust is a rock type called basalt, so the name basaltic crust can be used. 13 14 GEOLOGY AND THE SCIENTIFIC METHOD Like any Science discipline, Geology employs an orderly way of solving problems The Scientific Method is a problem solving process that involves the following steps: Collection of scientific facts through observation and measurements; Formulation of questions that relate to the facts and the development of one or more working hypotheses that may answer these questions; Development of experiments to test the hypotheses (the verification process requires that predictions be made and tested by comparing them against objective observations of nature); and Acceptance, modification, or rejection of the hypotheses based on extensive testing. If a hypothesis survives the testing and competing models are eliminated, it becomes a scientific theory. 15 THEORY OF PLATE TECTONICS Observations: Earthquakes, volcanoes and mountain building take place in certain areas of Earth, e.g. the Pacific Rim, along the Ocean Ridges, etc. Continents move. Rock evidence shows that continents have had very different climates, e.g. the Arabian peninsula was once at the south pole The sea floor has a measurable movement that is about 40 mm/year Trenches are formed near the continents, e.g. the Peru- Chile trench close to the Andes mountain chain in South America Theory (explaining the observations): The outer part of Earth, the Lithosphere, consists of a number of rigid interlocking plates that are in constant movement relative to each other. 7 major plates are; North and South American, Pacific, Eurasian, African, Australian and Antarctic plates 16  The Asthenosphere (weak sphere) is a weak and semi-plastic layer of the upper mantle. It supports the more rigid plates of the Lithosphere and allows them to move relative to the lower and more solid mantle. Active edges, boundaries of the plates, are marked by earthquake zones and can be of three types: divergent, convergent, or transform boundary. 17 DIVERGENT BOUNDARIES Also known as Constructive Boundary because new ocean floor is generated in these boundaries. Occurs at continental rift zones or oceanic ridges Two adjacent lithospheric plates are moving apart under tension (ß|à) producing long narrow fracture: o Heat flow causes magma to rise through the fractures (breaks) and cools to form new seafloor. o When new plate material is produced as the plates are pulled apart causing seafloor spreading and explaining the movement of the plates at an average of 50mm/year (like a fingernail growth). Oceanic Ridges Elevated areas of the seafloor characterized by high heat flow and volcanism. When magma forces itself through cracks or faults in a rock (volcanic extrusion) in slow spreading oceanic ridges, pillow lavas form, Continental Rift Zones A continental rift occurs when plate motions produce opposing forces (tension) that pull and stretch the lithospheric plate. o The stretched lithosphere is thin and develops deep cracks (normal fault). o Plate sections between these cracks drop into deep depressions called a rift valley. Further rifting will cause lengthening and deepening of the rift valley leading to the transformation to a narrow sea. 18  Examples of divergent boundaries include Mid-Atlantic ridge Red Sea Rift / East African Rift 19 CONVERGENT BOUNDARIES The expanding seafloor moves sideways away from the oceanic ridge to form new oceanic crust mainly of basalt rock. The size of Earth remains constant and new crust is being formed at divergent boundaries, so somewhere some material must be used up to keep things constant. Because oceanic crust is being used up, it is also called a Destructive Boundary Oceanic-Continental Convergence: The moving oceanic crustal plate is cooling and with increased density is dragged downward and under the lighter continental crust (subduction) The dragged down part of the crust forms the trench and explains its presence close to the continental crust The crust is being consumed as it is dragged under into a high pressure and high temperature environment Subducted materials, plus some of the asthenosphere above the downward moving slab, sometimes melt to form magma which can rise to produce volcanic eruptions such as Mount St. Helens and build mountain chains (continental volcanic arcs). Example: the Andes mountain volcanoes, where the Nazca plate meets the South American plate in Chile Oceanic-Oceanic Convergence: One oceanic slab descends another oceanic slab by the same mechanism that works at oceanic-continental boundaries. Arc-shaped chain of volcanic islands is formed called volcanic island arc. Continental-Continental Convergence: Two continental fragments collide, resulting in the formation of high mountain ranges like the Himalayas and the Alps. Continental - Oceanic - Continental - Oceanic Oceanic Continental 20 TRANSFORM FAULT BOUNDARIES Parts of the same plate grow while others are quiet fractures (splits) form with a sideways movement as part of the same plate (or even the whole plates) slip past each other. This is a Transform Fault Boundary e.g. the San Andreas fault Transform Fault Boundaries are also known as Shear Fault Boundaries One of the most famous transform boundary is the San Andreas Fault in California 21 WHAT DRIVES THE PLATES? Heat from the inside of Earth is thought to set up Convection Currents o Material is rising (upwelling) to replace cold rock, which sinks, warms up and again rises (a convection current) o Earth’s internal heat is transferred to the surface Speed of convection movement is equal to that of seafloor spreading, i.e. about the same rate of growth as finger nails A. Slab pull Subducting slabs of oceanic crust are more dense (colder) than the underlying warm asthenosphere which allows it to sink. Major driving force of plate motion. B. Ridge Push ‘Uplift’ of the mantle material is situated just below the Mid-Ocean ridge Slabs of lithosphere slides down the flanks of the oceanic ridge C. Mantle Drag Flow of the mantle can affect plate motion. DISTRIBUTION OF CRUST ACTIVITY The theory of Plate tectonics can now explain that: Production of earthquakes, volcanoes and mountain building is situated near to: o Convergent boundaries o Divergent boundaries Trenches are situated near to Convergent Boundaries 22 Recall: Chemistry Concepts MATTER Anything that occupies space and has mass. It can be classified based on state or composition CLASSIFICATION OF MATTER BASED ON COMPOSITION Matter Pure Mixtures Substances Elements Compounds Heterogeneous Homogeneous Pure Substances Particles behave as a single unit (as an atom, molecule, or formula unit) Fixed composition – does not change from sample to sample Characterized by sharp melting and boiling points Components can be separated by chemical means ELEMENTS Made up of particles (atoms or molecules) with only one type of atom COMPOUNDS Made up of particles with two or more elements o Molecular or Covalent – made up of molecules o Ionic – made up of ions in formula units 23 Mixtures Composed of two or more pure substances Variable composition – ratio of components can change from sample to sample. Characterized by broad melting and boiling points Components can be separated by physical means HOMOGENEOUS MIXTURES Made up of particles (atoms or molecules) with only one type of atom HETEROGENEOUS MIXTURES Made up of particles with two or more elements o Molecular or Covalent – made up of molecules o Ionic – made up of ions in formula units STATES OF MATTER Gas Liquid Solid Crystalline Amorphous Crystalline Solid Solids with particles arranged in a highly ordered, repeating three dimensional pattern. Amorphous Solid Solids with particles lacking a long-range order. 24 Mineralogy WHAT ARE MINERALS? “Inorganic building blocks of rocks and are characterized by a particular chemical compositions and a defined crystal structure.” (International Mineralogical Association) Characteristics of a Mineral: Naturally occurring Crystalline solid Formed by geological processes Generally inorganic. Definite chemical composition that allows for some variations. WHAT ARE ROCKS? A natural aggregate (collection) of one or more minerals. When a liquid is cooled slowly, the particles (atoms, ions) have enough time to reach the ordered arrangement forming a crystalline solid. On the other hand, if the liquid is cooled very fast (quenching), the particles solidify without the ordered arrangement forming an amorphous solid. 25 A MINERAL IS A CRYSTALLINE SOLID Particles (atoms , ions) of a mineral is arranged in a highly ordered, repeating pattern in three- dimensional space. The arrangement of the particles is called a crystal structure or crystal lattice. Fluorite (CaF2) crystal lattice shows how Ca2+ and F- ions are arranged in 3D-space. Smallest repeating unit of the CaF2 crystal lattice. The smallest repeating unit of a crystal is known as a unit cell. CRYSTAL SYSTEMS The shape of a crystal are determined by the length of the edges and the angles between them. External shape of a crystal is often related to the internal arrangement of its atoms. Crystal structures are classified into different crystal systems 26 27  Halite (NaCl) Diamond (C) Halite (sodium chloride) and diamond (carbon) both exhibit a cubic crystal system. In halite, the sodium ions (Na+) and chloride ions (Cl-) are alternating in the crystal lattice held together by ionic bonds. In diamond, each carbon atom is covalently bonded to four other carbon atoms. PHYSICAL PROPERTIES OF MINERALS A. Crystal Form and Habit Crystal form refers to a group of identically shaped faces on a crystal. Habit refers to the external shape of an individual crystal or aggregates of crystals When grown freely, minerals follow the shape of the internal structure of the unit cell. o Examples: Pyrite (FeS2) and Halite (NaCl) with six sided cube and 12 sided Garnet dodecahedron Free growing conditions can be in cavities (holes), called geodes, left by gases in molten rock Most rock crystals do not show perfect shapes because: o Of too little space due to other growing crystals o Different crystal faces grow at different rates 28 COMMON CRYSTAL AGGREGATES Fibrous (hair-like) Rosette (like flower petals) Asbestos Barite (BaSO4) Botryoidal (like grapes) Acicular Hematite (Fe2O3) Quartz (SiO2) B. Cleavage and Fracture Both cleavage and fracture describe how mineral crystals break. Cleavage is the tendency of a mineral to break along planes of weak atomic bonding. Graphite (carbon) is made up of layers of hexagonal rings of carbon atoms held together by strong covalent bonds. Between layers, there are weak van der Waals forces. 29  Graphite can cleave (split) into flat layers along these weak cleavage planes. Number of Cleavage Planes Cleavage Type Mineral Example 1 Sheets (tabular) Muscovite Mica Elongated 2 fragments (prismatic) Feldspar 3 Equidimensional fragments Galena (cubes, dodecahedron, etc.) Calcite 30  Fracture refers property of a mineral breaking in a more or less random pattern with no smooth planar surfaces. Minerals with no weak planes in the structure fracture (break) with an irregular surface. E.g. quartz (silica, SiO2) has no cleavage but breaks like glass along a curving surface: a conchoidal fracture. Glass can also break with an even flat surface or maybe splinter C. Hardness Hardness refers to the measure of the resistance of a mineral to abrasion or scratching. Relative hardness is determined using a scratch test. A harder substance is capable of scratching a softer substance. Mohs Hardness Scale 31 D. Density Density measures the amount of matter present given its volume. 𝑚 𝜌= 𝑉 Relative Density (Specific Gravity) is the ratio of the density of a substance to the density of a reference (typically water) 𝜌!"#!$%&'( 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑎 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛 𝑖𝑛 𝑎𝑖𝑟 𝑅𝐷 = = 𝜌)%$(* 𝑤𝑒𝑖𝑔ℎ𝑡 𝑖𝑛 𝑎𝑖𝑟 − 𝑤𝑒𝑖𝑔ℎ𝑡 𝑖𝑛 𝑤𝑎𝑡𝑒𝑟 Pure minerals can be described using relative density Light colored minerals generally have densities between 2.5 – 3.0 (e.g. orthoclase RD = 2.6) Dark colored minerals generally have higher relative densities (RD > 3.0) (e.g. hornblende RD = 3.0 – 3.5) Metallic minerals generally have RD > 5.0 (e.g. galena, PbS RD = 7 ) E. Twinning in Feldspars Twinning happens because there is a mistake during the crystal growing Twinning is very important in identifying K-feldspar and plagioclase feldspar; o In hand specimen o In thin section Twinning also occurs in many other minerals such as; amphibole (hornblende) and pyroxene (augite) !"#$$#$% #$ &'(F*+,-* 32 F. Color The overall structure (including physical defects) and elemental composition of a mineral crystal contributes to the color. It may be used for identification but can be misleading G. Streak Streak refers to the color of the mineral powder. This is determined by rubbing the mineral on a hard surface (such as an unglazed porcelain) called a streak plate Streak Color Mineral Black Magnetite Red / Brown Hematite Yellow / Brown Limonite Greenish / Black Iron pyrites H. Luster Luster refers to the way the mineral reflects light. This can be classified as metallic, or nonmetallic. Metallic luster have an appearance of a polished metal surface. Typically seen in minerals of pure metals, metal oxides, or metal sulfides. Pyrite (FeS) 33 Nonmetallic lusters Mineral Vitreous topaz Amber Opal Luster Vitreous Resinous Greasy Description like glass like resin like grease Mineral Satin spar (gypsum) Diamond Pearly muscovite Luster Silky Adamantine Pearly Description like silk Brilliant like pearl Mineral Hematite Luster Dull Description like earth 34 CHEMICAL PROPERTY OF MINERALS Reactivity with Acids Some minerals are capable of reacting (dissolving) in the presence of acids. Acid test is performed by adding dilute hydrochloric acid (HCl) to the specimen. 𝐶𝑎𝐶𝑂+ (𝑠) + 2𝐻𝐶𝑙(𝑎𝑞) → 𝐶𝑎𝐶𝑙, (𝑎𝑞) + 𝐻, 𝑂(𝑙) + 𝐶𝑂, (𝑔) Mineral Classification Product Observation Calcite, CaCO3 Carbonate CO2(g) Dissolution with Dolomite, (Mg, Ca)CO3 bubble formation Galena, PbS Sulfide H2S(g) 35 Silicates WHAT ARE SILICATES? Minerals that are made up of SiO4 groups. Main elements of the Earth’s crust: Element Chemical Symbol Percentage by Mass Oxygen O 46.6% Silicon Si 27.7% Aluminum Al 8.1% Iron Fe 5.0% Calcium Ca 3.6% Sodium Na 2.8% Potassium K 2.6% Magnesium Mg 2.1% Total: 98.5% 75% of the Earth’s crust is made up of Silicon and Oxygen Silica tetrahedron is the basic building block of minerals and rocks 95% of minerals in Earth’s crust are made up of silicates SILICATE STRUCTURE A. Single (Isolated) Tetrahedra A single group of (SiO44-) forms a four-sided tetrahedron Mineral: Olivine, (Mg, Fe)2SiO4 36 Crystal structure for olivine shows individual silica tetrahedra held together by electrostatic interactions with Mg2+ or Fe2+. Olivine family includes a spread of compositions: Forsterite (Mg2SiO4) to Fayalite (Fe2SiO4) Magnesium ions (Mg2+) lie between Iron II ions (Fe2+) lie between separate separate SiO44- tetrahedra SiO44- tetrahedra Polymerization Joining of silicate (SiO44-) groups to form a chain of (SiO3)n groups. 37  B. 1-D Single Chain Silicate (SiO44-) groups form a chain Crystal structure of Diopside (a pyroxene) shows two silica tetrahedra in a chain. The chains are held together by Mg2+ and Fe2+. Mineral: Pyroxenes, (Mg, Fe)SiO3 Pyroxenes have two cleavages at 90o angles View on z-axis View on x-axis 38 C. 1-D Double Chain Crystal structure of Hornblende (amphibole) shows formation of a double chain. Mineral: Amphibole (Hornblende), Ca2Mg5Si8O22 Large hexagonal holes are occupied by (OH_) Double chains are held together by Mg2+ or Ca2+ Amphiboles (Hornblende) have two cleavages at 120o and 60o View on z-axis View on x-axis 39 D. Flat sheets Crystal structure of Micas showing a 2D flat sheet. Mineral: Mica, KMg3AlSi3O10(OH)2 Double layers have the tetrahedron pointing inwards and large hexagonal holes are occupied by (OH-) Sheets are held together by weak attractions giving one cleavage parallel to the sheets. 40  E. Framework silicates Most abundant silicates in the Earth’s crust Each silicate (SiO44-) tetrahedra is fully connected to four others giving a fully polymerized, 3-dimensional (3-D) framework. Mineral: o Quartz, SiO2 o Feldspar, NaAlSi3O8 Silicate Arrangement of Mineral Mineral Formula Al/Si : O ratio Structure Tetrahedra Example Olivine (Mg, Fe)2SiO4 Isolated R3R2(SiO4)3 1:4 tetrahedra Garnet [R3: +2 metal ion; R2: +3 metal ion] 1- D single chain Pyroxene (Mg, Fe)SiO3 1:3 1- D double Amphibole Ca2Mg5Si8O22(OH)2 4 : 11 chain 2- D sheet Mica KMg3AlSi3O10(OH)2 2:5 Quartz SiO2 1:2 3- D framework Feldspar NaAlSi3O8 1:2 * Al/Si:O ratio can be used to identify the silicate structure (and mineral) 41 SOLID SOLUTIONS Solid solutions takes place when two ions have similar sizes and charges, that allows them to fit in the same site in a crystal structure. The spread of composition of olivine between forsterite (Mg2SiO4) to fayalite (Fe2SiO4) shows that magnesium ion and iron (II) ion can replace each other due to their similar ionic sizes (ionic radii) This mixing is indicated by the formula for olivine, (Mg, Fe)2SiO4. Ionic Radii of Rock forming Ions Negative Ion Positive Ion (anion) (cation) SI+4 Al+3 0.39 0.51 Fe+3 Mg+2 Fe+2 0.64 0.66 0.66 O-2 Na+ Ca+2 K+ 1.40 0.97 0.99 1.33 42 Optical Properties of Minerals ELECTROMAGNETIC RADIATION Formed by the vibrations in electric and magnetic field. EM waves are classified based on certain wave characteristics. CHARACTERISTICS OF A WAVE For optical mineralogy, we focus on the EM radiation between 400 – 700 nm, known as the visible spectrum (visible light). VISIBLE LIGHT White light (polychromatic) contains the spectrum of wavelengths of visible light. 45 PROPERTIES OF VISIBLE LIGHT 1. Refraction Refraction refers to the changing of the speed of the light wave as it travels through different medium. This is indicated by the refractive index (RI) which shows the ratio of the speed of light in vacuum and the speed of light in the crystal. 𝜈-%'"". 𝑛 𝑜𝑟 𝑅𝐼 = 𝜈'*/!$%0 Since light travels faster in vacuum than in any other substance, refractive indices are always greater than 1. In refraction, the light beam bends towards the medium with a higher refractive index. 46  2. Interference Interference refers to the changes in the amplitude of a wave due to wave combinations. When two waves are in-phase, it results in a wave with a increased amplitude, while out-of- hase waves result in a decreased amplitude. POLARIZATION OF LIGHT Light waves travel forward in waves, that vibrate along all 360o direction. A polaroid is a material that allows only light vibrating in one plane to pass through. This plane is known as plane of polarization. Light passing through the polaroid is called plane polarized light (PPL). With a second horizontal polaroid in placed, no light passes through. Two polaroid placed such that their plane of polarizations are perpendicular to each other is called crossed polar position. 47 P L A N E P O L A R I Z E D L I G H T (PPL) P R O P E R T I E S Opacity Cleavage Color Pleochroism Relief 1. Opacity Opacity is the ability of a substance to prevent light from passing through. An opaque substance will prevent light from passing through. An example of which are substances with metallic luster such as magnetite as shown in the image below. Nonopaque substances allow light to pass through. Biotite and hornblende are examples of nonopaque substances. Bio – Biotite; Hbl – Hornblende (amphibole); Mag – Magnetite 2. Cleavages Cleavages refer to the tendency of minerals to break along flat surfaces due to the arrangements and chemical bonds between its atoms. This can give an insight to the silicate structure present in the mineral. (left) Crystal structure for olivine shows individual silica tetrahedra held together by electrostatic interactions with Mg2+ or Fe2+. (right) Olivine in PPL showing no cleavage 48 Diopside (pyroxene), with a 1-D single chain silicate structure, shows two sets of cleavage lines that are at a 90o angle to each other. View on z-axis View on x-axis 49 3. Color As light passes through a material, some wavelengths are absorbed while others are not. The color of the mineral as viewed from the microscope is a result of the wavelengths that were not absorbed. This depends on the arrangement of atoms in the crystal and its interaction with light. Biotite crystals viewed through plane polarized light absorb blue/green light leaving a red/brown color 4. Pleochroism The color of a mineral depend on the arrangement of atoms that the light waves meet. Same crystals in different sections or orientations would show different atom arrangements which will result in different colors. Pleochroism refers to the ability of a mineral to change color under PPL when the stage is rotated. Biotite exhibits different colors in different crystal position (pleochroic). Biotite changes color when the stage is rotated. Brown grains turn light yellow. 50 5. Relief Relief refers to how well the mineral grains stand out from a background. This occurs due to the differences in the refractive index of the mineral and its background. When mineral slides are prepared, these are ground to a very thin plate (thin section) of a thickness of 35 mm. The mineral plate is mounted on a glass slide for viewing under a polarizing microscope. Canada balsam, a common mounting medium, is used to hold the thin mineral into the glass slide. The refractive index for Canada Balsam is 1.54. High relief: good contrast and difference in refractive index is high. Low relief: poor contrast (not sharp) Negative relief: refractive index is less than that of Canada balsam (not sharp) 51 C R O S S P O L A R I Z E D L I G H T (XPL) P R O P E R T I E S Birefringence Isotropy Interference Color Extinction Twinning 1. Isotropy Isotropic minerals have the same atomic arrangements in all directions which results in the same refractive index (plural: indices) in any direction. This typically include minerals belonging to the cubic (isometric) system and glass (an amorphous solid). This results in the mineral having the same refractive index in all directions which means light travels in a single velocity regardless of the direction of propagation. Since the optical properties are uniform in all directions, there are no permitted direction of vibrations that differ from one another. When plane polarized light passes through an isotropic mineral, the light is transmitted with the same polarization. Since the plane of polarization of the analyzer is at a 90o angle with the polarizer, no light can be transmitted in XPL. 52 Anisotropic minerals have different atomic arrangements in different directions. This results in the mineral having different properties, such as refractive indices, in different directions. Due to this difference, there exist permitted directions of vibrations. When the plane polarized light is parallel to the permitted direction, the light is transmitted with the same polarization. When the plane polarized light is not parallel to the permitted direction of vibration, the light splits into two rays. The transmitted light consist of two rays that are (1) perpendicular to each other; (2) at an incline with both the polarizer and analyzer; and (3) emerge from the crystal with a phase difference. This is known as double refraction. 53 DOUBLE REFRACTION 1. Plane polarized ray enters the anisotropic crystal. Wave splits into two rays (e1 and e2), vibrating at planes perpendicular to each other. Both waves are in-phase (in-step) upon entering the crystal. 2. Inside the crystal, the two waves travel at different speeds (different refractive indices) with different wavelengths (𝜆) and develop a phase difference. 3. After time t1, the path difference is described by the retardation (Δ). 54 2. Birefringence Birefringence refers to the optical property occurring in anisotropic minerals where light splits into two rays upon entering the crystal. These rays travel at different velocities, resulting in different refractive indices. This is calculated by obtaining the difference in the refractive indices of the slow and fast ray produced by double refraction. For quartz, n1=1.544 (fast ray) and n2=1.553 (slow ray). Low birefringence (𝐵𝑖𝑟𝑒𝑓𝑟𝑖𝑛𝑔𝑒𝑛𝑐𝑒 = 𝑛, − 𝑛1 = 1.553 − 1.544 = 0.009) It is only when the birefringence is high that the effects of double refraction can be seen without a microscope. For calcite, n1=1.486 (fast ray) and n2=1.658 (slow ray). Since birefringence is high, calcite shows two images due to double refraction. 55 3. Extinction When the stage is rotated, the permitted directions of vibration will also rotate. This will affect whether the plane polarized light is transmitted or split into two. When the plane polarized light is parallel to the permitted direction, the light is transmitted with the same polarization. Since the polarizer and analyzer are perpendicular to each other no light will pass through and the crystal will appear in extinction (dark) As the stage is rotated by 45o angle, the plane polarized light is no longer parallel to the permitted direction of vibration and will split into two. Since the rays are not perpendicular to the analyzer, light can pass through. At this angle, the amount of light that can pass through the analyzer is at its maximum and the crystal will appear brightest. When the stage is rotated to a 90o angle from its original position, the plane polarized light will become parallel again to the permitted direction of vibration in the crystal and will be transmitted. Since the polarizer and analyzer are perpendicular to each other no light will pass through and the crystal will appear in extinction (dark) 56 EXTINCTION ANGLE A given grain of mineral changes its brightness as it is rotated on a microscope slide. The crystal goes into extinction 4 times in a complete rotation The angle of extinction is the angle between: 1. the position of extinction AND 2. some prominent (important) crystal direction, e.g. the cleavage Finding the extinction angle of plagioclase feldspar. Rotate the crystal to each side of the cross wire (crosshair) untill the twin lamellae go into extinction. Average the two readings. Example1: mica has an extinction angle of zero to a few degrees compared to the cleavage. Example 2: plagioclase feldspar shows inclined extinction, or oblique extinction as shown in the figure above. Parallel extinction occurs when the cleavage is parallel to the ‘cross wires’(crosshairs) of the microscope. 57 4. Interference Colors Interference colors refer to the colors that appear in XPL. When the plane polarized light is not parallel to the permitted direction of vibration, the light is split into two rays that travel at different speeds inside the crystal. Upon exiting the crystal they become out-of-step (out-of- phase) to an extent measured by the retardation ( Δ ), also known as the optical path difference. The transmitted light is now the combination of the two rays. The colors result to the interference effects when the two waves recombine. The optical path difference depends on (1) the birefringence of the crystal in its particular orientation; and (2) the thickness of the crystal. The observed colors in relation to these factors are depicted in the Michel Levy chart. Interference colors can be classified as: First order colors: Strong colors that do not give a complete rainbow. Second order colors: strong colors that give a complete rainbow. Third order colors: weak, “washed out”, pale colors, but gives a complete rainbow. 58  5. Twinning Twinning results from an error during crystallization. Instead of the crystal growing in a single orientation, different domains of the crystal have different orientations appearing to be one crystal growing out of or into each other. Potassium feldspar Plagioclase feldspar exhibits Potassium feldspar (orthoclase) exhibits simple lamellar twinning (microcline) exhibits cross- twinning hatch (tartan) twinning OPTICAL CLASSIFICATION OF MINERALS Opacity Isotropy Examples of Common Minerals Opaque Gold, Copper, Pyrite, Magnetite Isotropic Garnet, Diamond, Halite Non opaque Quartz, Calcite, Feldspar, Mica, Pyroxene, Anisotropic Amphibole 59 Igneous Rocks Schematic Diagram for the Rock Cycle IGNEOUS ROCKS First formed rocks Comprise most of the Earth’s crust along with a thin cover of sedimentary and metamorphic rocks. Formed from cooling of magma or lava. MAGMA Refers to completely or partially molten rock inside the earth. Formed by partial melting of rocks in the crust and in the upper mantle. Its low density causes it to rise upwards to the Earth’s surface. o Magma that reaches the surface is called lava. Most magmas contain materials in different states: o Liquid component: This is referred to as melt. It is composed mainly of silicate materials (Si + O) and mobile ions (Si4+, Al3+, Fe2+/Fe3+, Mg2+, Ca2+, Na+, K+) 60  o Solid component: Magma may contain crystals of minerals that have started to crystallize out but not yet fully solidified. Other inclusions, such as foreign or unmelted fragments of surrounding rocks might also be present. o Gaseous component: Magma dissolved materials that form gases on reaching the surface (volatiles), i.e. CO2, H2O and SO2. Cooling and Crystallization of Magma 1. At elevated temperatures, rocks exist in their molten form (magma or lava). 2. Cooling causes atoms to become organized into crystals formed around nucleation centers which can be impurities from the magma. 3. Each nucleus grows as ions in the magma lose their mobility and join the crystalline network. 4. Finally, a complete interlocking texture of crystals is formed when the igneous rocks hardens. When all the crystals are similar in sizes, it is called equi-granular. The size of the crystal grains depend on the speed of cooling. Magmas reaching the surface cools quickly as lava, etc. and form fine-grained extrusive, or volcanic rocks. Rocks cooled very rapidly (quenching) form volcanic glass with no ordered lattices (no crystals formed), i.e. Obsidian. Magmas deep in the crust cool slowly to form coarse (large) grained intrusive or plutonic rocks. 61 CLASSIFICATION OF IGNEOUS ROCKS 1. Igneous texture: the overall appearance of the rock based on size, shape, and arrangement of its interlocking crystals. 2. Igneous composition: is determined from the chemical composition of the magma and the environment of crystallization. IGNEOUS TEXTURES Factors affecting crystal size: The rate at which magma cools The amount of silica present The amount of dissolved gases in the magma. A. Aphanitic Texture Fine-grained, crystals are < 1mm. Fast cooling on the Earth’s surface. Individual minerals can only be distinguished using a polarizing microscope. B. Phaneritic Texture Coarse-grained, crystals are 2 – 5 mm. Slow cooling in intrusions below the surface. C. Porphyritic Texture Large crystals (phenocryst) embedded in a matrix of smaller crystals (groundmass). Two stages of cooling: o Slow cooling at depth o Fast cooling on the surface D. Glassy Texture No crystals are formed due to rapid cooling of the magma (quenching). o Ions are unable to organize themselves in a crystal lattice. E. Vesicular Texture Rocks that contain spherical openings called vesicles resulting from voids left by gas bubbles that escape as lava solidifies. F. Pyroclastic Texture Formed from consolidation of individual rock fragments ejected during explosive volcanic eruptions 62 63 IGNEOUS COMPOSITION Nearly 98% of magma and igneous rocks are composed of eight key elements: O, Si, Al, Ca, Na, K, Mg, and Fe. Si and O are the most abundant elements in the Earth’s crust form silicates. o Amount usually represented as %silica. Two main groups of silicates based on composition. Light silicates Dark silicates Non-ferromagnesian silicates Ferromagnesian silicates Felsic Minerals Mafic Minerals Rich in K, Na, Ca Rich in Fe and/or Mg High in silicates Low in silicates e.g. Quartz, Muscovite Mica, Feldspars e.g. Olivine, Pyroxene, Biotite Mica Quartz Olivine SiO4 (Mg,Fe)2SiO4 Feldspars KAlSi3O8 Pyroxene NaAlSi3O8 (Mg,Fe)SiO3 CaAl2Si2O8 (Ca, Mg, Fe)Si2O6 Muscovite Mica Biotite Mica KAl2(AlSi3O10)(OH)2 K(Fe,Mg)3(AlSi3O10)(OH)2 64 CLASSIFICATION OF IGNEOUS ROCKS BASED ON COMPOSITION Basis: proportion of light and dark colored minerals present. A. Granitic (Felsic) Composition Composed entirely of light silicate minerals (quartz and feldspars) Contain about 10% dark silicate minerals (biotite mica and amphibole) Rich in silica (around 70%) Major constituent of the continental crust. B. Basaltic (Mafic) Composition Contains at least 45% dark silicate minerals and calcium-rich feldspar (but no quartz). High iron content makes these rocks darker in color and denser than granitic rocks. Basaltic rocks make up the ocean floor. C. Andesitic (Intermediate) Composition Contains at least 25% dark silicate minerals (amphibole, pyroxene and biotite mica with plagioclase feldspar. D. Ultramafic Composition Mostly olivine and pyroxene Includes peridotite (rocks from upper mantle). 65 66  Felsic Intermediate Mafic Ultramafic (granitic) (andesitic) (basaltic) Intermediate in Contains a lot of Nearly all Dominated by Si composition Composition ferromagnesian ferromagnesian and Al between Felsic minerals minerals and Mafic Usually light in Little darker in Rare on Earth’s Appearance Grey to black color color than felsic surface Continental crust Continental crust Oceanic crust Main rock of the Environment environment environment environment mantle Found as Forms viscous Forms viscous Low viscosity Lava type xenoliths in (stiff) lava (stiff) lava (runny) lava basaltic lavas Silica, SiO2 66-75 % 55-65 % 45-55 % 40-45 % content Peridotite Granite (coarse) Diorite (coarse) Gabbro (coarse) Rock Types (coarse) Rhyolite (fine) Andesite (fine) Basalt (fine) Komatiite (fine) 45% K-feldspar 20% K-feldspar 50% Ca- 30% Quartz 20% Quartz plagioclase 90% Olivine / Minerals usually 15% Na- 50% Na- 40% Pyroxene Pyroxene present and plagioclase plagioclase (Augite) 10% Ca- approximate % 10% Mica + 10% Biotite + 10% Olivine / plagioclase Hornblende Hornblende Amphibole 67 68 FORMATION OF MAGMA Magmas form from melting rocks in the crust and upper mantle. Three factors influence the generation of magma from solid rock. 1. Heat Earth’s temperature increases with increasing depth at an average of 25oC per km. This is known as the geothermal gradient. This temperature alone is not sufficient to melt rock at the mantle or lower crust. o Partial melting occurs since igneous rocks are a mixture of minerals in which melting occurs within a range of temperatures. o Produces magma with a higher silica content. Additional heat is generated by: o Friction in subduction zones o Crustal rocks heated during subduction o Rising, hot mantle rocks entering crustal rocks (intrusions) 69 2. Pressure At greater depths within the Earth, the increased pressure forces molecules closer together, which raises the melting temperature of rocks and minerals. o Lowering the pressure decreases the melting point of the rock Convection currents cause an upwelling (rising) of solid mantle rock starts to melt as the pressure decreases (decompression melting). This forms the magmas that are stored in magma chambers. 70 3. Volatiles Substances that can easily vaporize at relatively low temperatures (less than melting temperature of rocks) Water (H2O), Carbon dioxide (CO2), Sulfur dioxide (SO2), Hydrogen sulfide (H2S) Presence of water (and other volatiles) lowers the melting point of rocks, like how salt lowers the melting point of ice. At convergent boundaries, where wet oceanic lithosphere subducts beneath the continental crust, the released water from the subducting plate lowers the melting point of the overlying mantle, leading to partial melting and magma formation. 71 EVOLUTION OF MAGMA: BOWEN’S REACTION SERIES Observation: Different minerals tend to coexist in specific combinations within igneous rocks. Hypothesis: Minerals in magma crystallize at different temperatures How it works: Crystallization starts with the highest melting point minerals: i.e. olivine, pyroxene and Ca- rich plagioclase feldspar. Crystallization happens over a temperature range because minerals are not pure compounds. They are solid solutions. On further cooling the solid parts react with the remaining melt to produce the next mineral, resulting in two branches: 1. Discontinuous series – left of the Bowen’s reaction series where minerals of different silicate structures are formed one after the other. 𝑜𝑙𝑖𝑣𝑖𝑛𝑒 → 𝑝𝑦𝑟𝑜𝑥𝑒𝑛𝑒 → 𝑎𝑚𝑝ℎ𝑖𝑏𝑜𝑙𝑒 → 𝑏𝑖𝑜𝑡𝑖𝑡𝑒 𝑚𝑖𝑐𝑎 2. Continuous series – right of the Bowen’s reaction series where the mineral structure (framework silicate) remains the same but with different cations forming different solid solutions. 𝐶𝑎 − 𝑟𝑖𝑐ℎ 𝑝𝑙𝑎𝑔𝑖𝑜𝑐𝑙𝑎𝑠𝑒 𝑓𝑒𝑙𝑑𝑠𝑝𝑎𝑟 → 𝑁𝑎 − 𝑟𝑖𝑐ℎ 𝑝𝑙𝑎𝑔𝑖𝑜𝑐𝑙𝑎𝑠𝑒 𝑓𝑒𝑙𝑑𝑠𝑝𝑎𝑟 During crystallization, the cooling magma; o Changes composition of the liquid part o Loses ferromagnesian elements: Fe, Mg, and Ca o Is enriched in the elements: Al, Na, and K o Is enriched in SiO2 towards the end of crystallization 72 HOW DO MAGMAS EVOLVE? Observation: There are different kinds of igneous rocks. Hypothesis: As minerals crystallize at different temperatures, the magma evolves in composition. Magma Differentiation It refers to the formation of one or more secondary magmas from a single parent magma. It is crucial for the formation of diverse igneous rocks. Several mechanisms result in magma differentiation. A. CRYSTAL SETTLING Earlier formed high density crystals settle to the bottom of the magma chamber. Removal of the crystal components leaves the remaining magma with a different composition (richer in Na, K, and silica) When this new magma solidifies it will form a different igneous rock. (e.g. the palisades Sill, New York is 300 m thick and shows a gradual change in composition from top to bottom) At any stage in the cooling of a magma the liquid and solid can separate into different units and produce different rocks of increasing % of silica; Rock types : 𝑀𝑎𝑓𝑖𝑐 → 𝐼𝑛𝑡𝑒𝑟𝑚𝑒𝑑𝑖𝑎𝑡𝑒 → 𝐹𝑒𝑙𝑠𝑖𝑐 B. ASSIMILATION Change in composition of magma brought by incorporation of the surrounding rock (country rock) into the magma. Magma Mixing Occurs during the ascent of two chemically distinct magma bodies. Once they are joined convective flow stirs the two magmas, generating a single mass that has intermediate composition Partial Melting Process by which only a portion of the rock melts to form magma due to the differences in melting points of the minerals. Occurs due to either decompression melting or subduction. o Bowen’s reaction series predict that quartz, muscovite mica and orthoclase feldspar would melt first. Main process that produces magma 73 MAGMA COMPOSITION Basaltic (Mafic) Magma Called primary or primitive magma as they have not yet evolved. Originates from the partial melting of mantle rocks. Partial melting of mantle rocks may be triggered by: o Decompression melting o Release of water from subduction zones promoting partial melting of overlying mantle rocks. Andesitic (Intermediate) Magma Called secondary magma as they are derived from basaltic magmas. Formed by magmatic differentiation of basaltic magmas. o Crystallization of basaltic magma o Assimilation of silica-rich crustal rocks Granitic (Felsic) Magma Formed by crystallization of andesitic magma Formed by partial melting of continental crust 74