Geology Notes and in class reviews from week 3
Geology Notes and in class reviews from week 3 Geos 1113
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This 4 page Class Notes was uploaded by Brandon Notetaker on Monday September 19, 2016. The Class Notes belongs to Geos 1113 at University of Arkansas taught by Mohamed Aly in Fall 2016. Since its upload, it has received 10 views. For similar materials see General Geology in Geology at University of Arkansas.
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Date Created: 09/19/16
Week 3 Geology Class Notes and In Class Reviews (Week of 9/4/16) Matter and Minerals Minerals: Building Blocks of Rocks 1. Naturally occurring with no Synthetic materials 2. Generally inorganic. Such as table salt, but not sugar. 3. Solid substance with the exception of Mercury 4. Orderly crystalline structure 5. Definite chemical composition Rock A solid mass of minerals or minerallike matter that occurs naturally (coal, pumice, obsidian) Atoms: Building Blocks of Minerals 1. Smallest particles of matter that cannot be chemically split 2. Composed of: Protons (charge of +1), Neutrons (charge of 0), and surrounded by electrons (charge of –1). Electrons exist as a cloud of negative charges surrounding the nucleus of protons and neutrons, called principal shells. The outermost shell contains valence electrons, which interact with other atoms to form chemical bonds 3. The nucleus contains virtually all of the mass of the atom. The remainder of the atom is the space occupied by negatively charged electrons. 4. The number of protons in the nucleus, and determines the atom’s chemical nature. 5. Element (defined by their number of protons) – A group of the same kind of atoms. Approximately 90 natural elements and several synthesized in a laboratory. Organized in a periodic table so that those with similar properties line up Why Atoms Bond 1. Ionic bond: the attraction of oppositely charged ions to one another Ex: Halite (table salt, NaCl) 2. Chemical Bonding – Formation of a compound by combining two or more elements. Transferring or sharing electrons that allow each atom to attain a full valence shell of electrons 3. Octet Rule: Atoms tend to gain, lose, or share electrons until they are surrounded by 8 valence electrons Types of Bonding 1. Ionic Bonding (electrons transferred) – Atoms gain or lose outermost (valence) electrons to form ions (positively and negatively charged atoms). 2. Ionic compounds consist of an orderly arrangement of oppositely charged ions 3. Covalent Bonding (electron sharing) – Atoms share a pair of electrons 4. Metallic Bonding (electrons free to move) – Valence electrons are free to migrate among atoms. Accounts for the high electrical conductivity of metals 5. Hybrid Bonding – Many chemical bonds are actually hybrids that exhibit some degree of electron sharing and some degree of electron transfer How Do Minerals Form? 1. Precipitation of Mineral Matter – Ions dissolved in an aqueous solution reach saturation and start forming crystalline solids. A drop in temperature or water loss through evaporation can cause ions to reach saturation 2. Crystallization of Molten Rocks – Similar to water freezing – When the magma is hot, the atoms are mobile. When the magma cools, the atoms slow and begin to chemically combination 3. Deposition as a Result of Biological Process – Marine organisms secrete calcium carbonate (CaCO3) Ex: corals are organisms capable of creating large quantities of marine limestone (a rock that is mainly calcite) Properties of Minerals 1. Primary Diagnostic Properties – Determined by observation or performing a simple test. Several physical properties are used to identify hand samples of minerals 2. Optical Properties – Luster, Ability to transmit light, color, and streak Luster Appearance of a mineral in reflected light. Metallic – Nonmetallic (includes vitreous or glassy luster, dull or earthy luster, pearly luster, silky luster, and greasy luster) Ability to transmit light – Opaque (no light transmitted), Translucent (light, but no image is transmitted), Transparent (light and an image are visible through sample) Color – Generally unreliable for mineral identification. Often highly variable due to impurities or slight changes in mineral chemistry Streak – Color of a mineral in its powdered form. Not every mineral produces a streak when rubbed across a streak plate 3. Crystal Shape or Habit – Characteristic shape of a crystal or aggregate of crystals Thin, rounded crystals that break into fibers Elongated crystals that are flattened in one direction Minerals that have stripes or bands of different color or texture Groups of crystals that are shaped like cubes 4. Tenacity the mineral’s resistance to breaking or deforming Brittle minerals (such as those with ionic bonds) will shatter into small pieces Malleable minerals (such as those with metallic bonds) are easily hammered into different shapes Sectile minerals, such as gypsum and talc, can be cut into thin shavings – Elastic minerals, such as the micas, will bend and snap back to their original shape 5. Hardness – Resistance of a mineral to abrasion or scratching. All minerals are compared to a standard scale called the Mohs scale of hardness 6. Cleavage – Tendency to break along weak bonded planes producing a smooth, flat surface Described by Number of planes. Angles between adjacent planes. Resulting geometric shapes 7. Fracture minerals that have chemical bonds strong in all directions show absence of cleavage when a mineral is broken Irregular fractures – Conchoidal fractures, Splintery fractures, and Fibrous fractures Other Properties of Minerals 1. Density and Specific Gravity – Density is defined as mass per unit volume. Specific gravity is ratio of the weight of a mineral to the weight of an equal volume of water (Most minerals have a specific gravity between 2 and 3) 2. Taste (Halite tastes like salt) 3. Feel (Talc feels soapy, Graphite feels greasy) 4. Magnetism (Magnetite can be picked up by a magnet, Lodestone is a natural magnet) 5. Optical properties (Calcite has double refraction) 6. Reaction to dilute hydrochloric acid (Carbonates will effervesce (fizz) in acid) Mineral Structures and Compositions 1. All mineral samples are crystals or crystalline solids 2. Unit cells (Atomic arrangement that results in the basic building blocks of a mineral crystal) 3. Minerals can be constructed of the same unit cells and have different external forms 4. Steno’s Law (Law of Constancy of Interfacial Angles) Regardless of crystal size, the angles between equivalent crystal faces of the same mineral are consistent 5. Compositional Variations in Minerals – Ions of similar size can substitute for one another without disrupting the mineral’s internal framework. Other minerals have trace variations in their chemical compositions which can significantly influence the mineral’s color 6. Structural Variations in Minerals – Polymorphs and Minerals with the same composition but different crystalline structures. Transforming one polymorph into another is called a phase change Mineral Structures and Compositions How Minerals Are Classified 1. Nearly 4000 minerals have been named 2. Rock Forming Minerals – Only a few dozen. Common minerals that make up most of the rocks of Earth’s crust. Composed mainly of the 8 elements that make up most of the continental crust 3. A collection of specimens that exhibit similar internal structure and chemical compositions are called mineral species. Mineral species are then further divided into mineral varieties. Mineral species are assigned to mineral classes (silicates or nonsilicates) The Silicates 1. Silicate minerals are the most common type of minerals (account for >90% of Earth’s crust) 2. All silicate minerals contain oxygen and silicon (the two most abundant elements in Earth’s crust) 3. Silicon–oxygen tetrahedron – (Fundamental building block) Four oxygen ions surrounding a much smaller silicon ion 4. Single tetrahedron linked together to form various silicate structures Common Silicate Minerals 1. Light (Nonferromagnesium)Silicates – Generally light in color, but Lack iron and magnesium. Have a specific gravity of 2.7 Containing varying amounts of aluminum, potassium, calcium, and sodium • Feldspar group –The most common silicate group and make up >50% of Earth’s crust. Forms under a wide range of temperatures and pressures. Exhibit two directions of perfect cleavage at 90 degrees Most common members Orthoclase (potassium feldspar), and Plagioclase (sodium and calcium feldspar) • Quartz –The secondmost abundant mineral in the continental crust and the only common silicate composed entirely of silicon and oxygen. Hard and resistant to weathering. Conchoidal fracture –Often forms hexagonal crystals –Colored by impurities (various ions) • Muscovite –Common member of the mica family. Excellent cleavage in one direction. Thin sheets are clear (used as glass during the Middle Ages). Produces the “glimmering” brilliance often seen in beach sand • Clay minerals – “Clay” is a general term used to describe a variety of complex minerals that have sheet structure. Clay makes up a large percentage of soil. Most originate as products of chemical weathering, and Kaolinite is common clay mineral used to manufacture fine china Common Silicate Minerals 2. Dark (Ferromagnesium) Silicates – Contains iron and/or magnesium in their structure. Generally dark in color, and have a specific gravity between 3.2and 3.6 • Olivine group –Hightemperature silicates. Black to green in color. Glassy luster and conchoidal fracture. Forms small, rounded crystals • Pyroxene group – Important components of darkcolored igneous rocks. Augiteis the most common mineral in the pyroxene group, Black in color, Two distinctive cleavages at nearly 90 degrees, and Dominant mineral in basalt Common Silicate Minerals • Amphibole group – Hornblende is the most common mineral in this group. Two perfect cleavages exhibiting angles of 120 and 60 degrees • Biotite – Ironrich member of the mica family. Excellent cleavage in one direction • Garnet –Composed of individual tetrahedra linked by metallic ions (similar to olivine). Glassy luster and conchoidal fracture Common Silicate Minerals Important Nonsilicate Minerals 1. Nonsilicate minerals are not as common as the silicates but important economically 2. Divided into groups based on the negatively charged ion or complex ion that the members have in common 3. Make up ~8%of Earth’s crust 4. Carbonates – Composed of the carbonate ion (CO32−) and a positive ion. Most common carbonates are calcite (CaCO3) and dolomite CaMg(CO3)2.Primary constituents in limestone and dolostone Common Nonsilicate Mineral Groups 1. Many nonsilicate minerals have economic value • Ex: Halite (mined for salt), Gypsum (used to make building materials), Hematite and magnetite (mined for iron ore), Native elements (gold, silver, and diamonds) Important Nonsilicate Minerals: Calcite, Dolomite, Halite, Gypsum, Hematite, Magnetite, Galena, chalcopyrite, fluorite. Plate T ectonic Review 1. Oceanic Ridges are high, because they are less dense than the sea floor away from the ridges. 2. The Oceanic lithosphere is beneath the continental lithosphere because it is more dense than the continental lithosphere. 3. Rocks that form in an oceanic environment are found in the highest parts of the Himalayan mountains, because the oceanic sediments were trapped and deformed during the continental collision that formed those mountains. 4. The sulfur spreads and magma rises up to fill the gap, forming underwater features like oceanic ridges and submarine volcanoes at divergent boundaries. 5. There was no Atlantic Ocean 200 million years ago. 6. Sediments on the ocean floor get thicker with increasing distance from the ridge because the seafloor gets older as it moves away from the ridge, so more sediments have time to accumulate from the ridge. Matter and Minerals Review 1. Deposition of materials by biological process is most likely to form minerals. 2. Some minerals break along one or more specifically oriented planes while others do not because some minerals have weak bonds in some directions and other minerals have strong bonds in other directions. 3. Steno’s law – Crystals of a given mineral will have angels between equivalent faces being equal. 4. Oxygen is the most common element in the continental crust, but 90% of the earth’s crust is silicon. 5. The silicon to Oxygen bonds in Quartz is stronger compared to the bonds in silicate minerals. 6. Quartz also has no cleavage and is highly resistant to weathering.
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