Chapter 3 Notes
Chapter 3 Notes GEOL 1301 - 001
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Chapter 3 Earth Materials: Minerals and Rocks Geology 1301-007 Professor John Wickham Purple- important people Blue-vocabulary Green-important dates/time periods • What are minerals? o Minerals are the building blocks for rocks o Mineralogy: the branch of geology that studies the composition, structure, appearance, stability, occurrence, and association of minerals § Few rocks are made up of only one kind of mineral • Ex. Limestone § Other rocks are made of several different minerals • Ex. Granite § We must understand how minerals are formed to classify the types of rocks on earth and understand how they are made § Mineral: naturally occurring, solid crystalline structure with a specific chemical composition • Usually inorganic • Homogenous: can’t be divided into smaller components by mechanical means • Naturally occurring o Must be found in nature o Synthetic substances produced in labs don’t count § Neither do thousands of laboratory products invented by chemists • Solid crystalline structure o Solid substances- neither liquid or gas o Crystalline b/c its atoms are composed in an orderly, repeating, three dimensional array § Solids that don’t have this are referred to as glass or amorphous (without form) • Not conventionally called minerals • Ex. Windowpane glass • Usually inorganic o Excludes materials that make up plant and animal bodies o Organic matter is composed of organic carbon § Found in all organisms living or dead o Many minerals are secreted by organisms § Ex. Calcite • Contains inorganic carbon • Forms from shells of oysters and other marine organisms---> shells accumulate on the ocean floor--->they can be geologically transformed into limestone • With a specific chemical composition o Key to understanding composition of Earth materials is how chemical elements are organized into minerals § Each mineral is made unique by chemical composition and arrangement of atoms in internal structure • Chemical composition is either fixed or varies within defined limits • Ex. Quartz has a fixed ratio of two atoms of oxygen to one atom of silicon o The ratio never varies • The Formation of Minerals o Orderly forms of minerals result from chemical bonds o Minerals can be viewed in two complementary ways § assemblages of submicroscopic atoms organized in an ordered three- dimensional array § as crystals we can see with the naked eye o The Atomic Structure of Minerals § Minerals form by the process of crystallization: atoms of gas or liquid come together in proper chemical proportions and arrangement to form a solid substance • Ex. Bonding of carbon atoms in a diamond o Under high pressures and temperatures, carbon atoms bond in a tetrahedra, and each one attaches to another to build a 3-D shape o Covalently bonded • Ex. Sodium and chloride atoms crystallize in orderly, 3-D array o Ionically bonded § Most cations are small while most anions are large • Most of crystal space is occupied by anions due to their size and cations fit into the spaces between them o Crystal structure largely determined by this arrangement § Cations of similar sizes and charges tend to substitute for one another and form compounds with the same crystal structure but different chemical compositions • This is known as cation substitution • Ex. Aluminum and Silicon ions o The Crystallization of Minerals § Starts with microscopic formation of single crystals: orderly 3-D arrays of atoms in which the basic arrangement is repeated in all directions • Boundaries are naturally flat (planar) surfaces called crystal faces: external expressions of a mineral’s internal atomic structure o During crystallization microscopic crystals grow larger, maintaining crystal faces as long as they are free to grow o Large crystals with well-defined faces form when growth is slow and steady and space is adequate to allow growth without interference from other crystals nearby § most large mineral crystals form in open spaces in rocks, such as fractures or cavities § more often spaces between growing crystals fill in or crystallization proceeds rapidly---> crystals grow over one another to become a solid mass of crystalline particles, or grains • few or no grains show crystal faces • large crystals that can be seen with naked eye are unusual • glassy materials don’t for crystals with linear faces o instead they are masses with curved, irregular surfaces § most common natural glass is volcanic glass o How do minerals form? § Lowering temperature of liquid below freezing point is one way to start crystallization • Ex. Ice crystals begin to form at 0 degrees Celsius • Magma (hot, molten rock) crystallizes into solid minerals when it cools o as it falls below its melting point, which may be higher than 1000 degrees Celsius, crystals of silicate minerals (such as olivine and feldspar) begin to form • can also occur as liquids evaporate from a solution o solution: homogenous mixture of one chemical substance with another § Ex. salt and water § A saturated solution occurs when the solution can hold no more of a particular chemical • If this continue the chemical will precipitate: (n.) crystals that drop out of a saturated solution (v.) to drop out of a saturated solution as crystals o Table salt and halite form under these conditions o Polymorphs: minerals with alternative structures formed form same chemical element or compound § Ex. Diamond and graphite • Both formed from carbon but have different chemical structures and appearances o Diamond forms under high pressure and has a closely packed structure and density: mass per unit volume of a substance (g/cm3) o Graphite has a lesser density • Classes of Rock-Forming Minerals o All minerals on earth have been grouped into seven classes based on chemical structure § Native Elements: naturally, un-ionized pure elements § Minerals are classified by their anions Class Defining Anions Example Native elements None: no charged ions Copper metal (Cu) Oxides Oxygen ion (O ) 2− Hematite (Fe O ) 2 3 − Halides Chloride (Cl ), fluoride Halite (NaCl) (F ), bromide (Br ), − − iodide (I ) Carbonates Carbonate ion (CO ) 2− Calcite (CaCO ) 3 3 Sulfates Sulfate ion (SO ) 4− Anhydrite (CaSO ) 4 4− Silicates Silicate ion (SiO ) 4 Olivine (Mg, Fe) SiO 2 4 Sulfides Sulfide ion (S ) 2− Pyrite (FeS ) 2 • Silicates o Most abundant class of minerals in Earth’s crust o Composed of oxygen and silicon ions § Mostly in combination with cations of other elements because of its negative charge § Tetrahedron § Typically bonds to sodium, magnesium, calcium, iron • Can bond with other silicate tetrahedra and oxygen o The simplest silicate is silica § Found in the mineral quartz • Linked sharing two oxygen ions for each silicon ion so the formula is SiO2 o Crystal structures § May be isolated (linked to only cations) • Linked by bonding each oxygen ion to each of the tetrahedra to a cation o Cations bond to other oxygen ions o Tetrahedra is isolated by cations on all sides § Ex. olivine § Liked to other tetrahedra in links, single chains, double chains, sheets or framework • Single-chains o Formed by sharing of oxygen ions § Two oxygen ions bond to adjacent tetrahedra in an open ended chain o Linked to other chains by cations o Ex. Minerals of pyroxene group • Double Chains o Single chains may come together to form double chains linked by oxygen ions o Adjacent double chains linked by cations form the structure of minerals in the amphibole group o Ex. Hornblende • Sheets o Each tetrahedron shares three of its oxygen ions with adjacent tetrahedral to build stacked sheets of tetrahedral § Cations may be interlayed with sheets § Ex. Mica and clay minerals • Framework o 3-d frameworks form as each tetrahedra shares its oxygen ions with another tetrahedra § Ex. Feldspar and quartz § • Carbonates o Minerals composed of carbon and oxygen § In combination with calcium and magnesium § Ex. Calcite o The carbon ion is surrounded by and covalently bonded to three oxygen ions in a triangle o Arranged in sheets like sheet silicate with cation layers in between o Calcite is one of the most common minerals found in Earth’s crust § Constitutes Limestone • Oxides o Compounds of the oxygen anion and metallic ions, such as iron o Ex. Hematite o Ionically bonded o Structure vary with side of metallic ions o This class includes ores containing minerals such as chromium and titanium, which are used to manufacture metallic devices and materials § Chief ore is iron • Sulfides o Compounds of the sulfide anion and metallic ions o Ex. Pyrite o Chief ores of other valuable materials such as zinc, copper, and nickel 2− o The basic building block of this class is the sulfide ion (S ) § a sulfur atom that has gained two electrons. § the sulfide ion is bonded to metallic cations. Most sulfide minerals look like metals, and almost all are opaque § most common sulfide mineral is pyrite (FeS )2 • often called “fool’s gold” • Sulfate o Compounds of sulfate anions and metallic ions o Ex. Anhydrite o tetrahedron made up of a central sulfur atom surrounded by four oxygen ions (O ) o one of most abundant minerals of this class is gypsum § primary component of plaster § stable at low temperatures and pressures o anhydrite differs from gypsum because it contains no water § stable and high temperatures and pressures • Native elements, hydroxides, and halides are less common as rock minerals • Physical Properties of Minerals o In the 19 and early 20 cen. geologist carried around field kits of rough chemical analyses that would help in their identification § The acid test • Geologists carried around diluted solutions of HCL to determine whether a particular rock had calcite o If it fizzled, the rock contained calcite o Hardness: measure of ease that the surface of a mineral can be scratched § Diamond, the hardest mineral known, scratches glass § Quartz can scratch a feldspar crystal because it is harder than feldspar § In 1822 Friedrich Mohs, an Austrian mineralogist devised a scale • Mohs Scale of Hardness: ascending scale of hardness based on one minerals ability to scratch another mineral o One of the best practical tools for identifying an unknown mineral o Covalent bonds are generally stronger than ionic bonds § The stronger the bonds, the harder the mineral § Hardness can vary with crystal structure as well • Within groups that have similar crystal structures, hardness is related to other factors as well o Size § the smaller the distance between them and the greater the electrostatic attraction—and thus the stronger the bond. o Charge § The larger the charge of ions, the greater the attraction between them, and thus the stronger the bond. o Packing § The closer the packing of atoms or ions, the smaller the distance between them, and thus the stronger the bond o Cleavage: the tendency for a crystal to split along planar surfaces § Also used to describe geometric pattern of such breakage § varies inversely with bond strength: • strong bonds produce poor cleavage • weak bonds produce good cleavage § Because of strength, covalent bonds produce poor cleavage • Ionic bonds produce good cleavage • Ex. Diamond can be cleaved along weaker planes, but not so much on the rest of the mineral § Classified according to two primary sets of characteristics • Number of planes and patterns of cleavage o Crystal structure determines cleavage planes and crystal faces § Cleavage occurs along any plane where bondage is weak § All crystals have characteristic cleavage planes • Only some have display particular faces • Ex. Distinctive angles help identify two important groups of silicates: pyroxenes and amphiboles • Quality of surfaces and ease of cleaving o Assessed as perfect, excellent, good, fair or none according to the quality of the surface and ease of cleaving § Ex. Muscovite is cleaved easily with a smooth surface so it is considered perfect o Fracture: tendency of crystal to break along irregular surfaces other than cleavage § This shows across the cleavage plane or with no cleavage in any direction § related to how bond strengths are distributed in directions that cut across cleavage planes § may be conchoidal, showing smooth, curved surfaces like those of a thick piece of broken glass • also described as fibrous or splintery • depend on structure and composition of mineral o Luster: the way the surface of a mineral reflects light § Depends heavily on visual perception of reflected light § Controlled by kinds of atoms present and their bonding • Ionically bonded have a glassy or vitreous luster • Covalently bonded are more variable • Gold and many sulfides have a metallic luster • pearly luster results from multiple reflections of light from planes beneath the surfaces of translucent minerals o color: property of mineral imparted by transmitted or reflected light § may be distinctive but not a reliable clue to a mineral’s identity § may show distinctive color on freshly broken or weathered surfaces § many ionically bonded crystal are colorless § streak: color of finely deposited mineral powder left on an abrasive surface • good identification tool § determined by ions and pure minerals as well as trace colors • ions and mineral color o color depends on presence of certain ions • trace elements and mineral color o instruments can now measure small quantities of minerals o trace elements: elements that make up less than 0.1% of a mineral § used to deduce origins of minerals in which they are found § others contribute to local natural radioactivity § color can come from trace elements dissolved in the solid crystal o density § specific gravity: weight of mineral divided by weight of equal volume of pure water at 4 degrees Celsius § depends on atomic mass of minerals atoms or ions, and how closely they are packed in its crystal structure § increases in density transform the way minerals transmit light, seismic waves, and heat § temperature also affects density • the higher the temperature, the lower the density o crystal habit: shape of which individual crystals or aggregate of crystals grow § often named after common geometric shapes • Ex. Blades, plates, and needles o Needle grows quickly in one direction and very slowly in another o Plate grows fast in all directions o Fibers shape as multiple, long narrow fibers § some minerals have easily recognizable crystal habits • What are rocks? o Rock: naturally occurring solid aggregate of miners or nonmineral solid matter § aggregate, minerals are joined in such a way that they retain their individual identity § Rocks vary in color, in the sizes of their crystals or grains, and in the kinds of minerals that compose them. § The identity of a rock is determined partly by its mineralogy and partly by its texture § Texture: describes the sizes and shapes of a rock’s mineral crystals or grains and the way they are put together • coarse-grained: If the crystals or grains are large enough to be seen with the naked eye, • fine-grained:If they are not large enough to be seen • The mineralogy and texture that determine a rock’s appearance are themselves determined by the rock’s geologic origin § igneous rocks: All rocks formed by the solidification of molten rock, such as basalt and granite • latin for fire • form by crystallization from magma o magma forms microscopic crystals when it cools o Intrusive igneous rocks crystallize when magma intrudes into unmelted rock masses deep in Earth’s crust § Large crystals § Ex. Granite o Most of the minerals of igneous rocks are silicates § partly because silicon is so abundant in Earth’s crust and partly because many silicate minerals melt at the high temperatures and pressures reached in deeper parts of the crust and in the mantle • The silicate minerals most commonly found in igneous rocks include quartz, feldspars, micas, pyroxenes, amphiboles, and olivines § sedimentary rocks: All rocks formed as the burial products of layers of sediments (such as sand, mud, or the calcium carbonate shells of marine life) • the precursors of sedimentary rocks, are found at Earth’s surface as layers of loose particles, such as sand, silt, and the shells of organisms • Weathering: all of the chemical and physical processes that break up and decay rocks into fragments and dissolved substances of various sizes • particles are then transported by erosion: the set of processes that loosen soil and rock and move them downhill or downstream to the spot where they are deposited as layers of sediment • Siliciclastic sediments: are made up of physically deposited particles, such as grains of quartz and feldspar derived from weathered granite o sediments are laid down by running water, wind, and ice • Chemical sediments and biological sediments: new chemical substances that form by precipitation • Lithification: the process that converts sediments into solid rock • FROM SEDIMENT TO SOLID ROCK o Occurs in two ways § In compaction, particles are squeezed together by the weight of overlying sediments into a mass denser than the original. § In cementation, minerals precipitate around deposited particles and bind them together • LAYERS OF SEDIMENT o Sediments are compacted and cemented after they are buried under additional layers of sediments o Bedding: the formation of parallel layers of sediment as particles are deposited • COMMON MINERALS OF SEDIMENTARY ROCKS o most common minerals in siliciclastic sediments are silicates because silicate minerals predominate in the rocks that weather to form sedimentary particles o The most abundant silicate minerals in siliciclastic sedimentary rocks are quartz, feldspar, and clay minerals o most abundant minerals of chemical and biological sediments are carbonates, such as calcite, the main constituent of limestone § metamorphic rocks: All rocks formed by the transformation of preexisting solid rock under the influence of high temperatures and pressures • greek for change and form • The temperatures of metamorphism are below the melting point of the rocks (about 700°C), but high enough (above 250°C) for the rocks to be changed by recrystallization and chemical reactions • REGIONAL AND CONTACT METAMORPHISM o Regional metamorphism: occurs where high pressures and temperatures extend over large regions, as happens where plates collide § Accompanies plate collisions that result in mountains and folding and breaking of sedimentary layers § Many regionally metamorphosed rocks, such as schists, have characteristic foliation, wavy or flat planes produced when the rock was folded o contact metamorphism: Where high temperatures are restricted to smaller areas, as in the rocks near and in contact with a magmatic intrusion, rocks are transformed • COMMON MINERALS OF METAMORPHIC ROCKS o Typical minerals of metamorphic rocks are quartz, feldspars, micas, pyroxenes, and amphiboles—the same kinds of silicates characteristic of igneous rocks o These minerals form under conditions of high pressure and temperature in the crust and are not characteristic of igneous rocks o They are good indicators of metamorphism • The Rock Cycle: Interactions Between Plate Tectonics and the Climate System o rock cycle: explains how each type of rock is transformed into one of the other two types § known to be the result of interactions between two of the three global geosystems: • the plate tectonic system and the climate system o Interactions drive transfers of materials and energy among Earth’s interior, the land surface, the ocean, and the atmosphere § Ex. the formation of magmas at subduction zones results from processes operating within the plate tectonic system. When these magmas erupt, materials and energy are transferred to the land surface, where the materials (newly formed rocks) are subject to weathering by the climate system. The eruption process also injects volcanic ash and carbon dioxide gas high into the atmosphere, where they may affect global climates. As global climates change, perhaps becoming warmer or cooler, the rate of weathering changes, which in turn influences the rate at which materials (sediments) are returned to Earth’s interior. o Turn of the rock cycle § beginning with the creation of new oceanic lithosphere at a mid- ocean ridge spreading center as two continents drift apart § ocean gets wider and wider, until at some point the process reverses itself and the ocean closes • As ocean basin closes, igneous rocks created at the mid- ocean ridge are eventually subducted beneath a continent • Sediments that were formed on the continent and deposited at its edge may also be dragged down into the subduction zone o the two continents, which were once drifting apart, may collide o As the igneous rocks and sediments go deeper into Earth’s interior, they begin to melt to form a new generation of igneous rocks o The great heat associated with the intrusion of these igneous rocks, coupled with the heat and pressure that come with being pushed to levels deep within Earth, transforms igneous rocks into metamorphic rocks § When the continents collide, these igneous and metamorphic rocks are uplifted into a high mountain chain as a section of Earth’s crust crumples, deforms, and undergoes further metamorphism o Concentrations of Valuable Mineral Resources o The rock cycle is crucial in creating economically important concentrations of the many valuable minerals found in Earth’s crust o Minerals are not only sources of metals § they also provide us with stone for buildings and roads, phosphates for fertilizers, cement for construction, clays for ceramics, sand for silicon chips and fiber-optic cables, and many other items § In most places, any given element will be found homogenized with other elements in amounts close to its average concentration in the crust. An ordinary granitic rock, for example, may contain a small percentage of iron, close to the average concentration of iron in Earth’s crust. § When an element is present in concentrations higher than the average, it means that the rock underwent some geologic process that concentrated larger quantities of that element than normal § concentration factor of an element in a mineral deposit is the ratio of the element’s abundance in the deposit to its average abundance in the crust. High concentrations of elements are found in a limited number of specific geologic settings. These settings are of economic interest because the higher the concentration of a resource in a given deposit, the lower the cost to recover it. § Ores: rich deposits of minerals from which valuable metals can be recovered profitably • The minerals containing these metals are referred to as ore minerals o include sulfides (the largest group), oxides, and silicate § compounds of metallic elements with sulfur, with oxygen, and with silicon and oxygen, respectively o Hydrothermal Deposits § Many of the most valuable ores are formed in regions of volcanism by the interaction of igneous processes with the hydrosphere § Very large ore deposits can be formed in such plate tectonic settings when hot water solutions—also known as hydrothermal solutions—are formed around bodies of molten rock • happens when circulating groundwater or seawater comes into contact with a magmatic intrusion, reacts with it, and carries off significant quantities of elements and ions released by the reaction o usually happens as the solution cools o VEINS § Hydrothermal solutions moving through rocks often deposit ore minerals • These fluids flow easily through fractures in the rocks, cooling rapidly in the process. o Quick cooling causes rapid precipitation of the ore minerals. § resulting tabular (sheetlike) deposits of precipitated minerals in the fractures are called veins. • Some ore minerals are found in veins; others are found in the rocks surrounding the veins, which are altered when the hydrothermal solutions heat and infiltrate those rocks • As the solutions react with the surrounding rocks, they may precipitate ore minerals together with quartz, calcite, or other common vein-filling minerals o Vein deposits are a major source of gold. • Hydrothermal vein deposits are among the most important sources of metallic ores. o Typically, metallic ores exist as sulfides, such as iron sulfide (pyrite), lead sulfide (galena), zinc sulfide (sphalerite), and mercury sulfide (cinnabar) • Hydrothermal solutions reach the surface as hot springs and geysers, many of which precipitate metallic ores— including ores of lead, zinc, and mercury—as they cool. o DISSEMINATED DEPOSITS § disseminated deposits: Deposits of ore minerals that are scattered through volumes of rock much larger than veins • In both igneous and sedimentary rocks, minerals are disseminated along abundant cracks and fractures o Among the economically important disseminated deposits are the copper deposits of Chile and the southwestern United States. § Some geologists speculate that the ores were deposited by groundwater that was driven out of the ancestral Appalachian Mountains when they were much higher • A continent-continent collision between North America and Africa may have created a continental- scale squeegee that pushed fluids from deep within the collision zone all the way into the continental interior of North America. • Groundwater may have penetrated hot crustal rocks at great depths and dissolved soluble ore minerals, then moved upward into the overlying sedimentary rocks, where it precipitated the minerals as fillings in cavities. In some cases, it appears that these solutions infiltrated limestone formations and dissolved some carbonates, then replaced the carbonates with equal volumes of sulfide crystals. • The major minerals of these deposits are lead sulfide (galena) and zinc sulfide (sphalerite). o Igneous Deposits § The most important deposits of ore minerals in igneous rocks are found as segregations of ore minerals near the bottoms of magmatic intrusions • form when minerals with relatively high melting temperatures crystallize from a body of cooling magma, settle, and accumulate at the base of the magma • Geologists believe that these sulfide deposits formed from the crystallization of a dense, sulfide-rich liquid that separated from the rest of a cooling magmatic intrusion and sank to the bottom before it congealed § As the magma in a large granite-forming intrusion cools, the last material to crystallize forms pegmatites: extremely coarse-grained rocks in which minerals present in only trace amounts in the magma are concentrated • Pegmatites may contain rare ore minerals rich in elements such as beryllium, boron, fluorine, lithium, niobium, and uranium, as well as gem minerals such as tourmaline. o Sedimentary Deposits § Sedimentary deposits include some of the world’s most valuable mineral sources § Many economically important minerals, such as copper, iron, and other metals, segregate as an ordinary result of sedimentary processes • deposits are chemically precipitated in sedimentary environments to which large quantities of metals are transported in solution. • Some of the important sedimentary copper ores, such as those of the Permian Kupferschiefer (German for “copper slate”) beds of Germany, may have precipitated from hydrothermal solutions rich in metal sulfides that interacted with sediments on the seafloor. • Here, rifting of the continental crust led to development of a deep trough, where sediments and ore minerals were deposited in a very still, narrow sea. § Many rich deposits of gold, diamonds, and other heavy minerals such as magnetite and chromite are found in placers: sedimentary ore deposits that have been concentrated by the mechanical sorting action of river currents • originate where uplifted rocks weather to form grains of sediment, which are then sorted by weight when currents of water flow over them o Because heavy minerals settle out of a current more quickly than lighter minerals such as quartz and feldspar, they tend to accumulate on streambeds and sandbars o ocean waves preferentially deposit heavy minerals on beaches or shallow offshore bars. • Some placers can be traced upstream to the location of the original mineral deposit, usually of igneous origin, from which the minerals were eroded o Erosion of the Mother Lode, an extensive gold- bearing vein system lying along the western flanks of the Sierra Nevada, produced the placers that were discovered in 1848 and led to the California gold rush o Placers also led to the discovery of the Kimberley diamond mines of South Africa two decades later.