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Solid Earth materials Rocks and minerals Chapter 17 Minerals • Earth science definition: a naturally occurring, inorganic solid element or compound with a crystalline structure – Cannot be synthetic – Not directly produced by a living organism – Must have regular, repeating pattern • Example: halite (NaCl) • Nonuniform distribution of matter • Molten core – Contains most heavy elements – Iron, nickel • Thin surface crust – Mostly lighter elements – 8 elements make up 98.6% of crust – Rocks and minerals make up solid crust materials Crystal structures • Can be made up of one or more kinds of element – Diamond - carbon only – Quartz - silicon and oxygen • Classification – Based on surface symmetries – Six major systems: isometric, hexagonal, tetragonal, orthorhombic, monoclinic and triclinic Silicates and nonsilicates Silicates • • • • Contain mostly silicon and oxygen Make up 92% of Earth’s crust Based on silicon-oxygen tetrahedral unit Four major arrangements 1. 2. 3. 4. Nonsilicates • No silicon-oxygen tetrahedrons in crystalline structure • Make up remaining 8% of Earth’s crust • Eight groups of nonsilicates 1. Carbonates – 2. 3. 4. 5. 6. 7. 8. Most abundant Sulfates Oxides Sulfides Halides Phosphates Hydroxides Native elements Isolated tetrahedrons Chain silicates Sheet silicates Framework silicates Physical properties of minerals • Color – Not very useful – Can be influenced by trace impurities • Streak – Color of the mineral when finely powdered – More consistent than color • Hardness – Resistance to scratching – Mohs hardness scale • Uses ten test minerals with increasing hardness More physical properties • Crystal form – Related to the internal geometric arrangement of atoms – Six basic groups already mentioned • Cleavage – Tendency of minerals to break along smooth planes • Fracture – Irregularity in broken surfaces • Luster – Surface sheen – Types include metallic, pearly and vitreous • Density – Mass to volume ratio – Alternative: specific gravity (ratio of mineral density to that of water) – Two factors: kinds of atoms making up the mineral; packing of the atoms into the crystal lattice Minerals formed at high temperatures • Bowen’s reaction series – Silicate crystallization sequence – Ferromagnetic silicates crystallize at higher temperatures – Minerals crystallizing later are progressively richer in silicon Mineral forming processes Formation in two liquid environments most common 1. Water solutions – Crystals form from highly concentrated ions 2. Magma – – – Molten rock from which minerals can crystallize Can happen below or above Earth’s surface Lava: magma forced out to the Earth’s surface • Important factors – – – – Temperature Pressure Time Availability and concentration of ions in solution • Glass – Rapidly cooled solid lacking a crystalline structure Minerals formed at normal temperatures • Formed at the Earth’s surface in contact with oxygen, carbon dioxide and water • Carbonates, sulfates, oxides, halides and sulfides • Except for oxides, have substantially lower hardnesses and specific gravities than minerals formed at high temperatures Altered minerals • Exposure to different environmental pressure, temperature or chemical solutions can change minerals into new ones • Generally occurs above 150ºC and 2000 atmospheres of pressure • Examples: garnet, epidote, talc, graphite and serpentine (asbestos source) Rocks • Aggregation of one or more minerals and perhaps other materials • Minerals are physically combined to make rocks • Most rocks are silicate minerals • Ore minerals • • • • Mineral deposits with economic value Often found in veins Thin, flat bodies of mineral material Left over from crystallizing magma and flushed away in hot water solutions • Examples: pyrite (iron sulfide,“fool’s gold”), calcopyrite (sulfide of copper and iron) Igneous rocks Classification scheme – Based on how rocks were formed – Three main groups 1. Igneous 2. Sedimentary 3. Metamorphic • Formed from magma above or below Earth’s surface • All rocks were at one time igneous rocks • Cooling rate determines the texture • Intrusive igneous rocks – Formed beneath surface – Slow cooling produces large crystals – Coarse grained • Extrusive igneous rocks – Formed from lava on surface – Rapid cooling produces small crystals Igneous rock classification • Two factors 1. 2. • Nonferrous composition on left of figure – – – • Mineral composition Texture – Accumulated from rocks at various stages of breaking down Low in density Light in color Granite: most common example Greater density Darker color Example: basalt Sedimentary rocks • – Accumulations of silt, sand or other materials that settled out of water • Clastic sediments Ferromagnetic composition on right – – – • • Sedimentary rocks • Formed from particles or dissolved materials from previously existing rocks • Sediments Chemical sediments Formed from dissolved rock materials Three sedimentation paths 1. Chemical precipitation from solution 2. Crystallization from evaporating water 3. Biological sediments Lithification • • The rock-forming process Two main parts 1. Compaction • • Reduces thickness of deposit Squeezes out water 2. Cementation • • Spaces between sediment particles filled with chemical deposit Chemical deposit binds particles together Metamorphic rocks • Previously existing rocks changed by heat, pressure or hot solutions into distinctly different rock • Causes associated with geologic events – Movement of the crust – Heating and hot solutions from magma intrusion – Temperatures must be high enough to cause recrystallization, but not melting The rock cycle • Rocks transformed into new types by Earths’ interior and exterior dynamical processes – Moving continents – Seas advance and retreat – Weathered and eroded by wind and rain Classification of metamorphic rocks • Foliation – Alignment of flat crystal flakes into sheets – Caused by pressure on parent rocks – Rock cleaves along planes between aligned grains • Nonfoliated – Parent rocks consist mainly of one mineral – Grains not aligned into sheets