Solid Earth materials. Rocks and minerals

Transcription

1 Solid Earth materials Rocks and minerals Chapter 17 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 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) 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 Crystal structures

2 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. Isolated tetrahedrons 2. Chain silicates 3. Sheet silicates 4. Framework silicates No silicon-oxygen tetrahedrons in crystalline structure Make up remaining 8% of Earth s crust Eight groups of nonsilicates Nonsilicates 1. Carbonates Most abundant 2. Sulfates 3. Oxides 4. Sulfides 5. Halides 6. Phosphates 7. Hydroxides 8. Native elements 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

3 More physical properties Mineral forming processes 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 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 high temperatures Bowen s reaction series Silicate crystallization sequence Ferromagnetic silicates crystallize at higher temperatures Minerals crystallizing later are progressively richer in silicon 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

4 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) 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) Rocks Igneous rocks Aggregation of one or more minerals and perhaps other materials Minerals are physically combined to make rocks Most rocks are silicate minerals 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

5 Igneous rock classification Two factors 1. Mineral composition 2. Texture Nonferrous composition on left of figure Low in density Light in color Granite: most common example Ferromagnetic composition on right Greater density Darker color Example: basalt Sedimentary rocks Formed from particles or dissolved materials from previously existing rocks Sediments Accumulations of silt, sand or other materials that settled out of water Clastic sediments Accumulated from rocks at various stages of breaking down Chemical sediments Formed from dissolved rock materials Three sedimentation paths 1. Chemical precipitation from solution 2. Crystallization from evaporating water 3. Biological sediments Sedimentary rocks 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

6 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 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 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