O 2 Si 4 Al 3 Fe 2 Ca 2 Na 1 K 1 1 40 0 26 0 63 1 00 0 99 1 37 0 72 Dynamic Earth Test One September 25th Minerals Eight most abundant elements in the Earth s Crust Abundance by weight Most common Ionic Form Most Common Coordination Numbers Relative Ionic Size Oxygen O 46 6 Silicon Si 27 7 Si 4 Aluminum Al 8 1 Al 4 or 6 0 39 Iron Fe 5 0 Calcium Ca 3 6 Sodium Na 2 8 Fe 6 Ca 8 Na 8 Potassium K 2 6 K 8 12 2 1 Mg 2 Magnesium Mg Distinction between crystalline and non crystalline solids o Crystalline term refers to the ordered symmetrical arrangement or the atoms that make up the structure Minerals are naturally occurring solid chemical compounds with crystalline structure Exhibits cleavage o Non Crystalline Ex Glass Soften as the temperature increases and have no sharply defined melting point The Most common silicate minerals Structure and Composition o Olivine Mg Fe 2SiO4 Independent Tetrahedral o Pyroxenes Mg Fe 2Si2O6 Single Chain o Amphiboles W X Al 7 8 Z4O11 2 OH 2 double chains o Biotite mica K MgFe 3 AlSi3O10 OH 2 Sheet silicates o Muscovite mica KAl2 AlSi3O10 OH 2 Sheet Silicates Dynamic Earth Test One September 25th o Plagioclase feldspar NaAlSi3O8 CaAl2Si2O8 solid solution series framework silicates o Alkali Feldspars KalSi3O8 CaAl2Si2O8 solid solution series Framework silicates o Quartz SiO2 Framework Silicates W represents the large cations Ca Na and K That can substitute for one another X Represents the smaller Mg and Fe Z Represents the cations in the tetrahedral sites Si and Al Pairs of elements that commonly substitute for one another in Silicates o Si and Al o Mg and Fe and also Al o Na and K o Na and Ca Factors that control substitution The size of the ions and the size of the crystallographic sites into which they substitute The charges on the ions that are substitution for one another if charges are the same then the crystal structure can remain electrically neutral if charges not the same then other substitutions must take place to maintain charge balance Temperature and pressure at which the substitution takes place Greater amount of substitution that takes place at higher temperature Pressure can change the size of both the site and the ion resulting in different substitutions Degree of substitution Relationship of Cleavage to Structure in Silicates o Cleavage is the breaking along a planar surface of a silicate and it forms along the planes of ionic bonds Definitions o Mafic silicate minerals magmas and rocks which are relatively high in the heavier elements and rich in magnesium and iron o Felsic silicate minerals magmas and rocks which have a lower percentage of the heavier elements and are correspondingly enriched in the lighter elements such as silicon and oxygen aluminum and potassium Dynamic Earth Test One September 25th Earth s Heat How is heat transferred Radiation conduction convection o Radiation heat moves as electromagnetic radiation such as heat transferred from the Sun to the Earth o Conduction enhanced vibrational motion of atoms in materials is induced in neighboring atoms and this motion diffuses through the material o Convection Heat is carried by matter which is flowing Warmer and less dense matter rises while cooler and more dense matter sinks Original Source of Heat o During the early history of the solar system the Earth and other planets grew in size and mass as comets asteroids and other smaller masses fell into them heating up the earth o Today meteorites that reach Earth have velocities of approximately 30 70 km sec the large ones containing a large amount of kinetic energy that is converted to thermal energy and the planet heats up a bit Definitions o Geothermal gradient the rate at which the temperature increases with depth in the Earth varies from place to place o Geo Barometric gradient The change in atmospheric pressure per unit of horizontal distance in the direction in which the pressure changes most rapidly Magmas Melting of Silicate Rocks o Magmas from by the melting of pre existing silicate Major mechanisms that can cause a rock to melt are heating pressure decrease addition of water How composition of magmas are influenced by the degree of partial melting of source rock o Partial melting is when only a portion of the parent rock melts always produces a magma that is less mafic than the parent rock way of differentiating material making new rocks that differ in composition from the parent rock Role of pressure and volatiles water and gases on melting Dynamic Earth Test One September 25th o Pressure Temperature required to melt minerals increases with increasing pressure so that minerals which are heated to temperatures sufficient to melt them at atmospheric pressure can remain sold under the high pressure in the Earth Some rock s in the Earth s interior that are solid are so hot that if the pressure on these were released or they are convected into a lower pressure zone they could begin to melt o Volatiles The melting temperatures of minerals are reduced under high water pressure Consequently wet rocks rocks containing water melt at lower temperatures that do dry rock containing identical mineral assemblages Factors influencing the viscosity of magmas o Higher temperatures cause atoms to spread out and decrease in density so magma at higher temperature is less viscous and will flow more easily o Dissolved gases make the magma more fluid and buoyant The higher the temperature the more the gases remain dissolved and the less viscous the magma o The amount of silica in the magma Silica has close jointing and packing of atoms and makes flow slow and it is more difficult for gas to escape Crystallization of magmas o Mineral sequence with decreasing temperature If as magma is cooling the crystallizing minerals are removed then these minerals will deplete the system in those elements that are concentrated in mafic minerals thus becoming more felsic in composition o Equilibrium vs fractional crystallization Slow cooling allows previously formed crystals to react with melt to bring them into equilibrium as melt composition changes Rapid cooling crystal settling or mineral overgrowth inhibiting diffusion prevents melt reacting with crystals Where do magmas originate and how to they rise to shallow crustal levels o Magmas originate in the lower park of the Earth s crust and in the upper portion of the mantle There high temperatures and pressure cause some rocks to melt and form magma Dynamic Earth Test One September 25th Igneous
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