TEST 1 MINERALS eight most abundant elements in the Earth s crust O Si Al Fe Ca Na K Mg their most common ionic form Si 4 Al 3 Fe 2 Mg 2 Ca 2 Na 1 K 1 O 2 relative ionic sizes O 1 4 Si 0 26 Al 0 39 Fe 0 63 Ca 1 0 Na 0 99 K 1 37 Mg 0 72 and the most common coordination numbers of these ions Si 4 Al 4 or 6 Fe 6 Mg 6 Ca 8 Na 8 K 8 12 the distinction between crystalline and non crystalline solids The term crystalline refers to the ordered symetrical arrangement of the atoms that make up the structure the most common silicate minerals olivine pyroxenes amphiboles biotite muscovite plagioclase feldspar alkali feldspars quartz their structure and composition note while exact chemical formulae are not required knowledge of their relative silica contents and other major cations present is expected olivines Mg Fe 2SiO4 independent tetrahedra pyroxenes Mg Fe 2Si2O6 single chain amphiboles W X Al 7 8 Z4O11 2 OH 2 double chains biotite mica K Mg Fe 3 AlSi3O10 OH 2 sheet silicates muscovite mica KAl2 AlSi3O10 OH 2 sheet silicates alkali feldspars KAlSi3O8 NaAlSi3O8 solid solution series framework plagioclase feldspars NaAlSi3O8 CaAl2Si2O8 solid solution series silicates framework silicates quartz SiO2 framework silicates atomic or ionic substitution in silicates factors controlling substitution commonly substituting ion pairs and degree of substitution Ionic Substitution variations in composition resulting from a systematic substitution of ions There are several minerals which display solid solution solids which act like solutions during crystallization and melting The olivine group forsterite Mg2SiO4 fayalite Fe2SiO4 is a good example Both Mg 2 and Fe 2have the same charge 2 and about the same ionic size so that either can fit into the olivine crystalline structure Coupled Ion Substitution the plagioclase series NaAlSi3O8 to CaAl2Si2O8 is an example of a solid solution with coupled ion substitution The ions don t have the same charge but are able to substitute for one another when coupled with another ion Within the plagioclase crystal structure Ca 2and Al 3 with a combined charge of 5 substitute for Na 1 and Si 4 which also have a combined charge of 5 Variations in Composition and Crystalline Structure Most minerals contain impurities and several also display ionic substitution There are other minerals which have identical chemical compositions but different crystalline structures due to the conditions under which they crystallized relationship of cleavage to structure in silicate minerals Cleavage tendency of minerals to break along parallel planes of weaknesses cleavage planes within the crystal forming parallel planar surfaces along broken fragments Cleavage results from weaker bond strengths along the certain planar directions within the mineral The number of cleavage planes and the angles between the cleavage planes are important characteristics used in identifying minerals example micas excellent cleavage in 1 direction halite good cleavage in three directions each at 90o to each other and sphalerite 6 good directions not at 90o Difference between cleavage and crystal form crystal form is only an external reflection of atomic structure which is lost when the crystal is broken In contrast cleavage is related to planes of weakness which are found throughout the mineral Cleavage planes will form no matter how finely the crystal is broken meaning of mafic felsic Mafic mineral are relatively rich in Mg and Fe and relatively poor in silica and the alkali K and Na elements Felsic minerals have relatively high silica and alkali contents and lower Mg and Fe EARTH S HEAT how heat is transfered radiation conduction convection radiation heat moves as electromagnetic radiation such as heat transfered from the Sun to Earth conduction enhanced vibrational motion of atoms in materials is induced in neighboring atoms and this motion diffuses through the material if you could actually see the atoms a very crude analogy might be a crowd in a football stadium doing the Wave convection heat is carried by matter which is flowing Warmer and less dense matter rises while cooler and more dense matter sinks sources or origin of the Earth s heat 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 This process continues today albeit at a much slower rate Under the intense bombardment of the Earth during its earliest history the planet s temperature must have risen significantly perhaps to the point where some part of the planet melted Since that early history the Earth has been cooling but some of that original heat remains the geothermal gradient and geobarometric gradient The rate at which the temperature increases with depth in the Earth is the geothermal gradient and it varies from place to place The geobarometric gradient is the change in pressure with depth MAGMAS melting of silicate rocks effects of temperature pressure and water in general the 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 solid under the high pressure in the Earth It follows that some rocks 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 Another important factor in the melting and crystallization of magma is the presence of volatiles especially water The melting crystallization temperatures of minerals are reduced under high water pressure Consequently wet rocks those containg water melt at lower temperatures that do dry rock containing identical mineral assemblages how composition of magmas are influenced by the degree of partial melting of source rock Partial melting of rocks produce magmas that are more felsic than the rocks that are melted factors influencing the viscosity of magmas The viscosity a measure of a fluid s resistance to flowing of magmas is due to the polymerization of silicon and oxygen in the magma that is to the chemical bonding of Si O Si O Si ect into unordered chains and fragments of various shapes The higher the silica content the higher the viscosity Recall that mafic magmas such as those that crystallize basalts have a significantly lower silica content than do the more silica rich felsic magmas such as those that crystallize rhyolites and granites Another
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