Magma Differentiate magma based on it s chemical composition felsic vs mafic Melt Composition freezing T Liquid magma freezes into crystals the composition of what freezes first is governed by the melt s composition Analogous to the composition of seawater ice icebergs are composed of pure water pure water freezes first leaving the concentrated brine behind In magmas More silica lower T more Ca Mg higher T Silica polymerization also affected by T and how much Si there is Ca 2 O2Si4 O2 Mg2 Na Fe2 O Liquid hot O2MAGMA 2O O2 Si4 O2 O2O2 Si4 O2 Discontinous series Structures change harder to re equilibrate Continuous Series plag re equilibrates quicker and if not is a continuum in composition rather than a change in mineral as T decreases Mg2 2 cooling rock Mg2 Fe2 Silicate structures nesosilicates sorosilicates cyclosilictaes phyllosilicates inosilicates tectosilicates Mineral Structures Silicates are classified on the basis of Si O polymerism SiO4 4 Isolated tetrahedra Nesosilicates Examples olivine garnet Si2O7 6 Paired tetrahedra Sorosilicates Examples lawsonite n SiO3 2 n 3 4 6 Ring silicates Cyclosilicates Examples benitoite BaTi Si3O9 axinite Ca3Al2BO3 Si4O12 OH beryl Be3Al2 Si6O18 Mineral Structures Chain Silicates single and double SiO3 2pryoxenes single chains pyroxenoids Inosilicates Si4O11 4 Double tetrahedra amphiboles Mineral Structures Sheet Silicates aka Phyllosilicates Si2O5 2 Sheets of tetrahedra micas talc clay minerals serpentine Phyllosilicates Mineral Structures Framework silicates aka Tectosilicates low quartz SiO2 3 D frameworks of tetrahedra fully polymerized quartz feldspars feldspathoids zeolites Tectosilicates Characterizing minerals WITHIN classes like the silicate classes Minerals put into groups based on similar crystal structures differing typically in chemical substitution Groups usually named after principle mineral Feldspar group mica group feldspathoid group Sites designated M1 M2 etc designate spots where cations go into structure different site designations have different characteristics see different charge have different sizes etc and accommodate different ions based on this Tectosilicates Feldspars Substitute Al3 for Si4 allows Na or K to be added Albite Orthoclase Substitute two Al3 for Si4 allows Ca2 to be added Albite Anorthite Albite NaAlSi3O8 Equilibrium Need a description of a mineral s equilibrium with it s surroundings For igneous minerals this equilibrium is with the melt magma it forms from or is a representation of the Temperature and Pressure of formation NASA News 03 15 06 Scientists say the minerals found in Stardust aerogels include magnesium olivine forsterite In the coldest part of the solar system we have found samples that have formed at extremely high temperatures So the hottest samples in the coldest place Melt crystal equilibrium 1 When crystal comes out of melt some ions go in easier more Ca rich crystals form 1st Precipitated crystals react with cooling liquid eventually will reequilibrate back totallly cooled magma xstals show same composition Magma at composition X 30 Ca 70 Na cools first xstal bytownite X s u d i sol us d i liqu Melt crystal equilibrium 1 Magma at composition X 30 Ca 70 Na cools first crystal bytownite 73 Ca 27 Na This shifts the composition of the remaining melt such that it is more Na rich Y What would be the next crystal to precipitate Finally the last bit would crystallize from Z X Y Z Melt crystal equilibrium 1b Precipitated crystals react with cooling liquid eventually will re equilibrate back totally cooled magma xstals show same composition UNLESS it cools so quickly the xstal becomes zoned or the early precipitates are segregated and removed from contact with the bulk of the melt Why aren t all feldspars zoned Kinetics segregation IF there is sufficient time the crystals will reequilibrate with the magma they are in and reflect the total Na Ca content of the magma IF not then different minerals of different composition will be present in zoned plagioclase or segregated from each other physically Melt crystal equilibrium 2 miscibility 2 component mixing and separation chicken soup analogy cools and separates Fat and liquid can crystallize separately if cooled slowly Miscibility Gap no single phase is stable SOUP of X composition cooled in fridge Y vs freezer Z 100 SOUP Temperature C X 50 0 Y ice 20 10 Z fats Miscibility Gap 30 50 Water fat in soup 70 90 Fat Melt crystal equilibrium 2 miscibility 2 component mixing and separation chicken soup analogy cools and separates Fat and liquid can crystallize separately if cooled slowly Miscibility Gap no single mineral is stable in a composition range for x temperature monalbite anorthoclase 1100 Temperature C high albite 900 700 500 sanidine intermediate albite orthoclase low albite microcline Miscibility Gap 300 10 Orthoclase KAlSi3O8 30 50 NaAlSi3O8 70 90 Albite NaAlSi3O8
View Full Document
Unlocking...