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Magma

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

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Magma

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  1. Magma • Differentiate magma based on it’s chemical composition  felsic vs. mafic

  2. 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!

  3. Liquid hot MAGMA Mg2+ Na+ Ca2+ Fe2+ O2- O2- O2- O2- O2- O2- Si4+ O2- Si4+ O2- O2- 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 rock Mg2+ Fe2+ cooling Mg2+

  4. Silicate structures: nesosilicates phyllosilicates sorosilicates inosilicates cyclosilictaes tectosilicates

  5. Mineral Structures [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] Silicates are classified on the basis of Si-O polymerism

  6. Mineral Structures Chain Silicates – single and double [SiO3]2- single chains Inosilicates [Si4O11]4- Double tetrahedra pryoxenes pyroxenoids amphiboles

  7. Mineral Structures Sheet Silicates – aka Phyllosilicates [Si2O5]2- Sheets of tetrahedra Phyllosilicates micas talc clay minerals serpentine

  8. Mineral Structures Framework silicates – aka Tectosilicates low-quartz [SiO2] 3-D frameworks of tetrahedra: fully polymerized Tectosilicates quartz feldspars feldspathoids zeolites

  9. Characterizing minerals • WITHIN classes (like the silicate classes) Minerals put into groupsbased 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

  10. Tectosilicates Substitute Al3+ for Si4+ allows Na+ or K+ to be added Albite-Orthoclase Feldspars Substitute two Al3+ for Si4+ allows Ca2+ to be added Albite-Anorthite Albite: NaAlSi3O8

  11. 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

  12. 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."

  13. Melt-crystal equilibrium 1 liquidus X • 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 re-equilibrate back, totallly cooled magma xstals show same composition • Magma at composition X (30% Ca, 70% Na) cools  first xstal bytownite solidus

  14. Melt-crystal equilibrium 1 X • 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 Y Z

  15. 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

  16. Why aren’t all feldspars zoned? • Kinetics, segregation • IF there is sufficient time, the crystals will re-equilibrate 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

  17. 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 X Temperature (ºC) 50 Y 0 fats ice Miscibility Gap Z -20 10 70 30 50 90 Water Fat % fat in soup

  18. monalbite anorthoclase 1100 high albite sanidine 900 intermediate albite Temperature (ºC) 700 orthoclase low albite microcline 500 Miscibility Gap 300 10 70 30 50 90 Orthoclase KAlSi3O8 Albite NaAlSi3O8 % NaAlSi3O8 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

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