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Magma Differentiate magma based on it’s chemical composition felsic vs. mafic
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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!
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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 Liquid hot MAGMA Ca2+ Na+ Mg2+ Fe2+ Si4+ O2- rock Mg2+ Fe2+ cooling Mg2+
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Silicate structures: nesosilicates phyllosilicates sorosilicates
inosilicates cyclosilictaes tectosilicates
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Silicates are classified on the basis of Si-O polymerism
Mineral Structures Silicates are classified on the basis of Si-O polymerism [SiO4] Isolated tetrahedra Nesosilicates Examples: olivine garnet [Si2O7] Paired tetrahedra Sorosilicates Examples: lawsonite n[SiO3]2- n = 3, 4, Ring silicates Cyclosilicates Examples: benitoite BaTi[Si3O9] axinite Ca3Al2BO3[Si4O12]OH beryl Be3Al2[Si6O18]
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Chain Silicates – single and double
Mineral Structures Chain Silicates – single and double [SiO3] single chains Inosilicates [Si4O11] Double tetrahedra pryoxenes pyroxenoids amphiboles
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Sheet Silicates – aka Phyllosilicates
Mineral Structures Sheet Silicates – aka Phyllosilicates [Si2O5] Sheets of tetrahedra Phyllosilicates micas talc clay minerals serpentine
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Framework silicates – aka Tectosilicates
Mineral Structures Framework silicates – aka Tectosilicates low-quartz [SiO2] D frameworks of tetrahedra: fully polymerized Tectosilicates quartz feldspars feldspathoids zeolites
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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
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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
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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
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NASA News 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."
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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 re-equilibrate back, totallly cooled magma xstals show same composition Magma at composition X (30% Ca, 70% Na) cools first xstal bytownite X liquidus solidus
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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
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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
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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
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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 Need figure 5-11 or equivalent showing miscibility Y fats ice Miscibility Gap Z -20 10 30 50 70 90 Water Fat % fat in soup
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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 Miscibility Gap microcline orthoclase sanidine anorthoclase monalbite high albite low albite intermediate albite Orthoclase KAlSi3O8 Albite NaAlSi3O8 % NaAlSi3O8 Temperature (ºC) 300 900 700 500 1100 10 90 70 50 30 Need figure 5-11 or equivalent showing miscibility
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