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NASA News 03-15-06

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." . Tectosilicates.

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NASA News 03-15-06

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  1. 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."

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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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