Chapter 8: Major Elements

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(No Transcript)
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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."

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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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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 arent 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

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SOUP
X
Temperature (ºC)
50
Y
0
fats
ice
Miscibility Gap
Z
-20
10
70
30
90
50
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