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
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
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
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 reequilibrate 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 (ºC)

X
50
Y
0
fats
ice
Z
-20
10
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 (ºC)

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