Carbonates solid solution

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Carbonates/ solid solution
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Carbonate phase diagram
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• Solid solution - compositional variation with
  swapping of cations in various sites
• CaCO3 (CaMg)CO3 MgCO3
• Not T-P phase diagrams but T-composition phase
  diagrams

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Carbonate solvus phase diagram

• Solvus
• Forbidden zone
• Exsolution - unmixing

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Solid solution phase diagram for
plagioclase/olivine.

• Liquidus line above which only liquid stable and
  along which melt and xtals coexist in equilibrium
• Solidus line below which all is solid

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• Bulk composition at a is 60 An (anorthite)
• Or 40 Al (albite)

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(No Transcript)
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At 1450oC, liquid d and plagioclase f coexist at
equilibrium
A continuous reaction of the type liquidB
solidC liquidD solidF
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The lever principle
Amount of liquid
ef

Amount of solid
de
where d the liquid composition, f the solid
composition and e the bulk composition
d
f
e
D
liquidus
de
ef
solidus
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When Xplag h, then Xplag Xbulk and, according
to the lever principle, the amount of liquid
0 Thus g is the composition of the last liquid
to crystallize at 1340oC for bulk X 0.60
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Final plagioclase to form is i when
0.60
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Note the following 1. The melt crystallized
over a T range of 135oC 2. The composition of
the liquid changed from b to g 3. The
composition of the solid changed from c to h
The actual temperatures and the range depend
on the bulk composition
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• Equilibrium melting is exactly the opposite
• Heat An60 and the first melt is g at An20 and
  1340oC
• Continue heating both melt and plagioclase
  change X
• Last plagioclase to melt is c (An87) at 1475oC

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Fractional crystallization Remove crystals
as they form so they cant undergo a
continuous reaction with the melt At any T Xbulk
Xliq due to the removal of the crystals
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How can you tell fractional vs. equilibrium
crystallization?

• Fractional crystallization leads to zoned minerals

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How can you tell fractional vs. equilibrium
crystallization?

• equilibrium crystallization leads to
  homogeneous/unzoned minerals

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2. The Olivine System
Fo - Fa (Mg2SiO4 - Fe2SiO4) also a solid-solution
series
Fig. 6-10. Isobaric T-X phase diagram at
atmospheric pressure After Bowen and Shairer
(1932), Amer. J. Sci. 5th Ser., 24, 177-213.
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Equilibrium Crystallization of the Plagioclase
Feldspars
1. Liquid of composition X (An61) cools to the
liquidus
2. Crystals of approximately An87 begin to form
3. Crystals have higher Ca/Na than liquid ppt of
crystals causes L composition to become more
sodic.
X
P1 atm
4. Ratio of Ca/Na in both crystals and liquid
decrease with decreasing temperature proportion
of crystals increases as liquid decreases
5. Crystals of An61 cool without further change
in composition
Composition of 1st crystal
Composition of last crystal
PowerPoint presentation by Kenneth E. Windom
Composition of last liquid