Geochemistry
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Understanding the Fosterite enstatite phase diagram
I’m having a bit of trouble understanding how a liquid at about 20% is going to travel along this phase diagram as it cools. I understand that we hit the liquidus line and then travel along that line to the eutectic where enstatite begins forming. Then I believe a chemical reaction happens and we travel horizontally to the left along the line at 1557 degrees Celsius. If the system is in equilibrium don’t we end up with a solid with a similar composition to the liquid, or not? Any help understanding this phase change would be greatly appreciated.
Submitted October 17, 2015 at 05:12AM by ToddGilbert
via reddit http://ift.tt/1kb3pKL
If we start at 20% SiO2 (the marker 20) we have a liquid that is a 50/50 mixture of forsterite and enstite. If we begin cooling that liquid, the first line hit is the liquidus. At the liquidus, solid forsterite begins forming. The liquid composition will move down the “liquid” line while forsterite will always have the pure forsterite composition.
As temperature decreases, more forsterite will crystallize until 1557 degrees C and the liquid composition will get more silica rich.
At 1557 degrees C, some enstatite must be made. However, you’ll note that the liquid composition is now more silica-rich than enstatite. The final mixture has to be the 50/50 mixture of enstatite and forsterite represented by the starting point, but the system has 60% forsterite and 40% high-SiO2 liquid. In other words, some of the forsterite needs to react back to make enstatite. At that point, the peritectic, a bit of forsterite back reacts and the rest of the liquid cools, giving a final mixture that is at the 20 point or is 50/50 enstatite/forsterite.
Understanding the Fosterite enstatite phase diagram
I’m having a bit of trouble understanding how a liquid at about 20% is going to travel along this phase diagram as it cools. I understand that we hit the liquidus line and then travel along that line to the eutectic where enstatite begins forming. Then I believe a chemical reaction happens and we travel horizontally to the left along the line at 1557 degrees Celsius. If the system is in equilibrium don’t we end up with a solid with a similar composition to the liquid, or not? Any help understanding this phase change would be greatly appreciated.
Submitted October 17, 2015 at 05:12AM by ToddGilbert
via reddit http://ift.tt/1kb3pKL
If we start at 20% SiO2 (the marker 20) we have a liquid that is a 50/50 mixture of forsterite and enstite. If we begin cooling that liquid, the first line hit is the liquidus. At the liquidus, solid forsterite begins forming. The liquid composition will move down the “liquid” line while forsterite will always have the pure forsterite composition.
As temperature decreases, more forsterite will crystallize until 1557 degrees C and the liquid composition will get more silica rich.
At 1557 degrees C, some enstatite must be made. However, you’ll note that the liquid composition is now more silica-rich than enstatite. The final mixture has to be the 50/50 mixture of enstatite and forsterite represented by the starting point, but the system has 60% forsterite and 40% high-SiO2 liquid. In other words, some of the forsterite needs to react back to make enstatite. At that point, the peritectic, a bit of forsterite back reacts and the rest of the liquid cools, giving a final mixture that is at the 20 point or is 50/50 enstatite/forsterite.
