Based on the figure, the anorthite content of first formed plagioclase crystals from a liquid composition having 50 mole % anorthite is ......... mole % (In integer).
Correct Answer: 74 mole \% Solution:
Show Hint
When analyzing phase diagrams, follow the lines to interpret compositional changes at different temperatures. The intersection points provide key insights into the crystallization process.
Step 1: Use the plagioclase loop.
The plagioclase phase diagram has a liquidus and a solidus. A melt cooling down first grows crystals that are richer in anorthite than the melt.
Step 2: Locate the starting melt.
We begin with a liquid of 50 mole percent anorthite and follow it up to the liquidus where the first crystals appear.
Step 3: Read across to the solidus.
Drawing a level line across at that temperature to the solidus gives the composition of the first solid. It is shifted toward the anorthite rich side.
Step 4: Read the value.
That first crystal composition is about 74 mole percent anorthite.
Step 5: State the answer.
So the first plagioclase has 74 mole percent anorthite.
\[ \boxed{74} \]
