Which one of the following represents the compositional change in plagioclase during the crystallization of mafic magma?
Show Hint
- Na/Ca ratio decreases; Al/Si ratio increases
- Both Na/Ca and Al/Si ratios increase
- Na/Ca ratio increases; Al/Si ratio decreases
- Both Na/Ca and Al/Si ratios decrease
The Correct Option is C
Solution and Explanation
Plagioclase forms a solid solution between albite (Na-rich) and anorthite (Ca-rich) end members. As mafic magma cools, the composition of plagioclase shifts from Ca-rich (anorthite) to Na-rich (albite).
Step 2: Compositional trends.
During this process, the Na/Ca ratio increases (more sodium-rich), while the Al/Si ratio decreases, since sodium-rich plagioclase contains relatively less aluminum compared to calcium-rich plagioclase.
Step 3: Conclusion.
Therefore, the correct trend is Na/Ca ratio increases and Al/Si ratio decreases.
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