Question

The following reaction takes place in the amphibolite grade of metamorphism of pelitic rocks:
\[ \text{kyanite + chlorite} \leftrightarrow \text{staurolite + quartz + H}_2\text{O} \] Which of the following is a CORRECT statement on this reaction?

• A. The reaction can be represented as a sharp univariant boundary.
• B. Initially chlorite and staurolite are Fe-rich and will gradually become Mg-rich with increasing temperature.
• C. With increasing temperature chlorite becomes Mg-rich and staurolite becomes Fe-rich.
• D. The reaction is independent of fugacity of H\(_2\)O.

Hint:

In metamorphic reactions, mineral compositions change with temperature and pressure, and the balance between Fe and Mg in minerals is important in determining the type of rock.

Answer 1

The Correct Option is B

The reaction provided is a metamorphic equilibrium reaction between kyanite, chlorite, staurolite, quartz, and water. The reaction occurs during the amphibolite facies metamorphism of pelitic rocks, and the mineral compositions change with temperature.

Step 1: Evaluate Option (A).
The reaction is not a sharp univariant boundary, as it represents a reaction between two minerals that occurs over a range of temperatures, not at a single point. Therefore, Option (A) is incorrect.

Step 2: Evaluate Option (B).
Initially chlorite and staurolite are Fe-rich at lower temperatures. As the temperature increases, they gradually become Mg-rich, which is typical of progressive metamorphism. This statement is correct. Option (B) is the correct answer.

Step 3: Evaluate Option (C).
Chlorite becomes Mg-rich and staurolite becomes Fe-rich is incorrect. The opposite is true: chlorite becomes Mg-rich, and staurolite becomes Fe-rich with increasing temperature.

Step 4: Evaluate Option (D).
The reaction is not independent of the fugacity of H\(_2\)O, as water plays a crucial role in driving metamorphic reactions. The presence of water is essential to the reaction's progress. Therefore, Option (D) is incorrect.

Step 5: Conclusion.
Thus, the correct answer is (B).