Question

Compared to Fe, Mg, and Ca, the content of K is extremely low in igneous clinopyroxene. Which of the following CANNOT explain its low abundance?

• A. K\(^+\) has a larger ionic radius than Fe\(^{2+}\), Mg\(^{2+}\), and Ca\(^{2+}\)
• B. K is incompatible and hence, enriched in the continental crust
• C. K is fluid mobile and hence, easily leached out of clinopyroxene
• D. K has multiple oxidation states

Hint:

Understanding the ionic radii, fluid mobility, and behavior of elements in crystallization processes is key to explaining their presence or absence in minerals.

Answer 1

The Correct Option is B, C, D

Step 1: Understanding why K is low in clinopyroxene
Potassium (K) has a relatively low concentration in igneous clinopyroxene compared to other cations such as Fe, Mg, and Ca. Let's examine the reasons:

✅ Option (A) K\(^+\) has a larger ionic radius than Fe\(^{2+}\), Mg\(^{2+}\), and Ca\(^{2+}\):
This is a valid reason. Potassium (K\(^+\)) has a larger ionic radius compared to Fe\(^{2+}\), Mg\(^{2+}\), and Ca\(^{2+}\), making it less compatible in the clinopyroxene structure. Minerals like clinopyroxene prefer smaller cations to fit into their crystal structure, which is why K is present in low amounts.

❌ Option (B) K is incompatible and hence, enriched in the continental crust:
This statement is incorrect. While K is incompatible during crystallization, it does not become enriched in the continental crust. Instead, it remains in the melt during crystallization, meaning that it is less likely to be incorporated into clinopyroxene. This is one of the reasons for its low abundance in clinopyroxene.

✅ Option (C) K is fluid mobile and hence, easily leached out of clinopyroxene:
Potassium is mobile in fluids and can be leached from minerals like clinopyroxene over time, further reducing its concentration. This is a valid reason for the low abundance of K in clinopyroxene.

❌ Option (D) K has multiple oxidation states:
This statement is incorrect because potassium (K) typically exists in only one oxidation state, K\(^+\), and does not exhibit multiple oxidation states. This factor does not affect its low abundance in clinopyroxene.

Step 2: Conclusion
The correct answers are Options (A) and Option (C), as these factors contribute to the low abundance of K in clinopyroxene.