Question

Based on the given mineral proportions, which one of the following statements is CORRECT? 

Rock X: \(\text{Olivine : Orthopyroxene : Clinopyroxene :: 50 : 30 : 20}\) 

Rock Y: \(\text{Plagioclase : Alkali feldspar : Quartz :: 25 : 45 : 30} \)

Rock Z: \(\text{Biotite : Plagioclase : Alkali feldspar : Quartz :: 20 : 25 : 35 : 20}\)

• A. Y is more felsic compared to X & Z
• B. X is more felsic compared to Y & Z
• C. Z is more felsic compared to X & Y
• D. Y is the most felsic and Z is the most mafic

Hint:

When comparing felsic vs mafic, look at quartz and feldspar content: higher \(\Rightarrow\) more felsic; higher olivine/pyroxene \(\Rightarrow\) more mafic.

Answer 1

The Correct Option is A

Step 1: Recall mafic vs felsic classification. - Mafic rocks are rich in olivine, pyroxenes, amphiboles. - Felsic rocks are rich in quartz, alkali feldspar, plagioclase, and biotite.

Step 2: Analyze Rock X. Olivine + pyroxenes dominate (100% mafic minerals). \(\Rightarrow\) Rock X is ultramafic.

Step 3: Analyze Rock Y. Contains quartz (30%), alkali feldspar (45%), plagioclase (25%). \(\Rightarrow\) High quartz and feldspar content → typical granite-like (felsic).

Step 4: Analyze Rock Z. Contains biotite (20%), plagioclase (25%), alkali feldspar (35%), quartz (20%). \(\Rightarrow\) Intermediate composition (granitic but less quartz-rich than Y).

Step 5: Compare. - X → mafic. - Y → felsic (highest quartz + alkali feldspar). - Z → intermediate (felsic, but less than Y). Thus, Y is the most felsic among X, Y, Z.

Final Answer: \[ \boxed{\text{Y is more felsic compared to X \& Z}} \]