Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : In a hand specimen quartz can be easily distinguished from orthoclase. Reason (R) : Orthoclase has lower refractive index than quartz. In the light of the above statements, choose the most appropriate answer from the options given below :
Evaluating Mineral Identification Statements
This question asks us to evaluate two statements about distinguishing quartz from orthoclase, a common task in mineralogy.
Analyzing Assertion (A): Distinguishing Quartz and Orthoclase
- Assertion (A) states that quartz can be easily distinguished from orthoclase in a hand specimen.
- This is correct. Key physical properties observable in hand specimens help differentiate them:
- Hardness: Quartz (Mohs 7) is significantly harder than orthoclase (Mohs 6).
- Cleavage: Quartz typically shows no distinct cleavage, breaking with a conchoidal fracture, whereas orthoclase exhibits two directions of cleavage at approximately 90 degrees.
- Other properties like crystal habit and specific gravity also differ but hardness and cleavage are often sufficient.
Analyzing Reason (R): Refractive Index Comparison
- Reason (R) states that orthoclase has a lower refractive index (RI) than quartz.
- This statement is also factually correct.
- The refractive indices for quartz range roughly from $n \approx 1.544$ to $1.553$.
- The refractive indices for orthoclase range roughly from $n \approx 1.518$ to $1.526$.
- Thus, orthoclase does indeed have a lower refractive index compared to quartz.
Connecting Assertion and Reason
- While both statements (A) and (R) are true, Reason (R) does not correctly explain Assertion (A).
- The reason quartz and orthoclase are *easily distinguished in a hand specimen* is primarily due to observable differences in hardness and cleavage, which do not require specialized equipment like refractometers.
- Although refractive index is a fundamental property used in mineral identification, it's not the main reason why these two minerals are *easily* separated just by looking at or feeling a hand specimen. The RI difference is subtle and usually requires specific testing methods not typically performed during basic hand specimen identification.
Therefore, both statements are correct, but the difference in refractive index is not the primary explanation for the ease of distinguishing these minerals in hand samples.
Final Answer Choice: Both (A) and (R) are correct but (R) is not the correct explanation of (A).
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