Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).

Assertion (A) : Free silica occurs in soil s quartz and opal.
Reason (R) : Quartz consists of a continuous framework of silica tetrahedra.

In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Quartz is highly resistant to chemical weathering, making it the most dominant mineral in the sand and silt fractions of highly weathered soils.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Silica (\(\text{SiO}_2\)) is a major mineral constituent of the earth's crust and soils, occurring in both highly ordered crystalline forms and disordered amorphous forms.

Step 2: Detailed Explanation:
Assertion (A) is correct because free silica (silicon dioxide that is not bound to other metals) occurs in soils as crystalline quartz and as amorphous hydrated silica, known as opal.
Reason (R) is correct because quartz is chemically and structurally defined as a tectosilicate, composed of a continuous 3D network of silicon-oxygen tetrahedra where every oxygen atom is shared between adjacent tetrahedra.
While both statements are scientifically true, the 3D tetrahedral framework of quartz (R) does not explain why free silica exists in both quartz and opal forms (A) in soils.
Thus, (R) is not the correct explanation of (A).

Step 3: Final Answer:
Both (A) and (R) are true, but (R) is NOT the correct explanation of (A).