Question

A particular Index of Alteration (IA) is defined as the molar concentration ratio (expressed as weight percentage) of fluid immobile element(s) to fluid mobile element(s) and is expressed by \(100 \times \left(\frac{[Al_2O_3]}{[Al_2O_3] + [Na_2O] + [K_2O]}\right)\). For chemical weathering of silicate rocks, which one of the following statements is correct?

• A. High IA values (\(>\) 85) indicate intense chemical weathering
• B. Low IA values (\(>\) 15) indicate intense chemical weathering
• C. The IA values do not vary for silicate rocks
• D. The minimum IA value for an unweathered granite is 0

Hint:

The IA index is a useful indicator of weathering intensity, where high values indicate intense weathering and low values suggest minimal alteration.

Answer 1

The Correct Option is A

Step 1: Understanding the Index of Alteration (IA).
The IA value measures the relative concentration of immobile elements (like Al\(_2\)O\(_3\)) versus mobile elements (like Na\(_2\)O and K\(_2\)O) during the chemical weathering process. A high IA value suggests that immobile elements have been preferentially retained during weathering, indicating a high degree of chemical weathering.

Step 2: Evaluating the options.
Option (A) is correct because high IA values (greater than 85) are indicative of intense weathering, where a large proportion of mobile elements (Na\(_2\)O and K\(_2\)O) have been removed, and immobile elements (Al\(_2\)O\(_3\)) dominate.
Option (B) is incorrect because low IA values (< 15) would suggest little to no weathering, where the mobile elements are still abundant.
Option (C) is incorrect because IA values do vary significantly with the degree of weathering and the type of rock.
Option (D) is incorrect because even an unweathered granite would have an IA value greater than 0, given that some immobile elements are always present.