Question

Given below are two statements :
Statement (I) : Olivine is least stable.
Statement (II) : Has independent silica tetrahedron and held together by forming bonds with hydolysable Magnesium or oxidisable Iron.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both Statement (I) and Statement (II) are correct.
• B. Both Statement (I) and Statement (II) are incorrect.
• C. Statement (I) is correct but Statement (II) is incorrect.
• D. Statement (I) is incorrect but Statement (II) is correct.

Hint:

Olivine is a nesosilicate with independent $[SiO_4]^{4-}$ tetrahedra. Because these tetrahedra are not polymerized (shared) and are held together only by vulnerable $Fe^{2+}$ and $Mg^{2+}$ ionic bonds, olivine has the lowest chemical stability and weathers first.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Evaluating mineral stability requires analyzing their chemical formulas, crystalline structures, and susceptibility to environmental weathering agents like water, oxygen, and carbonic acid.

Step 3: Detailed Explanation:
Let us analyze both statements individually:
- Statement (I) is correct: According to Goldich's Weathering Series, olivine is the least stable (most easily weathered) mineral among the primary rock-forming silicates.
It crystallizes at very high temperatures and pressures deep in the Earth's crust and is highly unstable under surface atmospheric conditions.
- Statement (II) is correct: Structurally, olivine is a nesosilicate with a chemical formula of $(\text{Mg, Fe})_2\text{SiO}_4$.
It consists of independent, individual silica tetrahedra ($SiO_4^{4-}$) that do not share oxygen atoms with each other.
These independent tetrahedra are held together in the crystal lattice purely by ionic bonds with divalent cations, namely hydrolyzable magnesium ($Mg^{2+}$) and easily oxidizable ferrous iron ($Fe^{2+}$).
Because these ionic linkages are highly vulnerable to chemical weathering processes (such as hydration, hydrolysis, and oxidation of $Fe^{2+}$ to $Fe^{3+}$), the crystal structure collapses rapidly when exposed to water and oxygen.
Therefore, both Statement (I) and Statement (II) are correct.
This corresponds to Option (A).

Step 4: Final Answer:
Both Statement (I) and Statement (II) are correct.