Question

Which one of the following minerals is NOT formed by chemical weathering?

• A. Limonite
• B. Kaolinite
• C. Graphite
• D. Gibbsite

Hint:

Think about which mineral forms deep in the crust through metamorphism rather than at the surface through hydrolysis or oxidation.

Answer 1

The Correct Option is C

Step 1: Understanding the Question.
The question asks which mineral among the four options is NOT a product of chemical weathering. Chemical weathering breaks down primary rock forming minerals through hydrolysis, oxidation, hydration, and dissolution to form new, more stable secondary minerals.

Step 2: Key Concept.
Common secondary minerals produced by chemical weathering include clay minerals such as kaolinite, formed by hydrolysis of feldspar, and iron oxide-hydroxide minerals such as limonite, formed by oxidation of iron bearing minerals like pyrite and magnetite. Gibbsite, an aluminium hydroxide, is the end product of intense lateritic weathering, as in bauxite formation. Graphite, on the other hand, is a native carbon mineral that forms through metamorphism of carbon rich material or from magmatic and hydrothermal carbon, not through surface chemical weathering.

Step 3: Detailed Explanation.

(A) Limonite: This is a hydrated iron oxide formed when iron bearing minerals oxidise and hydrate near the surface, a classic chemical weathering product.

(B) Kaolinite: This clay mineral forms by hydrolysis of feldspar and other aluminosilicates under humid conditions, a well known chemical weathering product.

(C) Graphite: Graphite crystallises from carbon under high temperature and pressure during regional or contact metamorphism of organic rich sediments, or from magmatic and hydrothermal carbon sources. It does not form at the surface through hydrolysis or oxidation, so it is not a chemical weathering product.

(D) Gibbsite: This aluminium hydroxide forms in the final stage of intense chemical weathering, called laterisation, when almost all silica and bases are leached away, leaving an aluminium rich residue. It is a chemical weathering product.

Step 4: Final Answer.
Since limonite, kaolinite, and gibbsite are all secondary minerals produced by chemical weathering, while graphite forms through metamorphic or magmatic processes, the mineral that is NOT formed by chemical weathering is
\[ \boxed{\text{Graphite}} \]