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

This question checks which mineral does not come from chemical weathering. Chemical weathering changes primary minerals into new stable minerals at the surface, mainly through oxidation, hydrolysis, and leaching.

1. Limonite: forms when iron minerals like pyrite or magnetite oxidise and pick up water at the surface. This is a weathering product.
2. Kaolinite: forms when feldspar breaks down through hydrolysis in a humid climate. This is also a weathering product, a common clay mineral in soils.
3. Graphite: forms deep in the crust when carbon rich sediment is heated and squeezed during metamorphism, or crystallises from carbon bearing magmatic and hydrothermal fluids. No surface weathering reaction makes graphite.
4. Gibbsite: forms at the last stage of strong chemical weathering, once almost all silica and bases have leached out, leaving pure aluminium hydroxide behind, as seen in bauxite.

Three of the four minerals, limonite, kaolinite, and gibbsite, are all end products of chemical breakdown at the surface. Graphite alone needs heat and pressure at depth or magmatic carbon, not weathering.

Let's summarize:

• Chemical weathering makes clay minerals like kaolinite and oxide or hydroxide minerals like limonite and gibbsite.
• Graphite is a metamorphic or magmatic mineral, not a surface weathering product.

So the mineral that is NOT formed by chemical weathering is graphite.