Which clay mineral forms from weathering of feldspar in temperate environments?

• A. Kaolinite
• B. Illite
• C. Montmorillonite
• D. Chlorite

Answer: (C)

Montmorillonite is the best answer because the weathering of feldspar in temperate climates tends to produce smectite clay minerals when alteration is moderate and water is plentiful but leaching isn’t extreme. Montmorillonite is a 2:1 clay with expandable layers and a high cation-exchange capacity, formed when feldspar breaks down and exchangeable cations like Ca2+ or Na+ remain in the interlayers without being fixed as potassium. This combination—moderate weathering, adequate hydration, and available cations that don’t promote illitization—favors the formation of montmorillonite rather than the other clay types.

Kaolinite forms under intense leaching typical of tropical environments, illite forms when potassium released from feldspar becomes fixed in illite structures under cooler, less leached conditions, and chlorite develops under metamorphic or hydrothermal settings or from mafic mineral alteration, not from feldspar weathering in temperate soils.