Which one of the following is a 2:1 non-expanding clay mineral?

Correct option is D

The correct answer is (D) Illite

Explanation:
• Clay minerals are classified by the ratio of tetrahedral (silica) sheets to octahedral (alumina) sheets. A 2:1 mineral has two tetrahedral sheets sandwiching one octahedral sheet.
• Illite is a 2:1 mineral, but it is non-expanding. This is because the interlayer space is occupied by Potassium ($K^+$) ions, which fit perfectly into the hexagonal holes of the silica sheets, acting as a "strong bridge" that prevents water molecules from entering and expanding the layers.
• Montmorillonite (C) and Vermiculite (B) are also 2:1 minerals, but they are expanding because their interlayer forces are weak, allowing water to enter.
• Kaolinite (A) is a 1:1 mineral (one silica sheet, one alumina sheet) and is non-expanding due to strong hydrogen bonding between layers.

Information Booster:
• Cation Exchange Capacity (CEC) order: Vermiculite > Montmorillonite > Illite > Kaolinite.
• Illite is common in the alluvial soils of North India.
• Montmorillonite is the dominant mineral in Black (Regur) soils.

Additional Knowledge:
• Isomorphous Substitution: The process where one ion is replaced by another of similar size but different charge (e.g., $Al^{3+}$ for $Si^{4+}$), creating the negative charge of clay minerals.
• Chlorite: Sometimes called a 2:1:1 mineral because it has an extra magnesium hydroxide (brucite) layer between the 2:1 units.