Montmorillonite

Structure

Montmorillonite is a common member of a group of 2:1 clay minerals called smectites.

There are no exposed OH groups on the outside of the crystal layers to form hydrogen bonds. However, stacks of layers still form, with weak van der Waals forces providing the bonding. Montmorillonite layers carry a negative charge.

Why are montmorillonite layers negatively charged? Layer charge.

In montmorillonite, about a quarter of the octahedral Al³⁺ ions are replaced with divalent cations, commonly Mg²⁺ (isomorphous substitution). This gives the layer a net negative charge which is balanced by exchangeable cations. Look at the Adsorbed Ions module for more explanation.

In this model, the Mg-centered octahedrons are shown as green and are randomly distributed in the octahedral sheet.

What happens when montmorillonite gets wet? Interlayer swelling.

The weak interlayer attraction allows water molecules to penetrate between the layers, hydrating the interlayer cations and expanding the crystal. The degree of expansion depends on the interlayer cation. Look at the Aggregation module in Soil Structure.

Properties

Occurrence - Montmorillonite is common in arid and semi-arid soils subject to weak leaching, especially in soils derived from basic rocks like basalt. Typically abundant in black earths and grey clays.

Crystal size - Small

Cation exchange properties - CEC = 1 mole charge per kg. Montmorillonite has a high permanent charge due to isomorphous substitution, and exchangeable cations can easily penetrate between crystal layers.

Swelling when wet - The degree of swelling is high. Soils with a high proportion of montmorillonite clay develop deep cracks as they dry.