Illite

Structure

Illite (or clay mica) has a 2:1 layer structure based on the primary mineral muscovite. It can be formed in soil by weathering of primary micas. Here, muscovite crystals are used to illustrate the properties of illite, the structure of which is less well-defined.

This is the structure of muscovite. Layer charge arising from isomorphous substitution is balanced by unhydrated interlayer K⁺ ions, shown as spheres. These bond the layers tightly together. In illite, some of the K⁺ ions have been replaced by hydrated exchangeable cations.

Why are illite layers negatively charged? Layer charge.

A large negative layer charge results from about one-sixth of the tetrahedral Si⁴⁺ ions being replaced by Al³⁺. This is balanced by the interlayer K⁺ ions. The true size of K⁺ ions is larger than shown. They fit snugly in the holes in the tetrahedral sheet.

Properties

Occurrence - Illite is mainly formed from parent materials containing micas (granite, shale, schist, and many other rocks). Being a 2:1 clay mineral, it is only stable in soils that are not strongly leached.

Illite is often the dominant clay mineral in the agricultural soils of southern Australia (e.g., red-brown earths).

Crystal size - Medium.

Cation exchange properties - Cation exchange properties - CEC = 0.3 moles charge per kg. Illite has a high level of isomorphous substitution, but non-exchangeable interlayer potassium ions balance most of the layer charge, resulting in only a moderate CEC.

Swelling when wet - Illite swells moderately. Some water penetrates between layers where interlayer potassium ions have been displaced, allowing swelling to take place. However, where the unhydrated interlayer K⁺ ions occur, they bond the layers tightly and allow no swelling.