Image credit: Josh Lory
Clay Minerals and Related Properties
There are many different types of clay minerals, each with unique chemical and physical properties which arise from the structure. But nearly all clays contain just two basic components which occur in different arrangements. These two basic building blocks of all clay minerals are the silica tetrahedron and the aluminum octahedron.
When scientists talk about ‘1:1’ or ‘2:1’ clays, they refer to the ratio of silica tetrahedron sheets to aluminum octahedron sheets. A 1:1 clay has one of each sheet (think of an biscuit with icing on top); 2:1 clays have two tetrahedral sheets on either side of an aluminum octahedron sheet, (like an Oreo biscuit). These tetrahedral and octahedral sheets are variously arranged and modified during mineral formation to create different types of clay minerals.
Examples of Clay Minerals
Kaolinite is an example of a 1:1 clay mineral. It does not shrink when dry or swell when wet. Sheets of kaolinite are arranged like pages in a book; this affects the amount of surface area available for holding water or cations like Ca2+ or K+. Imagine a closed book with many pages; each page has surface area on the front and back, which could be a lot of surface area. But if the book is closed the pages are tightly packed so the surface area is not available. The only practical surface for water or cations to attach (or “adsorb”) would be the outside of the book. Because of this arrangement, kaolinite has less external surface area than other clay minerals, with no internal surface area, and hence less capacity for holding water and cations.
The 2:1 clay minerals look much different; Montmorillonite (one of the smectite clay minerals) looks like a sponge. The Figure above shows the differences in spacing between the 2:1 sheets. The larger interlayer spaces have the capacity to hold water molecules and a variety of cations (some of which cause dispersion, e.g. Sodium) with important consequences for plant growth. Also, with larger interlayer spaces comes a greater tendency for shrink/swell behaviour (note: not all 2:1 clays are expanding).
Image credit: The Cooperative Soil Survey