What is clay?
Since the earliest times, humankind has had a close association with clay. From use as a building material, in pottery, for treating human digestive ailments to a multitude of industrial uses, clay is a key ingredient in the material world we live in.
Kaolinite mineral structure
The basic unit of kaolinite is composed of a 2D layer of silicate groups tightly bonded to a 2D layer of aluminate groups.
Commercially, the most important clays are known as kaolin and bentonite
Where does clay come from?
Clay is a soft, loose, earthy material containing particles with a grain size of less than 4 micrometres (μm). It forms as a result of the weathering and erosion of rocks containing the mineral group feldspar (known as the ‘mother of clay’) over vast spans of time.
During weathering, the feldspar content is altered by hydrolysis (reaction with water) to form clay minerals such as kaolinites (the principal minerals in kaolin clays) and smectites (the principal minerals in bentonite clays).
Clay minerals
A mineral is a naturally occurring crystalline material that has a specific or limited range of chemical compositions.
Kaolinite crystals
Kaolinite is the principal mineral present in kaolin clays. It has a flat plate-like structure. This scanning electron microscope image of a sample of kaolinite at a magnification of 1,500 times clearly shows this.
Clay minerals have a sheet-like structure and are composed of mainly tetrahedrally arranged silicate and octahedrally arranged aluminate groups.
Kaolinite is the principal mineral in kaolin clays. It is a 1:1 clay mineral – the basic unit is composed of a 2-dimensional (2D) layer of silicate groups tightly bonded to a 2D layer of aluminate groups.
Throughout the mineral, there is a tetrahedraloctahedral (TO) layered structure with tight packing between the layers. This tight packing – like the pages of a closed book – results in kaolinite not shrinking when dry or swelling when wet.
Smectite mineral structure
Smectite is made up of bonded sheets of silicate and aluminate groups. The arrangement is known as TOT. Water molecules and cations invade the space between the TOT layers.
Smectite minerals are found in bentonite clays. Unlike kaolinite with its TO sheet arrangement, these minerals have a tetrahedral/octahedral/tetrahedral (TOT) sheet structure.
This results in a TOT TOT TOT TOT arrangement, with space between each TOT unit.
Water can invade the space between the layers, so bentonite clays swell when wet and shrink when dry.
New Zealand clay deposits
Deposits of clay are commonly found in New Zealand. The Matauri Bay (upper Northland) deposit produces high purity kaolin clay rich in the clay mineral known as halloysite. It is exported to over 20 countries for the manufacture of high-quality ceramics such as porcelain and fine bone china. The unique and exceptionally white primary clay deposits (reputably the whitest clay in the world) were formed from the alteration of volcanic rocks.
New Zealand clay deposits
This map shows the main deposits of clay found in New Zealand. The Matauri Bay deposit of a kaolin clay known as halloysite has a worldwide reputation for its purity and its whiteness.
The country’s largest bentonite clay quarry is in the Harper Hills near Christchurch. The quarry is worked in the dry summer months, and the clay is processed at the nearby township of Coalgate.
The processed bentonite clay is used in paper-making, for stabilising drill holes during drilling, in agriculture as a growing medium and stock food additive and in a range of geotechnical and environmental applications. It is being increasingly used in water treatment, where it helps to remove suspended silt that discolours water, and in wastewater management.
Nature of Science
In order to better understand the fine structure of clay minerals, soil scientists are increasingly dependent on high-tech instruments such as the scanning electron microscope and X-ray diffraction analysis equipment. Technical support is an essential part of science.
