clays

clays are one of mankind's oldest raw materials, used for making pottery since Neolithic times. The plasticity of clays enables them to be shaped; firing then converts them into rigid, impermeable articles which can be used in cooking and other purposes. Modern society uses a wide range of clays, ranging from common clays used for brick manufacture to speciality clays used to make high-grade paper and plastics. Clays are also important components of most natural soils.

In engineering or soil science a clay particle is usually defined as bring less than 2 or 4 microns (0.002 or 0.004 mm), although many geological formations described as clays contain a substantial proportion of particles larger than 4microns.

Sedimentary clays are common and are usually laid down in relatively still water: below the range of wave action or in lakes, for example. When buried by later sediments, clays can undergo changes in their mineralogy, a process known as diagenesis. Further increases in temperature and pressure will convert a clay into a shale and then a slate. Because of this, clay becomes progressively less common in older geological periods and is almost never found in Precambrian formations.

Clays are also be formed by hydrous fluids or gases altering harder rocks such as igneous rocks or slates. The altering fluids can be meteoric water derived from the surface (weathering), or hot water or gas associated with magmatic activity (hydrothermal alteration). Both processes usually involve the removal of silica and alkalis, and the addition of water.

The dominant minerals in a clay are usually layer silicates and fine silica (quartz), together with smaller amounts of iron sulphides and oxides, titanium minerals, various carbonates, and organic matter. The layer silicates (phyllosilicates) can be classified into various groups, according to their chemical composition and the layer structure of their crystal lattice. The layers are composed of various combinations of tetrahedral silica sheets and octahedral hydrated aluminium oxide sheets, frequently with appreciable amounts of potassium, calcium, magnesium, sodium, and iron. Some phyllosilicate particles are composite alternations of different clay minerals. When mica occurs in fine-grained form in a clay it is usually referred to as illite or clay mica and is similar in composition to muscovite, but there is usually some substitution of hydroxyl ions into the lattice. There are many other clay minerals, including chlorite, vermiculite, and various chain silicates such as attapulgite and pyrophyllite, some of which are valuable industrial materials.

Techniques commonly used to study clay minerals include X-ray diffraction for the crystal lattice structure, X-ray fluorescence spectrometry for chemical composition, and scanning electron microscopy for the morphology of clay particles.

The kaolin group of clay minerals includes kaolinite and halloysite, both of which are hydrated aluminium silicates. Kaolinite has a simple layer structure; a kaolinite layer is composed of one tetrahedral sheet combined with one octahedral sheet. Clays predominantly composed of kaolinite are commercially known as ‘kaolin’. The china clays of south-west England are ‘primary’ kaolins formed in situ from granites by a combination of hydrothermal and weathering action. Elsewhere in the world important deposits of sedimentary kaolin of late Cretaceous-early Tertiary age are found in Georgia, USA; others, of Neogene age, are in the Amazon Basin. The principal use for these kaolins is in the manufacture of paper, both as a filler and as a coating material to provide a receptive surface for printing ink. Other uses include ceramics, and as a filler in paint, rubber, and plastics.

Ball clays are sedimentary rocks, usually laid down in fresh water, which are composed of a special type of kaolinite known as b-axis disordered, together with clay mica (illite), which gives the clay good plasticity and strength, making it particularly suitable for ceramics. Fireclays have a similar mineral composition to ball clays, but are generally less pure and lack plasticity and strength. Large quantities of fireclay were formerly used in the iron and steel industry, but nowadays their main use is for making bricks and sanitary ware. Halloysite has the same basic layer configuration as kaolinite, but the layers are rolled up into scrolls. Deposits of halloysite are valued for use in high-quality ceramics, such as porcelain.

Another important group of layer silicates are the smectites, many of which have the property of absorbing water by expanding their layer structure. Their chemical composition is variable, owing to much substitution in the crystal lattice and to the presence of variable amounts of cations such as calcium and sodium between the layers. The purer deposits of smectite are known as bentonites and are valued for use in various applications such as drilling mud, iron ore pelletizing and foundry use, and in civil engineering, as well as for clarifying liquids used in the food and drink industry. Less pure deposits of smectite are known as fuller's earths and were originally used for removing oil and grease from wool and cloth. Nowadays their main application is as an absorbent (cat litter). Traditional sources of high-quality bentonite include Wyoming, USA and Mediterranean islands such as Milos. Fuller's earth deposits are widespread, but the fuller's earth near Bath, which gave rise to the term in the late eighteenth century, is no longer exploited.

Common clays are usually mixtures of clay minerals such as illite, smectite, and kaolinite, together with fine silica and other minor constituents. Interlayered clay minerals (see above) also frequently occur. Formations such as the Oxford Clay or the London Clay typify this type of clay. Although widespread, these clay formations are important economically, for they provide the basic material for brickmaking and for heavy clay products such as sewer pipes and clay floor tiles.

Colin Bristow