INTRODUCTION AND LITERATURE REVIEW
1.0 General Introduction
Clay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried or fired. Clay deposits are mostly composed of clay minerals (phyllosilicate minerals) and variable amounts of water trapped in the mineral structure by polar attraction. Organic materials which do not impart plasticity may also be a part of clay deposits. Clay is a widely distributed, abundant mineral resource of major industrial importance for an enormous variety of uses (Ampian, 1985). In both value and amount of annual production, it is one of the leading minerals worldwide. In common with many geological terms, the term “clay” is ambiguous and has multiple meanings: a group of fine-grained minerals which show plasticity through a variable range of water content, and which can be hardened when dried or fired i.e., the clay minerals; a particle size (smaller than silt); and a type of rock i.e., a sedimentary deposit of fine-grained material usually composed largely of clay minerals (Patterson & Murray, 1983; Bates & Jackson, 1987). Clays find wide range of applications, in various areas of science, due to their natural abundance and the propensity with which they can be chemically and physically modified to suit practical technological needs (Xi et al., 2005).
Clays are distinguished from other fine-grained soils by various differences in composition. Silts, which are fine grained soils which do not include clay minerals tend to have large particle sizes than clays but there is some overlap in both particle size and other physical properties, and there are many naturally occurring deposits which include both silts and clays. The distinction between silts and clay varies by discipline.Geologists and soil scientists usually consider the separation to occur at a particle size of 2µm (clays being finer than silts), sedimentologists often use 4-5µm, and colloid chemists use 1um. Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, ISO 14688 grades; clay particles as being smaller than 0.063mm and silts one larger.
There are three or four main groups of clays; kaolinite, montmorillonite-smecite, illite and chlorite. Chlorites are not always considered clay, sometimes being classified as a separate group within the phyllosilicates. There are approximately thirty different types of “pure” clays in these categories but most “natural” clays are mixtures of these different types along with other weathered minerals (Lagaly, 1984).
1.1 Clay Minerals
Clay minerals likely are the most utilized minerals not just as the soils that grow plants for foods and garment, but a great range of applications, including oil absorbants, iron casting, animal feeds, pottery, china, pharmaceuticals, drilling fluids, waste water treatment, food preparation, paint e.t.c.
Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays form flat hexagonal sheets similar to the micas. Clay minerals are common weathering products (including weathering of feldspar) and low temperature hydrothermal alteration products. Clay minerals are very common in fine grained sedimentary rocks such as shale, mudstone, and siltstone and in fine grained metamorphic slate and phyllite. Clay minerals are usually (but not necessarily) ultrafine-grained (normally considered to be less than 2µm in size on standard particle size classifications) and so may require special analytical techniques for their identification/study. These include x-ray diffraction, electron diffraction methods, various spectroscopic methods such as Mössbauer spectroscopy, infrared spectroscopy, and SEM-EDX or automated mineralogy solutions. These methods can be enlarged by polarized light microscopy, a traditional technique establishing fundamental occurrences or petrologic relationships.
Clay minerals can be classified as 1:1 or 2:1clays; this originates from the fact that they are fundamentally built of tetrahedral silicate sheets and octahedral hydroxide sheets, as described in Figure 1 below. A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, for example, kaolinite and serpentine. A2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, for example, talc, vermiculite and montmorillonite.
Clay minerals include the following groups:
- Kaolin group which includes the minerals kaolinite, dickite, halloysite, and nacrite (polymorphs of Al2Si2O5(OH)4). Some sources include the kaolinite-serpentine group due to structural similarities (Bailey 1980).
- Smectite group which includes dioctahedral smectites such as montmorillonite and nontronite and trioctahedral smectites, for example,saponite.
- Illite group which includes the clay-micas. Illite is the only common mineral.
- Chlorite group includes a wide variety of similar minerals with considerable chemical variation.
Typically, the structural formula for kaolinite is Al4Si4O10(OH)8 and the theoretical chemical composition given in Table 1.