The earth is a large and complex object. It is differentiated according to the densities of the minerals which compose the planet. The density differences along with the abundances of the elements making up the earth ultimately lead to an accumulation of iron in the center (the core) of the earth, surrounded by a thick layer of silicate minerals relatively rich in iron and magnesium (the mantle), overlain by a thin layer of silicate minerals with less iron and magnesium and more silicon and aluminum (the crust).
All of these materials, if placed on a laboratory table would be identified by a competent geologist as some sort of igneous or metamorphic rock, that is the product of heat along with pressure. Rocks at the surface of the crust are modified by contact with a very corrosive chemical: water in equilibrium with dissolved carbon dioxide to form carbonic acid. This weak acid coupled with an active atmosphere and moderate temperatures leads to the chemical
and physical breakdown of a wide variety of rock types.
Thus, a mineral (part of a rock) that formed and was at equilibrium under conditions of elevated temperature and pressure is unstable at earth surface conditions. The acidic water, possibly aided by temperature fluctuations, attacks the mineral and a series of chemical reactions ensue producing a series of new minerals which are at equilibrium under the new conditions. If there is sufficient time, then ultimately the end of the search for thermodynamic equilibrium produces clay minerals with lesser quantities of other colloidal materials. This is the process known as weathering and is the major source of sediments and soils at the earth’s surface on the continents and blanketing much of the oceans. Since the clay minerals form at low temperatures (in a geological sense), reaction rates are slow and crystals of these new phases form slowly and imperfectly resulting in very small particle sizes, far smaller than would result from mechanical abrasion of larger crystals.
We share the surface of the earth with these sediments, sedimentary rocks, and soils and our lives benefit greatly from the existence of these fine-grained materials. If weathering did not take place, the continents would be barren and unproductive places. The clay minerals form something like one third of the sediments (clay rich muds and silts), sedimentary rocks (principally shale) and soils.
Clearly a material that is so common at the surface of the earth would be of geological importance. In addition, clay minerals are of considerable technological importance as, for example, raw materials for ceramic ware, additives to a range of products including paints, inks and rubber. At one time, clay minerals were widely used as catalysts, but much of this market has been taken over by zeolites. The fertility of soils is largely due to the presence of clay minerals which contribute an ability to retain water and to exchange a variety of cationic species. These topics are discussed in some detail in Chapters 3 and 8.
The utility of clay minerals is strongly linked to their interfacial properties, especially with water. Thus, the ability to fabricate a complexly shaped ceramic body depends on the ability of the fine-grained raw materials, clay minerals, quartz and feldspar principally, to be formed while wet into the desired shape and retain that shape while drying.
Without the plasticity of the wet clay, the shaping would not be possible. In fact, a clay is a clay because it is plastic when wet with an appropriate quantity of water. The plasticity is a result of complex interactions between the water and the surfaces of the constituents of the clay, principally the clay minerals.
The ability of clay minerals to participate in ion exchange reactions is a mechanism for the release of transition elements, alkali and alkaline earth metals to plant roots. Inorganic cations are not the only exchangeable entities; a wide variety of organic cations also undergo exchange leading to the conversion of the clay surface from neutral or hydrophilic to hydrophobic. A fine-grained hydrophobic material has very attractive properties as a barrier for contaminated soils or dump sites, in that it has a large surface area coupled with an attraction for hydrophobic organic compounds such as halogenated aromatic molecules. This is an area of intense experimental and commercial interest at present. Untreated clay minerals are frequently transported by rivers and streams. The transport is easy because of the small grain size of the clays, typically of the order of a few p,m or less so that verage-sized clay particles are suspended in stagnant water. However, the usual turbulence of natural flowing water can keep these particles in almost indefinite suspension. When conditions change, as will happen if the chemistry of the water is modified by, for example, the entry of river water into the ocean (which is richer in electrolyte) can cause the clay mineral particles to flocculate upon encountering the higher salt content of seawater. Again, this is the result of competing forces between the clay particles and water molecule interacting with clay particles, and the interactions of water molecules with each other. The traditional explanation of flocculation rests on the electrostatic interactions of clay particles as modified by the electrolyte.
The real situation is much more complex and more interesting and will be discussed later (Chapter 8).
Source:
Rossman F. Giese and Carel J. van Oss 2002 . Colloid and Surface Properties of Clays and Related Minerals.
