Thematic Articles

The use of kaolins in health has its origins in prehistoric times. Humans and other animals consume kaolin for gastrointestinal ailments, digestive enhancement, and possibly nutritional supplementation. Kaolins are effective as hemostatic wound dressings, because they can clot blood from traumatic injury, with little damage to tissue. Various forms of kaolin have been shown to be antibacterial, and increasingly kaolins are being utilized in drug delivery. While nanoparticles of kaolin can have deleterious effects on human tissues, modern understanding of the mineralogy of kaolins and their interactions with human cells allows many health applications, reaching far beyond the prehistoric “first aid” uses.

Kaolinite is one of the most important industrial clay minerals, with a worldwide consumption in the millions of tons per year and applications in a wide range of industrial areas. Traditionally, its most important use has been in the paper and ceramic industries. New, innovative techniques are being developed that are based on intercalation and grafting of kaolinite’s unique dipolar layer structure. Such techniques are leading the way to the synthesis of kaolinite–polymer nanocomposites, including bionanocomposites that might have value-added properties benefiting industry and the health sciences.

The interactions of ions, organics, and microorganisms at the aqueous interface with kaolin-group minerals control many important geochemical processes in the environment. Kaolinite has both hydrophilic and relatively hydrophobic external surfaces that exhibit different adsorption phenomena. Our understanding of kaolin minerals in the environment is advancing as a result of molecular simulation and field studies. Molecular dynamics simulations reveal the structure and behavior of adsorbed ions and water molecules at the interface. The presence of microorganisms affects the formation and surfaces of kaolinite and halloysite. Mechanisms by which kaolin-group minerals complex, adsorb, and desorb radioactive pollutants in the subsurface can be understood by combining theory with observation.

Kaolin is used in many consumer products and as a functional additive and process enabler in manufacturing. It is typically extracted from open-pit mines that range from small to very large scale (tens to hundreds of thousands of dry metric tons produced per year). Ore processing consists of removing impurities, engineering particle size and shape, and enhancing certain properties through thermal and chemical treatment. In addition to the technical aspects of mining and processing, the social, environmental, and economic impacts of kaolin production are managed at each stage of the mining life cycle. Discussed herein are aspects of the history of kaolin mining, the classification of kaolin mines, the processing of kaolin, and the life cycle of mining.

Kaolin-group minerals typically form as a result of hydrothermal alteration and/or weathering processes. They occur in environments as diverse as tropical soils, continental sedimentary deposits, and altered crustal rocks. They have also been detected on the surface of Mars. Given their prevalence, they have attracted the attention of researchers in materials chemistry, environmental geochemistry, and high-pressure mineral physics. Their structure and related properties have been studied for about a century, and these studies reflect advances in experimental techniques, modeling approaches, and concepts in mineralogy. Among key features of their structure are the predominance of 2-D stacking defects and the peculiar role of H-bonding in the control of their polytypism.

Kaolin has played a prominent role in both Earth and human cultural history, and will continue to do so into the foreseeable future. Its abundance in the rock record has varied according to the waxing and waning of life faunal radiations, crustal differentiation, and climatic conditions. Kaolin-group minerals play an important environmental role in soils of tropical and temperate regions. They also occur in hydrothermal zones, where they form at the expense of felsic rocks through acid hydrolysis, and commonly in episodic geologic events. Kaolin deposits form the basis of many modern materials, but the first sophisticated human use was in porcelains from Jingdezhen, China. From the initial discovery of kaolin and its sculptural use 2000 years ago to today’s modern applications in nanocomposites, kaolin has been important throughout cultural history. Art and science are intertwined by the properties and uses of this amazingly small clay mineral.