The origin and formation of clay minerals in soils" past, present and future perspectives

It has long been recognized that the minerals in the clay (<2 gm) fractions of soils play a crucial role in determining their major physical and chemical properties, and inevitably, questions concerning the origin and formation of these minerals have assumed some prominence in soil science research. This review considers some important aspects of these questions and is confined to clay minerals in soils as they are generally understood, that is as a medium for plant growth. The occurrence of clay minerals in saprolites or weathered rock is not discussed to any great extent because many studies have shown that saprolites are characterized by physicochemical conditions that are different to those in soils. Intimate grain-tograin contacts promote a special chemical environment on a local scale, bringing about the formation of transient clay mineral phases which quickly disappear in the overlying soil. For the purposes of this review the perspective of the past will be taken to be that of the early 1960s, which is a convenient historical benchmark in that it represents a time when the overall structural characteristics of the layer silicate clay minerals were to a large extent known, even though much detail remained to be filled in, as well as a time that immediately pre-dated the widespread utilization in soil science of analytical techniques such as scanning electron microscopy, electron probe microanalysis, M6ssbauer spectroscopy, electron spin resonance spectroscopy and infrared spectroscopy. This review will therefore attempt to summarize the general conclusions that had been arrived at regarding the origin and formation of certain clay minerals in soils in the early 1960s, the developments since that time and the situation as it currently stands, and to consider the outstanding problems to be addressed in the future. In this context, particular attention will be paid to micas, vermiculites, smectites, chlorites, intcrstratified minerals and kaolin minerals. As in all such reviews, a certain amount of subjectivity and personal preference is perhaps inevitable. P A S T P E R S P E C T I V E S The reviews of Millot (1965) and Jackson (1964) represent a convenient starting point with regard to the origin and formation of soil clays as assessed principally by the X-ray diffraction and microscopic 9 1999 The Mineralogical Society