CRYSTAL STRUCTURE AND CHEMISTRY OF PHYLLOSILICATES: SOME RECENT PERSPECTIVES AND NEW TRENDS

The study of phyllosilicates, in particular the microcrystalline species that constitute clay minerals, was widely addressed in recent literature (for a review, BERGAYA et al., 2006). Many works related to layer silicates are not only published on dedicated journals, such as Clay Minerals, Clays and Clay Minerals and Applied Clay Sciences but also on numerous magazines oriented toward Mineralogy, Petrology, Geochemistry, Applied Chemistry, Environmental Sciences, Food and Soil Science, Ceramics, Applied Physics, Enginery, Water Science and Biology. During the beginning of 2007 (from January to April) more than 1200 contributions were published on clay minerals, also including 340 patents. Among these works, many address, or recall, the structure and crystal chemistry of clay minerals to account for different features and properties, also depending from chemical and physical experimental or environmental conditions. The great attention and relevance devoted to the crystal chemistry and structure of clay minerals are counterbalanced by a remarkable experimental complexity, characterizing most aspects in this field of research, thus also accounting for a significant lack of chemical homogeneity in the information available.In particular single crystal approaches on clay minerals are limited, in our current technological frameset, not only by the microcrystalline dimensions, common for clay minerals particles, and by their association with other phases, but also by their hydration, lack of homogeneity from layer to layer and stacking faults. A further complexity is also associated to the interlamination of different phyllosilicates. During last decades, transmission electron microscopy, X-ray powder diffraction techniques, spectroscopic techniques and theoretical or modeling approaches allowed a rather accurate description of the stacking sequence, at least when the basal spacing for the constituting layers is significantly different. Many recent contributions addressed interlaminated phases where this latter constraint is not fulfilled, both via experiment and theoretical approaches. A first example is constituted by the stacking of trans-vacant and cis-vacant illite layers, where DRITS et al. (1998, 2006) suggested a simple identification criterion based on the presence of d110 reflection violating the individual layer symmetry. A second example is constituted by 7Å serpentine and 14Å chlorite (e.g., ŚRODOŃ, 1999; XU & VEBLEN, 1996; BANFIELD & BAILEY, 1996). Moreover significant question marks still apply for the detailed crystallographic and crystal chemical description of the constituting layer, especially for what concerns cation exchange, the formation of high temperature phases and the establishment of different interlayer complexes. The relevance of all the above mentioned fields of investigation is not only speculative, but also deeply applicative.This brief introduction clearly depicts a frameset where an inherent complexity is well evident for most of the crystal chemical and crystallographic investigations on clay minerals, together with their remarkable speculative and applicative impact. A complete description is outside the scope and time-constraint of this document, where some examples and hot-spots will be addressed, together with some, nowadays substantially obscure aspects, that technical progress may render viable for investigation in the near future. In more detail the points that will be addressed are listed below:1.An historic background and recent advances on the structural description of a well characterized clay mineral: kaolinite.2.Crystal chemistry and structure of dioctahedral and trioctahedral micas: some recent advances and implication on clay structure. 3.Layer charge location and interlayer complexes.