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This chapter offers a not-exhaustive overview of structural mesoporosity in selected, natural mineral phases (carbon forms, chrysotile, gas hydrates). After a short introduction devoted to the introduction of the most general features of porosity, attention will be paid mostly to natural mesoporous mineral phases, leaving microporosity to other chapters of this volume. However, a few peculiar microporous structures will be also considered, because of their close resemblance with the mesoporous substances.
Conversely, no attempt will be made to report properties and structures of the many examples of man-made mesoporous materials including silica xerogels, mesoporous synthetic silica, calcium phosphate and the fantastic arrangements of shapes, surface patterns and channels that can occur (e.g., Yang et al. 1997; Maschmeyer 1998; Sayari 2003; Xia et al. 2003; Zheng et al. 2003; White et al. 2005). Similarly, the overview will not deal with crystal engineering processes, such as the fabrication of hollow porous shells of calcium carbonate from self-organizing media (oil-water-surfactant microemulsions supersaturated with calcium carbonate; Walsh and Mann 1995). Finally, one more aspect outside the scope of this article will be micro-to-mesoporosity conversion in synthetic materials, such as activated palygorskite and sepiolite, or the so-called pillared clays (e.g., Mass et al. 1997; Dékany et al. 1999; Salerno and Mendioroz 2002; Ferraris and Gula 2005). Evidently, these issues represent extremely important technological targets, often achieved following synthetic routes that may be totally different from those used by nature to form minerals.
Microporosity, mesoporosity and macroporosity
Porous structures are derived from a framework of linked atoms (“host”), that create volumetrically important voids (“pores”), possibly capable of including several different “guest” species. Three groups (micro-, meso- and macropores) are discriminated based upon pore size. According to International Union for Pure and Applied Chemistry (IUPAC) recommendations, pores with free diameters less than 2 …