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Structural aspects of natural nanomaterials
A large number of mineral species occur only as micron-sized and smaller crystallites. This includes most of the iron and manganese oxyhydroxide minerals, and other species whose formation processes and growth conditions limit ultimate size. Microscopic investigation of these species generally reveals sub-micron structure down to the nanometer level, including evidence of aggregation, agglomeration and assembly of nanometer units into larger crystals and clots. The bulk of studies in the literature dealing with nanoparticle structure and growth deal with metals, silicon, and other semiconductor materials. A great deal of attention has been given to the electronic properties of such solids, owing to both new commercial applications and new fundamental physics and chemistry tied to this area. Most applicable mineralogical or geochemical studies have not addressed the same issues, instead being more concerned with relatively bulk chemical properties. Very little has been done to understand how natural nanoparticulates (and related types of natural nanomaterials) form, how their microstructure is related to the growth process, and how their structure varies from larger crystallites or bulk material of the same composition. Magnetic and electronic properties of natural nanomaterials are similarly understudied.
In this chapter aspects of nucleation, aggregation and growth processes that give rise to specific microstructures and forms of nanomaterials are considered. Next the way in which the surface structure of nanoparticulates may differ from the interior, and how physical structure may be modified by reduced particle size is examined. The various techniques by which nanoparticle structure, size, microstructure, shape and size distribution are determined are then considered with examples. Finally some of the outstanding problems associated with nanoparticle structure and growth are identified, emphasizing natural processes and compositions.
Naturally occurring nanomaterials exist in a variety of complex forms. In this chapter a short set of definitions will be stated for clarity. …