Minerals are more complex than previously thought because of the discovery that their chemical properties vary as a function of particle size when smaller, in at least one dimension, than a few nanometers, to perhaps as much as several tens of nanometers. These variations are most likely due, at least in part, to differences in surface and near-surface atomic structure, as well as crystal shape and surface topography as a function of size in this smallest of size regimes. It has now been established that these variations may make a difference in important geochemical and
biogeochemical reactions and kinetics. This recognition is broadening and enriching our view of how minerals influence the hydrosphere, pedosphere, biosphere, and atmosphere.
Most physical, chemical, and biological processes on Earth are either influenced to some degree or fully driven by the properties of minerals. But with only about 4500 mineral species presently described, not many relative to millions of prokaryotic and eukaryotic species combined, their diversity and range of influence may seem, by comparison, relatively modest Minerals exert their influence by constituting the bulk of this rocky planet and having a wide range of composition and structure that is expressed in a marked diversity of physical and chemical properties. Now we are gaining a much better appreciation for another aspect of mineral diversity—that which is expressed in the nanoscale size range (1–3). Here, atomic and electronic structure of nanoparticles may vary with size even without a phase transformation, and surface-to-volume ratios change dramatically. Such particles are minerals that are as small as roughly 1 nm and as large as several tens of nanometers in at least one dimension. Limiting size in one, two, or three dimensions results in a nanofilm (or nanosheet), a nanorod, or a nanoparticle,respectively.
Minerals can be found in all of these shapes, although this review will concentrate on nanoparticles. Nanominerals are defined here as minerals that only exist in this size range; that is, one will not find their equivalent at sizes larger than this. Well-known examples include certain clays as well as iron and manganese (oxyhydr)oxides (with ferrihydrite,an iron oxyhydroxide, as a type example). Mineral nanoparticles are minerals that can also exist in larger sizes, and these probably include most of all known minerals.
The importance of certain types of nanominerals and mineral nanoparticles, namely clays and the smallest mineral colloids, has been known for a long time. What has been generally recognized more recently is that nanominerals and mineral nanoparticles commonly behave differently as a function of their size within the nanoscale size range.
Mineral nanoparticles also behave differently than larger micro- and macroscopic crystals of the same mineral. This observation violates aspects of the long-standing formal definition of a mineral. Although definitions vary somewhat, depending on the source, the general consensus is that minerals are naturally occurring, crystalline substances having a characteristic and defined chemical composition (or compositional range in the case of solid solutions). For any particular composition, each mineral expresses a set of specific physical and chemical properties.
Nanominerals and mineral nanoparticles satisfy these criteria, except that even with a fixed composition, they express a range of physical and chemical properties depending on their size and shape.