ABSTRACT
Naturally occurring phyllosilicates (silicate-based layer structures) are structurally diverse and some of these phases have the ability to form nanotube and tunnel forms. These tubes and tunnels are mesoporous because the tube center and the tunnels can potentially encapsulate ions and molecules. Chrysotile, ideally Mg3Si2O5(OH)4, is a common asbestos mineral that is nanotubular in form with potentially deleterious biological effects when inhaled, but the mineral has been considered as a nanowire substrate and synthetic analogs may avoid the human health issues (e.g., Roveri et al., 2006). In contrast, the nanotube mineral halloysite, ideally Al2Si2O5(OH)4 when H2O is not part of the structure, is of low toxicity and is potentially useful in many biologically relevant applications, such as in drug delivery systems (e.g., Veerabadran et al., 2007) and delivery systems of insecticides, herbicides, etc. (Lvov et al., 2008). The tunnel-like structures of palygorskite-sepiolite minerals are not believed to have health hazards, have excellent absorptive, rheological and catalytic properties, and are used in many industrial applications including nanomaterials (e.g., Galán, 1996, Álvarez et al., 2011). Imogolite and allophane (both Al-rich silicates) and hisingerite (an Fe-rich silicate) are nanotubular minerals that are poorly crystalline, and the structures are therefore poorly known. Duarte et al. (2012) have reviewed computer-modeling studies of imogolite, halloysite, and chrysotile.
