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Notes on a Molecular Nanotechnology of Silicates

S.L. Gillett*

University of Nevada

This is an abstract for a talk to be given at the Fifth Foresight Conference on Molecular Nanotechnology.

Theoretical studies of molecular nanotechnology (MNT) have focused on tetrahedral carbon ("diamondoid") frameworks as an ultimate goal. It seems natural to wonder, though, whether silicon, carbon's second row homolog in the periodic table, could form a reasonable basis for MNT, especially given the importance of Si in semiconductor technology. Although Si itself is unpromising--the Si-Si bond is not nearly so stable as is C-C, and Si shows minimal tendency to form the double and delocalized bonds so typical of C--silicates, compounds of Si and O, indeed show promise for MNT, because of:

• The strong and directional character of the Si-O bond, due to its partly covalent character.
• Their ease of polymerizing into 3D structures ("tectosilicates"). Silicates enter tetrahedral coordination at STP, even out of aqueous solution, in stark contrast to sp³ carbon.
• Their thermal stability: certain tectosilicates, such as quartz and feldspars, will even crystallize directly from silicate melts at hundreds of degrees C.
• Their stability to oxidizing conditions; whatever its other virtues, C burns.
• Their sheer abundance. O and Si make up, respectively, 60.4 and 20.5 atom percent of the Earth's crust. Nearly all common minerals are silicates.

This structural variety only in part accounts for the diversity of silicates, however, as other atoms can substitute for Si. In natural systems, Al substitution is ubiquitous; one class of alumino-tectosilicates, the feldspars,are the most common compounds in Earth's crust. Other alumino-tectosilicates,the zeolites, have great technological importance as "molecular sieves" and catalysts, due to their molecular-sized internal voids. Finally, other tetrahedral substitutions (e.g., B, Ga, P, transition metals) vastly increase the potential structures for MNT.

Siloxanes ("silicones"), "hybrids" of silicates and organic compounds that consist of siloxy (Si-O-Si) chains with H or organic side-groups attached to Si, show how silicate-based structures could be integrated with C-based structures. Cubosiloxane, H₈Si₈O₁₂, the siloxy analog of cubane, C₈H₈, in which each C-C bond is replaced by an Si-O-Si bond, shows their diversity; moreover, cubosiloxane is both considerably easier to synthesize and more stable than cubane.

Though silicate raw materials are literally everywhere, conventional mining waste seems especially attractive. Despite their ubiquity, silicates are usually unattractive as ore minerals due to the energy required to break the Si-O bond. However, most ores consist largely of silicate minerals in which the valued non-silicate ore minerals constitute only a minor fraction. Such ore rock must be crushed and ground merely to separate out the ore minerals; hence, not only is the energy needed to break the rock up mechanically been largely wasted, but the very finely comminuted waste from such mineral dressing operations (the "tailings") presents a serious disposal problem. Such material should furnish an excellent feedstock for silicate-based MNT, however, especially as it is relatively reactive due to its comminution.

Last, silicate-based MNT has major potential applications in space development, as many extraterrestrial bodies (e.g., the Moon) are dominated by silicates but have scant carbon. Indeed, the surface material of the Moon, which largely consists of silicate minerals comminuted by eons of meteorite impact, would also be an excellent feedstock to a silicate MNT.

*Corresponding Address:
Stephen L. Gillett, Dept. Geol. Sciences, Mackay School of Mines, University of Nevada, Reno, NV 89557, ph:702-784-4760, fax: 702-784-1833, email: gillett@seismo.unr.edu
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