The minimum size of biological entities. The width of DNA strands must be about an order of magnitude greater than ionic radii, the size of a bundle of DNA (a virus) must be yet an order of magnitude larger, and the size of an organism containing a bundle of DNA plus metabolic material (a small prokaryote) must be about another order of magnitude larger still. Organisms in the range of tens of nanometers are therefore improbable.

The minimum size of mineral particles. If repeat distances or monolayers in crystal structures are about 0.5 to 1 nm, particles at sizes less than 10 nm must have surfaces consisting entirely of steps and kinks and thus be extremely soluble. Such small particles are almost entirely "surface" and thus have properties unlike interiors of larger mineral particles (for more, see a related illustration). Furthermore, such particles may have electric double layers in the surrounding solution larger in radius, and much larger in volume, than the particles themselves.

Constraints on how we observe materials. The lower limit of light microscopy, for example, is limited by the wavelength of light. On the other hand, X-rays, and in particular CuKa radiation, are used in diffraction studies of minerals because CuKa radiation has a wavelength about the same as the d spacing of most minerals. X-ray lasers may let us image single molecules in the relatively near future (Kapteyn and Ditmire 2002).

The significance of quantum mechanics to big (nanometer-scale) things. The wave-particle duality of C₆₀ molecules demonstrated by Arndt et al. (1999) brings wave-particle duality to the scale of molecules and close to that of very small naturally occurring solids. This work in Anton Zeilinger's lab at the University of Vienna has now been extended to C₇₀ molecules (Brezger et al., 2002). Can sand grains be far behind?

Notes regarding small colloidal particles:
a: 2-450 nm particles in Glatt River, Switzerland (Waber et al. 1990).
b: 4-4000 nm particles in Rhine River (Newman et al. 1994).
For those and more, see Table 3 of Newman et al., 1994, Water Research v. 28, p. 107-118, Table 2 of Atteia and Kozel, 1997, Journal of Hydrology v. 201, p. 102-119, and Figure 1 of Buffle and Leppard, 1995, Environmental Science and Technology, v. 29, p. 2169-2175.
c: Thickness (1 nm) and diameter (60 nm) of smallest particles described by Kotlyar et al., 1998, Clay Minerals v. 33, p. 103-107.

Thickness of double layer in solution: Berner (1980) Chemical Sedimentology and Appelo and Postma (1994) Geochemistry, Groundwater, and Pollution.

Small magnetite and periclase crystals: http://www.psrd.hawaii.edu/May02/ALH84001magnetite.html.

Lipids: ridge.icu.ac.jp/gen-ed/ eukaryotic-cell.html.

Thickness of altered mineral surface layers: Wollast and Chou (1984. in Drever. ed., Chemistry of Weathering; Hellman et al. (1990) as cited in Hochella (1990) p. 99-100 in MSA Review in Mineralogy Volume 23: MIneral-Water Interface Geochemistry.

Viruses and Chromosomes: Campbell et al., Biology, p. 320 and 325.

Largest prokaryotic and eukaryotic cells: http://www.globaldialog.com/~jrice/algae_page/valonia.htm and http://www.eurekalert.org/pub_releases/1999-04/AAft-BBEF-160499.php

Tallest trees: www.forestrytas.com.au/forestrytas/pdf_files/ tall_trees_survey_report.pdf

Gold cluster compunds: J. F. Hainfeld, R. D. Powell, F. R. Furuya, and J. S. Wall , 2000, Gold Cluster Crystals: Microsc. Microanal., 6, (Suppl. 2: Proceedings) (Proceedings of the Fifty-Eighth Annual Meeting, Microscopy Society of America); Bailey, G. W.; McKernan, S; Price, R. L.; Walck, S. D.; Charest, P.-M., and Gauvin, R., Eds.; Springer-Verlag, New York, NY, 2000, pp. 326-327. (see http://www.nanoprobes.com/MSAXTALS00.html) The size estimate shown on the figure is Railsback's estimate for the Au₆ compound.

Responses of matter with absorption of electromagnetic radiation: Figure 1 of Calas, G., and Hawthorne. F.C., 1988, Introduction to spectroscopic methods, in Hawthorne, F.C. ed., Spectroscopic Methods in Mineralogy and Geology: Mineralogical Society of America, Reviews in Mineralogy v. 18, p. 1-9.

The filtration spectrum: http://www.osmonics.com/products/refpage/zoomfiltspec.htm

Buckyballs and wave properties:
    Arndt et al. (1999) Wave-particle duality of C₆₀ molecules: Nature v. 401, p. 680-682;
    Björn Brezger, Lucia Hackermüller, Stefan Uttenthaler, Julia Petschinka, Markus Arndt, and Anton Zeilinger (2002) Matter-wave interferometer for large molecules: Physical Review Letters v. 88, 100404.

X-ray lasers: Kapteyn and Ditmire (2002) Ultraviolet upset: Nature v. 420, p. 467-468.

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Send comments and suugestions to Bruce Railsback at rlsbk@gly.uga.edu.

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