“Liesegang banding” developed in a volcanic tuff (8.6 cm across).

Irregular, reddish or brownish or orangish-brown iron banding is commonly seen in many porous rocks, particularly sandstones and pebbly sandstones. These have been almost universally referred to as “Liesegang banding”, representing precipitation lines of iron-rich minerals (e.g., hematite, limonite, goethite, etc.) at & along groundwater chemical interfaces. But, according to Neil Wells of Kent State University, the original concept of Liesegang banding (Liesegang, 1896) does not match up with what is seen in the rock record (see Wells et al., 2003).

True Liesegang banding refers to parallel bands of precipitate formed by diffusion along a single chemical gradient during one event. What's seen in the rock record often consists of sets of irregularly concentric iron bands, with different sets of bands quite frequently oriented in different directions, and showing cross-cutting and dissolution of older sets. Iron banding in the rock record is clearly the result of numerous precipitation events over long periods of time by moving groundwater (Wells et al., 2003). Iron mineralization along these bands appears to be induced by the presence of either a redox interface (change from reducing to oxidizing conditions in the groundwater) or a pH interface (change in acidity).

Since Neil Wells is the first (apparently) to point out that what geologists have been calling Liesegang banding really isn’t, a renaming seems to be in order. It was jokingly suggested in 2003 that the iron banding discussed above be called “Wells Banding”. I’m all for that.

References:

Liesegang, R.E. 1896. Ueber einige Eigenschaften von Gal-lerten [On some properties of gelatin]. Naturwissenschaftliche Wochenschrift 11: 353-362. (see also: Liesegang, R.E. 1945. Geologische Bänderungen durch Diffusion und Kapillarität [Geologic banding by diffusion and capillarity]. Chemie der Erde, Zeitschrift der Chemischen, Mineralogie, Petrographie, Geologie und Bodenkunde 15: 420-423.)