“LIESEGANG BANDING”
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 (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.
“Liesegang banding” developed in a
volcanic tuff (8.6 cm across).
“Liesegang banding” developed in a
quartzose sandstone (= “Scenic Sandstone”) (~20.5 cm across).
References:
Neil Wells (pers. comm., 2003)
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.)
Wells, N.A., D.A. Waugh & A.M. Foos.
2003. Some notes and hypotheses concerning iron and iron remobilization
features in the Sharon Formation (Summit County, Ohio). in
Pennsylvanian Sharon Formation, past and present: sedimentology, hydrogeology,
and historical and environmental significance, a field guide to Gorge Metro
Park, Virginia Kendall Ledges in the Cuyahoga Valley National Park, and other
sites in northeast Ohio. Ohio Division of Geological Survey Guidebook
18: 33-37.