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Liesegang rings (geology)
Liesegang rings () (also called Liesegangen rings or Liesegang bands) are colored bands of cement observed in sedimentary rocks that typically cut-across bedding.〔Jackson, Julia A., 1997. "Glossary of Geology." American Geological Institute, Alexandria, Virginia. 4th edition. P. 366〕〔Stow, A.V., 2009, Sedimentary rocks in the field. A color guide (3rd ed.), Pp. 103, 107.〕 These secondary (diagenetic) sedimentary structures exhibit bands of (authigenic) minerals that are arranged in a regular repeating pattern.〔Middleton, Gerard V.; Church, Michael J.; Coniglio, Mario; Hardie, Lawrence A.; Longstaffe, Frederick J.; 2003. "Encyclopedia of Sediments and Sedimentary Rocks." Kluwer Academic Publishers, Dordrecht. Pp. 221, 224.〕 Liesegang rings are distinguishable from other sedimentary structures by their concentric or ''ring-like'' appearance. The precise mechanism from which Liesegang rings form is not entirely known and is still under research,〔Krug, H.-J,. Brandtstadter, H., and Jacob, K.H., 1996. Morphological instabilities in pattern formation by precipitation and crystallization processes. ''Geologische Rundschau'', 85: 19-28.〕 however there is a precipitation process that is thought to be the catalyst for Liesegang ring formation referred to as the Ostwald-Liesegang supersaturation-nucleation-depletion cycle.〔Decelles, P.G., and Gutschick, R.C., 1983. Mississippian wood-grained chert and its significance in the western interior United States. ''Journal of Sedimentary Petrology'', 53: 1175-1191.〕 Though Liesegang rings are considered a frequent occurrence in sedimentary rocks,〔Merino, E., 1984. Survey of geochemical self-patterning phenomena. In Nicolis, G., and Baras, F. (eds.), Chemical Instabilities. Dordrecht: D. Reidel Publishing Company, pp. 305-328.〕 rings composed of iron oxide can also occur in permeable igneous and metamorphic rocks that have been chemically weathered.〔McBride, E. F. (2003), Pseudofaults resulting from compartmentalized Liesegang bands: update. Sedimentology, 50: 725–730. 〕
== History ==
In 1896, a German Chemist named Raphael E. Liesegang first described Liesegang ''banding'' in his observations from the results of an experiment, although it is important to mention that Wilhelm Ostwald provided the earliest explanation for the phenomenon.〔Steefel, Carl I., 2008, Geochemical Kinetics and Transport: ''in'' Brantley, Susan L; Kubicki, James d; White, Art F. (eds), Kinetics of Water-Rock Interaction, Springer New York, p. 545-589.〕 The purpose of Liesegang's experiment was to observe precipitate formation resulting from the chemical reaction produced when a drop of silver nitrate solution was placed onto the surface of potassium dichromate gel. The resultant precipitate of silver dichromate formed a concentric pattern of rings. Liesegang and successive other workers observed the behavior of precipitates forming rings in sedimentary rocks, hence these features became known as ''Liesegang rings''.〔
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