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Liesegang rings () (also called Liesegangen rings or Liesegang bands) are colored bands of
cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...
observed in
sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s that typically cut across
bedding Bedding, also known as bedclothes or bed linen, is the materials laid above the mattress of a bed for hygiene, warmth, protection of the mattress, and decorative effect. Bedding is the removable and washable portion of a human sleeping environm ...
.Stow, A.V., 2009, Sedimentary rocks in the field. A color guide (3rd ed.), Pp. 103, 107. These secondary (
diagenetic Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...
)
sedimentary structures Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different particl ...
exhibit bands of (
authigenic Authigenesis is the process whereby a mineral or sedimentary rock deposit is generated where it is found or observed. Such deposits are described as authigenic. Authigenic sedimentary minerals form during sedimentation by precipitation or recrys ...
) 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. but 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 Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
and
metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
s that have been chemically
weathered ''Weathered'' is the third studio album by American rock band Creed, released on November 20, 2001. It was the last Creed album to be released until '' Full Circle'' came out in October 2009, with Creed disbanding in June 2004. It is the only Cr ...
.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, and
Wilhelm Ostwald Friedrich Wilhelm Ostwald (; 4 April 1932) was a Baltic German chemist and German philosophy, philosopher. Ostwald is credited with being one of the founders of the field of physical chemistry, with Jacobus Henricus van 't Hoff, Walther Nernst, ...
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 Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...
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''.


Mechanism for development

upA close view of a Liesegang ring present on a natural arch of sandstone, found at a beach near Khayelitsha, South Africa. The process by which Liesegang rings develop is not completely understood. Liesegang rings may form from the chemical segregation of iron oxides and other minerals during weathering. One popular mechanism suggested by geochemists is that Liesegang rings develop when there is a lack of convection (advection) and has to do with the inter-diffusion of reacting species such as oxygen and ferrous iron that precipitate in separate discrete bands which become spaced apart in a geometric pattern. A process of precipitation known as the Ostwald-Liesegang supersaturation-nucleation-depletion cycle is known by the geologic community as a probable mechanism for Liesegang ring formation in sedimentary rocks. In this process the "...diffusion of reactants leads to supersaturation and nucleation; this precipitation results in localized band formation and depletion of reactants in adjacent zones." As Ostwald suggests, there is a localized formation of crystal seeds that occurs when the right level of supersaturation is reached, and once the crystal seeds form, the growth of the crystals is believed to lower the supersaturation level of fluids in pore spaces surrounding the crystals, thus mineralization that occurs after the initial crystal growth in the surrounding areas develops in bands or ''rings''. One classic example based on the Ostwald-Liesegang hypothesis is observed in water and rock interactions where iron hydroxide precipitates in sandstone through pore space.


Occurrence in the environment

Liesegang ring patterns are considered to be secondary (diagenetic) sedimentary structures, though they are also found in permeable igneous and metamorphic rocks that have been chemically weathered. Chemical weathering of rocks that leads to the formation of Liesegang rings typically involves the diffusion of oxygen in subterranean water into pore space containing soluble ferrous iron. Liesegang rings usually cut across layers of
stratification Stratification may refer to: Mathematics * Stratification (mathematics), any consistent assignment of numbers to predicate symbols * Data stratification in statistics Earth sciences * Stable and unstable stratification * Stratification, or st ...
and occur in many types of rock, some of which more commonly include
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...
and
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...
. Though there is a high occurrence of Liesegang rings in sedimentary rocks, relatively few scientists have studied their mineralogy and texture in enough detail to write more about them. Liesegang rings are referred to as examples of geochemical self-organization, meaning that their distribution in the rock does not seem to be directly related to features that were established prior to Liesegang ring formation.Chen, W., Park, A., and Ortoleva, P., 1990. Diagenesis through coupled processes: modeling approach, self-organization, and implications for exploration. American Association of Petroleaum Geologists Memoir 49, ''Prediction of Quality Through Chemical Modeling''. pp. 103-130. For instance, in certain types of sedimentary rocks such as carbonate siltstones (
calcisiltite Calcisiltite is a type of limestone that is composed predominantly, more than 50 percent, of detrital (transported) silt-size carbonate grains. These grains consist either of the silt-size particles of ooids, fragments of fossil shells, fragments of ...
s), Liesegang ring patterns can be misinterpreted for faults; the rings may appear to be "offset," however the laminae in the rock exhibit an unbroken pattern, therefore the ''observed'' offset is attributed to pseudofaulting. Pseudofaults are the result of Liesegang rings developing within areas of the rock that are adjacent to each other but at varying stratigraphic levels. Liesegang rings can have the appearance of fine lamination and can be mistaken for laminae when parallel or subparallel to the bedding plane, and are more easily differentiated from laminae when the rings are observed cutting across beds or lamination.


References

{{reflist Sedimentary structures