page_title: Liesegang rings (geology)
section_title: Mechanism for development
hierarchical_section_title: Liesegang rings (geology) / Mechanism for development
caption_attribution_description: English: Excellent example of Liesegang banding in sandstone. Taken near Garden of the Gods in IL
context_page_description: Liesegang rings are colored bands of cement observed in sedimentary rocks that typically cut-across bedding. These secondary sedimentary structures exhibit bands of minerals that are arranged in a regular repeating pattern. 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, 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. Though Liesegang rings are considered a frequent occurrence in sedimentary rocks, rings composed of iron oxide can also occur in permeable igneous and metamorphic rocks that have been chemically weathered.
context_section_description: 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.
caption: Anvil rock in the Shawnee National Forest, Illinois
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