Models of iron oxide concretion formation: field, numerical, and laboratory comparisons

Abstract

Well-exposed Jurassic Navajo Sandstone iron oxide concretions preserve important diagenetic records of groundwater flow and water-rock interactions. Field relationships, precipitation patterns, and geometries of the Navajo concretions provide the basis for input parameters in numerical computer simulations and laboratory chemical bench tests. Although field geometries are very difficult to replicate, numerical simulations and laboratory experiments examine end results such as nucleation and growth of iron oxide concretions, produced from known input parameters. Three numerical simulations show the development of periodic self-organized nucleation centers through Liesegang-type double-diffusion of iron and oxygen. This numerical model simulates a scenario where oxygen is provided by shallow fresh water and iron is sourced from deeper reduced formation water. Concretions form in the region where the two waters interact with each other. Model sensitivities show that advection of water is an important mechanism for supplying the iron, and that acidic conditions in the iron-charged water can cause iron to stay in solution longer to produce nucleation centers that are farther from the input source. Laboratory bench tests with reactions of FeSO 4 or Fe(NO 3 ) 3 with KOH show how the precipitation of hydrated iron sulfates or iron-hydroxides may form rinds around an initial, spherical source of iron (i.e. nucleation center). These rinds may show inward growth depending on the concentration of the iron source in relation to the surrounding fluid. A number of complex factors such as concentration and flux, time, and multiple events can create banded patterns during rind growth. Comparisons of the terrestrial examples with numerical and laboratory models have strong implications for understanding similar hematite concretions on Mars.

Jarosite dissolution rates and nanoscale mineralogy

“…Reaction products have been suggested to play a significant role in determining jarosite solubility and dissolution rates, however, the mineralogy and texture of jarosite dissolution products remains speculative. Multiple studies have investigated the incongruent dissolution reaction products (Golden et al, 2005;Barron et al, 2006;Chan et al, 2007) and reported schwertmannite, goethite, hematite, ferrihydrite, and lepidocrocite as the dominant reaction products under varying dissolution conditions. However, in studies examining jarosite dissolution rate, the reaction products were inferred based on the aqueous solution chemistry (Welch et al, 2008), morphology , or reported based upon unpublished X-ray diffraction patterns (Gasharova et al, 2005).…”

Hematite-Rich Concretions from Mesoproterozoic Vindhyan Sandstone in Northern India:A Terrestrial Martian 'Blueberries' Analogue with a Difference

“…However, the respective ages of concretion formation are still unknown. Experimental and numerical simulation studies have suggested that 'blueberries'-like concretion development requires a nucleus 5 , and the multi-rimmed VHC samples are nucleus-bearing unlike MC and UC structures. However, the presence of hematite in the VHC is similar to that MC and UC, and this has been verified by various analytical techniques.…”

“…MB are believed to have formed from acidic standing water or groundwater, whereas UC have formed by the interaction of hydrocarbon-rich briny fluids with oxygen-rich groundwater. Numerical simulations and experimental studies have shown that the concretions form around a nucleation centre when shallow freshwater interacts with iron-charged deeper and reduced formation water 5 . Other reported terrestrial analogues include hematite concretions occurring within evaporite-siliciclastic sequences of Permian age in the central United States 6 , and of modern age in Western Australia 7,8 .…”

Rhizogenic Fe–C redox cycling: a hypothetical biogeochemical mechanism that drives crustal weathering in upland soils

“…The deep, acidic Ultisol is derived from granitic-gneiss bedrock, and has depthdependent horizons that contrast in texture, mineralogy, structure, and biogeochemical processes. (b) The striking colorcontrasts of rhizospheres and inter-rhizosphere microsites suggest a Liesegang-banding phenomena (Chan et al 2007) horizons at 0.8-1.5 m at the Calhoun Forest (Table 2) and 0.9-1.5 m at the Duke Forest (Table 3). Fedepleted low-chroma rhizospheres from the Bt-horizons had Munsell colors when field moist to be 5Y 8/2 and 10 YR 8/1 or ''light gray'' for the Calhoun and Duke Forest soils.…”