MicromorphologyAccumulations of Fe and/or Mn forming nodules, mottles (similar to nodules but much larger and generally with diffuse boundaries) and linings (coatings, hypo-coatings, quasi-coatings). It comprises crystalline, cryptocrystalline and amorphous pedofeatures with intense red, brown yellow and black colors.
Fe/Mn nodules
Fe/Mn linings
Nódulos de Fe/Mn
These are local accumulations of Fe/Mn with more or less rounded shapes (sometimes irregular), with dark colors.
They are dark colors or even opaque (yellow, reds, brown or black). Generally they seem isotropic or cryptocrystalline under the microscope (individual crystals are not observed, but as a whole they show anisotropy). Their borders are sometimes sharp, sometimes diffuse and their degree of impregnation is very variable (low or strong).
Frequently they are irregulars, sometimes rounded and concentric.
Occasionally they are pseudomorphs.
The Mn nodules are always opaque. The Fe nodules shown variable colors.
They occur always in soils subject to intense seasonal changes. During humid phases Mn and Fe is reduced and mobilized, during phases of aeration Mn and Fe are oxidized and immobilized.
The periodic growth of the nodules can be reflected by a well expressed concentric fabric, which frequently presents a succession of black layers (in which Mn predominates) and yellow or brown ones (fundamentally with Fe). The nodules are practically always composed of both elements, but depending upon the conditions of formation one of them dominates.
Generally Mn is concentrated in the nucleus of the nodule, and its concentration decreases towards the periphery. The Fe components show an opposite trend and are more concentrated towards the exterior of the nodule.
Otherwise there seems to be a striking direct correlation between the intensity of the hydromorphism and the Mn/Fe ratio of the nodules, even as their size and abundance.
In general, Mn and Fe nodules as such are indicators of an incipient hydromorphism.
Goethite and hematite are systematically mentioned as the most important Fe-components, whereas for Mn oxides birnesite, lithiorphorite, todorkite, pyrolusite, psilomelane and cryptomelane have been observed.
Fe/Mn coatings
Frequently oxidation of Mn++ and Fe++ induces accumulation associated to natural surfaces.
Depending upon the localisation of the accumulation: in the pore covering the walls, impregnating the groundmass or immediately below the surface or at some distance, one distinguishes respectively coatings, hypo-coatings and quasi-coatings.
Fe/Mn coatings are formed in soils subject to an alternation of wet and dry conditions. As in the case of nodules, long periods of saturation are needed to cause a sufficient mobility of Fe and Mn, but also periods of intense drying to produce the accumulation.
During the dry periods the large (interpedal) voids are completely desiccated, whereas the small voids in the aggregates (intrapedal) preserve a given degree of humidity. As desiccation proceeds, the water of the interior of the aggregates (which contains Fe++ and Mn++) moves to the surface where it meets an oxidizing environment producing an accumulation on the surface of the aggregate.
When the desiccation is very strong, the surface of the aggregate will become dry and the air will penetrate the aggregate, producing oxidation and corresponding accumulation of Fe and Mn each time more remote from the surface.
Depending upon the intensity of the dryness, following features are formed:
--> coatings: restricted drying with only at the surface an oxidizing environment;
--> hypo-coatings: intense drying with oxidation at a short distance from the surface;
--> quasi-coatings: very strong drying: the oxidation invades the interior of the aggregate
Goethite and hematite are systematically mentioned as the most important Fe-components, whereas for Mn oxides birnesite, lithiorphorite, todorkite, pyrolusite, psilomelane and cryptomelane have been observed.
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