Duricrust
Duricrust is the generic term for all counted to the group of terrestrial soils crusts that occur beyond the influence of groundwater. It is to soil horizons that to little more than 10 meters can be powerful few millimeters, and in which by precipitation of mineral substances from the pore water parts of such a horizon or even the entire horizon is solidified rock -like. Duricrusts occur not infrequently in earlier geological eras on rock sequences and provide information about the environmental conditions that existed at its creation time.
Term
The word duricrust is composed of the Latin words for durus hard and crusta for the crust together. It was first used in Australia in connection with related or located at the surface layers of laterites, bauxites and quartzites. The word component crust here is somewhat misleading, because it is not, as in " crust " or " bread crust " for a solid shell or ceiling and also therefore referred to no landscape form. Instead, the processes that lead to the formation of Duricrusts, usually below the actual terrain surface run off. Most reaches a duricrust only by erosion directly to the surface.
Some forms of Duricrusts call in a broader sense, not only the " fossilized " soil horizon, but are broad chemical changes within the soil and are therefore regarded as so extreme soil types
Classification and Nomenclature
For Duricrusts exists a nomenclature illustrates the mineralogical differences in the first place:
- Prefix Al or aluminum for Duricrusts in which aluminum minerals such as gibbsite or boehmite dominate
- Prefix - Cal for Duricrusts in which calcite (calcium carbonate, CaCO3) dominates (eg, caliche )
- Prefix Dol or dolomite for Duricrusts in which dominated dolomite (calcium -magnesium Doppelkarbonat, CaMg ( CO3) 2 )
- Prefix ferri- for Duricrusts in which iron minerals such as hematite or goethite dominate (eg bog iron )
- Gyp - for Duricrusts where gypsum ( calcium sulfate, CaSO4 • 2H2O) dominates prefix
- Sal - prefix for Duricrusts where halite (sodium chloride, NaCl) dominated
- Sil - prefix for Duricrusts in which quartz (silicon dioxide, SiO2) dominates.
These prefixes are combined with the Nachsiben - crete or crust. A weakness of this nomenclature is that - cretes and crusts on the one hand the same can my other hand, but also each for a more specialized, although quite stand similar phenomenon. So the names Ferricrete and Ferricrust are on the one hand both for Duricrusts with a high iron content, but Ferricrete can also specifically a cemented with iron minerals clastic sediment and Ferricrust denote the hard crust of a Eisenkonkretion. Both the cementation of sediment with iron minerals and the formation of iron concretions can go back to a duricrust, but can also run under other geological processes.
In addition, it should be noted that there are transitional forms between different mineralogical Duricrusts that arise under similar conditions (eg between Silcretes and Ferricretes or Ferricretes and Alucretes or Calcretes, Gypcretes and Salcretes ).
In a recent study of metal-containing Duricrusts ( " Orecretes " called, Eng. Ore Ore ) is a slightly different nomenclature used. In this first only the anion of the dominant minerals in the duricrust is crucial, which is combined with the suffix- crete:
- Oxicrete: oxides and (oxide ) hydrates
- Carbocrete: carbonates
- Silicacrete: silicates (mainly quartz and opal )
- Halcrete: halides (chlorides, iodides, fluorides and bromides)
- Sulcrete: Sulfate and APS minerals ( aluminum phosphate - sulfates )
- Phoscrete: Phosphate
- Arsenocretes: arsenates
- Vanadocrete: vanadates
These names are then combined with the formula or the signs of it occurring metal, respectively, of abundant metals, such as Silicacrete (Fe -Al ).
Formation
Generally arise Duricrusts always involving water and primarily related to soil formation in areas with tropical to subtropical climate with at least locally flat relief. However, the water involved is by definition no ground water, but ground water, which comes from rainfall or nearby waters.
Formation in humidem climate
In humidem climate weathering of rocks to a considerable extent by the chemical route is done. The area between floor or ground surface and unweathered starting material of a soil is referred to as weathering profile. In terms of chemical weathering harsh climate of the humid tropics weathering profiles of up to 120 meters in thickness can occur.
In the course of chemical weathering are numerous minerals of the starting material (eg feldspars ) decomposed to various chemical elements and compounds dissolved in the soil water ( mobilized ) are. The chemically modified starting material is now referred to as saprolite.
After a longer or shorter vertical or lateral transport (migration) of the solutes, they are reprecipitated when the solution reaches an area of land, providing the necessary chemical conditions, which is often on a change from reducing to oxidative milieu. This leads to the formation of initially small, bulbous concretions, which, at constant conditions, are larger and to form a continuous layer can grow together over time, the previously existing there loose material is completely displaced. In other cases, the precipitation of the minerals results in a loose sediment is impregnated and cemented characterized.
Typical of Duricrusts in weathering profiles humid climates are Alucretes, Ferricretes and Silcretes. Here, among others, go Alucretes from the weathering of nepheline syenite and Ferricretes and tiallitische Alucretes among others from the weathering of basic rocks out. Alucretes thus forming deposits of aluminum ore bauxite, where they occur together with Ferricretes.
The bound in Silcretes SiO2 may well be mobilized again and redeposited elsewhere, either on the same site or. This deposition process can proceed at the same location at different times to a certain extent different from one another chemical environment, and then leads to the formation of complex, PolyPhaser SiO2 Duricrusts, referred to as multiple Silcretes.
Formation in semi-arid and arid climate
In warm dry climates, chemical weathering plays only a very minor role. Due to the high rate of evaporation occur at longer Sedimentationsunterbrechungen primarily Calcretes, Gypcretes and Salcretes by deposition of calcite, gypsum or saline ( Geolog.: rock salt ) from supersaturated solution when the water evaporates in the pore space of the upper soil layers.
Even in the development of Calcretes initially form tubers which then grow together to form a closed layer. Not infrequently, but also roots of plants are encrusted with calcite. Through regular alternation of Sedimentationsphasen and Sedimentationspausen series can arise with multiple in certain vertical distance consecutive calcrete horizons. Calcretes that arise in arid climate, are also called Caliche.
Gypcretes and Salcretes are especially typical of Playa and Sebcha sediments. In the bottom, near the shore of a oversalted marine lagoon or a salt lake, rises through the capillary pore water from the water-saturated area of the floor in the direction of the surface, thereby deposited there as a result of the high evaporation rate highly gypsum and halite. Due to the high water solubility of halite Salcretes be solved at increasingly becoming humid conditions but quickly in the soil.
Importance
Duricrusts have particular significance for geomorphology, in geology and in geology as an indicator for a certain paleo- environment.
Silcretes own in dry air high weathering resistance. So it can u.U. happen that a Silcreteschicht forms in a humid period in the bottom of a trough, with the solutions that provide the SiO2, come from the mountains, which rise above the valley and are exposed to a relatively strong chemical weathering. The climate changes towards drier conditions dominated physical weathering, which the now quite strongly challenged material of the mountains can oppose less than the valley floor, the now well protected by the Silcreteschicht, the only bad in dry climates weathered against further erosion ( " reinforced " ) is. Progressive erosion in the region now ensures that transform the mountains into valleys and the former valley floor forms a mesa. Such a process is commonly referred to as relief inversion.
Particular are Alucretes of importance for the economic geology and thus the mining industry because they form bauxite deposits, which are mined for the production of aluminum.
In the exploration of Mars, a rock sequence was discovered by the Opportunity rover in Eagle crater of Meridiani Planum, which contains gypsum and hämatitreiche concretions and cementation in which it could be to Duricrusts. This would be a clear evidence for a longer-lasting presence of liquid water in surface sediments of Mars. However, these rocks are probably already several billion years old.