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Silt particles are so fine-grained that, like clay particles, they can remain in suspension for a long time even in low turbulence . They can therefore be found in typical alluvial sediments at a relatively great distance from the actual river bed. The photo shows the interior of a car in the east (9th ward) of New Orleans , covered with silty clay material , which was affected by a breach of the Mississippi levee in connection with Hurricane Katrina .

Under silt (geologically also Silt ) means fine soils of different origin and unconsolidated clastic sediments whose mineral constituents predominantly (> 50%) has a grain size from 0.002 to 0.063  millimeters have. At the same time, silt and silt are the names for the corresponding grain size interval, which occupies a middle position between the coarser sand and the finer clay . Particles the size of silt are found to a large extent in the cohesive soils or sediments known as loam . A capital "U" is used as the abbreviation for the silt grain size. In the geosciences , clastic sedimentary rocks with a dominance of grain sizes in the silt area are called silt or silt stones .

Grain size determination in the field

Grains of silt are so small that they cannot be broken down individually either with the naked eye or with a magnifying glass. A distinction between silt and clay particles is therefore not possible optically in the terrain. However, due to the special properties of the respective minerals, silt particles usually consist of relatively hard minerals, mostly quartz , while clay particles consist of the much softer clay minerals . During the provisional investigation of weak to moderately solidified sedimentary rocks during geological and pedological terrain and field work, a distinction can therefore be made between siltstone or silty claystone and pure claystone by means of the crunch test. Here a very small corner of the rock in question is bitten off and chewed. A characteristic crunch of the rock between the teeth indicates the presence of silt in the sampled rock.

The delimitation of silty to clayey fine soils or unconsolidated sediments is carried out in the terrain by means of a finger test . In contrast to relatively pure clay, moist silt or clay rich in silt is only moderately malleable between the fingers. In addition, the larger grain size reflects the light more diffusely , so that the material does not have a greasy sheen when rubbed between the fingertips, as is typical for relatively pure clay.

Subdivision of the grain size silt

The grain size of silt or silt is defined by the EN ISO 14688 standard, which replaced DIN 4022 in 2002 . It is divided into fine, medium and coarse silt.

(Silt) (U / Si)
Coarse silt (PDO / CSi) 0.02 to 0.063  mm
Medium silt (mU / MSi) 0.0063 to 0.02 mm
Fine silt (fU / FSi) 0.002 to 0.0063 mm

Since in the grain size analysis of a sediment or soil in the laboratory, the separation of the silt from the clay fraction and the separation of the sub-fractions of the silt fractions cannot be carried out by sieving due to the small grain sizes , but rather by sludge , clay and silt particles are included in this context understood the term slurry grain.

The Wentworth scale used internationally to classify sediments is based on the logarithm to base 2 , whereas the classification according to EN ISO or DIN is based on the decadic logarithm . The latter standards thus achieve that on a logarithmic grain size scale the distances are the same in the important range from 0.002 to 20 mm. As a consequence, the limits of the grain classes of the two systems differ from one another. According to the Wentworth scale, the silt has a grain size of 0.004 to 0.063 mm (rounded). Therefore, when using literature from different sources, the respective underlying reference system must be observed.

Classification of silt-containing sediments

All clastic sediments, the majority of whose grains (> 50%) are in the size range of silt or silt, are called silts or silts. Depending on which of the silt grain size dominates, a distinction can be made between fine, medium and coarse silt, analogous to the corresponding grain size designations. Fine and medium silt belong together with the clay sediments to the pelites , while coarse silts together with the sands belong to the psammites . In sediments with a high proportion of clay and silt, nomenclature gradations such as silt clay (clay still dominates), clay silt (silt already dominates) and clayey silt (silt clearly dominates) are possible.

A relatively strong solidified clastic sedimentary rock, which consists mainly of silty grain, is called siltstone (siltstone). In the English nomenclature, all clastic sedimentary rocks in which grain sizes below that of fine sand dominate are summarized as mudstones or mudrocks ("mud [ge] stones").

Fine-grained, dense carbonate rocks can be classified as lime silt, dolomite silt, etc., whereby it should be noted that carbonate particles of this size are usually not clastic, but are generated in the deposition area (" in situ ") by organisms ( bioerosion , active separation of calcium carbonate by protozoa ), although these particles can of course be rearranged later.

Web links

Commons : Silt  - collection of images, videos and audio files
Wiktionary: Schluff  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. see e.g. B. Amr A. El-Sammak: Nature and genesis of silt-size carbonate sediments; Northern Red Sea, Egypt. Carbonates and Evaporites. Vol. 16, No. 1, 2001, pp. 37-45, doi: 10.1007 / BF03176225