Silicification

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Blocks of silicified and therefore very verwitterungsresistentem, late Cretaceous sandstone ( " tertiary quartzite ") form erosion relics on the Frankish Alb .
A slice of silicified wood from the Arizona Triassic , Petrified Forest National Park , USA
Silicified brachiopod leading limestone from the Devonian of Indiana, USA. In an enlarged view, so-called silification rings can be seen on the surface of the brachiopod shell

Silicification , also called silification , is the impregnation or conversion of a naturally occurring mineral or organic material with or to microcrystalline silicon dioxide (SiO 2 ). Silicification can take place through the precipitation of dissolved SiO 2 in the pore space of sediments or soils or through metasomatic or diagenetic displacement of other minerals by the SiO 2 (dissolution of the previous mineral with simultaneous precipitation of SiO 2 ).

The term silicification is derived from the word silica , which denotes various forms of SiO 2 dissolved in water and is often used synonymously for the microcrystalline form of this chemical compound. In order for the process to take place, an aqueous solution that contains large amounts of silica is required. This solution must also be able to circulate freely in the fracture or pore space so that it can reach areas with suitable chemical conditions for the precipitation.

Certain sediments as Radiolarien sludges, silicification during diagenesis due to their high primary SiO 2 -content almost inevitably. The same applies to quartz sandstones , although these are much less often silicified. Lime sludge often also contains certain amounts of SiO 2 particles, for example sponge needles . In the alkaline environment ( pH value  > 7) that generally prevails in carbonate sediments, these particles are dissolved and the solutions circulate in the pore space of the sediment. Where there is a locally acidic environment (pH value <7) in the sediment, the SiO 2 can precipitate again. Acidic milieu is often created by bacteria that decompose organic matter, for example a dead marine animal or the mucus on the wall of a dwelling of such an animal. Lime sludge and animal remains can then silicify there. This is how the flint lumps and layers are created in fine-grain limestones.

Areas with active volcanism or thermal springs, such as Yellowstone National Park, are typical places for silicification processes.

Silicifications in soils and paleo soils are known as silcretes . They arise from the desilification of silicate rocks such as granite or gneiss and the subsequent precipitation of SiO 2 , i.e. as a result of weathering processes .

Artificial silicification

To seal the building , the pore space is often filled with water glass against rising damp, and silicic acid esters are used to consolidate natural stone and plastering mortar. These techniques are also called silicification . Artificial silicification is also used on concrete floors (e.g. in industrial halls), as the lime particles in the concrete core, the so-called concrete dust, reach the surface. Smoothed floors have a micro-roughness (the surface is porous ), which is removed during silicification. In addition, the lime particles are chemically bound inside and the core is compacted.

See also

Individual evidence

  1. ^ Hans Murawski, Wilhelm Meyer: Geological dictionary . 11th edition. Elsevier / Spektrum, Heidelberg 2004, ISBN 3-8274-1445-8 , pp. 262 .
  2. ↑ Silicification. Retrieved April 29, 2017 .