Sandstone

Sandstone is a sedimentary rock with a share of at least 50 % sand grains. The sand grains are, by self-definition from 0.063 to 2 mm in size and consist primarily of quartz.

Definition, nomenclature and differentiation related rocks

In a sandstone, whose sand fraction consists of at least 90 % quartz, one speaks of a quartz sandstone or sandstone in the strict sense. As a share of 25 % feldspars one of Arkose speaks in a high proportion of clayey matrix and a fraction of less than 75 % quartz in the sand fraction of greywacke. The corresponding transitional forms ( quartz content in the sand fraction, respectively 90-75 %) or hot Subarkose Subgrauwacke. Sandstones with a high proportion of primary calcium carbonate are also called sand-lime bricks. These may not be the same as indicated by the, man-made sand-lime bricks are mistaken.

Predominates in the rock, the proportion of grains that are larger than 2 mm, it is a conglomerate (with rounded grains = cobbles ) or a breccia ( with angular grains = fragments ). Unlike arkose, greywacke and calcareous sandstones conglomerates and breccias are not considered sandstones also in the broader sense. However, the transitions from a coarse-grained sandstone to fine-grained conglomerate or a fine-grained breccia are fluid.

Formation

Sandstone is formed by cementing ( cementation ) of loose sand and admixtures of coarser or finer sediment particles (clay, silt, pebbles ). He is, in contrast to chemical and biogenic (mainly from the remains of living organisms combined) rocks, such as limestone or coal, clastic ( detrital ) origin, and thus consists of small debris weathered and worn rocks. Quartz comes in many igneous rocks before primary. Since quartz is also relatively resistant to chemical weathering, is sand -grain-sized quartz particles in the rock cycle ( exogenous cycle) can accumulate strong, while other potentially sandkorn forming minerals chemically decompose relatively quickly. Therefore, most sandstones consist predominantly of quartz.

The most important development areas for sandstones are the shallow coastal seas, the shelves. However, the sand comes mostly from the mainland and is transported by rivers and ocean currents to its final deposition site. In deep, offshore marine regions, however, are deposited predominantly fine -grained slurries, from which no sandstone can occur. Exceptions are the sandstones turbiditischer deposits. This can be seen from the sandy material shelf edge that the continental slope slides down. There are also numerous sandstone deposits, which arose on the mainland. In this case outweigh the fluvial, ie the deposited by rivers, sandstones well above the Aeolian so deposited by the wind.

The depositional environment of a sandstone can be determined, inter alia, using the information contained sedimentary structures. For this purpose, each with characteristic forms of cross-bedding or certain trace fossils. Also body fossils are important indicators of the depositional environment. For the distinction of fluvial aeolian sandstones is the grain size and especially the grain sorting ( textural maturity) of significance: AEOLIAN sandstones are always relatively homogeneous fine-grained. The mineral composition and grain rounding give clues about the formation. A relatively small proportion of quartz (low compositional maturity) and small grain rounding grades are typical of sandstones that were deposited in Gebirgsvortiefen or foreland basin as part of flysch or Molasseserien, greywacke and arkose.

Through the tamper head of the younger, overlying sediments, as well as by precipitation of minerals dissolved in the sea or ground water, often by the minerals contained in the sandstone itself it comes to solidify ( diagenesis ) of sand. This process is different from fast and lasts from a few decades and several million years.

Cementation

When cementation is called the natural hardening of the sand by the precipitation of minerals in the voids between the sand grains ( pore space ). The cement can be made of different minerals:

  • Quartz cemented sandstones ( quartzitic sandstones, " cement quartzite " ) owe their solidification, inter alia, the pressure solution and subsequent reprecipitation of quartz in the form of Anwachssäumen connecting the sand grains together. Previously the term pebble sandstone was common for a corresponding sandstone. Quartz is the most abundant mineral cement in sandstones.
  • Carbonate- cemented sandstones contain as binders usually calcite, ankerite or siderite rare. Calcite is also a very common mineral cement.
  • Ferritic cemented sandstones (iron sandstone ) as a binder iron oxides and oxide hydrates (hematite, goethite, limonite ).
  • Other possible cement minerals are feldspars, clay minerals ( kaolinite, illite, chlorite ), zeolite or plaster.

A high content of clay minerals or micas primary may impede cementation, since this reduces the porosity and formation water prevents the penetration. An even more effective Zementationsverhinderung done by a impregnation with oil.

Most sandstones have at least within a single bank to a single cement mineral. But Different layers in a sandstone sequence may be cemented in each case by different minerals, eg due to grain size differences. Thus, coarse-grained sandstones are cemented rather großporigere carbonate and feinkönigere rather quarzitisch. Several cement generations of different minerals within a bank is an exception and can be found only in larger pores. The younger generation cement is thereby enclosed by the elderly.

Dissemination

Sandstone is a very widespread on the Earth's surface rocks and occurs practically in all continents and is handed down from almost all ages in the earth. It occurs in various geological contexts, such as in the overburden of the weak subsidenten continental plates or (formerly ) more subsidenten epicontinental sedimentary basins. Most of these sandstones formed under marine cover. These include Germany in the sandstones of the Elbe Sandstone Mountains and the Zittau Mountains. In central and southern Germany are widespread, the fluvial sandstones of the Buntsandstein. Sandstones in the subsurface of the North German Plain and the North Sea are important reservoir rocks for oil and gas. In the Alpine region occur sandstones inter alia in the Molasse Zone and the greywacke zone. In general, the proportion of sandstones in the geologically young alpidischen fold mountains is rather low, which is due to the fact that these are the result of wide tropical Schelfen with predominantly carbonate sedimentation. Also be found in the geologically very old continental shields few sandstones, as these regions are in constant enhancement and therefore there today almost exclusively occurring rocks that have been developed deep in the crust or once sunk there and thereby greatly changed.

  • Examples of sandstone formations

Rocky gorge in the Elbe Sandstone Mountains, Tisa Walls

Of erosion in the Garden of the Gods in Colorado

Bungle Bungle in the Australian Purnululu National Park

Natural flushing in the Navajo Sandstone ( Antelope Canyon in Arizona)

Red sandstone at Petra

Soil formation on sandstone

Due to the prevalence of quartz occur in Central Europe sandstone generally nutrient-poor soils, which also tend to acidification. Depending on the water supply and development depth of soil you will find Ranker, brown earths or podzols. Most of these soils are used for forestry (for example, in the Thuringian wood country).

However, exceptions are sandstones with a high carbonate or clay and sandstone sequences are incorporated into the calcareous or clayey layers. The soils on these rocks are then usually richer in nutrients and can be used for agriculture. An example are sandstones of the Keuper ( Upper Triassic) in southern Germany.

Properties and Applications

Sand stone in architecture

Sandstone is a common building material and was often the patches used for sculptures and especially for facades. The cityscape formative sandstone can be found for example in Bern, Breslau, Dresden, Dusseldorf, Hamburg, Nuremberg and Prague. Even the Freiburg Münster is largely built of sandstone. In Dresden was used for the interior decoration and sculptures a soft sandy soils, the Cottaer sandstone, and for load-bearing or otherwise loaded architectural elements a quarzitisch bound sandstone of the Elbe Sandstone Mountains, as it is still mined at Pirna.

The color of sandstone can, just like the sand, vary, usual colors are gray ( without additions - such as Ruhr sandstone from Hohensyburgstraße ), yellow ( by contained limonite - Ibbenbürener sandstone), brown, red ( by hematite - as in red Weser sandstone ) and white ( as in gray, only the surface reflection is another - example Rackwitzer sandstone). Green is the earlier south of Soest degraded green sandstone; the now degraded at Anroechte glaukonithaltige limestone, however, falls somewhat darker than the green sandstone used since the Middle Ages.

Basically, break fresh sandstones can be technically easier than longer edit stored. Common to all sandstones is the lack of de-icing salt. The frost resistance is different depending on the variety. Due to the polarity of the deposition direction can be " in stock " are different (parallel to bedding ) and " against the camp " (orthogonal to the stratification ) cut sandstones. The resulting textures on the visible surfaces can be very different.

  • Using examples of significant sandstone places

The Cotton Exchange in Bremen White Main sandstone

The Old Königsberg exchange in Kaliningrad Oberkirchner sandstone

The Semper Gallery in the Zwinger from varieties of Elbsandsteins

The Berliner Dom with Silesian sandstone varieties Friedersdorf and Wünschelburger sandstone

The cladding of the Metz railway station consists of yellow Vosges sandstone

Reddish Vosges sandstone at the Strasbourg Cathedral

Sandstone buildings as biotopes

Dry stone walls, built of sandstone rock fragments or sandstone derived from a quarry nearby, today are important habitats for numerous succulent plants and important refuges for insects. In the ungrouted masonry find not only spiders and beetles shelter, but also larger animals such as lizards and slow worms. Specifically, in viticulture areas characterize dry sandstone walls the landscape of the rivers of southern Germany and the basins of the Rhine and Main.

Natural stone varieties

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