Sandstone

Sandstones constitute between 10 and 20 percent of the Earth’s sedimentary rock record. Sandstones are used as building materials, and as valuable sources of metallic ores. Most significantly, they are the single most useful sedimentary rock type for deciphering Earth history.


Sandstone is very often visibly layered. The width of the sample from Scotland is 7 cm.
Sandstone is very often visibly layered. The width of the sample from Scotland is 7 cm.

image source: http://www.sandatlas.org/sandstone/

WHAT IS SANDSTONE?

Sandstone is a sedimentary rock composed mainly of sand-size mineral or rock grains. Most sandstone is composed of quartz and/or feldspar, because these are the most common minerals in the Earth’s crust. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow, red, gray, and white.


Some sandstones are resistant to weathering, yet are easy to work. This makes sandstone a common building and paving material. Given the hardness of the individual grains, uniformity of grain size, and friability of its structure, sandstone is an excellent material from which to make grindstones for sharpening blades and other implements. Non-friable sandstone can be used to make grindstones for grinding grain (such as gritstone).

The grains of sand in a sandstone are usually particles of mineral, rock, or organic material that have been reduced to “sand” size by weathering and transported to their depositional site by the action of moving water, wind, or ice. Their time and distance of transport may be brief or significant, and during that journey the grains are acted upon by chemical and physical weathering.

If the sand is deposited close to its source rock, it will resemble the source rock in composition. However, the more time and distance that separate the source rock from the sand deposit, the greater its composition will change during transport. Grains that are composed of easily-weathered materials will be modified, and grains that are physically weak will be reduced in size or destroyed.

If a granite outcrop is the source of the sand, the original material might be composed of grains of hornblende, biotite, orthoclase, and quartz. Hornblende and biotite are the most chemically and physically susceptible to destruction, and they would be eliminated in the early stage of transport. Orthoclase and quartz would persist longer, but the grains of quartz would have the greatest chance of survival. They are more chemically inert, harder, and not prone to cleavage. Quartz is typically the most abundant type of sand grain present in sandstone. It is extremely abundant in source materials and is extremely durable during transport.

Source: http://geology.com/rocks/sandstone.shtml

 

Red sandstone formation under blue sky, The Wave, Coyote Buttes, Paria Canyon, Vermilion Cliffs National Monument, Arizona, Southwest, USA, America.
Red sandstone formation under blue sky, The Wave, Coyote Buttes, Paria Canyon, Vermilion Cliffs National Monument, Arizona, Southwest, USA, America.

image source: https://www.robertharding.com/index.php?lang=en&page=search&s=wave&smode=0&zoom=1&display=5&sortby=0&bgcolour=white

How is Sandstone Formed?

Sandstones are clastic in origin (as opposed to organic, like chalk and coal, or chemical, like gypsum and jasper). They are formed from cemented grains that may be fragments of a pre-existing rock or crystals of a single mineral. The cements binding these grains together are typically calcite, clays and silica. Grain sizes in sands are in the range of 0.1 to 2 millimeters. Rocks with smaller grain sizes include siltstones and shales and are typically called argillaceous sediments. Clays and rocks with larger grain sizes include both breccias and conglomerates and are termed rudaceous sediments.

The formation of sandstone involves two principal stages. First, a layer or layers of sand accumulates as the result of sedimentation, either from water (as in a river, lake, or sea) or from air (as in a desert). Typically, sedimentation occurs by the sand settling out from suspension, that is, ceasing to be rolled or bounced along the bottom of a body of water (such as a sea or river) or ground surface (such as in a desert or sand dune region).

Finally, once it has accumulated, the sand becomes sandstone when it is compacted by pressure of overlying deposits and cemented by the precipitation of minerals within the pore spaces between sand grains. The most common cementing materials are silica and calcium carbonate, which are often derived either from dissolution or from alteration of the sand after it was buried.

Deposition from sand dunes can be recognized by irregular and fluidly shaped weathering patterns and wavy coloration lines when sectioned. Water deposition forms more regular blocks when weathered. The regularity of the latter favors use as a source for masonry, either as a primary building material or as a facing stone, over other construction. Principal environments of deposition may be split between terrestrial and marine, as illustrated by the following broad groupings:

  • Terrestrial environments:
  1. Rivers (levees, point bars, channel sands)
  2. Alluvial fans
  3. Glacial outwash
  4. Lakes
  5. Deserts (sand dunes and ergs)
  • Marine environments:
  1. River deltas
  2. Beach and shoreface sands
  3. Tidal deltas and flats
  4. Offshore bars and sand waves
  5. Storm deposits (tempestites)
  6. Turbidites (submarine channels and fans)

Source: http://www.newworldencyclopedia.org/entry/Sandstone

Sand dune in Sahara (Morocco). Sand dunes are in constant motion as you can see here the sand grains blowing off the crest of the dune. But as soon as environmental conditions change enough for the dune to lose its mobility, sand grains will be slowly cemented together by minerals precipitating out from groundwater and the formation of sandstone begins.
Sand dune in Sahara (Morocco). Sand dunes are in constant motion as you can see here the sand grains blowing off the crest of the dune. But as soon as environmental conditions change enough for the dune to lose its mobility, sand grains will be slowly cemented together by minerals precipitating out from groundwater and the formation of sandstone begins.

image source: http://www.sandatlas.org/sandstone/

Types and Compositions of Sandstone

 

Sandstone is composed of sand-sized (0.0625…2 mm) mineral grains, rock fragments, or pieces of fossils which are held together by a mineral cement. It grades into siltstone, shale or mudstone (grains less than 0.0625 mm in diameter) and conglomerate (or breccia if the clasts are angular) if the average grain-size exceeds 2 mm1.
Sandstone and other clastic sedimentary rocks differ from the igneous rocks in possessing a framework of grains which only touch each other but are not in a continuous contact. Consequently, sandstone contains a network of pores which are at least partly filled with a mineral cement. However, sandstone does not need to contain open pores, they may be, and often are, completely filled with a cementing material. The definition of sandstone is based on the size of the framework grains.

Sandstone may even contain biogenic grains (shells, coralline algae, etc), but a rock that contains more than 50% of sand-sized carbonate grains is usually named calcarenite which is a type of limestone.

Source: http://www.newworldencyclopedia.org/entry/Sandstone

This coarse-grained rock from Cyprus shares both sand- and limestone properties. It is clearly clastic, but it is composed of carbonate grains of biogenic origin. Such rocks are known as calcarenite, they are considered to be a subtype of limestone. The width of the sample is 7 cm.
This coarse-grained rock from Cyprus shares both sand- and limestone properties. It is clearly clastic, but it is composed of carbonate grains of biogenic origin. Such rocks are known as calcarenite, they are considered to be a subtype of limestone. The width of the sample is 7 cm.

image source: http://www.sandatlas.org/sandstone/

Once the geological characteristics of a sandstone have been established, it can then be assigned to one of three broad groups:

  • arkosic sandstones, which have a high (greater than 25 percent) feldspar content and a composition similar to granite.
  • quartzose sandstones (also known as ‘beach sand‘), which have a high (greater than 90 percent) quartz content. Sometimes these sandstones are termed “Quartzites“—for example, the Tuscarora Quartzite of the Ridge-and-valley Appalachians.
  • argillaceous sandstones, such as greywacke, which have a significant clay or silt content.

Source: http://www.newworldencyclopedia.org/entry/Sandstone

Sandstone is usually known as layered and often cross-stratified sedimentary rock, but sometimes it may form even columns. These columns occur in the Negev Desert (Makhtesh Ramon), Israel.
Sandstone is usually known as layered and often cross-stratified sedimentary rock, but sometimes it may form even columns. These columns occur in the Negev Desert (Makhtesh Ramon), Israel.

image source: http://www.sandatlas.org/sandstone/

Sandstones are widespread and compositionally variable rocks which gives rise to a myriad of more narrowly defined varieties:

Sandstone or related rock type Description
Arenite A general term for all sandstones.
Arkose A feldspar-rich (>25%) variety.
Calcarenite A limestone variety composed of sand-sized non-terrestrial carbonate grains. Sandstone which is composed of terrestrial limestone fragments is calclithite.
Calclithite A variety of terrigenous sandstone consisting of carbonate grains (>50%) from disintegrated limestones.
Flagstone A sandstone that is readily split into thin flags suitable for paving. Fissility is given to the rock by mica.
Graywacke A term that has been defined in several ways which has caused confusion and ultimately lead to its use only as a field term. Graywacke is generally imagined to be dark-colored, coarse-grained, lithic, well-indurated, and immature sandstone.
Greensand A sandstone or sand that contains lots of green clay mineral glauconite.
Grit A coarse-grained variety with angular grains. The term has also been applied to fine-grained gritty rocks in the past.
Orthoquartzite A relatively pure light-colored quartz-sandstone. The term has been applied to well-indurated (quartzitic) rocks in the past, but nowadays it seems to involve all pure sandstones regardless of how friable they are.
Psammite A synonym of sandstone and arenite.
Quartz arenite An almost pure sandstone. Quartz content is above 90-95%.
Quartzite A metamorphosed sandstone. The term has been applied to hard sedimentary sandstones.

Source: http://www.sandatlas.org/sandstone/

This is what happens to sandstone if it gets buried deep enough. Sand grains fuse together to form a metamorphic rock known as quartzite. Telemark, Norway. Width of sample 9 cm.
This is what happens to sandstone if it gets buried deep enough. Sand grains fuse together to form a metamorphic rock known as quartzite. Telemark, Norway. Width of sample 9 cm.

image source: http://www.sandatlas.org/sandstone/

where can we find  sandstone today?

Sandstone is a rock type which has many uses. Strongly cemented rocks are used as a building material all over the world where the material is readily available. Sandstone is often used in construction.

The old town of Alghero, in the north-west of Sardinia, was built entirely of yellow sandstone coming from the coast, until the ’50s, including the walls and all the fortifications. It was used for this purpose since ancient times. Famous example: the Romans used it in the last quarter of the second century A.D. for the construction of the Porta Nigra (Gaul, Germany today).

Source: http://www.sandatlas.org/sandstone/

Porta Nigra Trier from the field north.
Porta Nigra Trier from the field north.

image source: http://www.treveris.com/porta_nigra_it.htm

Crushed sandstone (as sand) is a common filling material in road construction and sand is a principal component of concrete.

Pure quartz sand is a source of silica which is used to make glass, carborundum, and semiconductors. Some strong rocks with sharp grains (gritstone) are good for grinding.
Special types of crushed sandstones are used in agriculture as a soil conditioners (lime sand) or fertilizer (glauconite sand).

Chemical industry uses sandstone because it is very resistant to most acids (however, this is true if the sand is really almost pure quartz sand). Because the rock is porous, it is by far the most important reservoir rock of ground water and hydrocarbons (crude oil and natural gas).

This rock is also a very valuable material for geologists because it is abundant, resistant to diagenesis, and contains lots of information to reconstruct the Earth’s geologic history.

Source: http://www.sandatlas.org/sandstone/

The Main Quadrangle of the University of Sydney, a so-called Sandstone university.
The Main Quadrangle of the University of Sydney, a so-called Sandstone university.

image source: https://en.wikipedia.org/wiki/Sandstone

 

info sources: http://www.sandatlas.org/sandstone/

http://www.newworldencyclopedia.org/entry/Sandstone

http://geology.com/rocks/sandstone.shtml

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