Sandstone

Sandstones are sedimentary rocks. They are considered to be the most useful type of rock for decoding Earth history. These rocks are also used as building materials, and as metallic minerals sources.

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 mineral or rock grains. The sand grains in a sandstone are generally 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.


Most sandstone is composed of quartz and/or feldspar, because these are the most common minerals in the Earth’s crust. Like sand, sandstone can be of any color, but the most common ones  are tan, brown, yellow, red, gray, and white.
Although some sandstones are resistant to weathering, they are easy to work with, which is why they are used in building and paving materials. Hornblende and biotite are the most chemically and physically susceptible to destruction, and they would be eliminated in the early stage of transport, unlike Orthoclase and quartz which would persist longer. They are more chemically harder, and not prone to cleavage, for this reason extremely durable during transport. Quartz is particularly the most abundant type of sand grain present in sandstone.

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; They are formed from cemented grains that may be fragments of a pre-existing mineral. The cement holding these grains together are typically: calcite, clays and silica. Grain sizes are in the range of 0.1 to 2 millimeters.

The formation of sandstone present two principal phases. First, a layer of sand accumulates as the result of sedimentation from water or air. 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.

Principal environments of deposition may be split between terrestrial and marine, as illustrated these 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
    1. Beach and shoreface sands
    1. Tidal deltas and flats
    1. Offshore bars and sand waves
    1. Storm deposits (tempestites)
  1. Turbidites (submarine channels and fans)
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 mineral grains, rock fragments, or pieces of fossils which are held together by a mineral cement. The definition of sandstone is based on the size of the grains:
It grades into siltstone, shale or mudstone (grains less than 0.0625 mm in diameter) and conglomerate ( if the 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 becouse sandstone contains a network of pores which are at least partly filled with a mineral cement. Sandstone may also include biogenic grains (shells, coralline algae, etc), but a rock that usually contains more than 50% of sand-sized carbonate grains is named calcarenite.

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/

Sandstones are divided in three main groups:

  • Arkosic sandstones, which have a high (greater than 25 percent) feldspar content and a composition similar to granite.
  • Quartzose sandstones (beach sand’), which have a high (greater than 90 percent) quartz content.
  • Argillaceous sandstones, which contein a significant livel of clay or silt.
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 also compositionally variable rocks which gives rise to a lot of more types:

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.
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.
Graywacke  Graywacke is usually dark-colored, coarse-grained, lithic, well-indurated, and immature sandstone.
Greensand A sandstone which contains lots of green clay mineral glauconite.
Grit A coarse-grained variety with angular grains.
Orthoquartzite A relatively pure light-colored quartz-sandstone. The term has been applied to well-indurated (quartzitic) rocks in the past, but today is refered to 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.
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 has many uses. Strongly cemented ones are used as a building material all over the world.
Sandstone is often used in construction.The old town of Alghero (the walls and all the fortifications), in the north-west of Sardinia, were built mainly of yellow sandstone coming from the coast.
Other 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).

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 is a common material in road construction and sand is a principal component of concrete.

Pure quartz sand is used to make glass, carborundum, and semiconductors.
Special types of crushed sandstones are used in agriculture as a soil conditioners or fertilizer.
Chemical industry also uses sandstone because it is very resistant to most acids.

Finally, this rock is also a very important material for geologists because it contains lots of information to reconstruct the Earth’s geologic history.

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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