Sandstone



What is sandstone?



Sandstone is a sedimentary rock that is composed of sand size grain particles such as minerals, rock fragments or organic material. Sandstone is the most common rock type found throughout the world. 
Most sandstone is composed of quartz or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow, red, grey, pink, white, and black. Since sandstone beds often form highly visible cliffs and other topographic features, certain colours of sandstone have been strongly identified with certain regions.
Rock formations that are primarily composed of sandstone usually allow percolation of water and other fluids and are porous enough to store large quantities, making them valuable aquifers and petroleum reservoirs. Fine-grained aquifers, such as sandstone, are better able to filter out pollutants from the surface than are rocks with cracks and crevices, such as limestone or other rocks fractured by seismic activity.
Quartz-bearing sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. It has large pore spaces and is often bind together by cementing material or clay and silt as a matrix. Sandstone is often mined to use as a construction material e.g. building block of a wall or as a raw material for other manufacturing. Due to the large pore spaces of sandstone, it is interconnected thus has permeability that allows liquid and gas to pass through it. For this reason sandstone mostly occur as aquifer for ground water and even a reservoir for oil and gas.

What is Sand?

To a geologist, the word "sand" in sandstone refers to the particle size of the grains in the rock rather than the material of which it is composed. Sand-size particles range in size from 1/16 millimetre to 2 millimetres in diameter. Sandstone are rocks composed primarily of sand-size grains.

Origin of sandstone

Sandstones are clastic in beginning (rather than either natural, similar to chalk and coal, or compound, similar to gypsum and jasper). They are framed from solidified grains that may either be pieces of a prior shake or be mono-minerallic precious stones. The concretes restricting these grains together are normally calcite, clays, and silica. Grain sizes in sands are characterised (in topography) inside the scope of 0.0625 mm to 2 mm (0.002–0.079 inches). Clays and residue with littler grain sizes not obvious with the bare eye, including siltstone and shale, are commonly called argillaceous dregs; rocks with more noteworthy grain sizes, including breccias and aggregates are named rudaceous residue. 
Red sandstone inside of Lower Antelope Canyon, Arizona, worn smooth by disintegration from blaze flooding over a large number of years. 
The development of sandstone includes two chief stages. Initial, a layer or layers of sand gathers as the aftereffect of sedimentation, either from water (as in a stream, lake, or ocean) or from air (as in a forsake). Ordinarily, sedimentation happens by the sand settling out from suspension; i.e., stopping to be rolled or bobbed along the base of a waterway or ground surface (e.g., in an abandon or erg). At last, once it has gathered, the sand gets to be sandstone when it is compacted by weight of overlying stores and established by the precipitation of minerals inside the pore spaces between sand grains. 
The most widely recognised solidifying materials are silica and calcium carbonate, which are frequently gotten either from disintegration or from adjustment of the sand after it was covered. Hues will as a rule be tan or yellow (from a mix of the unmistakable quartz with the dull golden feldspar substance of the sand). A prevalent extra colourant in the southwestern United States is press oxide, which confers rosy tints going from pink to dull red (earthenware), with extra manganese bestowing a purplish tone. Red sandstone are likewise found in the Southwest and West of Britain, and in addition focal Europe and Mongolia. The normality of the last supports use as a hotspot for brick work, either as an essential building material or as a confronting stone, over other development. 
The earth where it is stored is essential in deciding the attributes of the subsequent sandstone, which, in better detail, incorporate its grain size, sorting, and organisation and, in more broad detail, incorporate the stone geometry and sedimentary structures. Vital situations of testimony might be part amongst earthbound and marine, as showed by the accompanying general 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. Deltas
  2. Beach and shoreface sands
  3. Tidal flats
  4. Offshore bars and sand waves
  5. Storm deposits (tempestites)
  6. Turbidites (submarine channels and fans)

Components of sandstone

Framework grains

Framework grains are sand-sized (0.0625-to-2-millimetre (0.00246 to 0.07874 in) diameter) detrital fragments that make up the bulk of a sandstone.These grains can be classified into several different categories based on their mineral composition:
  • Quartz framework grains are the dominant minerals in most clastic sedimentary rocks; this is because they have exceptional physical properties, such as hardness and chemical stability.These physical properties allow the quartz grains to survive multiple recycling events, while also allowing the grains to display some degree of rounding. Quartz grains evolve from plutonic rock, which are felsic in origin and also from older sandstones that have been recycled.
  • Feldspathic framework grains are commonly the second most abundant mineral in sandstones. Feldspar can be divided into two smaller subdivisions: alkali feldspars and plagioclase feldspars. The different types of feldspar can be distinguished under a petrographic microscope. Below is a description of the different types of feldspar.
  • Alkali feldspar is a group of minerals in which the chemical composition of the mineral can range from KAlSi3O8 to NaAlSi3O8, this represents a complete solid solution.
  • Plagioclase feldspar is a complex group of solid solution minerals that range in composition from NaAlSi3O8 to CaAl2Si2O8.
  • Lithic framework grains are pieces of ancient source rock that have yet to weather away to individual mineral grains, called lithic fragments or clasts. Lithic fragments can be any fine-grained or coarse-grained igneous, metamorphic, or sedimentary rock, although the most common lithic fragments found in sedimentary rocks are clasts of volcanic rocks.
  • Accessory minerals are all other mineral grains in a sandstone; commonly these minerals make up just a small percentage of the grains in a sandstone. Common accessory minerals include micas (muscovite and biotite), olivine, pyroxene, and corundum. Many of these accessory grains are more dense than the silicates that make up the bulk of the rock. These heavy minerals are commonly resistant to weathering and can be used as an indicator of sandstone maturity through the ZTR index. Common heavy minerals include zircon, tourmaline, rutile (hence ZTR), garnet, magnetite, or other dense, resistant minerals derived from the source rock.

Matrix

Matrix is very fine material, which is present within interstitial pore space between the framework grains. The interstitial pore space can be classified into two different varieties. One is to call the sandstone an arenite, and the other is to call it a wacke. Below is a definition of the differences between the two matrices.
  • Arenites are texturally clean sandstone that are free of or have very little matrix.
  • Wackes are texturally dirty sandstone that have a significant amount of matrix.

Cement

Cement is what binds the siliciclastic framework grains together. Cement is a secondary mineral that forms after deposition and during burial of the sandstone. These cementing materials may be either silicate minerals or non-silicate minerals, such as calcite.
  • Silica cement can consist of either quartz or opal minerals. Quartz is the most common silicate mineral that acts as cement. In sandstone where there is silica cement present the quartz grains are attached to cement, this creates a rim around the quartz grain called overgrowth. The overgrowth retains the same crystallographic continuity of quartz framework grain that is being cemented. Opal cement is found in sandstone that are rich in volcanogenic materials, and very rarely is in other sandstone.
  • Calcite cement is the most common carbonate cement. Calcite cement is an assortment of smaller calcite crystals. The cement adheres itself to the framework grains, this adhesion is what causes the framework grains to be adhered together.
  • Other minerals that act as cements include: hematite, limonite, feldspars, anhydrite, gypsum, barite, clay minerals, and zeolite minerals.

Pore space

Pore space includes the open spaces within a rock or a soil. The pore space in a rock has a direct relationship to the porosity and permeability of the rock. The porosity and permeability are directly influenced by the way the sand grains are packed together.
  • Porosity is the percentage of bulk volume that is inhabited by interstices within a given rock. Porosity is directly influenced by the packing of even-sized spherical grains, rearranged from loosely packed to tightest packed in sandstones.
  • Permeability is the rate in which water or other fluids flow through the rock. For groundwater work permeability may be measured in gallons per day through a one square foot cross section under a unit hydraulic gradient.

Sandstone weathering and transport

Sandstone comprises of particles range from 1/16 to 2 millimetre. Sandstone is derived from pre-existing rock by their weathering. Rock is weathered and break into fine sand size particles which are then carried by transporting agent as water, wind or ice. The particles is transported to the site of deposition and during this transport it is subjected to chemical and physical weathering. When sand deposits near the source rock, the properties will be resembled to that of the source rock but in large time and space difference in transport, the sand particle which is durable will be highly modified and those less resistant can be reduced in size or completely destroyed.
The composition of the source rock matters much for example if the source rock is granite it will contain minerals of hornblende, orthoclase, biotite and quartz. Orthoclase and quartz are more resistant due to their high hardness so they have great chance of survival but biotite and hornblende are less resistible to chemical and physical weathering to they will be destroyed. 

Sand grain types

The grains of sandstone can comprise of minerals, rock or organic matter depend upon the source rock. The sandstone can be classified according to the Folk's classification. If a sandstone has above 90% quartz so it will be quartz arenite. If it has more feldspar it will be arkose and will increasing lithics and feldspar, changes from arkose to lithic arkose and then to feldspathic litharenite. Same is the case with quartz and lithic fragments.

Uses of sandstone

Sandstone has been used for domestic construction and housewares since prehistoric times, and continues to be used.

Sandstone was a popular building material from ancient times. It is relatively soft, making it easy to carve. It has been widely used around the world in constructing temples, homes, and other buildings. It has also been used for artistic purposes to create ornamental fountains and statues.
Some sandstone are resistant to weathering, yet are easy to work. This makes sandstone a common building and paving material including in asphalt concrete. However, some that have been used in the past, such as the Collyhurst sandstone used in North West England, have been found less resistant, necessitating repair and replacement in older buildings. Because of the hardness of individual grains, uniformity of grain size and friability of their structure, some types of sandstone are excellent materials from which to make grindstones, for sharpening blades and other implements. Non-friable sandstone can be used to make grindstones for grinding grain, e.g., gritstone.
A type of pure quartz sandstone, the orthoquartzite, with more of 90–95 percent of quartz, has been proposed for nomination to the Global Heritage Stone Resource. In some regions of Argentina, the orthoquartzite-stoned facade is one of the main features of the Mar del Plata style bungalows.

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