Everything about Sandstone totally explained
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. Since sandstone beds often form highly visible cliffs and other
topographic features, certain colors of sandstone have been strongly identified with certain regions.
Some sandstones are resistant to
weathering, yet are easy to work. This makes sandstone a common
building and
paving material. Because of 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, for example,
gritstone.
Rock formations that are primarily sandstone usually allow
percolation of water and are
porous enough to store large quantities, making them valuable
aquifers. Fine-grained aquifers, such as sandstones, are more apt to filter out pollutants from the surface than are rocks with cracks and crevices, such as
limestones or other rocks fractured by
seismic activity.
Origins of Sandstone
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 either be fragments of a pre-existing rock or be mono-minerallic
crystals. The cements binding these grains together are typically
calcite,
clays and
silica.
Grain sizes in sands are in the range of 0.1 mm to 2 mm (clays and rocks with smaller grain sizes including
siltstones and
shales are typically called
argillaceous sediments; rocks with larger grain sizes including
breccias and
conglomerates 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, for example, ceasing to be rolled or bounced along the bottom of a body of water (for example, seas or rivers) or ground surface (for example, in a desert or sand dune region). Finally, once it has accumulated, the sand becomes sandstone when it's
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. Colors will usually be tan or yellow (from a blend of the clear quartz with the dark amber feldspar content of the sand). A predominant additional colorant in the southwestern United States is
iron oxide, which imparts reddish tints ranging from pink to dark red (
terra cotta), with additional
manganese imparting a purplish hue. Red sandstones are also seen in the Southwest and West of
England and
Wales, as well as central
Europe and
Mongolia. Deposition from sand dunes can be recognized by irregular and fluidly shaped weathering patterns and wavy coloration lines when sectioned, while water deposition will form 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.
The environment where it's deposited is crucial in determining the characteristics of the resulting sandstone, which, in finer detail, include its
grain size,
sorting and
composition and, in more general detail, include the rock geometry and sedimentary structures. Principal
environments of deposition may be split between terrestrial and marine, as illustrated by the following broad groupings:
Marine environments
Deltas
Beach and shoreface sands
Tidal flats
Offshore bars and sand waves
Storm deposits (tempestites)
Turbidites (submarine channels and fans)
Types of sandstone
Once the geological characteristics of a sandstone have been established, it can then be assigned to one of three broad groups:
arkose or arkosic sandstones, which have a high (>25%) feldspar content and a composition similar to granite.
quartzose sandstones, also known as "beach sand", which have a high (>90%) quartz content. Sometimes these sandstones are termed "orthoquartzites", for example, the Tuscarora Quartzite of the Ridge-and-valley Appalachians.
argillaceous sandstones, such as greywacke or bluestone, which have a significant clay or silt content.
According to the USGS, U.S. sandstone production in 2005 was 192,000 metric tons worth $24.3 million, the largest component of which was the 121,000 metric tons worth $9.75 million of flagstone or dimension stone.
Further Information
Get more info on 'Sandstone'.
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