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 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 may be strongly identified with certain regions. For instance, much of the North American West is well-known for its red sandstones.
Amherst, Ohio is known as the Sandstone Capital of the World. Many buildings in Ottawa, Montreal, Halifax and Boston contain sandstone from Wallace, Nova Scotia.
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 (e.g.
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.
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 (rocks with smaller grain sizes include
siltstones and
shales and are typically called
argillaceous sediments, as too are
clays and rocks with larger grain sizes include both
breccias and
conglomerates and are termed
rudaceous sediments).
|
Sandstone with iron oxide bands |
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, i.e. ceasing to be rolled or bounced along the bottom of a body of water (e.g. seas or rivers) or ground surface (e.g. 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. 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, central
Europe and
Mongolia. Deposition from sand dunes can recognized by irregular and fluidly shaped weathering patterns and wavey 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 of deposition 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:
* Terrestrial environments# Rivers (
levees, point bars, channel sands)# Alluvial fans# Glacial outwash# Lakes # Deserts (sand dunes and ergs)
* Marine environments#
Deltas# Beach and shoreface sands# Tidal deltas, flats# Offshore bars and sand waves# Storm deposits (tempestites)#
Turbidites (submarine channels and fans)
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 (>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", e.g., the Tuscarora Quartzite of the
Ridge-and-valley Appalachians.
*
argillaceous sandstones, such as
greywacke, which have a significant
clay or
silt content.
Boggs, J.R., 2000,
Principles of sedimentology and stratigraphy, 3rd ed. Toronto: Merril Publishing Company. ISBN 0130996963
Folk, R.L., 1965,
Petrology of sedimentary rocks PDF version. Austin: Hemphill's Bookstore. 2nd ed. 1981, ISBN 0914696149
Pettijohn, F.J., P.E. Potter and R. Siever, 1987,
Sand and sandstone, 2nd ed. Springer-Verlag. ISBN 0387963502
Scholle, P.A., 1978,
A Color illustrated guide to constituents, textures, cements, and porosities of sandstones and associated rocks, American Association of Petroleum Geologists Memoir no. 28. ISBN 0891813047
Scholle, P.A., and D. Spearing, 1982,
Sandstone depositional environments: clastic terrigenous sediments , American Association of Petroleum Geologists Memoir no. 31. ISBN 0891813071
Image:USDA Mineral Sandstone 93c3955.jpg|Prepared sample of sandstoneImage:Sandstone Concretion.jpg|A natural sandstone formation composed of cemented quartz sandImage:Wall Patterns.jpg|Sandstone patterns on an chamber wall in PetraImage:Arbroath_Abbey1.jpg|Arbroath Abbey, showing distinctive sandstone colouring*
Old Red Sandstone*
Bargate stone*
Geology*
List of minerals*
List of stone*
sedimentary basins
*
Brownstone
