Potter and R. Siever, , Sand and sandstone , 2nd ed. Spearing, , Sandstone depositional environments: clastic terrigenous sediments , American Association of Petroleum Geologists Memoir no. Jenson Vanities James Martin Vanities. Design Services What's New? What is Limestone? What is Marble? What is Sandstone? What is Shale? What is Slate? What is Soapstone? What is Travertine? New Store. Sandstone worn smooth. Sandstone near Stadtroda, Germany. Quartz Sandstone. Gallery Sandstone. Sandstone is a type of rock made from sediment — a sedimentary rock.
The sediment particles are clasts, or pieces, of minerals and fragments of rock, thus sandstone is a clastic sedimentary rock. It is composed mostly of sand particles, which are of medium size; therefore, sandstone is a medium-grained clastic sedimentary rock.
The sand grains that makeup sandstone are aptly referred to as framework grains. Sandstone may include finer and coarser material and still be called sandstone, but if it includes more than 30 percent grains of gravel, cobble or boulder size it's classified instead as conglomerate or breccia together these are called rudites.
Sandstone has two different kinds of material in it besides the sediment particles: matrix and cement. Matrix is the fine-grained stuff silt and clay size that was in the sediment along with the sand whereas cement is the mineral matter, introduced later, that binds the sediment into rock.
Sandstone with a lot of matrix is called poorly sorted. If matrix amounts to more than 10 percent of the rock, it is called a wacke "wacky". A well-sorted sandstone little matrix with little cement is called an arenite. Another way to look at it is that wacke is dirty and arenite is clean. You may notice that none of this discussion mentions any particular minerals, just a certain particle size.
But in fact, minerals make up an important part of sandstone's geologic story. Sandstone is formally defined strictly by particle size, but rocks made of carbonate minerals don't qualify as sandstone.
Carbonate rocks are called limestone and given a whole separate classification, so sandstone really signifies a silicate-rich rock. A medium-grained clastic carbonate rock, or "limestone sandstone," is called calcarenite. This division makes sense because limestone is made in clean ocean water, whereas silicate rocks are made from sediment eroded off the continents.
Mature continental sediment consists of a handful of surface minerals , and sandstone, therefore, is usually almost all quartz. Other minerals—clays, hematite, ilmenite, feldspar , amphibole, and mica — and small rock fragments lithics as well as organic carbon bitumen add color and character to the clastic fraction or the matrix. A sandstone with at least 25 percent feldspar is called arkose. A sandstone made of volcanic particles is called tuff.
The cement in sandstone is usually one of three materials: silica chemically the same as quartz , calcium carbonate or iron oxide. These may infiltrate the matrix and bind it together, or they may fill the spaces where there is no matrix. Depending on the mix of matrix and cement, sandstone may have a wide range of color from nearly white to nearly black, with gray, brown, red, pink and buff in between.
Sandstone forms where sand is laid down and buried. Usually, this happens offshore from river deltas , but desert dunes and beaches can leave sandstone beds in the geologic record too. The gray and white grains are mostly quartz, the black grains are particles of impure coal and shale, and the brown material is stained clay minerals that probably formed when feldspar grains in the sandstone were broken down by weathering. 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. Of these minerals, 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 often the most abundant type of sand grain present in sandstone. It is abundant in source materials and is the most durable during transport. The best way to learn about rocks is to have specimens available for testing and examination. The grains in a sandstone can be composed of mineral, rock, or organic materials.
Which and in what percentage depends upon their source and how they were altered during transport. Mineral grains in sandstones are usually quartz.
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