Home » Rocks » Sedimentary Rocks » Sandstone Sandstone A clastic sedimentary rock composed of sand-size grains of mineral, rock, or organic material. Article by: Hobart M. Find Other Topics on Geology. Maps Volcanoes World Maps. Rock, Mineral and Fossil Collections. Hardness Picks. Flint, Chert, and Jasper. Tumbled Stones. Fluorescent Minerals. Lapis Lazuli. Rocks: Galleries of igneous, sedimentary and metamorphic rock photos with descriptions.
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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.
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.
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. 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. The environment where it is 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:. Once the geological characteristics of a sandstone have been established, it can then be assigned to one of three broad groups:.
Boggs, J. Toronto: Merril Publishing Company. 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 famous red rocks of the Grand Canyon, for instance, formed in a desert setting. Fossils can be found in sandstone, although the energetic environments where sand beds form don't always favor preservation.
When sand is deeply buried, the pressure of burial and slightly higher temperatures allow minerals to dissolve or deform and become mobile. The grains become more tightly knit together, and the sediments are squeezed into a smaller volume. This is the time when cementing material moves into the sediment, carried there by fluids charged with dissolved minerals.
Oxidizing conditions lead to red colors from iron oxides while reducing conditions lead to darker and grayer colors. The sand grains in sandstone give information about the past:.
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