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Sandstone (sometimes known as arenite) is a clastic sedimentary rock composed mainly of sand-sized minerals 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, grey, pink, white, and black. Since sandstone beds often form highly visible cliffs and other topographic features, certain colors 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 sandstones, 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. == Origins == Sandstones are ''clastic'' in origin (as opposed to either ''organic'', like chalk and coal, or ''chemical'', like gypsum and jasper).〔"A Basic Sedimentary Rock Classification", L.S. Fichter, Department of Geology/Environmental Science, James Madison University (JMU), Harrisonburg, Virginia, October 2000, (JMU-sed-classif ) (accessed: March 2009): separates clastic, chemical & biochemical (organic).〕 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 defined (in geology) within the range of 0.0625 mm to 2 mm (0.002–0.079 inches). Clays and sediments with smaller grain sizes not visible with the naked eye, including siltstones and shales, are typically called ''argillaceous'' sediments; rocks with greater 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 stream, 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 or ground surface (e.g., in a desert or erg). 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. Colours 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 colourant in the southwestern United States is iron oxide, which imparts reddish tints ranging from pink to dark red (terracotta), with additional manganese imparting a purplish hue. Red sandstones are also seen in the Southwest and West of Britain, as well as central Europe and Mongolia. The regularity of the latter favours use as a source for masonry, either as a primary building material or as a facing stone, over other construction. 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: *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 flats #Offshore bars and sand waves #Storm deposits (tempestites) #Turbidites (submarine channels and fans) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「sandstone」の詳細全文を読む スポンサード リンク
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