Sandstone
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Sandstone is an arenaceous sedimentary rock composed mainly of feldspar and quartz and varies in colour (in a similar way to sand), through grey, yellow, red, and white. Since sandstones often form highly visible cliffs and other rock formations, 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.
Sandstones are often relatively soft and easy to work which therefore make them a common building and paving material.
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 from seismic activity.
Origins
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Sandstones are clastic in origin (as opposed to organic, like chalk or coal). They are formed from the cemented grains that may be fragments of a pre-existing rock, or else just mono-minerallic crystals. The cements binding these grains together are typically calcite, clays and silica. Grain sizes in sands are in the range of 0.1mm to 2mm. (Rocks with smaller grainsizes include siltstones and shales and are typically called argillaceous sediments, as are also clays. Rocks with larger grainsizes include both breccias and conglomerates and are termed rudaceous sediments.).
The principal mechanism for the formation of sandstone is by the sedimentation of grains out of a fluid, such as a river, lake or sea. The environment of deposition is crucial in determining the characteristics of the resulting sandstone, which on a finer scale include its grainsize, sorting, composition and on a larger scale include the rock geometry. 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)
- Lakes
- Marine environments
- Shoreface sands
- Deltas
- Turbidites (submarine channels)
Types of sandstone
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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.
- quartzose sandstones 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.
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