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. Sandstones are often relatively soft and easy to work which therefore make them a common
building[?] and paving material.
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
silts and
clays and are typically called
argillaceous sediments. Rocks with larger grainsizes include both
breccias and
conglomerates and are termed
rudaceous sediments.).
The principle 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:
- Rivers (levees, point bars, channel sands)
- Lakes
- Shoreface sands
- Deltas
- Turbidites[?] (submarine channels)
Once the geological characteristics of a sandstone have been established, it can then be broadly divided between three groups:
- arkosic sandstones, which have a high (>25%) feldspar content
- quartzose sandstones, such as quartzite[?], which have a high (>90%) quartz content.
- argillaceous sandstones, such as greywacke[?], which have a significant fine-grained element
See also: Geology, list of minerals, List of stone -- sedimentary basins.