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:
- Terrestrial environments
- Marine environments
Types of sandstoneOnce 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