Sandstone
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 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.
Contents
Grain Size and Roundness
| Grain Size of Sand | Size of Grain (μ) | Size of Grain (ϕ) | Size of Grain (metric) |
|---|---|---|---|
| Very Coarse Upper | 1410 - 2000 | -0.5 - -1.0 | 1.4142 - 2.0 mm |
| Very Coarse Lower | 1000 - 1410 | 0.0 - -0.5 | 1.0 - 1.4142 mm |
| Coarse Upper | 710 - 1000 | 0.5 - 0.0 | 0.7071 - 1.0 mm |
| Coarse Lower | 500 - 710 | 1.0 - 0.5 | 0.5 - 0.7071 mm |
| Medium Upper | 350 - 500 | 1.5 - 1.0 | 0.35355 - 0.5 mm |
| Medium Lower | 250 - 350 | 2.0 - 1.5 | 0.25 - 0.35355 mm |
| Fine Upper | 177 - 250 | 2.5 - 2.0 | 0.17678 - 0.25 mm |
| Fine Lower | 125 - 177 | 3.0 - 2.5 | 0.125 - 0.17678 mm |
| Very Fine Upper | 88 - 125 | 3.5 - 3.0 | 0.08839 - 0.125 mm |
| Very Fine Lower | 62 - 88 | 4.0 - 3.5 | 0.0625 - 0.08839 mm |
Grain sizes for sandstones can be characterized in the Wentworth scale (Very Coarse, Coarse, Medium, Fine, Very Fine) or further classified by the Krumbein phi scale. The phi (φ) scale, created by W. C. Krumbein in 1937, is a logarithmic scale computed by the equation:
ϕ = -log2 D
where
ϕ is the Krumbein phi scale
D is the diameter of the particle or grain in millimeters
This equation can be rearranged to find diameter using φ:
D = 2-φ
Roundness and Sphericity of Grains
Roundness of a grain is characterized by the lack of angular portions on the outside of the grain. The overall roundness of grain is determined through the lack of sharp corners. Rounding of grain can occur due to transportation of the grain over distances through physical and chemical weathering.
Sphericity is determined through the comparison of the X, Y and Z axis of a grain to ascertain it's overall spherical nature. Grains which have large differences in the diameters of the X, Y and Z have a low sphericity.
Lithology
Sandstones contain many inclusions because its primary material, Quartz, is very resistant to weathering and will remain after everything else is weathered or sorted out. Since pure quartz sandstones (quartzarenite) tend to be rare due to the amount of weathering required to remove all of the other constituent grains. Usually quartz is mixed with one or more of the remaining three components. The classification of Sandstones use the QFL ternary diagram by identifying the composition of the matrix through the ratios of Quartz to Feldspars to Lithic Fragments.
Ternary End Members
Q (Quartz) includes all mono and polycrystalline quartz
F (Feldspar Grains) include feldspars and gneiss and granite lithic grains
R (Rock Fragments) include lithic grains such as chert, sedimentary rocks,
schist, shale, slate, and all carbonates
Minor Ternary End Members
MRF (Metamorphic Rock Fragments) includes low grade metamorphic rocks
V (Volcanic Rock Fragments)
SRF (Sedimentary Rock Fragments
Compaction, Lithification and Porosity
The compaction and lithification of sandstone occurs after deposition and during burial. The overburden pressure of more sediments on top can create a situation of increasing pressure causing the sand grains to pack closer together and lithify. The grains often can dissolve along grain boundaries forming sutured contacts decreasing the overall porosity of the sandstone.
The porosity of sandstone maintains after deposition makes great reservoirs. When compared to other sedimentary stones like shale, the framework of the grains maintains the porosity through lithification and burial. Shales and mudstones tend to lose their porosity during lithification. The main reason for this is due to grain size. Sand has a minimum size of grain because of the strength of the mineral, quartz. However, shales are made of smaller weaker grains such as clays which when packed together have less porosity.
