Sandstone is a consolidated sand. Sandstone is a very widespread and well-known sedimentary rock. It should be no surprise because sandstones make up 10…20% of all sedimentary rocks and sedimentary rocks are by far the most common rocks at the surface (see more interesting numbers pertaining to sand in a post brain games with sand grains).

Sandstone is composed of sand-sized (0.0625…2 mm) mineral grains, rock fragments, or pieces of fossils which are held together by a mineral cement. Sandstone grades into siltstone, shale or mudstone (grains less than 0.0625 mm in diameter) and conglomerate (or breccia if the clasts are angular) if the average grain-size exceeds 2 mm¹.

Sandstone feels rough and grainy. Weakly cemented sandstones are often fissile along the layering. The width of the sample from Estonia (Old Red sandstone) is 14 cm.

Sandstone is very often visibly layered. The width of the sample from Scotland is 7 cm.

Weakly cemented Devonian sandstone outcrop in Estonia.

This is a clastic sedimentary rock from a turbidite sequence in Spain but the framework grains are less than 1/16 mm in diameter which means that it is siltstone, not sandstone. Siltstone grains are so small that human eye can not distinguish one grain from another. The width of the sample is 12 cm.

This rock is also clearly composed of framework grains held together by a mineral cement but it is a conglomerate because the diameter of the grains is significantly larger than 2 mm. The sample is from Switzerland. The width of the sample is 9 cm.

Sandstone and other clastic sedimentary rocks differ from the igneous rocks in possessing a framework of grains which only touch each other but are not in a contnuous contact. Consequently, sandstone contains a network of pores which is at least partly filled with a mineral cement. However, sandstone does not need to contain open pores, they may be, and often are, completely filled with a cementing material. Sandstone definition is based on the size of the framework grains. No reference is made to the genesis.

Whether composition has any significanse is a tougher question to answer. It is generally not important whether it is composed of mineral grains or lithic fragments and what is the origin and proportion of these particles. Sandstone may even contain biogenic grains (shells, coralline algae, etc) but a rock that contains more than 50% of sand-sized carbonate grains is usually named calcarenite which is a type of limestone.

This coarse-grained rock from Cyprus shares both sandstone and limestone properties. It is clearly clastic like sandstone but it is composed of carbonate grains of biogenic origin. Such rocks are known as calcarenite, they are considered to be a subtype of limestone. The width of the sample is 7 cm.

Sandstone composition

Sandstones are composed of mineral grains or rock fragments that were once part of another rock. Therefore, it seems logical to assume that all rock-forming minerals have a chance to become sandstone constituents. Theoretically it is true but in reality minerals differ greatly in their ability to resist weathering.

Many rock-forming minerals are simply dissolved during the transport as sand grains. Pyroxenes and amphiboles are very abundant minerals in certain dark-colored igneous and metamorphic rocks but they are relatively rare in sandstone. Feldspars are even more widespread and also more resistant to weathering. Hence, feldspars are quite common in sandstone although significantly reduced in quantity. Sandstone that contains more than 25% feldspar is named arkose. Quartz, on the other hand, is a common rock-forming mineral (although not as widespread as feldspars) and it is almost insoluble in water and physically very hard. This is why quartz is so abundant in sand. Some sandstones (quartz arenite) are almost exclusively composed of quartz grains. Micas are common minerals in rocks and form a significant part of certain micaceous sandstones.

An arkose sample (contains more than 25% feldspar) from Estonia. The width of the view is 15 cm.

An outcrop of sandstone (quartz arenite) in Estonia consisting of almost pure quartz.

Poorly sorted micaceous sandstone (very rich in muscovite mica flakes) which glitters beautifully in sunshine. The sample is from Ireland. The width of the sample is 11 cm.

The framework grains in most sandstones are either mineral grains (composed of only one mineral) or rock fragments (mineral aggregates of one or several minerals). It is the lithology of the source area that decides which ones will dominate. Granite, gneiss, and other coarse-grained crystalline rocks yield mostly mineral grains but fine-grained rocks like basalt and shale can contribute mostly lithic fragments. Lithic and muddy sandstones (graywacke) tend to be darker in color than white or reddish “cleaner” quartzose sandstones.

Lithic coarse-grained sandstone (graywacke) from Germany. The width of the sample is 11 cm.

In addition to framework grains, sandstones also consist much smaller silt- or clay-sized clasts known collectively as matrix and a mineral matter between the grains that holds them together. This is known as cement. Sandstone cement is usually either carbonate (calcite and dolomite are very common) or silica (chemically precipitated material identical in composition to quartz grains). Small amount of iron oxides are very common also. These oxides are mostly all what is left of unstable iron-bearing minerals like aforementioned amphiboles and pyroxenes.

A coarse-grained Ordovician quartzose sandstone with a dolomitic cement from Estonia. The width of the sample is 5 cm.

Major sandstone components like quartz, feldspar, calcite, and iron oxides are usually accompanied by small amount of other minerals known as heavy minerals. Important heavy minerals are magnetite, garnet, ilmenite, epidote, and zircon. Overview of minerals often found in sand is here: sand minerals.

Sandstone with abundant phosphatic shells of brachiopods (Lingulata) from the Ordovician of Estonia. Width of view is 12 cm.

Sandstone formation

Sandstone forms when sand layers are buried under sediments. Ground water that moves through the sand layers carries dissolved mineralized matter which precipitates over time to bind individual sand grains into solid rock. The most common binding agents in sandstone are quartz, calcite, and iron oxides.

This outcrop consists of visibly layered and grainy rocks but it is not sandstone. This is a pyroclastic rock (volcanic sediment) known as tuff. The outcrop is in France (The Massif Central).

This is what happens to sandstone if it gets buried deep enough. Sand grains fuse together to form a metamorphic rock known as quartzite.

Structures in sandstone

Sandstone structures are easily visible to the naked eye and their study is usually possible only in outcrops. Their scale is simply too large to be studied microscopically. Sedimentary rocks are usually layered and sandstone is no exception. Individual layers are made visible mostly by the variation in grain size. Layers are often easily noticeable because they may be differently colored. This is often also the result of a grain-size variation because water flows more easily in coarser layers and leaves behind more iron oxides. However, layers may also differ in original mineral content right after the deposition.

Common structure in sandstone is graded bedding which means that the grain-size gets gradually smaller, usually from coarse at the bottom to the finer sand at the top. This indicates that the current that carried sand grains gradually lost its velocity. Cross-stratification or cross-bedding is a structure where parts of the sandstone sequence are deposited at an angle to the main sequence. Cross-bedding suggests that the sand form as a whole was slowly moving down-current. There are many types of cross-stratification which are not easy to differentiate because outcrops generally show us only a two-dimensional snapshot of the whole sandy dataset.

Layered sandstone from Germany. The width of the sample is 7 cm.

Unconsolidated dune sand in Estonia. Rust-colored layers are slightly coarser.

Cross-stratified sandstone from Switzerland with a light-colored vein of calcite. Such sandstones from the Alps are often named flysch. The width of the sample is 16 cm.

Alternating layers of conglomerate, fine-grained sandstone, and coarse-grained sandstone from Ireland.

Alternating layers of fine- and coarse-grained sandstone, and conglomerate in a pebble from Estonia.

Ripples in sandstone. The width of the sample is 16 cm.

Tilted muddy sandstone (wacke) layers on the eastern coast of Ireland.

This small strongly cemented sandstone from Estonia has a special importance to me. I already collected rocks as a young kid but only few samples of my early collection have survived. This is one of them. I remember that I found a pebble which was reddish on one side and yellow on the other. I breaked it with the help of bigger rocks as I always did (and still do although now I mostly use a hammer) to see a fresh surface and kept one of the resulting pieces. At that time (I was probably younger than 10) I had no idea how to name it. Now I know that this is strongly cemented sandstone (sometimes named quartzite) from the Proterozoic (it formed roughly 1.4 billion years ago). The width of the sample is 4 cm.

Sandstone is usually known as layered and often cross-stratified sedimentary rock but sometimes it may form even columns. These sandstone columns occur in the Negev Desert (Makhtesh Ramon), Israel.

Sandstone color

The color of sandstone is highly variable. The most common mineral in most sandstones is quartz which is colorless if pure. Hence, pure quartzose sandstone tends to be light-colored (picture below about quartz arenite). However, these sand grains are often covered with very fine-grained hematitic pigment which gives variable shade of reddish color to the rock. The cement is usually responsible for the color of sandstone although the main coloring agent may sometimes make up less than 1% of the rocks volume. This is a common situation with redbeds to which vivid red color is given by a small amount of iron oxide (mostly hematite). Sandstones that contain lots of rock fragments (lithic sandstones) are often dark-colored. Such sandstones are known as graywacke although this term is a bit old-fashioned nowadays.

Coarse-grained strongly cemented sandstone (gritstone) from Israel. Hematite gives it a reddish color which is very typical to sandstones. The width of the sample is 8 cm.

An outcrop of sandstone on the eastern coast of Scotland. Hematite is again responsible for the reddish coloration.

Sandstone types

Sandstone is a granular rock but usually it is assumed that these grains are mostly composed of silicate minerals. Clastic rock which is composed of carbonate shells is considered to be a special type of limestone (calcarenite or coquina), not sandstone or conglomerate.

Feldspar-rich sandstone (arkose) from Argentina. The width of the sample is 18 cm.

Poorly sorted lithic sandstone (graywacke) from Scotland. The width of the sample is 8 cm.

Calcareous sandstone from Switzerland containing lots of fossils. Such sandstones are known as molasse. The width of the sample is 8 cm.

Uses of sandstone

Sandstone is a rock type which has many uses. Strongly cemented sandstone is used as a building material all over the world where sandstone is readily available. Sandstone is often used in house building and fireplaces.

Crushed sandstone (as sand) is a common filling material in road construction and sand is a principal component of concrete. Pure quartz sand is a source of silica which is used to make glass, carborundum, and semiconductors. Some strong sandstones with sharp grains (gritstone) are good for grinding.

Special types of crushed sandstones are used in agriculture as a soil conditioners (lime sand) or fertilizer (glauconite sand). Chemical industry uses sandstone because it is very resistant to most acids (however, this is true if the sand is really almost pure quartz sand). Last, but not least, sandstone, because it is porous, is by far the most important reservoir rock of ground water and hydrocarbons (crude oil and natural gas).

Sandstone is also a very valuable material for geologists because it is abundant, resistant to diagenesis, and contains lots of information to reconstruct the Earth’s geologic history.

Sandstone is often strong enough to be used as a building material. This visibly grainy slab of sandstone contains lots of reddish feldspar grains. The width of the tile is 7 cm.

Glauconite is a green silicate mineral that usually occurs in marine sandstones and gives them greenish color. Glauconite sand may be used as a fertilizer. This sample of glauconite sandstone (greenbed) is from Estonia. The width of the sample is 5 cm.

Sandstone from Australia containing semi-precious mineraloid opal. The width of the sample is 4 cm.

Tar sand is a clastic sediment which contains sticky hydrocarbons (heavy fraction of crude oil). In the future we most likely use these rocks as an energy source more than we do at the present to satisfy our thirst for crude oil. The width of the sample is 5 cm.

References

1. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.