Sand is a natural unconsolidated granular material. Sand is composed of sand grains which range in size from 1/16 to 2 mm (62.5…2000 micrometers). Sand grains are either mineral particles, rock fragments or biogenic in origin. Finer granular material than sand is referred to as silt. Coarser material is gravel. Majority of sand is dominantly composed of silicate minerals or silicate rock fragments. By far the most common mineral in sand is quartz. Hence, the term “sand” without qualification is imagined to be composed of quartz mostly. However, sand is a natural mixture which means that it is never pure. By no means can one say that quartz and sand are the same thing. Consolidated sand is a rock type known as sandstone.

Formation of sand

Sand forms mostly by the chemical and/or physical breakdown of rocks. This process is collectively known as weathering. Physical and chemical weathering are usually treated separately, but in reality they usually go hand in hand and it is often difficult to separate one from another because they tend to support each other.

Chemical weathering is much more important sand-producing factor overall. It operates most efficiently in humid and hot climate. Physical weathering dominates in cold and/or dry areas. Weathering of bedrock which produces sand usually takes place in soil. Soil covers bedrock as a thin layer, providing moisture for the disintegration process of rocks.

Granite is a common rock type and serves as a great example of sand forming processes. Granite is composed of feldspar (pink and white) which decomposes chemically into clay minerals. Another important constituent of granite is quartz (gray). Quartz is very resistant to chemical weathering. It does not alter to any other mineral — quartz is quartz and will remain that way. It eventually goes into solutions but VERY slowly. Hence, disintegrated granite yields lots of quartz grains which will be transported mostly by running water as sand grains. The sample is from Italy. Width of view 21 cm.

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Here is an example of mature sand sample from USA (St. Peter Sandstone from the Ordovician Period). It is composed of almost pure and well-rounded quartz grains. This is what eventually happens if we give enough time for the nature to chemically destroy most other minerals that were present in the source rocks. St. Peter Sandstone has seen much increased demand in recent years because it is well-suited for fracking purposes. Width of view 20 mm.

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But what happens to other minerals? They are either converted to new stable minerals in atmospheric conditions (mostly clay minerals) or get carried away as ions in hydrous solutions and end up in the oceans. So these are freshwater rivers that carry ions to the sea and make it salty. There is a nice amount of irony in it. Clay minerals are carried by rivers also and we usually refer to this load of clay as mud. There is a muddy temporarily dry riverbed on the picture. Mud that covers these rocks is a mixture of clay minerals, fine sand, silt, and water. Barranco de las Augustias, Caldera de Taburiente, La Palma.

Composition of sand

Sand is a residual material of preexisting rocks. It is therefore composed of minerals that were already there in the rocks before the disintegration commenced. However, there is one important aspect — sand occurs in a harsh environment where only the strongest survive. By “strongest” I mean the most resistant to the weathering processes.

Quartz is one of these minerals (list of minerals in sand) but not the only one. It is so dominant in most sand samples because it is so abundant. 12% of the crust is composed of it. Only feldspars are more abundant than quartz. (Here is more information about the composition of the crust).

Relatively rare minerals like tourmaline, zircon, rutile, etc. are also very resistant to weathering, but they rarely make up more than few percents of the composition of sand. These minerals are collectively referred to as heavy minerals.

Heavy minerals may sometimes occur in sand in much higher concentrations. This is usually a result of hydrodynamic sorting. Either sea waves or river flow sort out heavier grains and carry lighter ones away. Such occurrences are known as placers and they are often used as a valuable mineral resource. Minerals that are often extracted from placer deposits are gold, cassiterite, ilmenite, monazite, magnetite, zircon, rutile, etc.

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Concentrate of zircon extracted from beach sand in South Africa. Width of view 12 mm.

Quartz definitely dominates in most sandy environments, but it is usually accompanied by feldspars. Feldspars are only moderately stable in atmospheric conditions, but their overall volume in common rocks is huge. More than half of the whole crust is composed of feldspars. Other common rock-forming minerals like amphiboles and micas also frequently occur in sand. Some common minerals in certain rocks like olivine and pyroxenes occur in sand in smaller volume because their resistance to weathering is nothing to brag about.

However, there are enough sandy beaches that are mostly composed of pyroxenes and olivine with magnetite. How can anything like that happen? Such beaches with a black sand occur in volcanically active areas where quartz-bearing rocks are missing. Pyroxenes and olivine are common minerals in mafic rocks like basalt. Black sand is a typical phenomenon of oceanic volcanic islands where granite is missing and felsic quartz-rich rocks rare.

Siesta Key beach sand in Florida, on the other hand, is composed almost exclusively of quartz grains and is therefore as white as it possibly can be.

Most sand samples consist of sand grains which are composed of a single mineral — quartz grains, feldspar grains, etc. But sand may also contain grains that are aggregates of crystals i.e. fragments of rocks (known also as lithic fragments). Lithic sand is usually immature and it also tends to form when rocks are very fine-grained. Granite usually disintegrates into distinct mineral grains, but phyllite and basalt for example are often so fine-grained that they tend to occur in sand as lithic fragments. Lithic fragments are also common in regions where erosion is rapid (mountainous terrain). You can find more about immature sand in this article: Sand that remembers the rock it once was.

Sometimes sand contains new minerals or mineral aggregates that were non-existent in the source rocks. Notable example is a clay mineral glauconite which forms in marine sand and gives distinctive dark green color to many sand samples. In some instances glauconite in sand may come from disintegrated glauconitic sandstone nearby, but eventually it is of marine origin anyway.

There are many other strange sand samples that require special formation conditions. One good example is sand in New Mexico that is composed of pure gypsum. I have written about it here: Gypsum sand. Sand with such a composition is odd and unexpected because gypsum is an evaporite mineral. It was precipitated out of hyper-saline water and it goes easily into solution again. Hence, it can only survive in dry conditions with no outlet to the sea. Halite, which is even more soluble than gypsum, is also known to form sand in special conditions.

Volcanic ash is usually treated separately, not as a type of sand. Probably because we humans tend to create artificial barriers and classification principles. We think that sand is a collection of sedimentary particles, but sedimentary and igneous rocks are two different worlds. In reality, this is more complicated because there is every reason to say that volcanic ash grains (and other pyroclastic particles like lapilli and volcanic bombs) are also sedimentary particles because they got deposited on the ground not much differently than sand grain in a dune does. Volcanic ash and sand have even comparable classification principles — volcanic ash is a pyroclastic sediment with an average grain size less than 2 millimeters. Hence, volcanic ash is a volcanic analogue of sand and silt.

Third major and versatile component of sand (two others were mineral grains and lithic fragments) are grains of biogenic origin. Biogenic sand is composed of fragments of exoskeletons of marine organisms. Common contributors are corals, foraminifera, sea urchins, sponges, mollusks, algae, etc. Such sand is usually known as coral sand although in many cases it contains no coral fragments at all. Biogenic sand is light-colored and widespread in low latitude marine beaches although there are exceptions. Corals indeed live only in warm water, but many other taxons can do well in colder climate (coralline algae, clams, some forams). Most biogenic sand grains are calcareous and provide material for limestone formation. Most limestones are former calcareous muds deposited on the seafloor.

Sometimes sand contains or is entirely composed of well-rounded carbonate grains that are not fragments of dead marine organisms. These grains are ooids that also require special formation conditions.

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Biogenic sand from Tuamotu is mostly composed of forams. Width of view 20 mm.
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Ooid sand from Cancún, Yucatán, Mexico. Width of view 5 mm.

Sand does not need to be a pure collection of either mineral, lithic, or biogenic grains. In many cases two of them and sometimes even three are mixed.

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Mixture of mafic volcanic rocks and various biogenic grains in a sand from the Azores archipelago. Width of view 20 mm.
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Mixture of dark-colored volcanic rocks, worn-out biogenic grains, and some silicate grains from Jeju-do Island, South Korea. Width of view 20 mm.

Texture and transport of sand

Geologists describe sand by measuring the roundness of grains and the distribution of grain sizes. By doing that they hope to shed some light on the origin of the grains being measured. Roundedness usually gives information about the length of transport route and distribution of grain sizes helps to determine from which environment these grains come from. River sand is usually poorly sorted and compositionally immature. Beach sand is more rounded and eolian dune sand is generally well sorted.

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Poorly sorted river sand from Sikkim, India. Width of view 20 mm.

The average size of grains is determined by the energy of the transport medium. Higher current velocity (either stream flow or sea waves) can carry heavier load. Coarse-grained sediments therefore reveal that they were influenced by energetic medium because finer material is carried away.

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Sometimes river flow is so energetic that sand grains are all carried away and only large rounded stones remain. Such lithified deposits of former riverbeds are known as conglomerates. Photo taken in Cyprus.

Sand is mostly transported by rivers, but average sand grains are too large and heavy for average river to carry them in suspension. Hence, sand grains tend to move in jumps. They are lifted up by more energetic current and settle out when current velocity decreases and then wait for the next jump. This mode of movement is known as saltation. Average silt grain moves differently. It is light-weight enough to be carried in suspension for a long time and this is actually one of the most important reasons why we treat silt separately from sand.

Most sand grains carried by the rivers are eventually deposited at the rivermouths where the current velocity suddenly drops. Then sea waves (longshore currents) take over and carry sand along the coastline. Sand grains carried by the rivers are also deposited on alluvial flood plains and point bars (inside bend of streams where current flow is the slowest).

Sand is also transported by wind, ocean currents, glaciers, turbidity currents, etc. Moving sand forms landforms like ripples and dunes.

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Wave ripples on a tidal flat in Ireland.
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Dunes of Maspalomas on Gran Canaria.
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Sand dunes near Stovepipe Wells, Death Valley (Mesquite Dunes).

Sand dune in Sahara (Morocco) on a windy day.

Further reading

Pettijohn, F. J., Potter, P. E. & Siever, R. (1973). Sand and Sandstone. Springer
Siever, R. (1988). Sand, 2nd Edition. W H Freeman & Co.

Sand can be unbelievably colorful. Most of us are familiar with tan, orange, yellow and red colors but the range of colors that occur in sand is almost limitless. In this post I will show some compositionally interesting sand samples and explain briefly which constituents are responsible for these colors.

If you love sausages, you should never try to find out how it is made. The same applies to many other things as well. If you love the idea of sunbathing in the Caribbean, you should consider skipping this post.

Biogenic or coral sands found on the seashores of tropic beaches are largely composed of bits and pieces of coral, calcareous algae, foraminifera, gastropoda, sea urchins, etc. Much of this material was rasped from the coral reefs by brightly colored parrotfish. They grind the pieces of corals which then goes through their digestive system to get deposited on the shallow seafloor as calcareous sand grains.

Most parrotfish actually do not feed on corals. They are looking for algae that are inhabiting coral reefs. One might think that parrotfish are a threat to the coral reef ecosystem but it is not true. They actually help corals by not letting algae to choke them.

Parrotfish are very important agents of bioerosion. One parrotfish can make 90 kg of coral sand a year. Parrotfish depend on coral reefs just as coral reefs depend on them. Coral reefs in many parts of the world are not doing well recently, and the same applies to parrotfish as well. They are not seriously endangered, but the future with overfishing, global warming, pollution, and ocean acidification promises not much good to them.

Here is a short video of sandmaking process:

Sand grains are not uniform in size. The minimum diameter of a sand grain is only 62.5 micrometers or 0.0625 millimeters while the upper limit of a sand grains diameter is 2 millimeters. It is common knowledge but why such numbers? One may say that you simply have to draw the border somewhere in order to be able to differentiate sand from silt or gravel. So are these numbers completely arbitrary? Yes and no. Exact numbers are definitely arbitrary. They are determined by the logarithmic scale which also determines the borders between fine, medium, and coarse sand.

The gray circle resembles the upper limit of a sand grains (very coarse) size while the smallest red circle resembles the smallest. Black, blue, green, and yellow are the upper borders of coarse, medium, fine, and very fine sand grains respectively. The graph is to scale.

However, this classification scheme is chosen to make as much sense in geology as possible. It reflects the movement of sand grains in water. In the river water, the sand grains are not carried in the suspension. They tend to move in jumps — running water occasionally lifts sand grains up but is not able to carry them far. Sand grains settle again and wait for the next jump. Such mode of movement is called saltation and it is especially characteristic to sand grains. Gravel just rolls on the river bed while silt is usually carried in the suspension.

Sure, it depends on the speed the river water is running. Sometimes (in the fast moving mountain streams) granules saltate as well. And sometimes the river water is not capable of lifting sand grains up even temporarily. Nature doesn’t classify. It has no need for it. But we humans desperately need the classification schemes in order to categorize things and try to make some sense of the world surrounding us. Therefore, no classification scheme is perfect and the one used now is by no means the only one possible.

It is perhaps rather difficult to imagine how different can two grains be if one of them has a diameter of only 62.5 micrometers while the other is 2000 micrometers or 2 millimeters thick. The first one is barely visible while the other is as big as the head of a match. How much is one bigger than the other? It should be simple, we just divide 2000 with 62.5 and get the result of 32. However, such a result may be mathematically correct but it makes no sense. The true measure of a grain size is its volume. After all, whether the river water is capable of carrying the grain depends on the mass and volume of the grains, not on the diameter.

If we assume that our grains are perfect spheres, then the bigger one has 32,768 times larger volume. That’s a huge difference and obviously has to significantly influence the behavor of the grains.

How much one sand grain weighs? Let’s assume that we are dealing with quartz grains. Quartz has a density of 2.65 grams per cubic centimeter. A grain with a diameter of 2 millimeters makes up only little more than four thousands of a cubic centimeter, and it weighs approximately 0.011 grams. I am not giving the mass of a smaller grain, the number would be ridiculously small but you can easily calculate it by dividing 0.011 with 32,768.

Now we know that even the largest sand grains are lightweight. How about the number of grains that we can fit into a container with a definite volume, let’s say 1 cubic centimeter? In order to calculate that, we need to know how many grains we can press into this container. Theoretical calculations show that if the grains are placed irregularly, you can not achieve better packing than about 63%. It means that about 37% of your container will be filled with air, water or something else. It makes up the pore space volume which is a very important metric if we try to calculate, for example, how much crude oil a sandstone layer can contain. Simple calculation yields a result that 1 cubic centimeter can contain 151 sand grains with a diameter of 2 mm and 4,959,645 sand grains with a diameter of 62.5 micrometers.

Most sand collectors prefer to have at least 30 ml of sand per sample. I am an exception because I am satisfied with much less than that. Here are some calculations why this is the case. Let’s assume that average sand grain has a diameter of 250 micrometers (this is a borderline between fine- and medium-grained sand). If you have 30 ml of such sand, then you have 2,324,833 sand grains. Do you really need that many if your goal is to get a general overview of the sand samples composition? Definitely not. Even one hundredth of that is good enough. That is the basis of my claim that if you have a very interesting sand sample but can only send one gram, I would still be happy. It is more than I need.

Can we try to estimate how many sand grains are there in the whole world? Well, no one ever counted them but I think we can make some very rough estimations. There are approximately 200 million cubic kilometers of continental sediments. Assuming that about fourth of it is sand, the total volume of sand is perhaps 50 million cubic kilometers. If we assume that the average sand grain has a diameter of 250 micrometers, then we have approximately 4 x 10²⁷ sand grains in the crust.

This is a really huge number. I remember Carl Sagan once said in his television series Cosmos that there are perhaps more stars in the Universe than there are sand grains on all of the beaches. That may be true but beaches are not the only places where sand can be found. If we calculate the number of all sand grains covering the Crust, I think the sand grains still have the last laugh.

Sand is usually composed of mineral grains. You’ll find a post about the sand-forming minerals here. Sand itself is not a mineral. It is a sediment just like clay, gravel and silt. Most common sand-forming mineral is quartz. There are two good reasons for that.

First, quartz is very important rock-forming mineral. It occurs in many igneous rocks, especially granite. It is also very common component of many metamorphic rocks (quartzite containing little else than quartz). And finally, quartz is a major component of sedimentary rocks, especially sandstone. As these rocks weather and disintegrate, quartz crystals are liberated as sand grains. Another reason why quartz is so widespread in sand is its extreme resistance to weathering. Weakly acidic rain water does not dissolve it effectively and it is also fairly resistant to abrasion.

Despite all of this, quartz is very rarely a sole component of sand. It is accompanied by very many mineral species. First group of them are in sand because they are simply very abundant in rocks. Second group of sand-forming minerals are almost never abundant as rock-formers, but they are, just like quartz, very resistant and therefore their concentration in sand slowly rises as time goes by.

First group contains such widespread minerals as feldspar (more than 50% of the Earth’s crust is composed of feldspars), pyroxene (11% of the crust), and amphibole (5%). They are all mineral groups with a varying chemical composition, unlike quartz, which is a single mineral with definite and very simple chemical composition (SiO₂). These minerals, when attacked by weathering agents, relatively quickly disintegrate and they mostly become particles of clay. If we imagine granite, a typical and widespread igneous rock, roughly one third of it is composed of quartz and two thirds is feldspar. When granite disintegrates, quartz becomes sand grains and feldspars are turned to clay. That is the very reason why we have so much of these two sediments and their usually unpleasant mixture with water which we call mud.

Second group consists of sand minerals that are quite unimportant in rocks but remain in sand fraction for a very long time. Such minerals are for example zircon, tourmaline and rutile. Here are some sand-forming minerals: olivine (least resistant), pyroxene, andalusite, sillimanite, amphibole, epidote, sphene (titanite), kyanite, staurolite, chlorite, spinel, garnet, apatite, rutile, tourmaline, zircon. Olivine disintegrates so quickly that we don’t know consolidated sediments older than Quaternary (last few million years) that contain it while zircon grains are the oldest minerals ever found on Earth. Some are almost as old as the Earth itself.

Sand is a very versatile mixture. We have much more than mineral grains in its composition. Third large and versatile group are lithic fragments. They are simply stones that have the size of sand grains. Common rock particles in sand are basalt, granite, schist, sandstone, and limestone which are all very common rock types as well. Therefore we can conclude that stone or rock fragments are an indication that the particular sand sample is not very mature. Otherwise, rocks would have had enough time to disintegrate into single minerals.

Fourth important component of many sand types are biogenic fragments. Many marine organisms build hard shells or tests which become sand particles after these organisms die. Most common sand producers are foraminifers, bivalves (clams), gastropods (snails), corals, sea urchins, etc. They are common in low latitude warm sea water, but many species exist in colder water too. In some regions they are so widespread that most beaches are almost entirely composed of these biogenic fragments.

Fifth component of sand are artificial or manmade objects like glass and concrete fragments and plastic pebbles. Definitely we don’t like the growing presence of such sand types although I have to say that some glass sands from former garbage dumps are pretty beautiful and definitely valued as a curiosity by many sand collectors.

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Sand composed of lithic fragments and rounded pieces of colorful glass. Glass Beach, Kauai, Hawai’i Islands. Width of view 20 mm.

Further reading

Pettijohn, F. J., Potter, P. E. & Siever, R. (1973). Sand and Sandstone. Springer.
Siever, R. (1988). Sand, 2nd Edition. W H Freeman & Co.