Sand types

Coral sand

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Coral sand is generally light-colored sand (or gravel) which is mostly composed of calcareous fragments of biogenic origin. The term “coral sand” is pretty loosely defined and understood in several different ways.

This sand sample is from Bermuda. Pink foraminifera Homotrema rubrum gives pink color to the beaches of Bermuda. Corals are light-colored but not all of the pieces are fragments of coral reefs. Molluscs, other foram species, and even echinoids (sea urchin spine in the lower left) are also present. Width of view 32 mm.

Scientists usually prefer to talk about biogenic sand instead of coral sand. Why? Because in many cases coral sand is not composed of fragments of coral reefs. Even if corals are present, they often form only a part and not necessarily a dominant part of the sand.

Real coral sand only occurs in the vicinity of coral reefs. These reefs are located roughly between 30° N and 30° S latitudes. Look at the map below to see that corals build reefs in warm waters. They are abundant in the Caribbean, around Hawaii, Polynesia, Indonesia, Indian Ocean, Australian north coast, the Red Sea, etc.

However, several other organisms also prefer to live there. Hence, pure coral sand is pretty hard to find. This is another reason why it is so difficult to draw a line between coral sand and biogenic sand.

http://picasaweb.google.com/107509377372007544953/Rocks#5799563696958491154
Locations of coral reefs are marked with red dots. Credit: University of South Florida Institute for Marine Remote Sensing.

Coral sand is composed of calcareous (few organisms have siliceous tests) bits and pieces of corals, foraminifera, molluscs, sea urchins, algae, sponges, etc. Most of them are no reef-builders. They are either benthic (live at the bottom of the sea) or planktonic (drift freely) marine organisms. Most examples of coral sand (even if they contain no coral fragments) are from low-latitude beaches. Carbonate sand forming in colder water is an interesting curiosity. Maerl, for example, is a type of biogenic sand that is composed of coralline algae and occurs in several beaches in Western Europe, especially Ireland.

Not all carbonate sands are biogenic, let alone made of corals. Carbonate sand may be composed of limestone fragments or ooids which are small spheroidal and well-rounded grains that may occur together with biogenic grains, especially in the Caribbean Sea and the Persian Gulf.

http://picasaweb.google.com/107509377372007544953/Rocks#5799610702421509186
Coral sand from Molokai (Hawaii) is composed mostly of well-rounded pieces of corals and foraminifera (Amphistegina). Width of view 30 mm.
http://picasaweb.google.com/107509377372007544953/Rocks#5799563684138800530
Gravel-sized pieces of coral reef from South Korea (Jeju-do Island). Only few grains are not coral fragments (one mollusc shell in the middle). Width of view 50 mm.
http://picasaweb.google.com/107509377372007544953/Rocks#5799563640568085282
Some coral fragments with a more characteristic shape picked from the Korean sand sample shown above. Width of view 7 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5799563632125395586
These grains look very much like coral fragments from Korea but they are not corals at all. These are pieces of coralline algae from Ireland (sand from the Mannin Bay). Width of view 6 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5798366154297834834
Close-up picture of Mannin Bay “coral sand”.
http://picasaweb.google.com/107509377372007544953/Rocks#5799563734787495218
Coral sand from the Caribbean (Tankah, Mexico) is composed of various biogenic grains (lots of gastropods in addition to corals). Width of view 25 mm.
http://picasaweb.google.com/107509377372007544953/Rocks#5799563751263675938
Foraminifera are very widespread organisms (amoeboid protists) that usually have calcareous tests. Baculogypsina sphaerulata is one of the most spectacular looking of them. Sand where such tests are abundant is known as star sand. Ryukyu Islands, Japan. Whether it is a coral sand is debatable, of course. Width of view 15 mm.
http://picasaweb.google.com/107509377372007544953/Rocks#5799563773379485458
These are also forams (Sorites), this time from Cyprus in the Mediterranean which is too cold for corals during the winter months. Width of view 20 mm.

This is ooid sand from the Persian Gulf (Abu Dhabi, The United Arab Emirates). Ooids have no direct relations with coral fragments. Ooids are generally very well-rounded which helps to identify them.
Biogenic grainsZakynthos biogenous sand grains
This is an example of “coral sand” from Zakynthos, Greece. These sand grains are clearly biogenic (forams, gastropods, sea urchins, ostracods, clams, coralline algae) but there are no corals. Want to know who is who on this picture, then check this post: Sand full of mysteries.

Remnants of bottle messages

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Trash thrown into the sea is a global and unfortunately ever-increasing problem. Most of the garbage is plastic because the decay of this material takes enormous time. Another type of resistant trash dumped into the sea is glass.

http://picasaweb.google.com/107509377372007544953/Coll#5852307320112952114
Glass Beach, Kauai, Hawaii Islands. Transparent, green, and brown glass pebbles. Dark grains are fragments of olivine basalt (without olivine which is due to weathering replaced by iddingsite). There are also biogenic grains (forams, mollusks, echinoids, and corals). Although I am somewhat reluctant to say so, but I think we have to admit that even trash can sometimes become beautiful. The width of the view is 20 mm.

A very small part of it may indeed come from castaway sailors sending bottle messages but mostly it is the result of people using sea as a huge garbage dump. Glass is not as visible and annoying in the sea as plastic because it is heavier than water and sinks to the bottom. And rounded glass pebbles can easily be mistaken for some minerals. It may look very similar to quartz, especially if the glass is transparent.

In our particular case the transparent grains are indeed glass pebbles, not quartz. It is easy to distinguish between the two with the help of a polarizing microscope. Unlike quartz, glass has no crystal structure and is therefore dark (showing no interference colors) when placed between the two crossed polarizers of the microscope.

Sea glass has lost its sharp edges because of the tumbling action of waves. It is frequently washed ashore but the concentration of seaglass in the beach sand is usually very low. There are some notable exceptions such as the Glass Beach in Kauai Island or the Glass Beach of Fort Bragg in California. Glass has concentrated there because these places were used as garbage dumps in the past.

Fort Bragg Glass Beach
Glass Beach, Fort Bragg, California. Transparent, blue, and green glass pebbles. There are some biogenic grains (mollusk shells, echinoids) and rock fragments (mostly lithic sandstone). The width of the view is 28 mm.

marine garbage on the Icelandic coast
The majority of trash on the coast is anything but beautiful. Plastic garbage is not only ugly sight but also a serious threat to marine life. The photo was taken in Iceland.

Sand that remembers the rock it once was

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Sand is usually composed of mineral grains that come from disintegrated rocks. Rocks are mineral aggregates, they generally contain several distinct minerals. If we have a sand that is compositionally close to the rocks it comes from, then we call it immature. The opposite situation is a sand that is almost exclusively composed of quartz and other ultraresistant minerals. Such sand is mature or evolved because it has lost significant part of its former components.

Sand sample collected near Bridalveil Fall, Yosemite National Park, California. This is river sand. Its source rocks are very near. Sand is composed of plagioclase (white), quartz (transparent), K-feldspar (yellow, reddish), and biotite (black, some are brown and green due to weathering). These minerals put together into one rock give us aigneous rock granodiorite (similar to granite but contains more plagioclase than K-feldspar). The width of the view is 10 mm.

Sand composed of rock fragments (lithic sand) is definitely the most immature sand type in existence but I’m not focusing on that here. This post is devoted to immature sand that is largely composed of mineral grains.

Many minerals are unstable in the weathering environment. Therefore the immature sands can not be located far from their parent rocks. In addition to time, the most important variables that determine the rate of weathering are temperature and the availability of moisture. Hotter climate makes chemical reactions faster and water is needed to generate these reactions. Maybe you have heard that weathering is the disintegration process of rocks which is driven by temperature changes, frost-thaw cycles, etc. That is true to some extent but chemical weathering is by far the most important agent of weathering.

So, where do we have the best chances to see this type of sand? We need cold and/or dry climate plus short transport distance. Our chances are pretty good if we start looking for this type of sand in the riverbanks because the transport distance there is often very short. Sand grains on the beach are often evolved, especially in hot and humid climates. Beaches of Florida, for example, are predominantly composed of almost pure quartz (some contain significant amount of biogenic grains also, but this is not important here). Beaches of Canadian Arctic on the other hand are not mature at all. They generally contain lots of feldspar and other minerals in addition to quartz.

Sand arctic Canada
This is beach sand from the Canadian Arctic. Coronation Gulf, Nunavut. Sand is composed mostly of quartz (transparent), feldspars (red), and hornblende (black). Its source area could be a metamorphic terrane composed of granitic gneiss and amphibolite. The width of the view is 10 mm.

Gypsum sand

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Gypsum is a relatively rare constituent of sand. An exception is a large dune field in New Mexico White Sands National Monument that is entirely composed of tabular gypsum grains.

Why is gypsum rare in sand? Because it is moderately soluble in water. Gypsum crystallizes out of concentrated solutions — it is an evaporite mineral. It can also quite easily go into solution again. Anything soluble is generally not going to last long in sand. Gypsum sands in New Mexico exist there because this state is not too famous for a wet climate, quite the contrary. The area also has no outlet to the sea which means that gypsum grains that are dissolved in rain water have no escape from the area and eventually may become sand grains again.

Ordinary sand grains made of quartz are the disintegration product of granite, sandstone, or other quartz-containing rocks. The crystals of quartz can be very old. Gypsum grains in the White Sands National Monument are different. They are not the product of disintegration of rocks. These grains are formed in the salty brines which get their high dissolved gypsum from the gypsum containing sedimentary rocks nearby.

White dunes in the
White gypsum dunes of the White Sands National Monument in New Mexico, USA. Photo: davebluedevil/Wikimedia Commons.

Gypsum sand from New Mexico White Sands National Monument
Sand from the White Sands National Monument in New Mexico that is composed entirely of gypsum grains. Width of view 5 mm.

Ooid sand

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Ooids are small rounded accretionary mineralized bodies. They could be called sand grains, but they are no ordinary sand grains. Just like normal sand grains, ooids have a diameter reaching up to 2 mm (usually less than 1 mm). Larger grains formed the same manner as ooids are called pisoids (just like sand grains larger than 2 mm are called granules). Rock type composed of ooids is oolite.

http://picasaweb.google.com/107509377372007544953/Rocks#5790668050221394914
Ooid sand from the Antelope Island, the Great Salt Lake. Width of view 5 mm.

Ooids are accretionary — it means that they have grown to the size they have now. Sand grains have usually had just the opposite story — they were once larger. Ooids grow in shallow wave-agitated water. Waves move fine sediment particles (quartz grains or biogenic fragments) which act as a crystallisation nuclei upon which mineralized matter starts to grow.

Most ooids are calcitic or aragonitic. They have a characteristic concentric layering which resembles the growth rings of trees. Most ooids have rounded morphology, but some are elongated or even tabular, reflecting usually the shape of the crystallization nucleus.

Ooids are usually marine. Well-known locations where ooid sands are forming are the Persian Gulf, the Gulf of Mexico near the Yucatán Peninsula and the Bahama platform. Non-marine ooid sands exist also in some saline and freshwater lakes, caliche soils, caves, and even in some rivers. A famous example of non-marine ooid sand is on the shores of the Great Salt Lake in Utah.

Ooids form in a wave-agitated water, which is usually warm. Why is it important? Agitation by waves matters because forming ooids need to be in motion to make them grow evenly on all sides. Increased water temperature leads to a loss of carbon dioxide (warmer water can hold a smaller amount of dissolved gases) and therefore enhances the precipitation of calcium carbonate which crystallises as mineral calcite or aragonite.

There is a long-held understanding that calcitic ooids were once aragonitic, but recent studies show that it is not necessarily true. Primary calcitic ooids do exist and some periods in the geologic past have even favored their formation. Sometimes original calcium carbonate has been replaced by hematite, silica, or dolomite, but ooids composed of a primary phosphatic composition or primary iron oxides exist as well. More about this can be found here: oolite.

Ooid sand from Abu Dhabi, United Arab Emirates. Width of view 5.5 mm.

http://picasaweb.google.com/107509377372007544953/Rocks#5790668012394173538
Ooid sand from Stansbury Island, The Great Salt Lake. Width of view 5.5 mm.
http://picasaweb.google.com/107509377372007544953/Rocks#5790668029504177410
Ooid sand from Cancún, Yucatán, Mexico. Width of view 5 mm.

Bahama ooid sand
Ooid sand from Bahama. Width of view 10 mm.

Desert sand

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Deserts cover huge areas, mostly between 10-30 degrees N and S of the equator. Many of these deserts are sandy, at least partly. Good example is Sahara — the largest desert in the world. To describe a desert sand, we first have to make clear what we are talking about. When we imagine desert sand, we probably think of sand dunes. That is what I am writing about here although dune fields cover only about 20 percent of modern desert areas.

Sand dune in Moroccan Sahara
Sand dune in Sahara (Morocco). Note how wind is blowing sand grains off the crest of the dune.

Dune sand is generally very well-sorted. It means that all the sand grains are roughly the same size. There is almost no dust. It is blown away and deposited elsewhere, possibly as a loess deposit far away from the source area. There are also no gravel or boulders because wind is not capable of carrying such a heavy load.

, Libya
Desert sand composed almost exclusively of rounded quartz grains. Sand sample is from the Sahara Desert (Erg Murzuk), Libya. Width of view 1 cm.


Rounded quartz grains (covered with rust-colored hematitic pigment) plus biogenic and lithic fragments from the Dubai Desert, United Arab Emirates. Biogenic grains clearly indicate that the sea can’t be very far away.


Dune sand from the Gobi Desert, Mongolia. Width of view 1 cm.

Lithic sand

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Lithic sand is predominantly composed of small fragments of rocks. Such sands and sandstones are said to be lithic. Lithic sand is unusually immature sand type. It means that the source rocks must be not very far and the weathering agents have not yet had time to break these rocks down to their constituents (individual mineral grains).

http://picasaweb.google.com/107509377372007544953/Rocks#5789446247782283426
Lithic sand from the West Beach, Whidbey Island, Washington, USA.

There are very many rock types and consequently almost an endless array of possibilities to categorize lithic sands into different types. Rock fragments that often occur in sand are amphibolite, sandstone, quartzite, chert, schist, phyllite, granite, and basalt. Most lithic sands are dark in color.

Basalt pebbles
Sand formation in progress. Pebbles and granules of basalt near the southern tip of La Palma slowly transforming into black sand.
Caption
Lithic fragments of mica schist in the sand. Width of view 20 mm.

Mixed carbonate-silicate sand

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Sand is usually composed of mineral grains but especially marine samples contain often biogenic fragments as well. There are many occurrences of either pure or almost pure mineral and biogenic sands but sometimes these two are mixed up in all proportions to form an interesting hybrid carbonate-silicate sand type.

http://picasaweb.google.com/107509377372007544953/Rocks#5789430272034997042
Mixture of yellow and white biogenic and black volcanic sand grains. Jungmun Beach, Jeju-do island, South Korea.

Biogenic sand

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Biogenic sand is composed of exoskeleton or bone fragments of dead sea (usually) creatures. This sand is widespread in low latitude (less than 35°) beaches. There are many different organisms that can contribute to the formation of this sand type and therefore one biogenic sand may greatly differ from another. Biogenic sand is usually light-colored and its components are usually made of carbonate material although some organisms prefer silica. Important source material are the remains of clams, sea snails, foraminifera, algae, corals, echinoids, sponges, etc. Sand may consist of up to 100 percent of biogenic grains but mixtures of mineral and biogenic grains are common also.

http://picasaweb.google.com/107509377372007544953/Rocks#5789419732868823874
A sand sample consisting of corals and foraminifera from Bermuda.

http://picasaweb.google.com/107509377372007544953/Rocks#5789419883905792914
Biogenic sand (forams, corals) from Molokai, Hawai’i.
http://picasaweb.google.com/107509377372007544953/Rocks#5789419818992905122
Biogenic grains from Majorca, Spain.
http://picasaweb.google.com/107509377372007544953/Rocks#5789419920337681522
Biogenic grains from Zakynthos, Greece.

Globigerina ooze covers large areas of ocean floor. These forams are gathered from the seafloor of the Weddell Sea (3500 meters below sea level) near Antarctica. Red circles are around Orbulina universa, green could be Rotaliida, and yellow ones are Globigerina but some of them may be Neopachyderma also. Width of view 5.1 mm.

I have also written about coral sand. Find out what is the difference between biogenic and coral sand.

Garnet sand

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Garnet is an important silicate rock-forming mineral and occurs in many rock types. However, it almost never makes up the majority of the rock’s mass. The same is true with a garnet-bearing sand. Garnet is a common mineral in sand but usually in low quantities. There are some hard to find exceptions – red and very beautiful sand where garnet is indeed the most abundant sand-forming mineral. Source rocks of garnet is usually either metamorphic (garnet schist, rarely eclogite) or igneous (aluminum-rich granite). Garnet sand is a sub-type of heavy mineral sand.

Garnet sand from Australia. Width of view 20 mm.

http://picasaweb.google.com/107509377372007544953/Rocks#5877446908198751970
Garnetiferous heavy mineral sand sorted out by running water near the coastline of Pfeiffer Beach, California.
http://picasaweb.google.com/107509377372007544953/Rocks#5878256035179681490
A macro photo of the Pfeiffer Beach sand. Most of it (pink mineral) is garnet. Width of view 8 mm.
Magnetite, garnet and quartz as a rock type
Garnet rich sand that has metamorphosed into hard rock. Magnetite and quartz are other notable minerals. Varanger Peninsula, Barents Sea, Northern Norway. Width of sample 18 cm.