Sand samples

Photos of sand may be beautiful but their educational value is probably pretty limited. We tend to overlook details if thousands of grains are visible at the same time. So I decided to try another approach with a volcanic sand sample from São Miguel Island in the Azores Archipelago.

This sand contains olivine, plagioclase, and two pyroxenes (diopside and aegirine). This clearly indicates that the sand is the disintegration product of mafic volcanic rocks. There are also lots of volcanic glass and analcime grains. I don’t know whether the latter is a primary mineral or an alteration product of volcanic glass. Analcime is the only zeolite that may directly crystallize from magma.

The presence of both analcime and aegirine give a strong hint that the volcanism in the Azores archipelago is probably alkaline. This term may easily create confusion. Here it means that the magma is enriched in alkaline chemical elements (sodium, Potassium). It has not much to do with brines or bases dissolved in water. Analcime and aegirine both contain sodium.

Most of the components this sand is made of are unstable in the weathering environment. Hence, it has to be pretty immature sand. Some olivine grains are clearly weathered. They are dull yellowish green while fresh olivines are bright green.

Pay attention to the fact that there is absolutely no quartz. Oceanic islands with mafic volcanism are one of the few places where you can see sand without this mineral. Quartz and olivine are often mutually exclusive. Sure, everything may be mixed up in sand but these minerals in normal circumstances do not crystallize from the same body of magma. Magnesium rich olivine forsterite (olivine in this sample is also forsterite) will react with free silica and form pyroxene: Mg₂SiO₄ (forsterite) + SiO₂ (quartz) → 2MgSiO₃ (enstatite, one of pyroxenes). This equation means that we may have olivine and pyroxene or pyroxene and quartz but no olivine and quartz together.

Sint Maarten is a sothern part of Saint Martin Island in the Caribbean. It belongs to the Netherlands. Needless to say, weather is generally very nice there and beach sand light-colored. What is the composition of this sand? It is a coral sand. It contains fragments of corals, forams, gastropods, sea urchins, clams, etc.

Carbonate biogenic beach sand from Sint Maarten. The width of the view is 10 mm.

Selected biogenic sand grains picked from the sand. The width of the view is 6 mm.

1. Foram. Homotrema?
2. Coral or calcareous algae?
3. Bivalvia
4. Crustacean shell (crab)?
5. Gastropod, family Caecidae
6. Octocoral spicule
7. Octocoral spicule
8. ?
9. Sea urchin spine
10. Gastropod?
11. Sea urchin spine
12. Sponge spicule
13. Foram

These identifications are by no means certain. I encourage you to let me know if you think you can identify some of the grains shown on the photo.

Sea urchins spines are very common biogenic sand grains. Sea urchins are sometimes called echinoids although it is not precise because sand dollars and few other groups are echinoids as well. However, sea urchins tend to leave behind the most visible and numerous traces. I don’t know how many spines one animal may have but this number definitely exceeds 100. Hence it is no surprise that there are lots of spines in the sand but precious little sea urchin tests. These of course tend to break into fragments which makes finding and recognizing them even more complicated.

Elongated colorful grains are sea urchin spines. The width of the view is 10 mm. Muizenberg, South Africa.

Sea urchin spines are often colorful. Green, white, and purple are very common shades. They are easily recognizable even if the fragments are not elongated because they have very well developed and characteristic strucure. Sea urchins inhabit all oceans. Hence they may be present in both low and high latitude beach sands.

Alive sea urchin on the seabed near Sardinia. Photo: Marco Busdraghi/Wikimedia Commons.

Sea urchin test without spines. Photo: NOAA.

My last post about the mysteries of sand was successful. I got some useful insight. Here is another photo of seashells picked from a single sand sample.

This time it is from Majorca, Spain. If You know more precisely to which organisms these seashells belonged or think that some of them may be misidentified, don’t hesitate to contact me.

Seashells picked from a sand sample collected in Majorca, Spain. The width of the view is 10 mm.

1. Echinoid spines
2. Echinoid spines
3. Echinoid spines
4. Gastropoda
5. Bivalvia
6. Foram
7. Bryozoan
8. Gastropoda
9. Ostracoda
10. Foraminifera
11. Serpulid
12. Foraminifera
13. Scaphopod
14. Foraminifera (could be Quinqueloculina)
15. Agglutinated foram maybe
16. Mollusk shell
17. Foraminifera
18. Bryozoan probably
19. Gastropoda
20. Bryozoan
21. Foraminifera
22. Sponge spicule
23. Foraminifera

It is much harder than I thought. I am no expert in marine biology but I would really like to know what my sand samples contain. I have turned to several biologists and paleontologists but so far I have received very little useful insight.

Well, now I am turning to You. If you happen to be a marine biologist or know someone who could help me, please let them know of me or my blog. I have some nice material and I am willing to share it here but pictures without explanations are not much worth.

Here are some biogenic fragments picked from a fine-grained sand sample collected in Zakynthos, Greece. I know most of them are foraminifera but there is some other stuff too.

Biogenic grains from a sand sample collected in Zakynthos, Greece. The width of the view is 7 mm.

1. Spiroloculina (foraminifera)
2. Clam or gastropod shell. Pelecypod?
3. Elphidium (foraminifera)
4. Peneroplis (foraminifera)
5. Ostracoda
6. Sorites (foraminifera)
7. Agglutinated foraminifera? Clavulina or Bigenerina.
8. Peneroplis (foraminifera)
9. Foraminifera
10. Calcareous coralline algae
11. Cibicides (foraminifera)
12. Elphidium (foraminifera)
13. Echinoid spine
14. Peneroplis (foraminifera)
15. Peneroplis (foraminifera)
16. Sorites (foraminifera)
17. Peneroplis (foraminifera)
18. Gastropoda
19. Foraminifera. Homotrema?
20. Echinoid spine?
21. Peneroplis (foraminifera)
22. Gastropoda
23. Mollusk shell. Clam?

Sand is full of wonders. There are lots of really nice and unusually large forams in a sand sample from Cyprus.

Forams (foraminifera) are amoeboid protists. They are benthic or planktonic sea creatures (they are not animals) who build calcareous (mostly) tests (or shells) which become biogenic sand grains after the owner of the test dies.

Tests of foraminifera are usually less than 1 mm in diameter but these are almost 2 mm. I am not sure about the genus. It could be Amphisorus but Sorites and Marginopora are possibilities as well.

Take a look at the star sand also. These are forams as well and look amazing.

Forams picked from the sand sample collected in Paralimni, Cyprus. The width of the view is 20 mm.

Foraminifera or more precisely their calcareous tests are very common components of many beach sands.

Lots of so-called coral sands are composed mostly (or in large part) of foram tests. These tests are usually worn out and barely recognizable. Still, some fresh examples of certain genuses look really spectacular. Probably the best example is a star sand from Ryukyu Islands, Japan.

This sand is actually composed of several foram species. Two of the most eye-catching genuses are Baculogypsina and Calcarina. Sometimes they are treated separately and called the Star sand and the Sun sand, respectively.

Below are pictures of selected foram tests picked from a sand sample collected in Hatoma Island (200 km east of Taiwan).

Foram species Baculogypsina sphaerulata. The width of the view is 15 mm.

Foram genus Calcarina. The width of the view is 20 mm.

Quartz grains in sand may be rounded or angular but they show their crystal faces extremely rarely. There are two good reasons for that.

Quartz is a tough mineral. It is resistant to both physical and chemical attack. But as millions of years pass they eventually obtain more and more rounded shape. Many quartz grains we encounter in beaches or desert dunes may be very old and their crystal faces, if they had them, are long gone.

But there is another and even better explanation. Most quartz grains never existed as beautiful crystals. Quartz is primarily an igneous mineral — it crystallizes out of magma. If the magma body cools, crystals start to form. Most of these crystals belong to the silicate minerals — they contain silicon and oxygen.

First silicates that start to crystallize take in addition to silicon and oxygen other elements such as iron, magnesium, and calcium into their crystal structure. If all these elements are removed from the melt and there is still silicon and oxygen available (it happens in most cases), then the rest crystallizes as pure silicon dioxide which is a mineral we call quartz. Therefore, quartz is among the last minerals to form in a cooling magma body and it has to take the space which is still available between the crystals that have already formed. This space is obviously irregular and provides no opportunity for the crystals to grow as they would like.

But sometimes quartz forms crystals with well developed faces. How is it possible? It can happen if the quartz crystals form in a vug — a crack or fracture in the rocks. It crystallizes out of water that circulates in such cracks. In this case there is nothing to stop quartz crystals to grow just like they want to as long as there is free room to do that.

Quartz from these vugs will be liberated as sand grains when the weathering reaches their host rocks. It should be obvious now that such quartz crystals are very rare in sand. There are some other environments where quartz crystals grow but none of them is a significant source. Below you can see one of these samples. These euhedral (with well developed crystal faces) quartz crystals grew in a gypsum deposit in New Mexico, USA. They are known as the “Pecos diamonds”.

Crystals of quartz (the Pecos diamonds) picked from a coarse-grained sand that comes from a gypsum deposit in New Mexico, USA. The width of the view is 15 mm.