Today I received three nice sand samples from Fuerteventura, Canary Islands. They were sent to me by Cathrene Rowell from England. One of the samples is taken from dunes which are supposedly made of sand from the Sahara Desert.

The sample is from the Corralejo Dunes (Dunas de Corralejo) which occupy large area of the NE part of Fuerteventura which is only 120 km away from the African coast. These dunes are a major tourist attraction of the island and as I can understand tourists are told that they are walking on a sand from the Sahara which is blown here by the wind.

There is one little problem with this nice story — it simply is not true. Why not? Dune sand of Sahara is composed of quartz grains mostly which are usually covered by fine hematitic powder that gives them reddish hue. The sand sample from the Corralejo Dunes, however, is composed of various fine-grained biogenic grains with no quartz at all. There are few silicate grains but these are mafic fragments of dark-colored volcanic rocks. This is the material the Canary Islands are made of.

I do not know whether locals genuinely believe the story of sand blown from the Sahara or are they just making it up to make the dunes more attractive to tourists. Corralejo Dunes are a remarkable place without this Sahara story. Approximately 15 square kilometers covered with biogenic grains of local marine origin on a relatively small volcanic island is a remarkable story in its own right.

This does not mean, however, that there is no sand blown there from the Sahara. Dry and dusty wind from the Sahara is well-known to the inhabitants of these islands but this is mostly very fine-grained dust, not sand. These tiny quartz grains from the Sahara are probably very common constituents of soil there but I am not aware of any mechanism that is able to separate this dust from the local material and pile it up into dunes.

But where is this sand coming from? It is believed that the source of the sand is seabed next to the island and between Lanzarote and Fuerteventura which are geologically the same structure. There is only a shallow strait separating them. The seabed was dry during the most recent glaciation and climate was very arid at the time around Fuerteventura which helped the prevailing trade winds to carry the sand near the present shoreline¹. Important conclusion of it is that the sand dunes of Corralejo are essentially non-renewable resource (unless new glacial episode arrives).

http://picasaweb.google.com/107509377372007544953/Tenerife#5810632695951968418
Dunes of Corralejo. Photo: Cathrene Rowell.
http://picasaweb.google.com/107509377372007544953/Tenerife#5810632700738477170
Close-up photo of sand that makes up these dunes. It is composed of tiny biogenic calcareous fragments of forams, molluscs, sea urchins, etc. It is a coral sand in the loose sense of the word. Width of view 8 mm.
http://picasaweb.google.com/107509377372007544953/Chert#5810642372312490050
This is how the real Sahara sand looks like. It is almost pure quartz with fine hematite powder with no marine shells. The sample is from the Erg Murzuk, Libya.

References

1. Carracedo, Juan C. & Day, Simon (2002). Canary Islands (Classic Geology in Europe). Dunedin Academic Press Ltd.

Mannin Bay is located in Connemara, in the west of Ireland. This place is noteworthy because the coast there is covered with light-colored and coarse grains that very much resemble calcareous pieces of corals. Is it really coral sand? In Ireland? That, I am afraid, is not possible. Corals prefer much warmer water.

This sand is locally known as maerl. Maerl is a type of coral sand (in the loose sense of the term) that is composed of calcareous pieces of coralline algae. Mannin Bay is only few kilometers away from the Dog’s Bay which is perhaps better-known because there is a nice white-colored beach and a parking lot for cars. Mannin Bay is somewhat harder to access. The road runs close to the coast but there is really no room for proper parking and one needs to climb over the fence to access the place. I, of course, did it because being a geologist who wants to see stuff means that you often need to park your car where you should not and go where you are really not supposed to go.

Mannin Bay is close to Dog’s Bay but it is even somewhat more special place. Sand of Dog’s Bay is finer and it is composed of many different biogenic grains (sponge spicules, clam shells, foraminifers, sea urchins, and gastropods). Sand of Mannin Bay is coarse and it is almost exclusively composed of pieces of coralline algae (Rhodophyta). Two of the most common species of coralline algae on the Ireland’s west coast are Phymatolithon calcareum and Lithothamnion corallioides. The algae lives less than 20 meters below the sea level. Living algae is reddish in color. Pieces of it get washed ashore during storms.

http://picasaweb.google.com/107509377372007544953/Rocks#5798366100965974386
Pieces of branched coralline algae of Mannin Bay. I picked grains with a more characteristic shape for this picture. Width of view 6 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5798366154297834834
Here is a close-up picture of Mannin Bay sand taken in situ.
http://picasaweb.google.com/107509377372007544953/Rocks#5798366180470636594
Mannin Bay is covered with coarse biogenic sand composed of coralline algae.
http://picasaweb.google.com/107509377372007544953/Rocks#5798366206364570690
A view towards the sea.
http://picasaweb.google.com/107509377372007544953/Rocks#5798366226939109874
These black rocks are metamorphic, probably metamorphosed mafic igneous rocks.
http://picasaweb.google.com/107509377372007544953/Rocks#5798366251003689154
Maerl forming terraced banks of a small stream.
http://picasaweb.google.com/107509377372007544953/Rocks#5798366136378597634
Metamorphosed and folded metabasite of Mannin Bay.

Lake Salda in SW Turkey seems to be a very spectacular place, especially geologically. It is a crater lake with sandy shores but this sand is not made of quartz or some other usual mineral. It is bright white and composed of hydromagnesite.

Hydromagnesite is a Mg-bearing hydrated carbonate mineral. It is usually an alteration product of brucite which is a magnesium hydroxide and occurs in dolomitic marble and serpentinite. In this case there is little doubt that this sand comes from serpentinite rocks because there are lots of serpentinite fragments among the sand grains. Serpentinite is a rock type that is composed of serpentine minerals which were formerly ultramafic igneous rocks. All these alterations are hydrothermal. It means that hot water circulating in the rocks carried out these reactions.

Sand from the southern shore of Lake Salda in Turkey containing hydromagnesite (white) and serpentinite (dark grains). The bright color of the sand is easily visible even in satellite maps. The width of the view is 15 mm.

This post is a follow-up to the article about sodalite. This time I am writing about the sand sample from which I picked these beautiful blue sodalite grains. It was an easy task to pick them because the grains are large and there are lots of them. Most of the sand is made of sodalite. Other major component is dolomite (gray crystals).

Sand sample from Namibia containing blue sodalite and gray dolomite. The width of the view is 14 mm.

As much as I know this sand sample is one of the most desirable gems sand collectors wish to have in their collections. The other highly sought after sample is probably the one from Japan that contains star-like tests of forams. I wrote a post titled star sand and sun sand where you can see how these forams look like.

The sodalite sand from Namibia is probably not a natural sand. I can’t say it for sure because I have never visited the collecting place but the composition (only dolomite and sodalite which are both rare in sand) and angularity of the grains do not leave much room for alternative explanations. The sample is from a sodalite mine in NW part of Namibia.

The sand itself may not be natural. It is probably what is left of crushed stones but for me this is no problem. I like them all, no matter whether they come from crushed rocks or natural sand.

It is of course interesting to know what type of rocks were the source material of the sand. Sodalite is usually magmatic mineral¹. Sometimes sodalite forms in contact metamorphosed carbonate rocks but magma is involved there as well. The most likely interpretation is that silicon-deficient and sodium-rich magma intruded into the dolomite formation and solidified there as an extremely feldspathoid-rich plutonic rock known as foidolite. This rock type contains more than 60 percent feldspathoids in a ternary diagram of alkali feldspar, plagioclase, and feldspathoids. Since I can’t find feldspars, I assume that it is foidolite. I did some googling as well and one of the sources confirmed that there is indeed a mine in Namibia where sodalite-bearing foidolite is mined.

It may come as a surprise but it seems to me that we have to thank the European Union and its regulations for that. In EU it is mandatory for dimension stone dealers to use scientific terminology. No more can they classify all of their rocks into marble and granite. It seems unbelievable but sometimes EU and its huge bureaucracy machine does something that seems to be genuinely useful, at least for me and other geologists. Rock dealers, I am afraid, would probably not agree with me.

References

1. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.

You may also like the gallery of colors in sand.

Globigerina ooze is a soft seafloor sediment composed of microscopic shells of calcareous foraminifera.

Globigerina ooze is white in color and is mostly composed of tests of globigerina. Most foraminifera are benthic (living on the seafloor), but globigerinida (order that contains genus globigerina) are planktonic organisms. They are very small. Picture below is only 5 mm in width. Their fragile tests are composed of hyaline (glassy) calcite.

Globigerina ooze is very common sediment on the seafloor (especially in the Atlantic and South Pacific Oceans), but it is not composed of globigerina tests only. There are other foram taxons as well like Orbulina, Neogloboquadrina, and Rotaliida.

Globigerina ooze is a calcareous sediment. Such sediments generally do not survive below the calcite compensation depth (CCD). The CCD in average is about 4500 meters below sea level. Below that level calcite dissolves in seawater. The forams on the picture below come from the Weddell Sea which is bordering Antarctica. The sample is taken 3500 meters below the sea level. I thank Michael Hesemann from foraminifera.eu for sending me this sample.

Volcanic ash is a fine mixture of minerals and rock fragments thrown out of a volcano during exposive volcanic eruption.

Here is an example of ash from the volcano that is responsible for the most deadly eruption in the history of USA. The volcano is of course Mount St. Helens and it happened in May 18, 1980.

Volcanic ash is so fine that with a naked eye you only see dark to light gray powder. What is it made of? Lots of it is glassy froth – pumice as a rock type (white grains). Mineralogically it is composed of plagioclase (gray blocky crystals), quartz (lots of silica is glassy, not quartz), amphiboles, and pyroxenes (black grains). All these components together in one rock is called dacite. This is a fairly silicic volcanic rock which explains why the eruption turned out to be so violent. Silicic magma is highly polymerized and holds the gases which escape from the rising melt but can not break free. So the whole thing expands like a fermented dough until KABOOM! In this particular case the story is more complicated. There was a flank collapse which triggered the eruption but that is not so important here.

This sample is collected just one day after the eruption and more than 100 kilometers away in Yakima County, Washington.

Volcanic ash collected in Yakima County 120 kilometers away from the Mount St. Helens in May 19, 1980 — just one day after the catastrophic eruption. The width of the view is 4 mm.

Herkimer diamonds are double terminated quartz crystals found in cavities of dolostone host rock in New York State, USA.

Herkimer diamonds are pretty famous. You will find lots of websites talking about these beautiful crystals. Many of these sites are about astrology and New Age but there is usuful stuff too.

The fact that these crystals are actually not diamonds doesn’t make them any less interesting. What makes these crystals so unique? It is the fact that they have nearly perfect crystal faces on every side. This is really remarkable. Quartz crystals grown in cavities are not that rare but they are attached to the wall of the cavity on one end. So they have only one end with perfect crystal faces. Herkimer diamonds had to grow so that they had minimal contact with the host rock. I really don’t know why and how they managed to do that but it should be obvious that crystals grown that way must be exceedingly rare. The extreme clarity of the crystals makes them very appealing as well.

The host rock of the crystals is approximately 500 million year old dolostone. This does not mean that the quartz crystals found inside the rock are that old. They are definitely younger. First there was lime mud on the bottom of the shallow sea. This mud was buried and compacted until it became hard rock which we know as limestone. This limestone had its composition altered by the magnesium bearing fluids which slowly turned it into dolostone. Dissolution vugs or cracks had to form somehow and only then quartz crystals had a chance to start growing very slowly out of the silica bearing water that was moving through the rock formation.

You may also want to take a look at the article about the Pecos diamonds which are also double terminated quartz crystals but they grew in gypsum deposit.

http://picasaweb.google.com/107509377372007544953/Rocks#5805071014597717554
These Herkimer diamonds (double terminated quartz crystals) were collected 40 years ago. The width of the view is approximately 10 mm.

I thank Bill Beiriger for sending me these beautiful crystals.

Tests or shells of Baculogypsina (foraminifera genus) look very spectacular. I wrote about them in an article of star sand. However, they are sand grains after the owner of the test dies and like all other sand grains as time goes by they become more and more rounded.

That’s why we usually do not sea Baculogypsina tests as beautifully preserved as are those collected in Hatoma Island, Japan. One often has to dive to collect the most beautiful specimens where waves can not break them.

Beach sand in Bali (Indonesia) contains Baculogypsina tests as well but at first sight they are hardly recognizable. It is really no “star sand” anymore. More appropriate name would probably be “sphere sand”.

Rounded tests of Baculogypsina from Bali, Indonesia. The width of the view is 6 mm.

Foram species Baculogypsina sphaerulata from Hatoma Island, Japan. The width of the view is 15 mm.

Heavy mineral sand is probably my favorite sand type. They are natural concentrates of many interesting minerals. These sands are beautiful to look at and very educational as well. Here is a sample taken from the shore of Halls Lake, Ontario, Canada. Thanks to Frances Vandervoort for this sample.

My photo equipment unfortunately is at its limit here. This sand and many other good samples are too fine-grained. I hope I will soon be able to take higher magnification images.

Most important components of this sand are quartz (which is hardly surprising) and actinolite (it isn’t rare either but usually not as abundant as here). Quartz is transparent and actinolite is black. Actinolite is a member of amphibole group. Other notable minerals are almandine, orthoclase, epidote, augite, pumpellyite, and magnetite.

Some of you might ask that how can you possibly be so sure about these minerals? How can you say that this feldspar is orthoclase and amphibole is actinolite? Indeed, this is difficult, and I do not claim to have an ability to do this with optical microscope only. X-ray diffraction is the keyword here. This technique significantly helped me in the identification process.

What is the possible source rock of this sand? To say that for sure, one really needs to go out and familiarize him/herself with the local geology. I have never been there, so I can only theorize. Most minerals seem to point to the metamorphic source. This part of Canada is a shield. There is lots of very old (Proterozoic) metamorphic and igneous stuff there.

Actinolite could form in metamorphosed calcareous sediments but it could be a product of regional metamorphism also (glaucophane schist facies). Pumpellyite also favors regional metamorphism and glaucophane schist facies but unfortunately there is no trace of glaucophane itself. Augite points to igneous source. Almandine usually comes from schist (regional metamorphism). This sand most likely has several if not many source rocks contributing to its composition but some of them can not be far away. Actinolite for example looks fresh and is very abundant here but its resistance to weathering processes is not good.

Sand sample from Ontario, Canada. The width of the view is 10 mm.

Update

Thanks to Howard I just discovered from where to download Canadian geological maps. Here is an excerpt from a geological map of Halls Lake area:

Geological map of Halls Lake area. The sample was probably collected at the NW corner of the lake. Photo courtesy: Department of Natural Resources Canada.

Pg — Irregular granitic gneiss
Pga — Regular granitic gneiss
Pp — Porphyroclastic gneiss
Pst — Granitic straight gneiss
Pta — Medium to fine-grained tonalitic straight gneiss
Pt — Coarse biotite-hornblende orthopyroxene tonalitic orthogneiss 1450-1300 Ma.

Actinolite probably comes from gneiss (Pp). Hopefully it is amphibolitic gneiss. It could be Pt also. Maybe this ‘hornblende’ there is a field term? Sometimes geologists have a habit to name it hornblende whenever they see black amphibole.