Ilmenite

June 12, 2025November 23, 2011 by Siim

Ilmenite is an iron titanium oxide. It is the principal ore of titanium. It is black (or dark gray) and has a metallic luster. It is usually weakly magnetic. The mineral itself is actually not magnetic, but it is often intergrown with magnetite, which very strongly responds to the magnetic force. You can use a hand-held magnet to test sand grains. If they are only lazily turning themselves but not jumping vigorously towards the magnet, then they are most likely ilmenite grains, not magnetite.

Another aspect that separates it from magnetite is the crystal structure. Magnetite is an isometric mineral, it forms double pyramids (octahedrons) just like diamond. These octahedrons are often worn off in sand, but in this case magnetite is rounded and still roughly isometric. Ilm. is at least theoretically different, its grains tend to be tabular. These minerals are both opaque minerals.

The crystal structure of is identical to hematite. It is compositionally pretty close to hematite as well. The only difference between them is that one Fe atom (in hematite) is replaced by titanium atom (in ilmenite). The chemical formula is FeTiO₃. This is ideal formula which often is pretty close to the reality, but some of the iron may be replaced with magnesium and manganese.

http://picasaweb.google.com/107509377372007544953/Coll#5851075733977440210
Beach sand from India containing lots of ilmenite. Brown grain in the lower left is leucoxene. Light blue elongated grain is kyanite. Transparent crystals are quartz grains. The width of the view is 5 mm.

Ilmenite crystal should ideally look like this.

Grains are often altered to leucoxene. Leucoxene is a mixture of several oxide minerals, it is not a mineral itself. It looks like a very fine-grained light-colored or brownish coating on ilmenite grains.

Ilmenite is found in igneous and metamorphic rocks although it very rarely forms a major component of a rock. If that happens (in iron- and titanium-rich magmatic cumulates) then it is usually mined for its titanium content. Titanium extracted from ilm. in the form of titanium dioxide is used as a white pigment in paints and sunscreens among other things. Millions of tonnes of ilmenite are mined every year, but the majority of it comes from sand, not from the hard rocks. It is resistant to weathering and therefore common in sand. It is usually accompanied by magnetite and possibly by a small amount of rutile and zircon as well (they are even more resistant to weathering).

http://picasaweb.google.com/107509377372007544953/Coll#5851075735049738098
Tellenes mine in Norway. It is fun to think that white paint and sunscreen come from this black hole. Image: Mikenorton/Wikipedia.
http://picasaweb.google.com/107509377372007544953/Coll#5851075703239069858
Ilmenite and leucoxene grains. Some grains demonstrate half-completed leucoxenisation process (few examples are annotated). The width of the view is 3.8 mm.
http://picasaweb.google.com/107509377372007544953/Coll#5851075705332924162
Beach sand containing ilmenite (black), leucoxene, quartz, almandine, and zircon. Calvert Cliffs State Park, Soloman Islands, Maryland.

Mysterious dunes in Estonia

June 11, 2025November 21, 2011 by Siim

There are weird landforms in the NE corner of Estonia which are called kriiva in Estonia. Most of them look like dunes and they are composed of fine sand but there are some difficulties.

First researchers almost 100 years ago thought that these landforms are marginal moraines because their NE-SW orientation match the orientation of the continental glaciers margin there some 12,000 years ago. However, we see no sign of deformations which should be there if we assume that these landforms are the result of a bulldozing work of an advancing glacier. This hypothesis is largely rejected now.

What is the problem then? First of all, there seems to be at least two morphologically different types of kriivas. First ones are straight and the other ones are curved. I visited several of these landforms about a month ago with some fellow geologists.

We first visited one of the straight kriivas and thought: what a heck, this is like a railway dam running straight through the forest. I never thought that a sand dune might look like this. Both sides of the landform had similar steepness. We made some excavations to look for a cross bedding but found only very subtle hints of it. The layers were mostly parallel and composed of fine well sorted sand. My belief in the dune hypothesis was quickly waning although I couldn’t figure it out what else it might be. All right, let’s assume we had a marginal crack in the glacier where the sediments were accumulating. But the sediments are too well sorted. Glaciers carry all kinds of material from clay particles to large boulders but there were only fine sand.

Then came the curved ones which were different. They are morphologically clearly resembling dunes (one side steeper than the other) and there were some cross bedding in the upper parts of the dune. However, interestingly the lower part of the dune had parallel alternating layers of silt and sand which is a characteristic to ice lake sediments (somewhat similar to varved clay sequences) and definitely not to dunes.

Well, our preliminary conclusion was that maybe these dunes were not dunes at the beginning. Maybe they were indeed some sort of “lake” sediments formed in a narrow crack? When the glacier retreated, some of these straight sandwalls were reworked and became curved dunes and some for some reason remained intact? I really don’t know the answer but if any one of you have experienced something similar, I and one of my friends who is writing his bachelor thesis on these dunes would appreciate it.

Google Maps is not very competent in this region but here is a link to that area.

LIDAR map of kriiva area
Here is an overview of the area taken from the Estonian LIDAR relief map. S — straight dune, C — curved dune. This map is a courtesy of Estonian land Board.

Straight kriiva cross section
There are many small and mostly abandoned sand quarries which made our job a lot easier. This is the cross section of a straight kriiva. It almost seems to be like a manmade landform.

Curved kriiva cross section
Curved kriiva. These dunes are composed of many smaller dunes that are sitting one on top of the other.

Alternating layers of silt and sand
Alternating layers of silt (darker) and sand (lighter) in the lower part of one of the curved dunes.

Slip side of kriiva
Here we see clearly that the layers (slip side of the dune) are inclined. I measured the maximum true dip which is 28 degrees. The dip of the same dunes windward side was approximately 8 degrees. These are quite characteristic numbers for a dune.

Kriiva sand
Here is an example of the material these dunes are made of. It is fine sand but surprisingly not too well sorted. Note how large are some quartz grains compared to the rest. Dune sands are generally very well sorted so it may be a sign that in this case the transport route of the grains have been very short. It is compositionally typical continental sand. Quartz and K-feldspar are the most important constituents. The width of the view is 5 mm.

Rapakivi

June 11, 2025November 19, 2011 by Siim

Rapakivi is a type of granite. Its type locality is the Vyborg batholith which is located in SE Finland and Karelia, Russia. “Rapakivi” is a term that comes from the Finnish language (rotten or crumbled stone) but this rock type is not restricted to this particular area. The magmatic plutons showing rapakivi texture are found in many places all over the world, including USA, Brazil, China, etc.

Large orthoclase ovoid (5 cm) mantled by the oligoclase (one of plagioclase feldspar minerals) rim. Not all of the orthoclase phenocrysts are mantled by the plagioclase rims, though. The large ovoid on the lower right lacks plagioclase rim.

This rock is well known for its interesting texture — large ovoidal orthoclase phenocrysts are surrounded by plagioclase mantles. However, not all the rocks that are described as rapakivi granites show this texture. To avoid confusion, rapakivi is now defined as a granite which comes from a pluton that shows rapakivi texture at least partly. The rock with a rapakivi texture from the Vyborg batholith has a special regional name — wyborgite.

Rapakivi from the type locality and its surroundings is approximately 1.6 billion years old. But they are not necessarily restricted to the Proterozoic eon. Rapakivis from the Archaean and the Phanerozoic eons are known as well. The Vyborg batholith is one of the many rapakivi plutons in Northern Europe but most of them are covered by younger layers of sedimentary rocks.

Rapakivi is clearly plutonic rock but the volcanic rocks associated with it most likely existed as well. We have even some examples of very old rhyolitic lavas but most of it is long gone. This volcanic episode must have been very violent and explosive but the evidence is very scarce. The chance for a survival is not very high for a supracrustal rocks of that age. The emplacement of rapakivi plutons and associated rocks (diabase, anorthosite, rhyolite) was probably caused by a rifting episode which subsequently failed. These rocks are therefore anorogenic, their emplacement was not caused by the mountain building episode.

Why is this rock “rotten”? Because it is quite susceptible to weathering. Plagioclase feldspar weathers easily and the inequigranular nature of the rock makes it easier to disintegrate by the temperature changes. The boulders are often so weathered that it takes only bare hands to crumble it into smaller pieces.

Here are some glacial erratics of this type now on the coast of the Baltic Sea near the Vyborg batholith in Karelia.

Some orthoclase ovoids are really big (the largest in the middle is 8 cm in diameter). Karelia, Russia (The Gulf of Finland).

Weathered boulder on the coast of Karelia, Russia (The Gulf of Finland).

Rapakivi granite
One more crumbled boulder on the Karelian coastline.

http://picasaweb.google.com/107509377372007544953/2015#6190951401996398930
Fresh plagioclase is greenish gray and orthoclase is more close to orange than pink tone. Luumäki, Finland. Width of view 35 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190951412460546738
Weathered surface looks different. Plagioclase is light gray which makes the texture more easily noticeable amd orthoclase is also clearly paler pink. Luumäki, Finland. Width of view 30 cm.

Sand from Bretignolles-sur-Mer

June 11, 2025November 18, 2011 by Siim

Sand from Bretignolles-sur-Mer (France, the Bay of Biscay) is famous among sand collectors because of its unusual mineralogy and appearance. The sand is composed of spinel, magnetite, epidote, garnet, staurolite, ilmenite, K-feldspar, quartz, zircon, corundum, apatite, titanite, rutile, etc. It is reddish or even purplish in color.

Many sand samples contain these minerals but in the vast majority of cases all other minerals besides quartz and feldspar occur in very small quantities. Here they clearly form the majority. Such sand type is called heavy mineral sand. These sands tend to be fine-grained. Getting sharp images which still show every single grain is a tough task but I hope you can get a general overview what kind of beautiful and colorful crystals such sands contain.

I recommend to check out the gigapan of the same sand sample.

Heavy mineral sand
Sand from Bretignolles-sur-Mer. There are more interesting minerals in this sand but here I numbered those that should be recognizable in this picture: 1 — spinel (iron-bearing), 2 — almandine (garnet), 3 — staurolite, 4 — may be titanite (sphene), 5 — K-feldspar, 6 — epidote, 7 — quartz, 8 — magnetite. The width of the view is 5 mm.

Lunar anorthosite

June 11, 2025November 17, 2011 by Siim

Anorthosite is a fascinating rock and sparks interest even among those who usually don’t care about rocks. The reason is simple. Anorthosite is often composed of mineral labradorite which is famous for an iridescent effect called labradorescence. You’ll find more in this article: anorthosite and labradorescence.

The Moon highlands seem to be composed of anorthosite. We have both indirect and direct evidence for that. Measurements made recently by the Japanese lunar orbiter SELENE suggest that the lunar anorthosite may in many cases be almost totally monomineralic — composed entirely of plagioclase with very high calcium content. We have direct evidence also — American astronauts who visited Moon in the early 1970s brought back 61 rock samples that were found to be anorthosites.

It is wonderful to think that large portion of the Moons surface (highlands surrounding the basalt lowlands or marias) is schillering like anorthosites here on Earth often do. However, it is likely not the case. There are several differences between terrestrial and lunar anorthosites. Terrestrial anorthosites contain more sodium (sodium and calcium can replace each other in all proportions in the crystal structure of plagioclase). Plagioclase must have the composition of labradorite — one of plagioclase minerals. It means that 50-70% of the sites in the crystal structure which are occupied either by calcium or sodium ions are occupied by calcium. In the lunar anorthosites Ca-content is close to 100%. In order to have a labradorescence, the percent of calcium needs to be in the range of 48-58%. The effect of labradorescence is the result of a breakup of plagioclase crystals into many alternating lamellae of different (calcium and sodium rich) composition. If there is very little sodium present, such exsolution simply can not take place.

There are more differences between terrestrial and lunar anorthosites. Lunar anorthosites are light-colored, while some terrestrial anorthosites are dark. Here on Earth the cooling of anorthositic magma bodies took very long time. The crystals which show labradorescence are often very large, even pegmatitic (more than an inch in length). Lunar anorthosites, however, are quite fine-grained. Only very few crystals are larger than 1 cm.

Lunar anorthosite is very old. It is believed that it formed when the lunar magma ocean solidified which probably took place in the first 100 million years of the existence of the Moon. Lunar anorthosite is believed to be the result of a gravitational differentiation. Plagioclase is lighter than most other minerals found there and therefore rose to the uppermost part of the magma ocean. However, the details of this process are still hotly debated.

Take a look at the NASA Lunar Sample Catalog if you want to see more images and general overview of the rocks collected by the astronauts of the Apollo program.

Ooid sand

June 12, 2025November 16, 2011 by Siim

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.

Anorthosite and labradorescence

June 13, 2025November 15, 2011 by Siim

Anorthosite is an igneous monomineralic rock that is composed of plagioclase feldspar (over 90% of the rock is composed of this mineral). Plagioclase is a very common mineral group, more than half of the Earth’s crust is composed of it. Therefore it isn’t really surprising that there is a rock type that is almost exclusively composed of plagioclase. Anorthosite is sometimes named plagioclasite or labradorite as well.

Anorthosite is usually light-colored because its main constituent plagioclase feldspar is normally white and most anorthosites have no iridescence. Rogaland, Norway. Width of sample 13 cm.

Because of linguistic similarities some people think that anorthosite is a rock composed of anorthite (one of plagioclase group minerals). This is not true. Anorthosite is usually composed of labradorite and sometimes bytownite or andesine as well (all plagioclase group minerals).

Anorthosite from Finland (known also as spectrolite) showing labradorescence (blue spot). The width of the rock sample is 17 cm.

http://picasaweb.google.com/107509377372007544953/2015#6190952891713510098
Another sample of anorthosite from Rogaland, Norway. Width of sample 12 cm.

The term ‘labradorite’ itself is a source of confusion as well. Maybe not so much in the USA because this trap was set by French and Russian geologists and creates trouble where their influence for historical reasons have been greater. The problem is that labradorite for them is a rock type containing lots of mineral labradorite. So the problem is just like the famous dolomite problem. One always has to think about what are we actually talking about – a rock or a mineral?

Anorthosite is usually defined as a leucocratic rock. Leucocratic means light-colored? Well, this is complicated. It needs to be careful here as well. Leucocratic can be defined as ‘felsic’ or ‘not mafic’. Leucocratic minerals are those that are relatively rich in silicon and aluminum but contain little iron and magnesium. Felsic minerals (quartz, feldspar, muscovite) are generally lighter than mafic minerals (pyroxene, hornblende, biotite, olivine) but not always. Some plagioclase feldspars are definitely darker than bright green olivine crystals. And some pyroxenes are colorful as well. Nature does not care about our classification schemes and always finds a way to laugh at us. Anorthosite in many cases is not light-colored at all. This is the result of tiny Fe-Ti oxide inclusions in plagioclase crystals that give them bluish-black hue.

One fascinating aspect associated with many anorthosites is an effect called labradorescence. It is a special form of iridescence. Plagioclase crystal is composed of many exsolution lamellae — minerals have broken up into many slabs of alternating composition. These slabs act like mirrors. Some light reflects back from the crystal surface but some portion of it penetrates the surface to be reflected back from the next lamellae which is 20…50 nanometers below the surface. Reflected lightwaves combine (this is called interference) and create peculiar colors which in our case seem to be mostly blue.

If you want to see anorthosite, you don’t have to go far – just look up at the night sky. It is a common rock type on the Moon (take a look at the post of lunar anorthosite). The highlands of the Moon surrounding the dark basaltic lava fields called maria are composed of this rock type.

http://picasaweb.google.com/107509377372007544953/2015#6191004927340681986
Anorthositic dike surrounded by metamorphic rocks. Hedmark, Norway.

http://picasaweb.google.com/107509377372007544953/2015#6191004345671387362
Anorthositic landscape in Rogaland, Norway.

Anorthosite is not nearly as common here as it is on the Moon. Anorthosites on the Moon are extremely old, almost as old as the Moon itself. Anorthosites on Earth are pretty old too, mostly from the Proterozoic Eon. Anorthosite is a plutonic rock just as granite and gabbro and it is usually associated with the latter. Therefore we can conclude that despite being officially leucocratic it actually is usually associated with mafic rocks.

Anorthosite is usually considered to be a cumulate rock. These are magmatic rocks that have crystallized from the magma which is enriched in low- or high density minerals. Anorthosite should therefore represent the upper portion of certain magma chambers where light plagioclase crystals have accumulated. However, there are lots of still unsolved questions. For example: if it really is a cumulate rock then where are the opposite mafic cumulates that are composed of pyroxene and olivine? I don’t want to say that these rocks don’t exist. They do in some other areas. These interesting rocks are collectively called peridotites. Peridotite is a common constituent of the Earth’s mantle but usually not associated with anorthosites.

One more interesting aspect is that most plutonic igneous rocks have extrusive equivalents. These pairs are gabbro-basalt, granite-rhyolite, etc. But there is absolutely no volcanic equivalent of anorthosite. Why not? And why did these rocks mostly form in the middle of the Proterozoic Eon?

Anorthosite is not an uncommon rock. It is present in nearly every continental landmass (exceptions are Greenland and Australia¹).

Update

It came as a nice surprise that this post inspired several other geobloggers to write about their experiences with this rock as well. Check out the blogs of Ron Schott, Garry Hayes, Dana Hunter, Ian G. Stimpson, Callan Bentley, Ryan Jackson, and Mika McKinnon who participated in this geomeme.

References

1. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.

Parrotfish makes sand

June 11, 2025November 14, 2011 by Siim

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:

Brain games with sand grains

January 11, 2016November 11, 2011 by Siim

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.

How the smallest sand grain compares to the largest
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.

Grain size (µm)	Aggregate name	Volume difference	No. of grains in 1 cm³
62.5	Very fine sand	1	4,959,645
125	Fine sand	8	619,956
250	Medium sand	64	77,494
500	Coarse sand	512	9687
1000	Very coarse sand	4096	1211
2000	Gravel	32,768	151

Staurolite

June 11, 2025November 9, 2011 by Siim

Staurolite is a common mineral in medium-grade aluminous metamorphic rocks. The composition is very rich in aluminum: Fe₂Al₉O₆[(Si,Al)O₄]₄(OH)₂. This indicates that the protolith had to be a clay-rich sedimentary rock. It is a hydrous mineral which makes it unstable at very high pressures.

Caption — Schist with a large twinned staurolite porphyroblast. Tohmajärvi, Finland. Width of sample 19 cm.

Staurolite is an orthosilicate: ratio of (Si,Al):O is 1:4 and the silica tetrahedra are isolated from each other. Structurally similar silicate minerals are kyanite and garnet which tend to occur often together with staurolite as they are all Al-rich minerals formed during metamorphism. Other common metamorphic minerals that often occur with them are kyanite, muscovite, biotite, chloritoid, and cordierite.

Crystals are elongated and dark brown. Edges of crystals may be lighter and even orange in color if the light is able to penetrate the crystal. Twinning is very common (crystals crossed with 90 or 60 degree angles between them) which are very characteristic and therefore useful in identification.

Width of sample is 7 cm — Dark, elongated staur. crystal in a metamorphic rock schist with garnet, kyanite and muscovite. Width of sample 7 cm.

This mineral is resistant to weathering and is therefore very common constituent in sand. It is part of the heavy mineral fraction, its density is 3.7…3.8 g/cm³. It may be easily mistaken for garnet because small grains are often not elongated and they are signifiantly lighter in color than large crystals in rocks. Staurolite as a sand grain is orange, garnet is usually pink (spessartine (Mn-rich garnet) may resemble staurolite in color, though). Polarizing microscope may be helpful to tell them apart. Garnet is an isometric mineral which will be dark in crossed polars while staurolite has interference colors.

Staurolite and garnet in sand — Blue circles mark garnet and green circles staur. grains. Sand sample is from the Mediterranean Coast of France (Rayol-Canadel-sur-Mer). Other grains are quartz, K-feldspar, and plagioclase. Width of view 10 mm.

Staurolite is mostly used as an abrasive (hardness 7-7.5) in sandblasting applications. So in this regard it is also very similar to garnet. Twinned crystals have a commercial value as they resemble crosses which can be used as earrings or pendants.