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I have a plan to do some geologising in Sri Lanka. Most likely very soon. This country has been on my list of places where I want to go for some time.
Unfortunately it is largely quite futile attempt to search for interesting outcrops and geolocations of Sri Lanka in the web.
So I am asking your help. If you have been there or live there or know some interesting places, just let me know. My email is siim.sepp at gmail.com
If you wish I may send you some interesting geological stuff from Sri Lanka in return. I am talking about ordinary beach sand but I promise that it will contain more than just quartz and these minerals will be identified.
By the way, the hard drive of my laptop crashed today. Such things seem to happen at the worst possible time. Friday after workday, so there is no hope to get professional help before monday and I have so much to do.
I had some material prepared for Sandatlas and that too is inaccessible now. I am writing this post with an iPhone. It is possible but not very enjoyable experience. But I try to take the best out of the situation. I have more time for books now which have been waiting me for some time.
Sandatlas is now out of baby age. It has together with the article you are reading now exactly 100 articles.
To celebrate that I prepared a patchwork quilt of sand which is composed of 100 photos of interesting sand samples. Unfortunately this is no Gigapan here. You can not zoom in to study every one of them in detail but I already have used many of these sand samples to illustrate the articles written so far. Just dig in if you want to know more about sand.
I hope that one day we can celebrate the 1’000th article here. I thank you all who have read my posts and hope that you will find something worth reading here in the future as well.
 100 interesting and versatile sand samples merged into one patchwork of sand.
While watching the horror movies of mineral destruction at Research at a snail’s pace I noticed that some minerals almost seem to explode. That is of course perfectly normal because these poor crystals got a serious blow from the hammer but there are more issues worth explaining that are deeply connected to the way minerals are built.
The first observation one can make while watching these videos is that minerals are brittle. This is hardly surprising, we all knew that before. But what makes them brittle? Is it universal among all minerals?
Minerals are naturally occurring inorganic crystalline solids. What is important for us here is that they are crystalline. It means that every atom or ion has a specific place in relation to neighboring ions. Everyone of them has neighbors and these neighbors are loyal to each other — they are not allowed to move freely in the lattice. The whole structure has to be electrically neutral (the sum of positive and negative charges or valence numbers is zero) and the distances and angles between the ions must form a stable three-dimensional network where the repulsive and attractive forces between the neighboring ions balance each other. There is usually not too many ways how a mineral with a given chemical composition can be put together to make such a structure. There is definitely nothing random here. People who like order should study crystallography. They would love it!
What happens to the crystal when we hit it with a hammer is that we deform the crystal structure and force the ions with a positive charge to be next to another ion with a positive charge and the same with anions (ions with a negative charge). Crystals can not take it because there are repulsive forces between the ions with the same sign. When that happens crystals simply break into pieces and often more violently than we would expect, leaving us an impression that they explode.
All right, that would explain why minerals with an ionic bonds can not tolerate hammer blows but what about the covalent bonds? These are bonds between atoms or atom groups that share electrons. Let’s take a look at diamond for example. It is composed of only one chemical element — carbon. There are no valence differences and hence the chemical bonds are 100% covalent. Is it safe to try the hardness of diamond with a hammer? No, I definitely do not recommend it. Diamond is hard but it is brittle as well. These are different things. Diamond is crystalline, every one of its countless atoms knows its neighbors and they still do not tolerate the idea that someone comes and tries to force things around. Remember, these atoms share electrons but they know with whom they share and never agree to change their mind.
But some minerals are not brittle, gold for example. What is the deal here? It is a special case. Gold atoms are bonded by a special type of covalent bonds which are called metallic bonds. Here again electrons are shared but this time atoms do not care much with whom they are sharing their electrons. Electrons are free to move around, forming an electron cloud which is among other things capable of carrying electric current. Gold is an excellent conductor of electricity. Gold atoms have specific locations in the lattice where they can be but they can change their locations because their free electrons allow them to move around. That’s why gold is malleable and we shouldn’t be afraid to break our gold rings when they happen to fall.
Leucoxene grains occur frequently in sand. Leucoxene is not a mineral because it lacks defined crystal structure and its chemical composition is far too variable to be expressed as a chemical formula. It is an alteration product of titanium-bearing minerals like ilmenite, rutile, and titanite (sphene).
Leucoxene is a mixture of several minerals, most important of them are rutile, pseudorutile, anatase, hematite, and goethite. The color of leucoxene is variable. It could be light gray, brown, yellow, orange, reddish, etc. It looks earthy because it is a mixture and therefore never forms beautiful crystals. Leucoxene is an economic mineral (titanium ore). It is usually mined with ilmenite.
There are black ilmenite and colorful leucoxene grains on the picture below. The width of the view is less than 4 mm, so the grains are really small and indistinguishabe to the naked eye. I like these grains for several reason. Leucoxene grains show different shades of color and some grains demonstrate the leucoxenisation process being halfway completed.
 Ilmenite and leucoxene grains. Some grains demonstrate half-completed leucoxenisation process (few examples are annotated). The width of the view is 3.8 mm.
Rutile is a common mineral in sand and one of the most important sources of titanium. The other important titanium-bearing mineral is ilmenite.
Rutile has very simple chemical composition (TiO2). So its an oxide like ilmenite, magnetite, quartz, etc. Yes, indeed, I mean it seriously. Quartz is traditionally and rightfully treated as a silicate mineral. But it is at the same time an oxide of silicon just as much as rutile is an oxide of titanium.
Rutile crystals are usually elongated and typically deep reddish brown although that color is best seen only in very small crystals. Larger grains are almost opaque and have a metallic luster. Smaller grains have intense adamantine luster because of extremely high refractive index. This index measures how much light bends (or its velocity slows down) when it enters the crystal.
Rutile is quite stable in the weathering environment and may therefore occur in sand as single crystals or it could be a part of a cryptocrystalline aggregate of several titanium and iron oxides and hydroxides. Such an aggregate is called leucoxene. Leucoxene forms as a weathering product of ilmenite. Hence, rutile as a main component of leucoxene and ilmenite are often mined together as a raw material of titanium. Rutile is sometimes concentrated enough to form a placer deposit. Most of rutile is mined from such placers. Rutile also occurs sometimes as fine needles in quartz or mica crystals.
Rutile occurs in many igneous (mostly plutonic) and metamorphic rocks (especially amphibolite, eclogite, and metamorphosed limestones) but usually as small crystals. Big rutile crystals may grow in pegmatites or hydrothermal veins with quartz and apatite. Rutile grains in sand are also small but their deep red color is pretty distinctive. It is noticed best if the light source is below the microscope slide.
Rutile has two polymorphs (same composition but different structure), anatase and brookite, which have similar properties but are not so widespread.
The rutile grains on the picture below are all smaller than 125 micrometers. I sieved the sand to remove the larger grains because grains as small as these need high magnification which makes the depth of focus very shallow. So the grains need to be almost uniform in size and the surface as planar as possible. Why did I chose smaller grains instead of larger ones? Because only the small grains or the edges of the bigger ones show the characteristic red color of rutile. Grains larger than a quarter of a millimeter are usually practically opaque. The sample is a rutile concentrate from Pooncarie, New South Wales, Australia.
 Rutile from Australia. The few transparent grains are zircon crystals. The width of the view is 4.2 mm.
The rock I am presenting today is clearly metamorphic but I am not sure whether I should call it mylonite or augen gneiss or mylonitic gneiss or something else. Perhaps it isn’t so important because these terms are often pretty vaguely defined and their meanings are overlapping. I encourage you to comment if you happen to know more about the subject.
This rock is a small glacial erratic about 30 cm in diameter near the coastline of NW Estonia. This rock was part of the Finnish bedrock and was carried south to Estonia by the advancing continental glacier during the Pleistocene. There is no doubt about that because such rocks are not exposed in Estonia. Estonia is a sedimentary platform composed of carbonate rocks and sandstone from the Paleozoic. The gray and yellowish stones surrounding the mylonite on the picture are limestone shingles from the local bedrock. Estonia is a paradise for fossil hunters (there are lots of trilobites, crinoids, bryozoans, cephalopods, etc.) but pretty boring for a hard rock geologists. However, there is lots of glacial debris which is mostly either high-grade metamorphic stuff (Svecofennian orogeny about 1.9 Ga) or rapakivi granites (anorogenic intrusions about 1.6 Ga).
The ovoidal reddish grains (porphyroclasts) are K-feldspar crystals or augens (“augen” is a German word that means “eye”) which have been subjected to some serious deformation. All this happened in a high temperature and high pressure conditions deep in the crust beneath a forming mountain belt where rocks behave like a very viscous liquid although they are still solid.
This rock is an example of a porphyroclastic texture. This is a texture where ovoidal and seriously strained but still largely survived crystals (porphyroclasts) are surrounded by finer foliated matrix. Rocks with a fabric like that are usually called mylonite or augen gneiss.
 Porphyroclastic metamorphic rock with K-feldspar augens. The rock is wet because the image was taken on a rainy day. The width of the rock is about 30 cm. This article is a part of Rocks from Fennoscandia series.
Serpentine is a mineral group that contains chrysotile, antigorite, and lizardite.
I chose to leave out chrysotile for now because this mineral has distinctly different appearance and properties from the other two. It has a fibrous habit and is the most widely used asbestos mineral. It definitely deserves a separate article. Here I will continue with antigorite and lizardite which are not fibrous and are therefore much better suited to become sand grains.
Serpentine (Mg3Si2O5(OH)4) is a metamorphic mineral group. It was made at an expense of mafic and especially ultramafic igneous rocks like peridotite and pyroxenite. Such rocks are common in the mantle but quite rare at the surface. When they do crop out, they are often altered by hydrothermal fluids. Hence, we often see meta-peridotite instead of real peridotite. The original mineralogy of the rocks is significantly changed. Rocks that are mostly composed of serpentine minerals are called serpentinites. It often contains all three mineral varieties — fibrous chrysotile veins often alternating with massive antigorite/lizardite. Serpentine may also form in contact metamorphosed carbonate rocks.
Antigorite and lizardite may have a very similar appearance. Thus, there is no reliable method to say which is which in a hand sample. Distinguishing between the two requires more sophisticated approach but even X-ray diffraction may give ambiguous results because they are structurally similar minerals and they tend to occur together.
Antigorite and lizardite are mostly green and they look greasy or waxy. Both light and dark green colors are possible. Mineral grains are darker when they contain more magnetite inclusions. It is easy to make sure that they mostly do contain lots of tiny magnetite crystals because serpentine grains are usually highly susceptible to magnetic field although the mineral itself is not magnetic at all.
Why do they contain magnetite inclusions? To understand that we first have to take a look at the chemical composition of pyroxenes and olivine which are the source material of serpentine. They both usually contain iron but serpentine contains very little iron (its chemical formula contains none but small amount of Mg ions may be replaced with Fe). Most of the iron ions of the precursor minerals are not incorporated into the crystal structure of serpentine, so they tend to form separate iron oxides, most frequently magnetite and hematite.
Antigorite and lizardite may form sand grains but they have only a local importance near the rocks containing these minerals. Serpentine usually alters to chlorite. Good place to look for serpentine are ophiolite sequences. Ophiolite is a piece of oceanic lithosphere that is tectonically pushed on top of the continental crust. The grains below are from a beach sample collected in Corse, France. An ophiolite sequence is exposed there.
 Serpentine or more precisely antigorite grains picked from a sand sample collected in Northern Corse near Albo. The darker the grains, the more magnetic they are. The wedge-shaped grain on the left that is significantly darker in the upper corner is an especially good example. It flips its darker side towards the hand magnet if the magnet is slowly moved towards the grain. The width of the view is 17 mm.
There seems to be an urban legend that the only beach in the world having green sand is near the southern tip of Big Island (Hawaii). An alternative version is that it is one out of two. The other being in Guam (Talofofo Beach).
I have to first make it clear that this article is about the green beach sands that are composed of mineral olivine. Lots of other minerals can form green sand beaches also but we exclude them for now.
 Olivine sand collected near the southern tip of Hawaii. It is from a tiny cove on the coastal trail, not from the beach itself where the grains tend to be duller green. The width of the view is 10 mm. I do not know for sure about Guam. I have a very nice sand from the Talofofo Beach but it is composed almost exclusively of magnetite. But there is no doubt that olivine sand could be there. Guam is volcanic island located on top of the Mariana Island Arc right next to the famous Mariana Trench that reaches almost 11 kilometers in depth and is the deepest part of the world’s oceans.
The famous green sand beach in Hawaii is called Papakolea or Pu’u Mahana Beach. As much as I know it really has few competitors in terms of purity and freshness of the olivine sand. However, it is definitely not correct to say that it is the only one. The absence of evidence is not the evidence of absence.
 Sand sample from Diamond Head Beach in Oahu. White grains are biogenic fragments (corals and forams). The sand is not as bright as are the sample from Papakolea but it is still clearly greenish. The width of the view is 11 mm. There is no need to go very far from the Big Island. Diamond Head is a tuff cone in Oahu that constantly feeds the sand beach right next to it with fresh olivine. The sand there is not as bright but it is definitely composed mostly of olivine and it is green.
It is interesting to take a look at the Pu’u Mahana and Diamond Head volcanoes. There are some striking similarities. They are both located right next to the beach. They both contain lots of olivine. They are both composed of easily erodable pyroclastic sediments. They are both high and steep which speeds up the erosion.
 Pu'u Mahana on the left and Diamond Head on the right. Images taken from Wikimedia Commons: jonny-mt (Pu'u Mahana) and Brian Snelson (Diamond Head). Why is this needed for the green sand beach to form? Because olivine is extremely unstable mineral in the atmospheric conditions. The transport route needs to be short and erosion fast to ensure that most of the olivine makes it to the beach and there is a constant supply of fresh material.
All right, we have Diamond Head Beach in addition to Pu’u Mahana but it is still only two and both of them in Hawaii. It may come as a surprise to many but olivine is not rare in sand. I would even say that it is an essential component of sand in many oceanic islands or volcanically active regions. Canary Islands, Iceland, Galapagos Islands, and Cape Verde are just a few names where olivine is a common constituent of many beach sands.
I especially love a sand sample from St. Lawrence Island in the Bering Sea. The climate there is not quite comparable to the Caribbean. That’s perhaps the most important reason why this island is not very popular among tourists. The local Yupik people are quite protective about their island as well but I am lucky to know two of them. They sent me a very nice sample from the northern coast of Sivuqaq (St. Lawrence Island in Central Siberian Yupik). This sand sample is not as green as are the samples from Hawaii because it contains lots of lithic material but olivine definitely dominates among the single minerals and gives greenish hue to the sand.
 Sand sample containing lots of olivine from Ivgaq, St. Lawrence Island, Alaska. The width of the view is 15 mm.
 St. George's church in Nördlingen on a postcard from 1919. Image from Wikipedia. Nördlingen is a small town in Bavaria with a population of 19,000. Towns in Southern Germany are very picteresque with their red roofs and timber-framed (fachwerk) houses, but Nördlingen with its history of more than 1,000 years is a real gem among them.
One of the most noteworthy aspect is the city wall which is complete and accessible to tourists. One can climb up to the wall from one of many guard towers and walk around the whole town which I did when I visited Nördlingen in the late summer of 2011. Such a journey takes a little longer than an hour. During that walk you can take a look at the Nördlingen church in the middle of the town from every direction.
St. George’s church is a really prominent structure because its tower, called Daniel, is 90 meters high. It was built in the 13th century. What is the most fascinating aspect about this church is the material that was used to build it. It is an impact breccia called suevite.
This is the very reason why I visited this town. No, I didn’t know that there is a church built of suevite but I did know that there is a famous impact structure in Southern Germany called the Nördlinger-Ries or just the Ries Crater.
The Nördlinger-Ries is a 14 million years old complex meteorite crater. Its diameter is 24 kilometers and it was created by an impactor 1500 meters in size that slammed into the Earth with an estimated velocity of 20 km/s. The explosion released an amount of energy thought to be equal to 1.8 million Hiroshima bombs.
What is left of the impact structure now is a circular depression about 150 meters deep and 24 kilometers wide. Nördlingen is just in the middle of that structure. The depression was originally thought to be volcanic in origin until one of the founding fathers of impact geology Eugene Shoemaker visited Nördlingen and noticed that the church in the middle of the town in not only notable because of its old age but also because it is built of a very unusual and rare rock type which is always associated with large impact events. Shoemaker published his finds in 1961.
 Nördlingen is surrounded by a city wall. The St. George's church is in the middle of the town. There is a nice geological museum in Nördlingen which is worth a visit. Among other interesting things is a piece of Moon rock. This rock is a gift from the Apollo-11 crew. Neil Armstrong and Buzz Aldrin had various training sessions in a nearby quarry prior to their mission to the moon. They trained skills to handle special tools for picking up samples of moon rock.
As if it isn’t already exciting enough, there is one more interesting aspect about the rocks that are so widely used to build houses in Nördlingen. The rocks that were hit by the meteorite contained graphite (graphite-bearing gneiss) which was turned into diamonds by the immense pressure (60 GPa) exerted by the impact explosion. These diamonds are microscopic (up to 0.3 mm) and have therefore no value as gemstones. However, there are trillions of them. Their estimated total mass is more than 70,000 tons (concentration in rocks reaches 0.7 ppm which is 0.7 grams per one ton of rocks). Hence, the houses in Nördlingen are built from diamonds and impact glass and breccia. I wonder whether people anywhere else in the world have such a noble taste for building materials.
This story is my contribution to the Accretionary Wedge #42.
 Suevite from Aumühle quarry which is also located inside the crater. Blue is glassy impact melt. The whole rock is a special type of breccia created by a powerful impact event. The width of the rock is approximately 10 cm.  Another piece of suevite from Aumühle quarry. Suevite from Ries is the original suevite. The place where this rock type was first described. The width of the rock is approximately 10 cm.  Suevite is still mined in Aumühle quarry inside the crater.  One of the guard towers (made of suevite) and city wall.  City wall of Nördlingen offers splendid views to the town from every direction.  Typical houses of Nördlingen. Can you see suevite?  Lovely and a bit out of shape fachwerk-house.  The doors of the church and suevite walls surrounding it. Image: Andreas Praefcke/Wikimedia Commons.
Sand sample from the Thassos Island in Greece is so rich in interesting minerals that it would be a shame to put it back to the drawer so soon. I used it before to illustrate the article of Kyanite. I mentioned there that this sand contains epidote among many other interesting minerals. Here are some of the epidote grains picked from the sand:
 Epidote grains handpicked from a sand sample collected in the Island of Thassos, Greece. The width of the view is 7 mm. Epidote is a very common mineral in sand. Green mineral found in sand is often epidote. However, epidote is definitely not the only one among green minerals. It could also be olivine, pyroxene, glauconite, chlorite, or pumpellyite among others that may be confused with epidote.
I will leave longer overview about this interesting mineral for another day. I have noticed that many sand samples from Greece seem to be especially rich in epidote. The grains on the picture are very small and the photo is not particularly rich in details but what I like about the picture is the color of the grains. This very well represents the dirty or pistachio green color that is so characteristic to epidote. Glauconite and chlorite are darker and fresh olivine tends to be “cleaner” green which makes identification easier.
Epidote has one good cleavage that runs parallel to the longer edges and controls the shape of the grains by making them elongated and the edges of the grains often relatively straight even in worn-out sand grains.
 The other important minerals besides epidote are kyanite, quartz, feldspar, and staurolite. Metalia Beach, Thassos Island, Greece. The width of the view is 9 mm.
Years ago as a first year geology student I photographed some of the hand samples we studied during the introductory geology courses. I also wrote articles to Estonian Wikipedia and uploaded some of my images.
It really was years ago. I was an undergraduate student then and now I’m finishing my masters studies. For three years I was away from active studies and worked as a science journalist. Yes, indeed, sensation-driven sexy-headline-writing journalist. This may explain why writing a blog post almost every day is not that difficult task for me. I am really used to it. Only thing that makes it harder now is that I write in a foreign language. But journalism I hope is history for me and I am back in geology trying to defend my masters thesis this spring.
I mentioned that I photographed some common rocks. I did so with a cheap point-and-shoot camera without a tripod but despite that I have seen one particular image in very many places. Perhaps you have seen it as well but don’t pay attention to it because there is no story connected with the image.
You can find this image illustrating the article about gneiss in Encyclopedia Britannica and Wikipedia among really many other places. I am pretty sure that this is the most famous gneiss in the Internet. Couple of days ago I was looking for information about the geology of Sri Lanka. I found one slideshow which was supposed to contain some images of rocks from Sri Lanka. The bedrock of this country is mostly composed of old metamorphic rocks, including gneiss. However, I did not believe what I saw there anymore since the moment I noticed my gneiss again. I don’t know from which country this particular hand sample is from but it is extremely unlikely that it is from Sri Lanka.
This particular image is not very sharp. It lacks location information. It even lacks scale. It was made by a first year geology student. What this story should teach us is that internet was then and still is quite poor of good geological information. We as geobloggers shouldn’t think that the Internet is already full of good information and we have nothing new to add. This is not true. Just take some pictures or write about something you know and show it to the world. Chances are pretty good that your modest contribution is the best Internet has to offer on this subject. And maybe one day you accidentally stumble upon your own image in Britannica.
 My gneiss photographed in 2005. No scale but I remember the rock was approximately 12 cm in length. No location information is known to me. This article is a part of Rocks from Fennoscandia series. I believe it could be from Karelia (part of a Fennoscanian shield) where I have seen similar gray and very old Archean gneisses.
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).
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 a magmatic mineral. 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 foid-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.
 Sand sample from Namibia containing blue sodalite and gray dolomite. The width of the view is 14 mm.
Just like spinel is both a single mineral and a mineral group, sodalite is also a mineral group that contains minerals sodalite, nosean, and haüyne. Lazurite also belongs to this group but it can be treated as a variety of haüyne. Sometimes lapis lazuli is erraneously also included. It is a beautiful blue gem but it is not a mineral. It is a rock type (unofficial) that contains lots of lazurite.
All sodalite group minerals are feldspathoids. What does it mean? These are minerals that somewhat resemble feldspars — both structurally and chemically and they have a similar role in igneous rocks. But they contain less silicon in relation to other ions in the crystal structure. Feldspathoids form in silicon deficient igneous rocks. Silicon deficiency means that there is not enough silicon for feldspars, let alone for quartz. Therefore, you will not find quartz together with feldspathoids in igneous rocks. Hence, no hope to find sodalite in granite for example. However, there is no restriction for feldspars to occur in the same rocks with feldspathoids. If there is silicon deficiency, some of the magma will crystallize as feldspathoids instead. Hence, they take the place which normally is reserved for feldspars.
Silicon deficient magma is not rare but then it usually contains lots of iron and magnesium. Such magma is called mafic or ultramafic. The latter also contains no quartz and the former may contain only very small amount of it. However, that is not enough for the feldspathoid minerals. Let’s take a look at the chemical composition of sodalite: Na8Al6Si6O24Cl2. It is clear that in addition to silicon deficiency one more important condition must be fulfilled — sodalite needs lots of sodium (Na). The combination of low silicon and high sodium is pretty rare and so are rocks that contain sodalite or other feldspathoid minerals.
Now it should become clear that we are dealing with minerals that are not easy to find. However, if the right conditions exist for the minerals to form, then they may form a significant portion of the rocks composition. They often are the most important minerals in the rocks that contain them, just like feldspar is usually the most important mineral in granite. Feldspathoid-bearing igneous rocks are usually called foid-bearing. This is officially accepted and agreed upon simplification.
 Sodalite grains from Namibia. The width of the view is 15 mm. Sodalite is the most common mineral of the sodalite group but not among feldspathoids. That honour goes to nepheline which often occurs together with sodalite. Sodalite is usually blue but other colors (green, yellow, pink, gray, colorless) are possible also. This mineral group is usually identified by its color. They are also optically isotropic. Hence, polarizing microscope is useful to confirm the identification. Sodalite, nosean, and haüyne are not easily distinguished from each other. More advanced methods like X-ray diffraction or chemical analysis is needed for that but it may be helpful to know that plutonic rocks (foid-bearing syenite for example) usually contain sodalite. Nosean and haüyne are normally restricted to volcanic rocks like phonolite and alkali basalt.
In addition to igneous rocks, sodalite group minerals also occur in contact metamorphosed carbonates. Lapis lazuli is such a rock. It usually contains calcite and pyroxene in addition to sodalite.
Sodalite is not a common sand constituent but may locally comprise a significant amount of some sand samples which are the disintegration products of foid-bearing rocks exposed nearby. Blue sand is highly sought after among sand collectors. It is usually sodalite that gives blue color to such sand.
Olivine basalt is basalt with olivine phenocrysts. Phenocrysts are mineral grains that are substantially larger than the groundmass surrounding them.
If the olivine grains are large enough they may be used as gemstones. Olivine as a gemstone is often named peridot(e). Olivine in basalt usually contains more Mg than Fe. Its chemical composition is (Mg,Fe)2SiO4
Olivine is denser than most other minerals in basalt and it is also one of the first to start crystallizing. Hence, it often sinks, if it has room to do so, and forms basalts that are abnormally enriched in olivine in the lower part of lava flows for example. Olivine crystals in this case were already formed before the lava poured out of the volcano because phenocrysts need time to grow. Rapid cooling in subaerial conditions would not allow it to grow as large as the crystals shown on the photo below which is a piece of olivine basalt from Oahu, Hawaii.
Olivine sand is a disintegration product of rocks with similar composition.
 Olivine basalt from Oahu, Hawaii. The width of the rock is 6 cm.
Scoria is a highly vesicular and glassy volcanic rock with mafic composition.
Scoria is very common rock type in volcanic areas. Sometimes it is not considered to be a distinct rock type. In this case it is a structural variety of basalt, andesite, etc. The problem is that volcanic rocks are officially categorized according to their chemistry. Scoria, however, is defined mostly by its vesicular nature.
Upper parts of basaltic lava flows may be scoriaceous for example. So the rock can easily be scoria and basalt at the same time. It may feel awkward and confusing to some but geology is full of such peculiarities which may seem strange at first but is actually necessary and really makes sense if you think about it.
Scoria often occurs as a single piece of rock that was thrown out of a volcano. Sometimes entire volcanic cones are made of scoria which in this case is also called cinder. ‘Scoria’ tends to be preferred term nowadays but ‘cinder cone’ is often used.
The composition of scoria is usually mafic. Similar rock with felsic composition is called pumice. Scoria, unlike pumice, does not float on water although it feels unusually lightweight for a dark-colored volcanic rock. Scoria contains smaller number of vesicles than pumice and these vesicles, because of less viscous magma, tend to be much larger. Here is an outcrop in Santorini that contains both pumice and scoria in different layers. Not every vesicular dark-colored volcanic rock is scoria. Occasional vesicles here and there are normal in basalt. It has to contain really many vesicles to be named scoria.
Scoria is sometimes reddish in color. This is iron, more precisely its oxide, that gives it such a rust-colored appearance. Scoria contains appreciable amount of iron, that is why it is called to be mafic (magnesium+ferric).
 Piece of scoria from Etna (volcano in Italy). Despite being 5 cm in width it weighs only 15 grams.
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