Overview and images of basalt as a rock type are here: Basalt
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 a 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.
I planned to write about sodalite. This is beautiful mineral and I think I have a nice photo of sodalite grains. I sat down and started writing: Sodalite is one of feldspathoid minerals.
And then I thought. What on Earth is this sentence giving to someone who has no idea what is feldspathoid? Who would like to read on when your brain is blocked since the first sentence because of this… What was it again? Yes, feldspathoid.
I am well aware of it. I also feel it is difficult, sometimes impossible to keep focusing if you barely understand half of the words. It used to be extra difficult for me because lots of ordinary English words were unknown anyway. It is now getting better but I still often struggle with scientific texts. I admit I don’t like reading scientific papers because they are too dense and also because their subject is usually so ridiculously narrow and hence hard to understand.
Mineralogy is especially hard because most of the minerals make not much sense to most people, geologists included. Geologists have gone through petrology courses during their studies. I am pretty sure that most of them understood igneous petrology much better than metamorphic petrology. Why? I think it is mostly because there are so many obscure metamorphic minerals (staurolite, pumpellyite, kyanite, omphacite, tremolite, glaucophane, etc.) and reactions between them. And then comes thermodynamics. Let’s not touch that now. Igneous processes seem to be much simpler. Minerals in the Bowen reaction series are not too hard to memorize and crystallization as a process is generally understandable.
I am writing this because I have recently written a series of posts about minerals and I have not even really started yet. It is probably needless to say that these posts are not very popular. I knew it is going to be so before I even started writing them and I said to myself that I am ok with it because I have a long term vision. This is Sandatlas here. It means that this website in my imagination should in the long run become a reference material for those who are looking information about sand as a geological phenomenon/material.
But now I am thinking that maybe there is something that is easy to do but makes the articles more interesting and easier to read (in the long run as well). I would appreciate your comments or proposals, either public or private. Is there anything you would like me to write about or is there anything I should change?
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.
Forams of globigerina ooze covering the seafloor of the Weddell Sea (3500 meters below sea level) near Antarctica. Red circles are around Orbulina universa, green could be Rotaliida, and yellow ones are Globigerina but some of them may be Neopachyderma also. The width of the view is 5.1 mm.
Kyanite (Al2SiO5) is a common mineral in aluminum-rich (pelitic) metamorphic rocks. Kyanite commonly forms large crystals (porphyroblasts) which usually have a distinct blue color. Its crystals are typically elongated and rectangular or with step-like features which is a typical sign of good cleavage in several directions. In addition to cleavage there is also a good basal parting. The lines running almost perpendicular (85°) to the longer edges are parting cracks. Parting is somewhat similar to cleavage but not every crystal has it and they are created by external stress.
Crystal aggregate with quartz. Kapteeninautio, Finland. Width of sample 11 cm.
Kyanite has an alternative name also — disthene. Both of these names have a meaning relating to the properties of the mineral. “Kyanos” is blue in Greek and disthene (also from Greek) means that its hardness varies. It is considerably weaker along the crystal (5 on Mohs scale like apatite) and stronger across it (7 like quartz).
Kyanite is typically light blue, but not always. It may be white, yellow, or gray. But even black, pink, and green colors are possible. Hence it is not very wise to identify kyanite by its color, although it is a common perception that kyanite is a blue mineral. Beautiful blue crystals may be used as gemstones.
The mineral has several polymorphs. These are minerals that have the same composition, but differ in the crystal structure. These are sillimanite and andalusite. Their composition is quite simple: Al2SiO5 and one may change to another as the metamorphism progresses.
http://picasaweb.google.com/107509377372007544953/2015#6196126624070396290 It is usually easy to identify because of elongated tabular crystals that are usually light blue in color. Kapteeninautio, Finland. Width of sample 15 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190951435257760754 Large sample of kyanite crystals with quartz. Kapteeninautio, Finland. Width of sample 28 cm.
Typical order is andalusite → kyanite → sillimanite, but not always because kyanite tolerates very high pressure but not too high temperature. So it may avoid the sillimanite phase if the temperature is not rising as fast as the pressure does. That may happen in subduction zones, for example. Kyanite is sometimes found in eclogites which are metamorphic rocks of very high pressure. The crystals on the picture below also come from eclogite.
Kyanite is almost always a metamorphic mineral (it is sometimes also found in pegmatites, kimberlites and veins). The rocks containing it were most likely once muddy seabed or something like that. Eclogite, however, is usually metamorphosed basalt or similar rocks. Rocks that most typically contain this mineral are schist and gneiss. It is one of the index minerals of metamorphism. It means that kyanite is used to roughly estimate the conditions (temperature, pressure) that prevailed when these minerals formed. It often occurs together with staurolite, sillimanite, garnet, andalusite, and other metamorphic minerals. It is quite common constituent in sand because its resistance to weathering is good and it is a common constituent of several rock types.
http://picasaweb.google.com/107509377372007544953/Rocks#5850107421377676002 Kyanite crystal picked from a beach sand of Thassos Island in Greece. Width of view 12 mm.
http://picasaweb.google.com/107509377372007544953/Rocks#5850107441132956786 Crystals shown above are picked from this very interestingly versatile beach sand. Other notable minerals it contains are quartz, feldspar, epidote, staurolite, and almandine. Can you spot a grain of eclogite in the middle? Yes, the one with green pyroxene and red garnet. Metalia beach, Thassos Island, Greece. Width of view 9 mm.
Schistose metamorphic rock with beautiful crystals of garnet, staurolite and kyanite. Light-colored micaceous mineral is muscovite. Width of sample is 7 cm.
Limburgite is a volcanic rock. This rock name is nowadays seldom used. It is one out of more than thousand rock names that were given to all kinds of igneous rocks during 19th and 20th century. Such a proliferation of names is understandable because at the time there were no uniformly accepted classification principles to use and everyone would be pleased to name a new rock type. But it definitely did not serve the best interests of geology. I am afraid no one fully understood what was going on at the time.
http://picasaweb.google.com/107509377372007544953/Rocks#5805071006844076034 Limburgite from Kaiserstuhl (the volcanism took place in the Miocene). The width of the rock is 11 cm.
Then came a man named Albert Streckeisen who just didn’t propose yet another scheme for the classification of igneous rocks. Instead he analysed all the work that was done so far, consulted with many other geologists, and became the first head of Subcommission of the Systematics of Igneous Rocks (under IUGS Commission on Petrology) in 1970.
Under his guidance were created QAPF diagram for the classification of plutonic rocks and TAS diagram for volcanic rocks which are both now widely used and considered to be a standard in igneous petrology. ‘Limburgite’ and many others were just dropped.
How to call this rock type now? Volcanic rocks are classified according to their chemical composition. Limburgite is similar to basalt but it contains less silica and more alkali metals. In TAS diagram it would fall in the field of basanite.
‘Limburgite’ may be dropped as an official rock name but geologists often stubbornly keep traditions alive and continue to use historical names in areas where these names were used before. So do I. This rock what I am presenting here is limburgite as much as one rock can be. It is from Kaiserstuhl (low volcanic range in Germany) where it was first defined by a German petrographer Harry Rosenbusch in 1872.
Typical limburgite contains lots of vesicules that are partly filled with secondary minerals (amygdaloidal texture). Limburgite contains phenocrysts of augite (black slender prisms) and weathered olivine (yellow spots) in a glassy groundmass. What makes this rock type special is the lack of feldspars.
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.
There are myriad of aspects one has to take into account to get good macro photos. I covered some basic guidelines here: How to take good macro photos. Now I am going through couple of simple to use but extremely helpful tools that helps to make the macro you just took look much better.
This tutorial works with black background photos. How to make such photos will be covered some time in the future.
Here is a photo of crystals without any kind of editing. There are lots of dust and halos around the crystals which we really don't need. The width of the view is 12 mm.
The same crystals with cleaned background. Looks much better, doesn’t it?
I do it with Photoshop. First tool is “Burn Tool”. The same tool is available in Photoshop Elements also and I am pretty sure that GIMP (free photo editing software) has analogous tool as well. You will find it in the main toolbox or if you just click “O”. Choose it, select appropriate radius and hardness, and all it takes is littlebit clicking on the spots you would like to remove. It works well if you just need to remove few dust spots.
Quicker method if there are more dust and you want the background to be absolutely black is to use “Quick Selection Tool” (W). Wipe or click on the background you want to clean. If you are satisfied with the selection, click Edit – Fill – Black. You can also add to the selection or subtract from it. Usually Photoshop does amazingly good job but some adjustments are probably needed.
Third method is to use “Clone Stamp Tool” (S). It works well after the background is already black but there are still few unwanted spots. Press “Alt” and click on the black background, release the key and click on the spot. It will be replaced with the background.
It is important to clean the glass (with ethanol for example) upon which you place the crystals. This is a serious issue here because it is macro photography and there is so much dust around us. The less dust you need to remove later, the quicker this job is done. Experimenting with lightning is useful also but some editing is probably inevitable anyway.
The grains used in the examples above are kyanite crystals.
Chromite is a black chromium-bearing mineral, FeCr2O4. It is a principal ore of chromium (Cr). Chromite is a member of the spinel group. These minerals share the same structure but differ in composition. Al and Mg in the lattice of spinel are replaced by Cr and Fe, respectively.
http://picasaweb.google.com/107509377372007544953/Rocks#5842261460204530978 Crystals picked out of a chromite concentrate from the Rustenburg mine, Western Bushveld, South Africa. Width of view is 5 mm.
The appearance of chromite may be similar to magnetite (which also belongs to the spinel group but has slightly different structure). Both are opaque and occur in octahedral crystals (double pyramids with eight crystal faces). However, both minerals occur more often as anhedral or granular masses. It isn’t hard to tell them apart because chromite may be only weakly magnetic while magnetite shows very strong affinity to magnets.
Pure chromite is rare. It usually contains magnesium which replaces iron. Other elements like trivalent Fe, Zn, Mn, and Al may be present as well. Chromite that contains more Mg than Fe is named magnesiochromite. There is continuous solid solution between the two and most commonly natural crystals are somewhere in the middle between the two endmembers but more on the side of magnesiochromite. Hence, most chromites are probably not chromites sensu stricto.
This mineral is relatively resistant to weathering. It may be important constituent of sand but not everywhere. It is found in mafic and ultramafic rocks like pyroxenite and peridotite (including dunite). These rocks are widespread deep below in the mantle, but they are not common rocks near the surface. Best chances to find it in sand is near ophiolites. These are assemblages of rocks which once formed ocean floor but are now tectonically pushed on top of the continental landmasses. It is also found in some meteorites.
Chromite in igneous rocks may form bands or layers which are very important mineral resources. These layers are the result of a crystal settling in a magma chamber. Such deposits are for example Bushveld complex in South Africa and the Great Dike in Zimbabwe. It is among the first minerals to crystallize out of magma. Important chromite mining countries are Kazakhstan, South Africa, Zimbabwe, and India. Chrome is needed for a variety of purposes, but it is mostly used to make stainless steel.
http://picasaweb.google.com/107509377372007544953/Rocks#5842261464434054482 Serpentinized ore from The Zhob Valley, Pakistan. Green is serpentine, black is chromite. This is a cumulate rock associated with ultramafic rocks. Width of sample is 6 cm.
You can take a much closer look into similar rock from the Stillwater complex in Montana. Here are thin section images and gigapans by Ron Schott.
I am testing an extreme macro lens Canon MP-E 65mm f/2.8 1-5X Macro Lens that allows to take shots with 5× magnification. Usually macro lenses have only 1× magnification. What is the meaning of these magnification numbers? It means that with this lens one can take photos of objects that measure only one fifth of the image sensor. My camera’s sensor has a width of 22.2 mm. So the photo taken with 5× magnification has a width of 22.2/5 = 4.44 mm which is covered by 4272 pixels. Smallest sand grains by the definition have a diameter of 62.5 micrometers. Hence with this camera this grain will be represented with 60 pixels.
That isn’t anything spectacular but it is much better than 12 pixels I had before which actually made it impossible to take sharp images of very fine-grained sand samples.
Here is an image taken with 4× magnification. The whole scene is approximately 1 mm in diameter. These crystals appear as dust grains to the naked eye without any details at all.
These are zircon crystals. Zircon is very important mineral for geologists and is a common component of sand also. However, zircon crystals are usually so miniscule that geologists often have heard lots about it but sometimes have never seen it. This image isn't yet as sharp as I'd like it to be but hopefully it just takes time and patience to get used to it. Some loss of sharpness is unfortunately inevitable because the depth of focus gets very narrow.
that allows to take shots with 5× magnification. Usually macro lenses have only 1× magnification. What is the meaning of these magnification numbers? It means that with this lens one can take photos of objects that measure only one fifth of the image sensor. My camera’s sensor has a width of 22.2 mm. So the photo taken with 5× magnification has a width of 22.2/5 = 4.44 mm which is covered by 4272 pixels. Smallest sand grains by the definition have a diameter of 62.5 micrometers. Hence with this camera this grain will be represented with 60 pixels.