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The classification of igneous rocks is largely based on two diagrams: QAPF diagram for plutonic rocks (formed in the crust) and TAS diagram for volcanic rocks. I made the schemes using the coordinates provided in the following book: Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks (2nd edition). Cambridge University Press.

These coordinates are shown on the second diagram which you can use to construct your own diagram if you wish. The use of TAS diagram is very simple and straightforward. You only need to know the major element chemical composition of the rock sample being studied. There are SiO₂ on the x-axis and the sum of K₂O and Na₂O on the y-axis. TAS stands for Total Alkali Silica.

http://picasaweb.google.com/107509377372007544953/Chert#5807632931777908018
Here are the coordinates of the intersections.

There is no doubt that the use of such a scheme is no rocket science but what is the biggest problem is that we need to know the chemical composition of the sample. There is really no way to do it reliably if you do not have an access to very expensive equipment which is used to analyse the chemistry of rocks.

Probably at least partly because of that, TAS diagram is actually not the first choice you should consider if you need to identify volcanic rocks. There is an analogue of QAPF diagram designed specifically for volcanic rocks which we should use if we can determine the mineralogical composition of the rock. In many cases we can not do it because volcanic rocks tend to be too fine-grained even for microscopic study and they often contain volcanic glass which may have very versatile composition. And here comes the second problem which for me is even more serious than the first one. QAPF diagram and TAS diagram are based on different criteria. There is absolutely no guarantee that a rock sample which was correctly identified as trachyte using QAPF diagram is still trachyte if we make a chemical analysis of it and plot the results on the TAS diagram.

But this is the reality we have to live with right now. The classification principles are not perfect but they are probably good enough because they have been in use for several decades already and nowadays they seem to have gained international recognition. There are some additional things which have to be accounted for if you’re going to use the TAS diagram. I recommend to consult the book mentioned above.

I have also written a post about how QAPF and TAS diagrams are related and how to make them match each other: A little fun with diagrams.

I hope that Simon Wellings won’t mind me using a nice poem about titanite he posted to Twitter some time ago:

There once was a mineral called sphene, whose edges were long straight and clean.
Then one day from spite, It was called titanite, which was silly, daft, stupid and mean.

I recorded it to my notes as I usually do with things that might be useful some day. Behind that poem is a story which has interested me for some time. How could it be that most mineralogists were calling a mineral sphene and all of a sudden someone comes and changes it according to the minority’s preference? At least that’s how I understood it was. It happened in 1982 when The International Mineralogical Association Commission on New Minerals and Mineral Names decided that from now on sphene as a mineral name is discredited and titanite should be used instead. Until then both were in parallel use.

I do agree that we need to work on the nomenclature and parallel names are actually a nuisance but in this case I would have definitely preferred ‘sphene’ over ‘titanite’. There are several reasons. First, I dislike the proliferation of these -ite endings of mineral names. There are way too many of them. Mineral names like olivine, hornblende, zircon, feldspar, etc. are like a breeze of fresh air. How fun would it be to name all of them olivite, hornite, zircite, and feldsite? Not fun for me. It would totally suck!

I’ve heard that titanite (CaTiSiO₅) was preferred term by some because the mineral contains titanium. This is true but titanite is not the only titanium-bearing mineral and it is not among the most important. Titanium is an important mineral resource but it is mostly extracted from rutile (TiO₂) and ilmenite (FeTiO₃). So, this argument really makes no sense.

‘Sphene’, on the other hand, comes from Greek word sphenos which means wedge. Wedge is exactly the word I would use to describe that mineral. Its idiomorphic crystals are indeed wedge-shaped which is a well-known identification aid.

I do sense that although 30 years have passed, many among geologists still do not like the decision and defiantly continue to, at least informally, use the old name (or include the old name in parentheses). I’ve tried to look for some background information which could explain the naming decision but so far haven’t found anything useful. If you happen to know more about the subject, or have strong feelings about the topic, do not hesitate to share your thoughts in the comments.

The mineral whose edges were long straight and clean. Brown and wedge-shaped titanite (sphene) phenocrysts in a granitoid igneous rock (probably granodiorite) from Austria. The width of the sample is 12 cm. The hand sample belongs to the Museum of Geology of the University of Tartu.

The classification of igneous rocks used to be a real mess. The proliferation of geological terms took especially enormous dimensions there. More than 1000 rock types were described, many of them had overlapping meanings and the whole collection of terms lacked any kind of systematic structure. It is different now. We have a systematic overview and a firm basis to guide the identification. I like it for several reasons. First of all, it makes our work easier. We have something to rely on. Second, we can be reasonably sure that my granite is granite for you as well. It makes communication between geologists easier. Third, geology is often too narrative and descriptive. Maybe partly for that reason geology is sometimes ridiculed and treated not as a real science or at least not as tough science as physics or chemistry.

http://picasaweb.google.com/107509377372007544953/Chert#5808891582074096306
QAPF diagram for plutonic rocks. The diagram was created by the Subcommission of the Systematics of Igneous Rocks in the latter part of the 20th century.

There are several reasons why this is the case. Geology is very complex, natural processes are often impossible to observe (metamorphism for example) or take place very slowly. That’s why we are unable to work like physicists, our problems are too complex to solve mathematically. We are still often forced to just describe and speculate instead of building models. However, we should strive to force ambiguity out of geology. One way to do it is to strictly define what we actually mean if we say that this rock sample I am holding in my hand is granite.

For these reasons I believe that the diagram below is probably my favorite geological illustration, or at least one out of several similar diagrams that are used to identify rocks. This article is my entry to the Accretionary Wedge #43 hosted by Hollis at In the Company of Plants and Rocks.

How to read this diagram. There are four minerals or mineral groups chosen as important cornerstones of the classification. These are quartz (Q), alkali feldspars (A), plagioclase feldspars (P), and feldspathoids (F). F and Q for chemical reasons can not exist together in one plutonic rock. Other minerals may and almost certainly occur in these rocks as well but they have no significance in this classification scheme. So, the whole diagram is actually composed of two ternary plots (QAP and FAP). To use the classification, the concentration (the mode) of these minerals must be known and recalculated to make their sum 100%.

Example: a plutonic rock that contains no alkali feldspar and no feldspathoids but contains lots of pyroxenes (neglected in this diagram), plagioclase feldspar and few quartz grains is probably gabbro (located at the right edge of the diagram, little bit up from P towards Q). The diagram doesn’t say whether it is gabbro, diorite, or anorthosite. There are another criteria used to decide that. Note that this diagram is not used for all plutonic rocks. Ultramafic rocks are the most important plutonics that have separate classification diagrams.

QAPF diagram is used to classify plutonic rocks. Although similar diagram exists for volcanic rocks as well but because volcanic rocks are typically too fine-grained for mineralogical analysis, they are classified according to their chemistry and TAS diagram is used instead of QAPF diagram. I have also written a post about how QAPF and TAS diagrams are related and how to make them match each other: A little fun with diagrams.

In 1978 Pierre Devismes wrote a book Atlas photographique des mineraux d’alluvions (Photographic atlas of detrital minerals). I believe it remains the best illustrated treatise on detrital minerals to this day. Unfortunately I do not own a copy of it. It is out of print and seems to be impossible to purchase. But I do have selected pages of it in scanned format.

There are 641 photographs depicting 183 alluvial minerals. This is really impressive number because I believe you can easily describe 99.99999 percent of the sand grains with 183 mineral species (if we leave out lithic fragments and biogenic grains).

But this is nothing unimaginable. What really strikes me is the number of sand samples/mineral concentrates his work group at BRGM mining division in Nantes described and analysed — 286,088 (most of them from France and collected during 8 years of field works). This number is beyond what I can imagine. I have more than 1,000 sand samples and even this is far more than I can handle. I have superficially looked (with microscope) upon all of them but I have more deeply studied maybe only one hundred or so.

The mineral concentrates shown on the photos of the book are selected from these samples. I don’t know how they identified all the mineral species but it definitely is lots and lots of work that takes enormous time. I wonder if someone now wants to underake writing such a book. How much would it cost? Well, I don’t want to think about that and it may well explain why there are no such books written in the last 30 years.

If such a huge number of samples is studied and a book written based on that, then it is no wonder that the book is out of print and no one considers selling it although it is quite ancient already.

Such a large number of sand samples seems to be a perfect database for some serious statistical conclusions. I am not sure about how much of it this book contains but I have a copy of a table from the book (repeated in another book) where mineral species have been categorized according to their frequency of occurrence in sand.

I hope I won’t commit a serious crime if I repeat part of it (the most common ones) here.

Quartz — essential component
Feldspar — essential component
Mica — very abundant
Tourmaline — very frequent
Staurolite — very frequent
Garnet — very frequent
Zircon — very frequent
Glauconite — very frequent
Magnetite — very frequent
Hematite — very frequent
Ilmenite — very frequent
Rutile — very frequent
Cassiterite — very frequent
Pyroxene — frequent
Sillimanite — frequent
Limonite — frequent
Leucoxene — frequent
Pyrite — frequent
Marcasite — frequent
Anatase — frequent
Monazite — frequent
Siderite — frequent
Amphibole — frequent enough
Epidote — frequent enough
Titanite — frequent enough
Andalusite — frequent enough
Kyanite — frequent enough
Goethite — frequent enough
Spinel — frequent enough
Chromite — frequent enough
Corundum — frequent enough
Apatite — frequent enough
Wolframite — frequent enough
Arsenopyrite — frequent enough
Topaz — frequent enough
Olivine — frequent enough
Perovskite — frequent enough
Xenotime — frequent enough
Columbite-tantalite — frequent enough
Galena — frequent enough
Sphalerite — frequent enough
Cinnabar — frequent enough
Scheelite — frequent enough
Barite — frequent enough
Beryl — frequent enough

According to Devismes, all other minerals are either rare or very rare in sand.

Callan Bentley and Robin Rohrback-Schiavone have gigapanned two more sand samples: pyrite sand and actinolite sand. Thanks!

The first one contains pyrite and silicate grains from Cyprus. The second one is from Ontario, Canada. I have already written about this sample: Actinolite sand from Ontario.

I also have to make a small correction. The sand sample with pyrite contains more than just pyrite and goethite. It seems to me that I made a mistake when packing these sands and sent the wrong sample. They are from the same place but with somewhat different composition. The other one I planned to send contained beautiful pyrite cubes, some of them with goethitic/limonitic cover but in this gigapan you also see green silicate grains. I am not sure but they could be epidote grains.

I also recommend to check a gigapan of olivine sand. There are lots of very beautiful bright green olivine grains.

Gigapanning is very useful technology in geology because it allows us to first get a general overview and then zoom in to see smaller structures. It is already used extensively by some members of the geoblogosphere. Thanks to their work we have seen many outcrops in such a detail that was unimaginable before.

I was interested to see how useful are gigapans in sand photography. Callan Bentley kindly offered to gigapan some of my sand samples. Here are two of them. I much prefer to watch gigapans in full screen. Unfortunately I can’t find a way to add such a feature to my website. To do that you probably just have to go directly to the gigapans website. Here are the links to these gigapans: gigapan.org/gigapans/98519 and gigapan.org/gigapans/98542.

The first one is from the La Paree Beach, Bretignolles-sur-Mer, France. The second one is from the Calvert Cliffs State Park, Soloman Islands, Maryland, USA. I am very pleased with the first one. The sand is interesting and beautiful and it shows its beauty and versatility quite well. I was afraid that these sands are perhaps too fine-grained but it doesn’t seem to be the case. The gigapans are sharp enough, almost comparable to the microscope view. The second gigapan is not as good. Probably because it is a mixture of transparent quartz and black ilmenite which need different exposure time. There are enough light for quartz but not enough for dark ilmenite. This is a common problem in macro photography if you have to take photos of light and dark objects at the same time.

There are several things that I like about these gigapans. It is good that I can first see the sand as I would see it with the naked eye and then smoothly zoom in to acquire microscope view. I’ve been thinking about that before when I show sand photos taken with macro lenses. These pictures may be pretty but they are somewhat detached from the reality. People who look them do not understand how this sand looks to them if they see it in a beach. This problem is eliminated here.

Taking macrophotos of sand is usually lots of work. I study sand under the stereomicroscope but I do take photos with camera and macro lens. Thus, I can not take a photo as soon as I spot something interesting. Sand grains are very small which means that the grains I was interested in are usually very hard or impossible to find with the camera. So, I just take a photo and hope that it has something interesting there. Usually it is not as good as I had hoped because one photo covers very small area. It is different with gigapans because they are composed of many photos. I can search large area and choose between many grains and take a snapshots of the best to illustrate my blog posts for example.

The results are very promising and I am sure that gigapanning will be much more widely used by geologists in the future although currently it is rather expensive and unfortunately affordable to research institutions only.

Erik Klemetti wrote about the reasons he loves volcanoes. I love volcanoes also but I am mostly blogging about sand. So I thought that I try to make a list of reasons why sand is interesting and why sand collecting is a great hobby.

1. It matches my interests
I am interested in almost all of physical geology. Sedimentology, petrology, and mineralogy are perhaps the most fascinating geological branches for me. Sand is a sediment but it is largely composed of minerals and it is a disintegration product of rocks. I can not imagine anything else that so nicely connects these three scientific disciplines.

2. Sand is beautiful
Sand is very beautiful under the microscope. Especially all sorts of heavy mineral sands that often contain gem minerals like tourmaline, spinel, garnet, etc. It is a delight to the eye to study sand. I also love to take photos and share the beauty of sand with others.

3. Puzzle solving
Studying sand is a great puzzle solving. I am always looking for something I haven’t seen yet. When I find it, I try to make sure what it is and step-by-step build my knowledge that way. Sometimes it is not easy at all and lots of problems are still waiting to be solved. It gives great satisfaction when I finally reach a breakthrough. Spinel and staurolite are minerals that I spotted in sand early on but it took some time when I finally reached a conclusion about their identity.

4. Interesting stories
There are lots of stories in sand just like there are stories in outcrops. Untrained eye will see just sand, it will not give its secrets to us easily. It takes lots of knowledge in petrology, mineralogy, local topography, regional geology, sedimentology, climate, and mineral associations among others to understand how this sand came to be. It is not easy and not always possible because my theoretical preparation could be much better and I often have only limited information available because most of my samples are not collected by myself. Nevertheless, it is always fun to try find out as much information as I possibly can.

5. Exploration
It may be hard to believe but there seems to be very few experts on sand. Sedimentologists study sandstones, there are textbooks written about sandstones, there are sandstone classifications. However, nothing like that seems to exist for sand. I know only few feeble and often amateurish attempts that barely scratch the surface. Let me know if you disagree with me. Some of my sand samples contain biogenic fragments but there seem to be no books with images that could guide me and help to identify these grains. I often feel like an explorer who have ventured into an area which have been visited by very few before. It makes my journey much more complicated but more fun as well. I have to discover things myself.

6. Lots of collectors
Sand collecting is a great hobby. I was and still am interested in rocks. I mean real rocks here, not minerals. I was looking for people who also collect rocks and would like to exchange this material with me. Unfortunately I have not so far found like minded people. Lots of people collect beautiful crystals but no one seemed to care about kyanite schist, rapakivi granite, and thin-bedded turbidite. But I did find out that there are fair number of people who collect sand. It got my attention immediately and my sand collection started to grow rapidly. However, if you read this and are a real rock collector then do not hesitate to contact me. I absolutely love all kinds of material our home planet is made of.

7. Easy to send and store
One of the reasons why rock swapping is not widespread may be that rocks are heavy. You need lots of money to regularly send fist-sized samples from Europe to Australia for example. Sand has almost the same density as rocks but we need much smaller quantity of it. Usually one sand sample is only 30 ml or even less and it isn’t very expensive to send for example ten samples in one package. Perhaps even more important aspect is that sand is easy to store. I keep my samples in small plastic micromount boxes which are kept in shallow drawers. Rocks on the other hand are much more problematic. Every one of them has different size and shape and lots of them are really big. There is no easy way to store them (it take lots of room). That’s why my rock collection is a real mess when compared to well-organized sand collection.

8. Sand is plentiful
There are lots of sand. Sand is a collection of small pieces of rocks. Upper part of the Earth’s crust is weathering, so there is really lots of this stuff to be found. Normal people don’t ask money for it and so far I haven’t heard anyone complaining when I take samples. It is different with rocks because taking samples from outcrops is often illegal and immoral. I usually don’t want to do it even if no one is seeing. And sometimes it is physically very difficult to do because many interesting outcrops are composed of extremely hard rocks. Our praised but ridiculously light-weight Estwings are usually not up to the task. Of course, you could collect pieces of rocks that have fallen off from the outcrop but these rocks are usually weathered and not as beautiful as the original fresh sample.

9. Sand is important
Sand is definitely very important material for mankind. We familiarize ourselves with it when we are still children playing in a sandbox. Lots of sand is used to make concrete. Sand is a raw material which we use to make glass and computer chips. Sand contains lots of ore minerals. Sand or sandstone is a reservoir of oil and gas and it also contains lots of our drinking water. Sand is component and source material of soil. Therefore it is important part of our food supply. So I am not exaggerating when I say that sand is absolutely vital material for the well-being of man and one of the cornerstones of modern society.

10. Blogging material
Last but probably not least. I like sand because it offers immense amount of material and stories for blogging. If I have nothing to write about, then I just need to take a look at some of my sand samples and interesting ideas start to emerge.

You probably already know that ice is a mineral. But did you know what is its most important use? According to the second edition of Introduction to Mineralogy by William Nesse its most important use is “to cool the drinks that slake the thirst of geologists after long hot days in the field”.

I am reading this book and highly recommend it if you want to strengthen your knowledge in mineralogy. It has good humor here and there but perhaps the most valuable aspect is that it is clearly written. It isn’t easy to make complicated topic understandable.

What I also like is the author’s unique and refreshing approach. I do not feel that I’ve read the same story many times from other books. Have you ever thought that ice as snow is sediment, ice in a glacier is a metamorphic rock and ice that covers lakes in winter is an igneous rock? Of course, this is there at least partly to provoke readers but such thoughts make the book entertaining and fun to read.

Nesse is especially strong in optical mineralogy. Almost 50 pages are devoted to this rather complex topic but in my experience this book or better yet “Introduction to Optical Mineralogy” from the same author are the best you can find if you are serious about getting to know this topic.

I have lots of this stuff outside at the moment but unfortunately it will melt as soon as I bring it in. So the only approach to make photos like this is to work outside which I am not tempted to do at the moment (‒20 °C). However, a man named Wilson Bentley photographed snowflakes more than a century ago. These are superb quality photos for its time. I doubt that I could add anything to it. So I am using his photos of beautiful hexagonal ice crystals and recommend to visit a website showing many more of his photos: snowflakebentley.com

To find out more about snowflakes, check this website.

I have a plan to do some geologising in Sri Lanka. Most likely in March. 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.

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.