|
|
Microcline is a very common rock-forming mineral.
Microcline belongs to the feldspar group. Microcline is one of three K-feldspars (K means potassium). The other two are orthoclase and sanidine. The chemical formula of K-feldspars: KAlSi3O8 but microcline usually contains sodium as well which replaces potassium.
It is often very difficult to distinguish between the three, especially between orthoclase and microcline. Sanidine isn’t usually colored while microcline and orthoclase are often salmon red or yellow. Sanidine occurs in volcanic rocks, microcline and orthoclase occur more frequently in deep-seated rocks, especially microcline which is absent in volcanic rocks. So, these three form like a series with their typical occurrences — microcline in deep intrusions, orthoclase in shallower intrusions but also in volcanics, and sanidine in volcanic rocks.
Microcline is often perthitic. It means that there are narrow bands of albite (sodium-rich feldspar) in the K-feldspar host crystal. Perthitic texture is often visible to the naked eye. This texture is the result of exsolution which happened after the cooling of originally homogenous microcline crystals which are not stable at lower temperatures. Perthitic feldspar is almost always microcline.
Some microclines (amazonite) are green or bluish green. The color of amazonite is caused by a small amount of lead. Amazonite is rare variety when compared to the total volume of all microcline in the crust but it is used as a decorative stone and therefore pretty well known.
Microcline is a common constituent of granite and granitic pegmatites. Microcline also occurs in other granitoids, syenite, and pelitic metamorphic rocks. Microcline is common mineral in detrital sediments. Microcline grains are commonly more fuzzy than generally clear quartz crystals and they tend to have rectangular blocky shape. This is the result of good cleavage in several directions. The cleavage planes are at right angles to each other.
 Large piece of perthite (perthitic microcline) from Quadeville, Ontario, Canada. The width of the sample is 22 cm.  Fragment of the sample above. This is how perthitic exsolution looks like. White veins are composed of albite.The width of the sample is 43 mm.  Pegmatite from The Austrian Alps. White mineral is sodium-rich plagioclase, green on the right is epidote, green in the lower part is amazonitic microcline, black is tourmaline (schorl), and gray is quartz. The width of the sample is 7 cm.  Perthitic and partly amazonitic microcline from Evje, Norway. The width of the sample is 7 cm.  Amazonite from near Larvik in Norway (Tönsberggranit Mine). The width of the sample is 10 cm.
Topaz is a silicate mineral.
Topaz belongs to the orthosilicate group. These are silicates that have isolated silica tetrahedra (isolated from each other like islands in a three-dimensional body). Topaz contains volatile components (F- and OH-) in addition to common chemical elements of most silicate minerals. The chemical formula of topaz: Al2SiO4(F,OH)2.
Volatiles suggest that topaz must form in an environment that contains lots of them. Such an environment forms when granitic magma is almost completely crystallized. Residual fluids are rich in incompatible and volatile elements which favors the formation of minerals containing them. Such minerals which are frequently associated with topaz are cassiterite (contains tin which isn’t compatible in main magmatic minerals), tourmaline (contains boron and volatiles), apatite (volatiles), lepidolite (lithium), fluorite (fluorine), beryl (beryllium), etc. The rocks that host these sanctuaries for unpopular chemical elements are known as pegmatites. Topaz may also form in cavities of rhyolite (volcanic equivalent of granite) and in greisen (granitic rock intruded by residual magmatic fluids). Common magmatic phases like quartz, feldspar, and mica are also usually present in topaz-bearing rocks.
Topaz is pretty famous mineral because it is extensively used as a gemstone. Topaz could find use as an abrasive (its hardness in the Mohs scale is 8, it is one of the ten defining minerals of the scale) but there are enough much cheaper synthetic alternatives.
Topaz forms elongated prismatic crystals which may be striated (striations parallel to the elongation). Topaz has one good cleavage perpendicular to the striations. It often determines the shape of the crystals which seem to be cut at right angles to the elongation of crystals. Topaz is resistant to the weathering processes and may be therefore found in sand.
 Topaz crystals from Ouro Preto, Brazil. The width of the view is 30 mm.  Topaz crystals (yellow) in pegmatite. Altenberg, The Ore Mountains, Germany. The width of the view is 21 cm. The specimen belongs to the Museum of Geology of the University of Tartu.  Topaz crystals in pegmatite, fragment of the rock above (another side). The width of the view is 35 mm.  Topaz (light blue) in a pegmatite. Pink flaky mineral is lepidolite. Viitaniemi, Finland. The width of the view is 9 cm. The specimen belongs to the Museum of Geology of the University of Tartu.
Molybdenite (MoS2) is a mineral (molybdenum sulfide).
Molybdenite is a sulfide mineral. This pretty much says it all about the occurrence of molybdenite because most sulfide minerals have very similar story about their formation. Molybdenite occurs in hydrothermal vein deposits and as disseminated grains in rocks which have been attacked by hydrothermal fluids (porphyry moly and copper deposits). Molybdenite may also occur in pegmatites and skarns.
Molybdenite has a metallic luster and steel gray color. It resembles graphite, both are very soft minerals and therefore have a greasy feel to the touch but molybdenite crystals have a clear bluish tone. Molybdenite has a slightly greenish streak on an unglazed porcelain while graphite is black.
Molybdenite is the principal ore of molybdenum which is mostly used to make high-strength steel. Molybdenum is also a micronutrient, so it is added to fertilizers and, yes, to your dietary supplement pills also.
 Molybdenite (metallic gray) with calcite (blocky gray), pyrite (greenish crystals in the upper part), and serpentinite (green in the background). Phillippsburg, New Yersey, USA. The width of the sample is 8 cm. The specimen belongs to the Museum of Geology of the University of Tartu.  Molybdenite with quartz from Altenberg, The Ore Mountains, Germany. The width of the sample is 8 cm. The specimen belongs to the Museum of Geology of the University of Tartu.
Charnockite is a granofels that contains orthopyroxene, quartz, and feldspar.
Charnockite is frequently described as an orthopyroxene granite. Granites are felsic rocks that usually contain no or very little pyroxene. There is actually an entire array of rocks (mostly granitoids but also syenite, monzonite, etc.) that may contain orthopyroxene plus quartz. These rocks are collectively referred to as charnockitic rocks or charnockitic suite. All of these rock names refer to igneous rocks which makes it very logical to assume that charnockite is just an igneous rock with somewhat unusual composition.
Such an interpretation (which seems to be prevalent) is very likely not true. Igneous rocks are formed from magma but charnockites are found in high-grade metamorphic terranes (granulite facies). The transformation from the protolith to charnockite had probably no magma phase which means that we are dealing with true metamorphic rocks which have nothing to do with igneous processes. Charnockitic rocks are sometimes described as granulites but this term seems to be somewhat out of favor nowadays. Partly because it may be confused with metamorphic facies with the same name and I also guess that partly because too many different rock types have been called that way which have created great deal of confusion in the past.
 Charnockite from Ubatuba, Brazil (known by its trade name Ubatuba Green). The width of the view is 10 cm. Well, can we conclude that charnockite isn’t granite then? Perhaps we should but we probably can not do it because the term “granite” isn’t reserved exclusively for igneous rocks. Some rocks that have been described as granites are almost certainly metamorphic rocks although they lack obvious foliation. Hence, we have to tolerate the situation that not all granites are igneous rocks and therefore we have no basis to demand that charnockite shouldn’t be named granite anymore. However, if we want to use metamorphic terminology, then we should call it granofels. Charnockite is coarse-grained, and it lacks foliation. This is the definition of granofelsic metamorphic rocks.
I have one more thing to say which disturbs me when it is said that charnockites have granitic composition. Yes, they have according to QAPF classification but only because we do not use pyroxenes in this classification scheme.
Charnockitic rocks are commonly green. Both feldspars and orthopyroxene tend to have a greenish or brown hue and quartz crystals may contain rutile needles which gives them bluish tinge. Charnockites are formed at high pressures in almost water-free conditions. That’s why we see only small amount of hydrous phases here (biotite, amphiboles) which are widespread in the rocks of amphibolite facies.
The name charnockite has an interesting origin. It was given to the rock type because it was first described as a tombstone of Job Charnock (1630–1692) in St John’s Churchyard in Calcutta (Kolkata). Job Charnock is known as the founder of the same city. Even today charnockite remains to be popular tombstone material.
 Charnockite from South Africa (known by its trade name Verde Fontaine). The width of the view is 7 cm.
Trachyte is a volcanic rock.
Trachyte is a fine-grained and generally light-colored rock that usually has a rough surface to the touch which is the reason it was given that name (trachys is ‘rough’ in the Greek language). It was either Alexandre Brongniart or René Just Haüy who first defined that rock type (different sources go against each other in this question). They were French mineralogists who both left a lasting mark in the history of geology.
Trachyte is a feldspar-rich volcanic rock. It has been defined several ways in the past. Rocks that are nowadays known to us as andesites or rhyolites have both been named trachytes in the past. Trachyte is chemically between these two. So, we can say that the definition of trachyte is narrower now and the rock type is not very abundant although its occurrences are widespread.
Trachyte usually contains lots of K-feldspar sanidine but plagioclase, anorthoclase, and sometimes feldspathoids are common as well. However, feldspathoid-bearing trachytes are often named foid-trachytes (if we use QAPF diagram instead of TAS diagram) and trachytes that contain lots of plagioclase may be more properly described as trachyandesite (known also as latite). Volcanic rocks that are similar to trachyte are rhyolite (contains more quartz), phonolite (more feldspathoids), trachyandesite (more plagioclase and dark minerals), and dacite (more quartz and plagioclase). See TAS diagram below to see how these rock types differ in their chemistry.
Trachyte is a volcanic equivalent of syenite. Syenite is a feldspar-rich plutonic rock which is similar to granite but lacks or contains very little quartz. Mafic minerals in trachyte are usually biotite, amphiboles (hornblende or arfvedsonite), and pyroxenes (diopside, augite, aegirine). Aegirine and arfvedsonite occur in the rock when it is rich in alkali metals (compositionally close to phonolite). Trachyte that is unusually rich in silica (>20%) is named trachydacite although in the QAPF diagram we would name it a rhyolite instead. So, these classification schemes are really not flawless.
Trachyte is probably a product of magmatic differentiation. Its parent magma was perhaps basaltic but it evolved (its composition became enriched in alkalies and silica) by the removal of mafic minerals. Trachyte may be associated with phonolite, latite, rhyolite, etc. which means that the same volcano has extruded magmas with slightly different composition. Trachyte is not necessarily volcanic in the strict sense. It may also form underground but still relatively close to the surface because its grain size is fine. Coarse-grained rocks with a trachytic composition are know as syenites as said before. Magma with a trachytic composition may also solidify as obsidian or pumice.
 Trachyte from the Drachenfels, Siebengebirge, Germany. Note large sanidine phenocryst in the lower right. The width of the rock sample is 10 cm.  The TAS diagram with the field of trachyte annotated.  Trachyte collected near the Laacher See caldera lake, Germany. The width of the rock sample is 7 cm.  Trachyte from the Eifel volcanic field, North of Mayen, Germany. Blue is feldspathoid haüyne (named after R. J. Haüy mentioned earlier). White grains are andesine (plagioclase) phenocrysts. So, the rock is compositionally close to foid-bearing latite (trachyandesite). The width of the rock sample is 11 cm.  Trachyte from the Eifel volcanic field, North of Mayen, Germany. Blue mineral is haüyne. Such a feldspathoid-rich trachyte must be compositionally close to phonolite (phonolitic trachyte). The width of the rock sample is 11 cm.
Here is an interesting pair of photos taken in the Spanish Pyrenees which illustrate several hugely important geological concepts.
 Thrust fault in the Spanish Pyrenees. Limestone below and siltstone on top of it. Siltstone layer is older than limestone layer despite being on top of it.  The same rock formations but this time in normal stratigraphical positions (younger on top of the older). So, what is going on here and how is it important? It is obvious that there are two distict rock formations on the first image. The gray layer is composed of limestone and on top of it is siltstone. We know that usually younger rocks are on top of the older ones but this isn’t true in this particular case. The limestone formation is younger than the siltstone formation. Note how it looks like the lower limestone formation is cutting the bedding of the siltstone formation. It shouldn’t be that way if the silt was deposited on top of the eroded surface of the limestone formation. There is a fault plane between them which means that the upper formation is pushed on top of the lower one. Such faults are called reverse or thrust faults and they are very common in mountainous areas.
Now if we move on to the second image (to do that I had to climb few hundred meters up in the mountains) we see that the order of rocks is reversed — limestone is on top of the siltstone formation. This is normal succession because younger rocks lay on the older ones. There is no fault plane between these formations but these rocks are not where they originally were — geologists say that they are allochthonous. It means that they have been moved from their original location but in this case they did so together as one block.
It is important to understand that the limestone formations on both photos are the same (more or less). We just have several copies of one crustal layer, one pushed on top of the other. The whole thing is hugely important in geology because that’s how mountains are mostly made and this particular set of outcrops in my opinion is especially good real-life illustration. In most cases it isn’t so easy to understand what is going on because rocks in mountainous areas are often severely folded and metamorphosed. Here you see only little folding on the second image and the rocks are not metamorphosed. Another very important aspect is that the blocks that were moved are relatively small. This is different in the Alps, for example, where it is much harder to to see and understand what is going on.
Augite is a very common rock-forming mineral that gives black color to many igneous rocks.
Augite belongs to a group of silicate minerals known as pyroxenes. By the way, ‘pyroxene’ comes from the Greek language and means that these minerals have nothing to do with fire (i.e. igneous processes). This is, of course, especially unfortunate misnomer but the name has stuck and nowadays very few people associate it with its original meaning.
Augite is the most common mineral of this group. Augite is usually black because of significant iron content but small crystals (in sand, for example) tend to be dark green. Augite is actually a member of a group of closely related minerals. There are continuous solid solutions between augite, diopside, and hedenbergite. The latter two occur mostly in metamorphic rocks, while augite is usual component of mafic igneous rocks. The chemical formula of augite is usually expressed the following way: (Ca,Mg,Fe2+,Fe3+,Al)2(Si,Al)2O6 Augite contains less calcium but more aluminium, magnesium and iron than diopside-hedenbergite pair. These three minerals together are usually called calcic clinopyroxenes or Ca-rich clinopyroxenes. Clino- means that they are monoclinic (some pyroxenes are orthorhombic, they are called orthopyroxenes). These terms refer to the crystal systems which are defined based on the combinations of the elements of symmetry.
Augite occurs in mafic igneous rocks (gabbro, basalt, andesite, pyroxenite, peridotite, etc.). Augite may also occur in metamorphic rocks (skarn, amphibolite, granulite) but in this case its composition is often fairly close to diopside. Augite crystals are prismatic but not as slender as the crystals of another pyroxene aegirine which also is connected with augite chemically but doesn’t fit into the diagram below along with several other members of the pyroxene group.
 Augite crystals from Kenya, The East African Rift. The width of the view is 23 mm.  Augite forms solid solution series with hedenbergite and diopside. There is no solid solution between augite and pigeonite. "Wollastonite" is written in italics because it isn't pyroxene. It is still included because of compositional considerations. Not all pyroxenes can be shown on this diagram. The same diagram is available in PDF version also.
 Basanite with olivine (green) and augite (black) phenocrysts. The rock sample is from Tenerife. The width of the specimen is 9 cm.  Diabase from Tenerife with black augite and white plagioclase phenocrysts. The width of the specimen is 14 cm.  Basalt from Germany. Augite gives black color to basalt but individual crystals are too small to see with an unaided eye. The width of the specimen is 11 cm.  Beach sand containing lots of green prismatic augite crystals. Black grains are mostly magnetite. Luciole Beach, Martinique. The width of the view is 10 mm.
Pegmatites are coarse-grained igneous rocks. Here is a sample from Norway which is composed of three minerals: spessartine (manganese-bearing garnet group mineral), muscovite (light-colored common mica), and white feldspar.
The sample was originally described simply as “spessartine”. It is obvious why, such a nice specimen which shows so many large garnet crystals is by no means common and it is easy to neglect the other components. I like individual minerals as well but perhaps even more I like the assemblages of minerals (which we call rocks) and the stories associated with them. That’s why I try to understand what could be the other minerals to see the broader picture.
The only instrument I have used to analyse the sample is my pair of eyes but I have no doubt that the greenish gray flaky mineral is muscovite. Muscovite is a very common mica, especially in pegmatites. The white mineral is a little trickier. It looks like feldspar but which one? White feldspar like this could be Na-rich plagioclase (albite) but K-feldspars (orthoclase, for example) may be very similar. I do not believe it could be Ca-rich plagioclase because muscovite is usually associated with felsic rocks which host K-feldspars and sodic plagioclase.
I think it is plagioclase because on the other side of the rock I saw that the crystal is in some places composed of narrow lamellae which reflect light differently when looked at a certain angle. I try to look for that if I suspect that the mineral might be plagioclase. It probably isn’t pure albite because these lamellae are not present in near-pure end-members of the plagioclase series. So, it could be oligoclase for example which is the next mineral in the plagioclase series after albite. In albite, up to 10% of the sodium is replaced with calcium. The percentage is 10…30 in oligoclase.
 Spessartine (red), muscovite (gray), and plagioclase (white) form this beautiful pegmatite from Ljosland, Norway. The width of the view is 9 cm. The specimen belongs to the Museum of Geology of the University of Tartu.  Another side of the same sample. The width of the view is 6 cm. Polysynthetic twinning is visible in the lower right which indicates that the feldspar is plagioclase.
Wollastonite (CaSiO3) is a silicate mineral that occurs in metamorphosed carbonate rocks.
Wollastonite belongs to a group of chain silicates named pyroxenoids. These are minerals that are similar to pyroxenes but their crystal chains are distorted (not straight). Compared to pyroxenes, these minerals are rare. There are only three common minerals among pyroxenoids (wollastonite, rhodonite, and pectolite) but even these are found in few rock types that are not very voluminous. Wollastonite as a most important rock-forming mineral occurs only in a handful of places. One of them is Willsboro in New York State (two photos below).
Wollastonite forms when limestone reacts with silicate fluids:
CaCO3 (calcite) + SiO2 (quartz) → CaSiO3 (wollastonite) + CO2 (carbon dioxide)
Wollastonite may form when impure (contains silica) limestone (or dolostone) gets buried deep enough for the necessary metamorphic reactions to take place (regional metamorphism) or when magmatic fluids intrude the limestone body (metasomatism which produces skarns). In either case many other minerals may form as well. Diopside, calcite, dolomite, tremolite, andradite, grossular, plagioclase, epidote, vesuvianite, etc. may be associated with wollastonite. Wollastonite may be very rarely found in some igneous rocks.
Pure wollastonite is white. Wollastonite usually is relatively pure and therefore white but gray and light green wollastonite is common also. Wollastonite is typically fibrous, columnar, or bladed. It may be very similar to tremolite (amphibole group mineral). Unfortunately these two love to occur together which complicates the identification process. Tremolite is light green (but usually darker than wollastonite) in color and forms columnar or acicular crystals.
 Wollastonite with diopside (green) from a wollastonite mine in Willsboro, New York State, USA. The width of the rock sample is 8 cm.  Calc-silicate minerals diopside (green), andradite (brown), and wollastonite (white) in a skarn from a wollastonite mine in Willsboro, New York State, USA. The width of the view is 5 cm.  Wollastonite from Lahore, Pakistan. The width of the specimen is 8 cm.  Wollastonite, tremolite (green), and actinolite (black) from Bastnäs, Sweden. The width of the specimen is 9 cm. The specimens shown above belong to the Museum of Geology of the University of Tartu.
Graphite is a mineral with a very simple composition — C (carbon).
Everyone is familiar with this mineral because pencil “leads” are made of graphite. It is named “lead” because centuries ago people thought that graphite is a form of lead. Real lead have never been used to make pencils. Early pencils were indeed made of a stick of pure graphite but such graphite deposits are very scarce. The best source at the beginning of the 19th century was in England. These deposits were unavailable to the French because of naval blockade during the Napoleonic Wars. So, they had to start thinking and very soon one clever man discovered that you can use impure graphite powder (which is quite plentiful) to make pencil leads if you mix it with various amounts of clay and burn the rods in a kiln. The hardness of the lead depends on the amount of clay (more clay adds hardness).
This story reminds me that humans tend to be most resourceful when they have some sort of trouble which needs to be solved. It also teaches us that it may not be the best idea to make your villains life difficult by limiting the resources available because they are then forced to find a way out. I remember a time when I was a small kid that there were huge lines in the petrol stations. It was so because newly independent Estonia was under economic blockade by Russia. They tried to break us. Our economy was in a very bad shape but their plans really didn’t work. We very quickly oriented towards west and slipped out of the Russian sphere of influence which was exactly the opposite they wanted to achieve. Right now China tries to play games with the rest of the world by limiting its rare earth metals output. I think we already know what will happen and who will win this battle. There is no real shortage of rare earts in the world, we just need to mine it.
Graphite occurs mostly in metamorphic rocks. It is basically metamorphosed organic matter. Most of the graphite occurs in slates, graphite schists, carbonate rocks, and metamorphosed coal beds (anthracite). Sometimes graphite occurs in skarn and rarely in igneous rocks.
Graphite has a layered structure. Very strong separate layers of carbon atoms are held together by weak chemical bonds which means that graphite as a mineral is very soft (its hardness in Mohs scale is 1…2). That allows it to be used in pencil leads — we can scrape off layer after layer by only applying a moderate amount of force when writing on a paper. The softness gives a characteristic greasy feel to the mineral which somewhat resembles talc (softest mineral in the Mohs scale). Graphite also conducts electricity which opens up many more industrial applications.
Graphite may be turned into diamond (which is also pure carbon) but it takes huge pressure to achieve that. Hence, natural diamonds form in the upper mantle and are very rarely brought up to the surface by violent gas-charged eruptions of kimberlite magma.
 Graphite flakes from Madagascar. The width of the view is 13 mm.  Graphite schist from Hohentauern, Austria. The width of the sample is 9 cm. The specimen belongs to the Museum of Geology of the University of Tartu.  Calcite crystals with graphite from Namibia. The width of the sample is 9 cm. The specimen belongs to the Museum of Geology of the University of Tartu.  Graphite schist with pyrite from Amstall Quarry, Austria. The width of the sample is 12 cm. The hand sample belongs to the Museum of Geology of the University of Tartu. Here is an article written about the rock sample shown above: Graphite schist with pyrite.
 Graphite layers in marble. Ruskeala, Karelia, Russia. I’ve also written about the marble sample above: Marvellous marble quarry in Karelia.
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 shown below 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. However, you may use the versions shown here. I release these two images and the pdf-file into the public domain.
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.
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.
 TAS diagram for volcanic rocks. Click on the image to access 4000 pixel version of the diagram.  Here are the coordinates of the intersections. Click on the image to access 4000 pixel version of the diagram.
Epidote is a common silicate mineral.
The color of epidote is often described as pistachio-green. This light-green color helps to distinguish epidote from chlorite which often occurs with epidote but is usually darker green. However, epidote may have various shades of green color, larger epidote crystals are almost black.
Epidote is a mineral but there are other structurally similar minerals which together are known as the epidote group. Hence, epidote has both wider and narrower meaning. Other epidote group minerals are clinozoisite, zoisite, allanite and piemontite. Epidote is the most common of these minerals and forms a solid solution series with clinozoisite. Clinozoisite can be described as epidote without or with little iron content. Zoisite shares the same composition with clinozoisite but has different crystal structure. Zoisite is not as common as clinozoisite. Allanite contains rare earth elements and piemontite is a relatively rare form of reddish epidote that contains manganese.
Epidote (I am now writing about the single mineral, not the epidote group) occurs mostly in various metamorphic rocks (greenschist and amphibolite facies) but it may also crystallize directly from felsic magma. Epidote in metamorphic rocks forms at the depth of 5…25 km and temperature between 300…650 °C. Epidote also occurs in a blueschist facies rocks and in metamorphosed carbonates. So, epidote is really widely distributed in metamorphic rocks. Epidote is a very common hydrothermal alteration mineral. This alteration process, if it happens with feldspars, is known as epidotization. Saussuritization is a hydrothermal alteration of plagioclase feldspar which yields epidote. Such epidote is usually found in mafic igneous rocks. Epidote sometimes forms very beautifully formed crystals in hydrothermal veins.
Epidote is pretty resistant to weathering and is therefore common mineral in sediments. The heavy mineral fraction of sands very typically contain green epidote grains with black magnetite and pink almandine garnet. However, there could be many different green-colored minerals in sand which seriously complicates the identification. Green minerals may also be olivine, pyroxene, chlorite, pumpellyite, etc.
 Sand-sized epidote crystals demonstrate that epidote forms elongated crystals. The width of the view is 38 mm. Nevada, USA.  Epidote vein in a granite. There is a crack in the middle which allowed the hydrothermal fluids to flow and metamorphose the rock. The width of the specimen is 11 cm. Arendal, Norway. The specimen belongs to the Museum of Geology of the University of Tartu.  Epidote and quartz (white) in a folded amphibolite. The width of the specimen is 16 cm. Flüela Pass, Switzerland. The specimen belongs to the Museum of Geology of the University of Tartu.  Hydrothermally altered granite from Evje, Norway. The width of the view is 6 cm. The hand sample belongs to the Museum of Geology of the University of Tartu.  Epidote grains handpicked from a sand sample collected in the Island of Thassos, Greece. The width of the view is 7 mm.
Natrolite is a zeolite group mineral.
Zeolites are minerals that are structurally somewhat similar to feldspars (they all have three-dimensional framework of silica tetrahedra) but their structure is much more open. Zeolites were once considered to be of minor importance because most zeolites are very fine-grained but X-ray studies in the last 50 years have revealed that zeolites are actually very common minerals in some sedimentary and metamorphic environments and form the largest single group of silicate minerals.
Zeolite group contains over 80 minerals. In addition to that there are more than 600 synthetic minerals which we haven’t found in nature. Obviously, there must be a good reason to make so many synthetic minerals and there is. The open structure of zeolites makes them ideal molecular sieves (they trap certain molecules which fit into the void). These voids are normally filled with water but water can be easily driven out by heating the material. Hence, zeolites can be used several times as a desiccant for example. Zeolites can be tailor-made for specific purposes like removing heavy metals from mine waste or cleaning up radioactively contaminated areas. Some zeolites are even useful in medicine as a blood-clotting agent (useful in battles or disaster areas). One zeolite mineral, erionite, may pose some risk to human health. Long term exposure to erionite fibers may cause similar lung pathologies as asbestos minerals do.
Most zeolites are too fine-grained to be identified without X-ray analysis but there are exceptions. Natrolite is probably the best-known zeolite because it forms beautiful white radiating crystals in the vesicles of mafic volcanic rocks. Other zeolite minerals that sometimes form large crystals are heulandite, chabazite, analcime, thomsonite, and stilbite. Analcime is sometimes treated separately from other zeolites because it may be considered to be one of the feldspathoids. Zeolites are usually either white or pastel shades of other colors.
 A zeolite (natrolite probably) in limburgite from Kaiserstuhl, Germany. The width of the crystal aggregate is 21 mm.  Natrolite in an alkaline (probably phonolite) volcanic rock. The sample is interesting because it demonstrates both the outer form and the internal structure of the aggregate of natrolite. The sample is from the Czech Republic. The width of the view is 8 cm.  Natrolite (white) and prehnite (green). Prehnite is not a zeolite group mineral. Unknown locality in Germany. The width of the sample is 12 cm. The specimens shown above belong to the Museum of Geology of the University of Tartu.
Seiland is an island in Northern Norway where really spectacular pegmatitic rocks are found. Pegmatites are often mineralogically unusual but this one is probably the weirdest I have seen. There seems to be only two minerals: biotite and zircon. Both biotite and zircon are common magmatic minerals but what is really striking is the size of the zircon crystals. Zircon usually occurs in very small grains because there is normally not enough zirconium in the magma to grow large crystals.
In this case things seem to be different. I don’t know what led to the formation of such a mineral assemblage but one thing that’s sure is that mineral collectors definitely love samples like this. Similar looking rocks are probably widely distributed in the collections worldwide because these rocks were mined there between 1980-1988. The rock body that contains large zircon crystals embedded in biotite is nepheline syenite dike (about 8 meters wide at the widest and several kilometers long).
 Zircon crystals (brown) surrounded by biotite (black). The length of the largest crystal is 14 mm. Seiland, Norway.  The length of the largest crystal is 17 mm. Seiland, Norway. The specimens belong to the Museum of Geology of the University of Tartu.
Vesuvianite is a calcium-bearing silicate mineral and occurs typically in metamorphosed limestones and skarns.
Vesuvianite is usually green, brown, or yellow. Vesuvianite is also known by the name idocrase. Vesuvianite is usually associated with other calc-silicate minerals like diopside, garnets, wollastonite, etc. Vesuvianite may occur in alkaline igneous rocks (nepheline syenite) and in metamorphosed mafic dikes which also contain serpentine, garnet (grossular), diopside, and epidote. Garnet and vesuvianite-bearing metamorphosed mafic igneous rocks are sometimes called rodingites.
Vesuvianite-bearing skarns, vesuvianite-bearing nepheline syenites and rodingites are rare rocks. Hence, vesuvianite is a borderline mineral which is sometimes treated as one of the rock-formers although its rare occurrence barely justifies such an approach. Vesuvianite is still reasonably well known mineral, probably mostly because it often forms beautiful crystals and is therefore sought after by mineral collectors.
 Vesuvianite crystals in a metamorphosed limestone from Eiker, Norway. The width of the view is 15 mm.  Vesuvianite crystal (6 cm) with garnet in a skarn from Kristiansand, Norway. The samples photographed belong to the Museum of Geology of the University of Tartu.
|
|
Recent Comments