Troctolite is an intrusive igneous rock consisting of plagioclase feldspar and olivine. It is a member of gabbroic rocks family. It is compositionally similar to gabbro. The main difference is that it does not contain pyroxene or contains very little while it is a major mineral in gabbro. It can be described as pyroxene-depleted gabbro.
Troctolite is an olivine-bearing gabbroic rock without pyroxene. Gray mineral is Ca-rich plagioclase, orange is olivine. Olivine has lost its original green color due to weathering. Orange spots are composed of various weathering products of olivine which is collectively known as iddingsite. Flakstadøya, the Lofoten Archipelago, Norway. Width of sample 15 cm.
http://picasaweb.google.com/107509377372007544953/2015#6195818050343685810 Classification diagram of olivine-bearing gabbroic rocks. The field of troctolite is annotated. The rock contains variable amount of plagioclase and olivine (10-90%) but very small amounts (less than 10%) of pyroxene1.
The genesis of troctolite is also strongly tied to gabbro. These two rocks usually occur together in the same magma intrusion because troctolite can not form directly as magma crystallizes. There is no way how magma can crystallize into olivine and plagioclase without pyroxene. Pyroxene crystals must be separated from the melt by a mechanism that leads to the formation of cumulate rocks. In these rocks some minerals occur in much higher (or lower) concentration than expected. Anorthosite is an example of cumulate rock which is also a member of the same gabbro family. Anorthosite is composed of almost pure plagioclase. Troctolite may be similar to anorthosite and these rocks may smoothly grade into each-other as demonstrated below.
http://picasaweb.google.com/107509377372007544953/2015#6190952353277858754 Anorthosite on the left and troctolite on the right. Flakstadøya, the Lofoten Archipelago, Norway.
http://picasaweb.google.com/107509377372007544953/2015#6195757889372414354 This sample can be described as coarse-grained leuco-troctolite. Flakstadøya, the Lofoten Archipelago, Norway. Width of sample 12 cm.
http://picasaweb.google.com/107509377372007544953/2015#6195757887502746626 Here is a sample of olivine gabbro from the same intrusion on Flakstadøya. The difference is that this rock contains lots of black pyroxene augite. Width of sample 12 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190952317337991970 An outcrop of mafic intrusion on Flakstadøya. The rock surface cropping out is troctolite. The layer of the rock is several hundred meters wide. It grades to anorthosite (in front) and gabbro (behind the scene).
http://picasaweb.google.com/107509377372007544953/2015#6190952319469711250 A closer look of the same outcrop reveals the spotted gray-orange appearance of the rocky surface.
Troctolite as demonstrated above has a speckled appearance which is the reason it carries such a name – troctolite means troutstone in Greek. ‘Troutstone’ is a synonym of troctolite in English and German (forellenstein) also2.
Granite is a crystalline igneous rock that consists largely of feldspar and quartz. These two are the most common minerals in the crust which means that granite too is among the most ubiquitous rock types, especially in the upper continental crust.
This picture of a granite pegmatite from northern Norway (Nyelv) is very coarse-grained for a normal granite and compositionally simpler than most granite samples, but it illustrates well what granite is. It is crystalline (composed of visible mineral grains) and always contains quartz (gray) and also feldspar (red). Width of view is about 50 cm.
Although it is indeed very widespread, not every rock that is named so is a true granite. This rock name may well be the most abused geological term in existence. This article aims to explain what really is granite and what it is not.
Classification
Granite is a coarse-grained quartzo-feldspathic igneous rock which contains 20-60% quartz of total quartz + feldspar (other minerals are neglected) and 35-90% of the feldspar is alkali feldspar (orthoclase or microcline). Rocks which fulfil the quartz content requirement but have a different ratio of feldspars have different names although they are known as granitoids or granitic rocks too. Granitic rocks which contain less than 10% plagioclase feldspar are named alkali feldspar granite and granitic rocks where the dominant feldspar is clearly plagioclase (over 65%) are named either granodiorite (65-90% of feldspar is plagioclase) or tonalite (over 90% of feldspar is plagioclase)2. See the diagram below:
http://picasaweb.google.com/107509377372007544953/2015#6193580045622566322 Classification of granite (red field) and granitic rocks (red+yellow) is based on the content of quartz (Q) (in relation to feldspar) and the mineralogy of the feldspar group minerals (A and P).
These rocks are known as granitoids (rocks resembling granite) because it may not be easy to distinguish different feldspar group minerals in the hand sample. But it should remain just a field term. It makes sense to classify granitic rocks based on the feldspars because rocks from different tectonic regimes tend to have different feldspar content. True granites, for example, are virtually absent in the oceanic crust although there are granitic rocks which are rich in plagioclase (tonalite).
Similarly coarse-grained igneous rocks that contain less quartz are named syenite (feldspar is mostly alkali feldspar), monzonite (contains both feldspars in roughly equal amounts) and diorite (plagioclase-rich). Rocks that contain more than 60% quartz as an average of voluminous igneous intrusion are uncommon because silicic magma from which granite crystallizes almost always contains enough potassium, sodium and calcium to form lots of feldspars in addition to pure silica (quartz). Feldspars actually precede quartz in the crystallization order from magma. So quartz can form only if there is free silica left after other cations have already satisfied their need for silica. In the vast majority of cases it contains more feldspar than quartz.
http://picasaweb.google.com/107509377372007544953/2015#6190953010689058114 Trondhjemite is a leucocratic variety of tonalite. It is a granitic rock, but not true granite because the feldspar it contains is sodic plagioclase, not alkali feldspar. Width of sample from Norway is 10 cm.
Syenite may be similar to granite, but it does not contain enough quartz. This sample is quartz alkali feldspar syenite from Estonia. Width of sample 8 cm.
Texture
Granite is a coarse-grained igneous rock with average grain size ranging from 1 to 25 millimeters1. These rocks crystallize from a very slowly cooling magma within the crust where they are well insulated. Time gives crystals a chance to grow. Volcanic rocks with a similar composition exist as well. These are known as rhyolite (volcanic equivalent of granite) and dacite (similar in composition to plagioclase-rich granitoids). The groundmass of these rocks is very fine-grained although they frequently contain phenocrysts (larger crystals that were already formed before the extrusion to the surface) embedded in the finer matrix. Granite that crystallized in a narrow dike may be fine-grained because the heat was rapidly lost to the adjacent rocks. Such granite is known as aplite. Sometimes granite is very coarse-grained. That too happens usually close to the margins of a granitic pluton, but the coarseness is mostly a result of volatiles in the magma which greatly reduce its viscosity and therefore enhance crystal growth. These coarse-grained granites are known as pegmatites.
It has often roughly uniformly sized crystals which show no preferred orientation but that is not always the case. Some granites just like rhyolites contain phenocrysts – crystals that are clearly bigger than the material surrounding it. These phenocrysts are usually feldspar crystals. The difference between granite and rhyolite is that in granite even the finer material is visibly crystalline, but in rhyolite the individual crystals within the groundmass are not visible to the naked eye. Elongated minerals may have a preferred orientation. Sometimes so much that it is no longer clear whether it is a granite or gneiss. Some granites have special textures like rims of one mineral around the other. Rapakivi granite is a notable example.
http://picasaweb.google.com/107509377372007544953/2015#6190951122739162034 Typical granite has visible mineral grains without any orientation. Orange mineral is alkali feldspar, gray is quartz. Picture taken in Norway. Width of view 20 cm.
http://picasaweb.google.com/107509377372007544953/2015#6194282136332693778 It may have a preferred orientation. Note how most of the biotite flakes are aligned sub-parallel to each-other. Lødingen, Hinnøya, Norway. Width of sample 15 cm.
http://picasaweb.google.com/107509377372007544953/Coll#5808110375348210322 Aplite in the lower half is much finer than granite above it. Their composition seems to be the same. There are larger phenocrysts of quartz and feldspar within the granite. Width of sample 12 cm.
Gneiss and granite may be the same compositionally, but elongated mineral grains are strongly oriented and segregated in gneiss. The sample of gneiss is from Karelia. Width of sample 11 cm.Rhyolite and dacite are extrusive versions of granitic rocks. Their groundmass is very fine-grained, but they often contain phenocrysts, mostly feldspars. The sample from Scotland is 8 cm in width.
http://picasaweb.google.com/107509377372007544953/2015#6194282159317086338 Rapakivi texture (plagioclase rims around ovoidal phenocrysts of alkali feldspar) in granite. Luumäki, Finland. Width of sample 17 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196126451189924914 Graphic texture (quartz intergrowth in feldspar that resembles runic script) is common in pegmatites. Quartz crystals are uniformly scattered in the feldspar crystals and not randomly oriented. These crystals grew simultaneously with alkali feldspar host and they are optically continuous (one crystal). The term “pegmatite” was originally used to describe graphic granites by Haüy in 1822. Graphic granites may indeed be described as pegmatites even according to current usage, but the term “pegmatite” nowadays has much wider meaning, encompassing also rocks which have no graphic texture nor do they have to be granitic in composition4. Evje, Norway. Width of sample 9 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190951568325813794 A contact between gneiss (granodioritic with migmatitic granite veins) and granite pegmatite. Nyelv, Norway.
Composition
Average chemical composition of granite determined by 2485 chemical analyses of granitic rocks3 (numbers are mass percents, recalculated volatile-free to total 100%):
Notable is a very high SiO2 which is the reason why it contains so much quartz. The amount of iron, magnesium and titanium are low when compared with basaltic rocks.
Minerals quartz and feldspar take most of the silicon, oxygen, potassium, sodium, aluminum, and calcium. Small amounts of other chemical elements will find their place in the crystal lattice of apatite (accommodates phosphorus), magnetite and ilmenite (iron and titanium), biotite and hornblende (iron, magnesium), muscovite (potassium, aluminum, fluorine), zircon (zirconium), titanite (titanium), monazite (rare earth elements, phosphorous). Some granites or granite-like rocks may contain pyroxene or even olivine which are rich in iron and magnesium.
Dark minerals are usually either biotite (mica) or hornblende (amphibole group mineral). It is usually biotite in potassium-rich true granites and more often hornblende in granodiorites and tonalites.
Pegmatites may be either simple (ordinary granite with large crystals) or complex. In the latter case granite has an elevated concentration of rare chemical elements which give rise to rocks with unusual composition. Minerals that often occur in granite pegmatites are tourmaline (contains boron), topaz (fluorine), spodumene (lithium), cassiterite (tin), fluorite (fluorine), lepidolite (lithium), zircon (zirconium). Complex pegmatites may be very valuable as mineral resources. Simple pegmatites may be mined as well, mostly because of large muscovite flakes they often contain.
http://picasaweb.google.com/107509377372007544953/2015#6194282129261240274 Granite pegmatite with tourmaline (schörl). Kaatiala, Finland. Width of sample 11 cm.
http://picasaweb.google.com/107509377372007544953/2015#6194282130920969538 Lepidolite (lithium mica) with cleavelandite (variety of albite) in a pegmatite. Haapaluoma, Finland. Width of sample 12 cm.
Types
Several classification have been propesed for granitic rocks. The most common way is to divide them into (I,S,M,A) types. Most of these letters refer to protoliths. I-type granite is the most common one which is believed to have an igneous protolith. S-type granites originate at least partly from a sedimentary source. M-type granites are rare and are supposed to have mafic protolith although this is somewhat problematic because mafic rocks do not yield granitic melt. ‘A’ means anorogenic. These are granites from plutons that were not associated with magmatism related to subduction processes and mountain-building. This classification scheme, although still widely in use, is somewhat problematic and inconsistent.
Newer and perhaps better or at least more easily quantifiable method is to use alumina saturation index. In this scheme the molecular ratio of Al2O3/K2O + Na2O + CaO is calculated. This ratio is useful because it is 1 in feldspars. So the excess or deficiency of aluminum must be accommodated by other minerals.
If granite is rich in aluminum, it contains muscovite in addition to feldspars plus other aluminum-bearing accessory minerals like corundum, tourmaline, cordierite, etc. These rocks are known as peraluminous granites.
Rocks which have Al2O3/K2O + Na2O + CaO below 1 but Al2O3/K2O + Na2O over 1 are metaluminous. These rocks are alumina deficient which means that minerals like biotite (Al-poor) and hornblende must crystallize in addition to feldspars.
Third category of granites (known as peralkaline) are even more deficient in alumina: Al2O3/K2O + Na2O < 1. These rocks contain alkali pyroxenes and amphiboles (aegirine, riebeckite) and even iron-rich olivine fayalite.
http://picasaweb.google.com/107509377372007544953/Rocks#5808533358262843506 Peraluminous granite with garnet phenocrysts. Granites that contain lots of aluminum may have a sedimentary protolith (S-type granites). Aluminum comes from pelitic (rich in clay minerals) sedimentary rocks. Width of sample from Estonia is 8 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5808533379571331314 Peraluminous granite from Estonia. The excess of aluminum is accommodated in light-colored mica muscovite. Width of sample 10 cm.
http://picasaweb.google.com/107509377372007544953/2015#6194386930081261026 Metaluminous granite containing lots of hornblende (black). Width of sample 9 cm.
http://picasaweb.google.com/107509377372007544953/2015#6194386929663937778 Peralkaline granite pegmatite. Black mineral is amphibole riebeckite. Width of sample 7 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190952952147907170 Peralkaline granite with iron-rich end-member of olivine group: fayalite. Southern Norway. Width of sample 18 cm.
Charnockite is an enigmatic rock which compositionally could be a subtype of orthopyroxene-bearing granite, but it probably formed as a result of metamorphic processes deep in the crust. It is perhaps better to classify such rocks as granulites. Flakstadøya, Lofoten Islands, Norway. Width of sample 9 cm.
Occurrence
Granite is one of the main ingredients of the continental crust. Although continental landmasses are composed of many different igneous, sedimentary and metamorphic rocks, originally they were derived from the mantle by a process called partial melting which will step-by-step lead to the generation of granitic magma. When rocks melt the liquid that forms has different composition than the original source rock. When peridotite melts, basaltic rocks form which contain more silicon and aluminum and less iron and magnesium. Basalt is because of its chemical composition more light-weight and moves upward and solidifies as a lava on the surface or as dikes in the crust. When basalt melts even lighter magma forms which will lead to the formation of granite. It is therefore a result of a remelting of the material from the continental crust.
Granite is so light-weight when compared to the original peridotite that it can not subduct back to the mantle. So it remains buoyant as a young continent. These continents will go through multitude of processes. Continents collide and metamorphose, they are worn down by weathering and igneous processes add new material. It has taken very long time. The process started probably right after the formation of the Earth as a planetary body. The cores of continents formed in the Archaean and they tend to grow bigger because the continental crust that has already formed can not sink to the mantle again. As a result continents are structurally complex mixtures of virtually every rock type known to us. Granite being perhaps the most important of them.
Not all of these rocks are granites in the strict petrological sense. Major part of this material is clearly metamorphosed and should be described as gneiss. However, our knowledge about the interior of the crust mostly comes from the seismic studies and for the seismic waves it does not make any difference whether it is granite or a granitic gneiss. This is why it is often assumed that the continental crust is granitic. Furthermore, the composition of this material is not necessarily granitic in the strict sense. Plagioclase feldspar is more common in the crust than alkali feldspar. So it is fair to assume that the dominant rock type in the continental crust is a metamorphic rock with a granodioritic composition.
True granites occur mostly in plutons which are pancake-shaped igneous intrusions in the upper crust. Sometimes they reach the surface when the material above them has been removed by the weathering processes. These plutons vary enormously in size (1-1,000,000 km3)1. The mechanism how they formed has been very controversial. The dominant hypothesis has been that they are the result of igneous diapirism – igneous material rises through the crust like a hot balloon. The formation of some granite plutons may be partly explained that way, but it seems more likely that the migration of granitic melt took place in the network of narrower cracks in the rocks. It seems rather difficult to understand how can very large balloon of hot magma move through the cold and rigid rocks of the upper crust. The formation process of granitic melt is known as migmatization and the rocks that contain metamorphic rocks mixed with magmatic veins are known as migmatites.
http://picasaweb.google.com/107509377372007544953/Coll#5776562547968632530 Much of the ‘granitic’ continental crust is actually made of granodioritic gneisses like the example above. Karelia, Russia (Archaean Fennoscandian Shield). Width of sample 16 cm.
http://picasaweb.google.com/107509377372007544953/2015#6194335090254198626 Migmatite erratic on the northern coast of Estonia. Migmatites are mixtures of metamorphic and igneous rocks. Igneous material usually has a granitic composition and is either a result of partial melting in situ or is a magma that formed elsewhere and intruded into the rocks where we see it now. Migmatization seems to be the mechanism how granitic magma is formed and how it migrates in the crust.
http://picasaweb.google.com/107509377372007544953/2015#6194337066073112130 Outcrop of a granite intrusion north of the High-Atlas mountain range in Morocco.
http://picasaweb.google.com/107509377372007544953/2015#6190952280371307266 Granite cutting through calc-silicate schist. Gimsøy, Lofoten Archipelago, Norway.
http://picasaweb.google.com/107509377372007544953/2015#6190952363734480450 Large granite dike (several meters wide). Å, Lofoten Archipelago, Norway.
Uses
Granite has several uses. It may contain valuable minerals. These may be either gemstones or industrial minerals. Pegmatites are especially rich source of both beautiful and rare crystals and minerals with useful properties. Zircon, beryl and tourmaline crystals come from pegmatites. Also industrial minerals like micas and feldspars are taken from pegmatites because large grain size makes the separation process easier. Zirconium and beryllium are extracted from granite pegmatites (minerals zircon and beryl, respectively). Granite may contain ore minerals. Tin and tungsten ores, for example, are hosted by granite.
Granite is an important construction material. It is hard and durable which makes it a very good material for aggregate. It is one of the most important dimension stones because of beautiful textures and colors. It is also massive and durable – ideal material for countertops. It must be said, however, that not all of these granites are true granites. Some of them are metamorphic although are granitic in composition. But sometimes rocks with much lower quartz content are named granite. For a geologists they may be diorite, monzonite, syenite, diabase, gabbro, etc. These rocks are also known as commercial granite.
http://picasaweb.google.com/107509377372007544953/2015#6190951401996398930 Rapakivi granite is a variety of granite widely used as a dimension stone. Luumäki, Wyborg batholith, Finland. Width of view 35 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190951563426090962 This ‘granite’ from Norway (Nyelv) is actually migmatitic gneiss, but such rocks are usually marketed as granite (commercial granite).
http://picasaweb.google.com/107509377372007544953/2015#6194393806570153730 Granite is an excellent material when hard and durable rocks are needed. These granitic rocks are used as a railroad ballast in Estonia.
Rhomb-porphyry is a porphyritic igneous rock with abundant wedge- or lens-shaped anorthoclase (feldspar) phenocrysts. Rhomb-porphyry is a rare rock type. The most well-known is the rhomb-porphyry from the Oslo Rift in Norway. Similar rocks are known to exist in only two other locations: The East African Rift Valley and the Antarctic1. They are all volcanic areas with active or former (when the rocks formed) continental rifting.
Rhomb-porphyry has an attractive appearance and is thus famous not only among geologists. The rock is from the Oslo Rift, Norway.
Rhomb-porphyry is compositionally trachy-andesitic (latite) which corresponds to plutonic rock monzonite.
Although rare globally, these rocks are very common in Norway just south-east of the capital Oslo. There are numerous lava flows (over 70) and many dikes. Lava flows cover over ten thousand square kilometers and dikes may reach thicknesses up to 80 meters. The most voluminous flow unit consists of about 1,000 cubic kilometers of lava. This is huge amount which has no historic analogues although other pre-historic lava flows of that magnitude are known. Thicknesses of a single lava flows vary between 4 and 130 meters1. The volcanism that produced rhomb-porphyry lava started about 295 million years ago and lasted for 20 million years2. Rhomb-porphyry lava also has a plutonic version known as larvikite which is highly valued dimension stone because of large feldspar crystals with bluish iridescence.
Rhomb-porphyry should not flow easily given its high silica content and abundance of phenocrysts. Yet it covers huge area as both subaerial lava flows and dikes. Field observations indicate that rhomb-porphyry lava behaved pretty much as basaltic lava is supposed to behave. This is in stark contrast to modern volcanoes with trachy-andesitic composition which tend to erupt violently and produce more ash than lava. To this day it remains somewhat of a mystery. However, we do know that rhomb-porphyry has abnormally high fluorine (0.2-0.45 %) content which might have lowered the viscosity of the magma. Continental rifts are known to produce odd volcanic phenomena. Perhaps the most well-known is a carbonatitic magma which compositionally is more like a limestone than normal igneous rock. Rhomb-porphyry is just another rock from a continental rift with unusual characteristics.
Different lava flows have different geochemical fingerprints and they also differ in appearance. Not all of them have rhomb-shaped phenocrysts. Width of sample 7 cm.
It is actually easy to see that rhomb is not the best analogue to describe the shape of the phenocrysts. They are highly variable but none of them seems to be rhomb-shaped. I would say that the most beautiful ones resemble lenses or wedges.
Fresh surface of a rhomb-porphyry sample from Norway. Width of sample 13 cm.
References
1. Ramberg, I. B. et al. (2008). The Making of a Land – The Geology of Norway. Geological Society of Norway. 2. Sundsvoll, B. et al. (1990). Age relations among Oslo Rift magmatic rocks: implications for tectonic and magmatic modelling. Tectonophysics. Volume 178, Issue 1.
Topics surrounding heavy minerals have been discussed here before but so far not as a rocky subject. I recently returned from Northern Norway, where I stumbled on a beautiful red-black-white folded metamorphic rock on a rocky coastline of the Varanger Peninsula. There were lots of interesting rocks, but something like this one really caught my eye.
http://picasaweb.google.com/107509377372007544953/2015#6191124032243014386 Magnetite, garnet and quartz in a metamorphosed placer deposit. Eastern coast of the Varanger Peninsula, Barents Sea, Northern Norway. Width of sample 18 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190951699030876802
http://picasaweb.google.com/107509377372007544953/2015#6190951639222326754 More rocks from the same formation.
The rock is composed of quartz (white), magnetite (black) and red garnet (almandine). These minerals are very common ingredients of heavy mineral (placer) deposits. I cannot imagine any other genesis so I assume that this rock is a former heavy mineral concentrate which got buried and finally metamorphosed to a rock-type similar to quartzite. But obviously it is no quartzite because quartz forms clearly less than half of the composition. How to name it? I don’t know. I would like to know if some of my readers have an idea. That rock would be a nice example of a BIF (banded iron formation), but these rocks normally contain no garnet. It is still a potential iron ore because it is very rich in magnetite (strongly attracts a hand magnet) although I am afraid it does not form a deposit which is extensive enough.
UPDATE: Pedro Castiñeiras (@PetroMet) suggested Grt-Mag-Qtz banded granofels or Grt-Mag-Qtz metaplacer as a name for the rock. I like the latter more as it also gives information about the genesis, which in this case seems to be important.
This is how the source deposit of this rock might have looked like. This is a modern heavy mineral rich beach sand from the Pfeiffer Beach, California, USA.
Monomineralic rocks, as the name suggests, are rocks that are composed of only one mineral. Majority of rocks are composed of several different minerals and absolutely pure monomineralic rocks do not exist in nature anyway. But some of them are reasonably ‘pure’ to be called monomineralic.
Dunite is composed of almost pure forsterite. Gusdal Quarry, Møre og Romsdal, Norway. Width of sample 9 cm.
How pure exactly do they need to be is something that is not set in stone but in several cases 10% is arbitrarily chosen as the lower purity standard. Especially among igneous rocks where the term ‘monomineralic’ is most often used. Examples of monomineralic igneous rocks are dunite (more than 90% olivine) and anorthosite (more than 90% plagioclase feldspar).
Common monomineralic metamorphic rocks are marble and quartzite although they do not need necessarily to be monomineralic. Similar is the situation with their sedimentary protoliths – limestone and sandstone which may be very pure. This is the most important difference between most igneous monomineralic rocks (which need to be more than 90% pure to earn its name) and sedimentary and metamorphic rocks. There are some exceptions, though. A rare igneous rock carbonatite may be almost monomineralic but is often not. It only needs to be more than 50% carbonate to be named that way. So it is very similar to sedimentary carbonate rock limestone that is also at least 50% carbonate. Pyroxenite is another example of often almost monomineralic igneous rock although it may contain up to 40% of olivine in addition to pyroxene group minerals.
So we can talk about ‘true’ monomineralic rocks that need to be pure and monomineralic rocks that only may be pure (their classification principles allow that) but do not have to be.
Dunite is an igneous rock made of olivine (over 90%). Dunite is relatively rare rock type on the surface but very common deep in the mantle. Dunite may be mined as a mineral resource because olivine has some industrial applications. Especially useful is its high melting temperature which makes it a good refractory material. The sample above is collected from a working olivine quarry.
Anorthosite. Rogaland, Norway. Width of sample 13 cm.
Anorthosite is a common rock type on the Moon where the highlands surrounding the darker maria are composed of this rock type. But it also occurs here on Earth although it is not distributed as widely here. The existence of this rock type is no accident because its constituent mineral plagioclase feldspar is the most common mineral in the Earth’s crust.
Carbonatite (sövite). Alnö Island, Sweden. Width of sample 8 cm.
Carbonatite is a rare igneous rock with an exotic composition. It is composed of carbonate minerals (>50%) which makes it compositionally very similar to sedimentary limestone and metamorphic marble. Its appearance may also very much resemble that of marble.
Pyroxenite. Aust-Agder, Norway. Width of sample 8 cm.
Pyroxenite is an ultramafic rock just as dunite. It may contain significant amount of olivine (up to 40%) but the sample above is mostly composed of almost pure clinopyroxene.
Quartzite. Telemark, Norway. Width of sample 9 cm.
Quartzite is a common metamorphic rock just as its sedimentary protolith sandstone. Many quartzite samples are composed of almost pure interlocking mass of fused quartz grains just as the sample above.
Marble. Baikal, Siberia. Width of sample 12 cm.
Marble is a metamorphosed limestone that may contain significant amount of impurities but almost pure marbles are common also.
Chalk. Width of sample 6 cm.
Chalk is a type of limestone that is composed of calcitic exoskeletons of coccolithophores.
Mylonite is a foliated metamorphic rock that is composed of intensely flattened minerals in a fine-grained streaked matrix. Mylonites form deep in the crust where temperature and pressure are high enough for the rocks to deform plastically (ductile deformation). Mylonites form in shear zones where rocks are deformed because of the very high strain rate. Mylonites contain porphyroclasts. These are crystals that existed before the process of mylonitization began, but have survived with lots of strain damage. They are ovoidal or stretched and are surrounded by finer and even more deformed mineral grains. Mylonitic rocks with porphyroclasts are often described as having a flaser fabric.
Mylonite is a fine-grained rock that containes intensely flattened minerals. A boulder on the coast of Varanger Peninsula in Norway.
Mylonites are foliated, but the foliation is generally passive. It means that the foliation is not controlling how the rock breaks. In this way mylonitic rocks resemble gneiss and are distinct from schist, slate, and phyllite which tend to break along the foliation plane.1
The rock is called protomylonite if the porphyroclasts make up more than half of the rock volume and ultramylonite if almost all the porphyroclasts are gone (less than 10% have survived). In the latter case the matrix grains are so small that the rock may even resemble chert.
Mylonitic fabric forms as a result of both destructive and constructive processes. The first is responsible for the plastic deformation and the latter for the recrystallisation which gives an impression that the rock has been semi-molten and flowing under very heavy stress. Mylonites may occur as thin sheets in relatively unmetamorphosed rocks (see the pictures below) or they may encompass broad zones several kilometers thick.
‘Mylonite’ as a term is not new. It was coined by Charles Lapworth in the late 1800s who described mylonitic rocks from the Moine thrust in Scotland. But the interpretation how these rocks were formed has changed. It was generally believed that mylonite is a result of intense brecciation (brittle pulverization). The name originates from the Greek language (mylon – to mill). Nowadays we believe that instead of milling mylonitization is a result of ductile deformation and the discovery of plate tectonics has given us a framework to understand where and how these rocks can form – deep in the crust where rocks are hot and there are movements between crustal blocks, but the rocks are too plastic to deform by faulting. The process higher in the upper crust where rocks are brittle is called cataclasis. That we can indeed describe as a certain type of brecciation that results in lots of broken rock fragments and generally increased volume of the fractured rock body because of induced voids between the fragments.
Mylonites with larger grain size are sometimes described as gneisses and very fine-grained ultramylonites may resemble slate. Mylonites usually have a quartzo-feldspathic protolith (like granite or gneiss) although they may originate from any rock type. The temperature has to be at least 250-300°C for the mylonitization to take place (for quartzo-feldspathic rock). This is enough for quartz to flow while feldspar still resists dislocation creep and slide and retains its former shape more successfully as strained porphyroclasts. 450°C is enough for feldspar to go through dynamic recrystallization as well.2
The photos below are from western Norway near the small village of Flatraket. There is an igneous intrusion surrounded by gneisses and also eclogites here and there. Especially eclogites were the reason why I visited this part of Norway and I managed to get some really nice pictures which I hope to demonstrate in the near future. The igneous intrusion is composed of mangerite according to the Norwegian bedrock map4. Mangerite is an orthopyroxene-bearing monzonite, but in this case it seems to contain a little too much quartz and alkali feldspar which makes it actually quartz syenite. It may be completely inappropriate to use the term mangerite in this case because according to Lappin et. al.3 this rock contains clinopyroxene, not orthopyroxene.
But in this article the composition of the protolith is not that important. What struck me when I walked into a roadside quarry (where I just hoped to find a good hand sample of mangerite) were narrow and very intense zones of mylonitization. In the end, I got no proper mangerite, but found something entirely different but no less interesting. Here are the pictures from the Flatraket quarry:
Large orthoclase ovoids in a matrix composed of plagioclase, quartz, clinopyroxene, garnet, and ore minerals.3 It turned out that the rock has a rapakivi texture. At least some orthoclase ovoids are surrounded by plagioclase rims – a texture very familiar to me as the type locality of these rocks is in southeastern Finland. Not far from my home. Here is a larger view with a hammer for scale. Something is going on here. The rock is clearly oriented and orthoclase porphyroclasts have tails (recrystallized mineral matter). The ovoids have even grown bigger which makes this rock a blastomylonite (recrystallization has increased the volume of porphyroclasts). More intense mylonitization… and even more. Now we are really talking about heavily stretched rocks.
http://picasaweb.google.com/107509377372007544953/2015#6190951215832700402 Some mylonite bands are so fine-grained that are approaching ultramylonite in appearance.
Mylonitic zones may be only few centimeters in width.
http://picasaweb.google.com/107509377372007544953/2015#6190951194612010802 Another narrow band of mylonite within an almost unstrained igneous rock.
And more ultramylonite (or approaching that) in the lower part of the deformed zone. Quartz syenite with mylonite.
Mylonites from other localities
http://picasaweb.google.com/107509377372007544953/2015#6191004916488584034 Mylonite with large (largest is over 10 cm) K-feldspar porphyroclasts. Sør-Trøndelag, Norway.
I recently returned from Norway with lots of rock samples which will be photographed in the near future. The photos will be added mostly to the old posts where appropriate but this way my regular readers will not see them. And I also miss an opportunity to learn something new from comments. So I thought that perhaps I should start writing shorter posts in addition to longer overviews of rock types.
Here is the first one of the new series which will contain mostly rock samples but also other interesting geological photos taken during my geotrips.
http://picasaweb.google.com/107509377372007544953/2015#6196126451189924914 Graphic granite from Evje, Norway. Width of sample 9 cm.
Width of view 14 cm.
Graphic granite is a graniticpegmatite where large alkali feldspar crystals contain angular, linear, or wedge-shaped intergrowths of quartz. These intergrowths may resemble cuneiform writings, hence the name.
Quartz crystals are uniformly scattered in the feldspar crystals and not randomly oriented. These crystals grew simultaneously with alkali feldspar host and they are optically continuous (one crystal).
The term “pegmatite” was originally used to describe graphic granites by Haüy in 1822. Graphic granites may indeed be described as pegmatites even according to current usage but the term “pegmatite” nowadays has much wider meaning, encompassing also rocks which have no graphic texture nor do they have to be granitic in composition1.
Another sample of graphic granite from the same locality. Evje, Norway.
References
1. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.
Pele’s hair is a term for volcanic glass fibers. Its formation is usually associated with fire fountaining when blobs of flying molten lava are stretched into very thin threads. Theoretically the same could be done with an ordinary molten glass because it is polymerized and can be stretched into very long and thin strands. Pele’s hair fibers may be several meters long while being only a fraction of a millimeter in width. These lightweight fibers are then transported away by wind. They can be carried several kilometers away from vents.
http://picasaweb.google.com/107509377372007544953/Rocks#5877446891106018450 Flying Pele’s hair can stick to the upper crust of a lava flow. Lava flow with Pele’s hair has a bronze-colored hue instead of ordinary black.
It is sometimes assumed that wind stretches these filaments out of a basaltic lava flow but I find it too hard to believe. Lava flow is too compact and thick for that. There must be more intense force involved that puts a real strain on the molten material. However, similarly stretched strands often form as lava tongues break out and stretch the already partly solidified outer crust of the flow. You can see such strand in the last picture, but the majority of Pele’s hair is most likely associated with fire fountaining.
http://picasaweb.google.com/107509377372007544953/Rocks#5877446878256230242 Pahoehoe fibers are golden brown. They may be found in crevices shielded from wind.
The term “Pele’s hair” comes from Hawai’i just as many other volcanological terms. Pele is a local volcano goddess there. For some reason her hair is usually imagined to be black and/or red by artists. There is an obvious inconsistency because these are not the colors of Pele’s hair. At least not the color of the material we call that way. Her hair is therefore not composed of lava flows and should be golden brown. Perhaps we should think again how to really draw her.
Pele’s hair is not associated with Hawaii only. Similar lava threads form in other places too, for example in Nicaragua (Masaya) and Ethiopia (Erta’ Ale)1.
http://picasaweb.google.com/107509377372007544953/Rocks#5877469849895990658 A macrophoto of Pele’s hair. Width of view is 16 mm.
http://picasaweb.google.com/107509377372007544953/Rocks#5877488061213480754 Thin and fragile strand extending from the glassy crust of a basaltic pahoehoe lava flow.
References
1. Francis, P. & Oppenheimer, C. (2003). Volcanoes, 2nd Edition. Oxford University Press.
Mushroom Rock is an interestingly shaped rock in Death Valley. This rock is fairly well known because it is located just by the road running from Furnace Creek to Badwater Basin (lowest point in the western hemisphere).
There are a number of similarly shaped rocks in the world. Most of them are shaped by wind-blown sand grains and are therefore ventifacts. There are some nice examples of ventifacts in Death Valley nearby (read my post about them: Ventifacts and dreikanters). However, this rock is probably shaped by salt erosion, not wind1. Crystallization of salt crystals can be an effective way to disintegrate rocks.
I read from several sources that the sign erroneously claims that Mushroom Rock is a ventifact (shaped by wind). There is no such sign anymore and not even a place to stop a car. Maybe because the rock was vandalized some years ago. It has been repaired and I did not notice anything wrong while I was there. So it must be a job nicely done.
http://picasaweb.google.com/107509377372007544953/California02#5874551929109845714 Small holes in the upper part of the rock (mushroom cap) resemble tafoni to me and may also indicate that salt is responsible for eroding this rock.
http://picasaweb.google.com/107509377372007544953/California02#5874551921375279730
http://picasaweb.google.com/107509377372007544953/California02#5874551925840941986
http://picasaweb.google.com/107509377372007544953/California02#5874551945812586962
Mushroom Rock is carved from igneous rock diabase with a nice porphyritic texture. White phenocrysts are plagioclase crystals. Width of the view is about 25 cm.
I don’t remember when and where I found this pebble. It was probably several years ago somewhere in Estonia. It is very small (about 4 cm across) and composed of limestone. I picked it up because it must have a remarkable geological story.
It had to be a part of some limestone formation. Most likely in northern Estonia where such rocks from the Ordovician Period are exposed. It has a rather distinct appearance and seems to be partly composed of clastic fragments but I am not familiar enough with Estonian bedrock to locate its exact source. I believe it comes from the coast because it is a pebble. Something had to break it from the limestone bed and then polish it to a nicely rounded shape. I guess it was done by sea waves.
What happened after that is harder to explain but it is obvious that something had to crush it. The pebble is now composed of four distinct parts. However, these fragments were not separated from each other which needs some sort of explanation. They are only slightly displaced. Most likely the pebble was surrounded by other rocks which held the fragments in place. What was the crushing force is impossible to tell. It was hardly an earthquake because these occur in Estonia very rarely and are weak. Maybe some bigger rock fell onto it? Maybe the event was associated with glacial activity during the last glacial epoch?
Anyway, the four main fragments of the pebble stayed next to each other and were cemented together again as a single rock and were later liberated from the surrounding material. So the pebble was crushed and then the pieces were glued together again and the same pebble, although seriously wounded, formed again.
http://picasaweb.google.com/107509377372007544953/Rocks#5873362125056612866 Small brecciated limestone pebble with an interesting history.
, 10th Edition. McGraw-Hill. Volume 8. 202-204.
, 2nd Edition. Wiley-Blackwell.
. Geological Society of Norway.
, 2nd Edition. Oxford University Press.
, 2nd edition. Kendall Hunt Publishing.