Syenite

June 11, 2025August 20, 2012 by Siim

Syenite is an igneous rock that solidified slowly in the crust in a similar manner to granite. A true syenite (sensu stricto) is also compositionally resembling granite. The most notable difference is the absence or very low quantity of quartz while it is an essential component of granite. The dominant mineral is alkali feldspar, usually orthoclase. This rock type is found in a wide variety of colors.

The classification on the QAPF diagram. Different syenitic rock types cover large part of the diagram. The true syenites are rocks that fit into the area annotated in red. Syenitic rocks are mostly composed of alkali feldspar (A) with minor amounts of quartz (Q). Plagioclase feldspar (P) is clearly less important than alkali deldspar. Foid syenites contain significant amount of relatively rare silicate minerals known as feldspathoids (F) which are called foids for simplicity. Note that Q and F are mutually exclusive. Syenitic rock that contains quartz can not contain feldspathoids and vice versa — foid-bearing and foid syenites contain no quartz.

The term “syenite” in a wider sense involves similar rock types like quartz syen., alkali feldspar syenite, foid syen., foid monzosyenite, etc. The classification principles are shown on the diagram below.

The diagram above may leave us an impression that the true syenite is a relatively insignificant part of the whole family. Actually, this is not true. Syenite sensu stricto and quartz syenites as well are the most common of the whole group, although syenite is a relatively rare rock type anyway, especially when compared with granite or other granitoids. Rocks like nepheline or sodalite syenites are actually curiosities that please the eye of a geologist, but are by no means common rocks we are likely to see all around us.

Alkali feldspar forming the majority of most syenitic rocks is usually intergrown with sodium-rich plagioclase feldspar (usually oligoclase). Such feldspar intergrowth is named perthite and this is the reason why syenite is more common rock type than alkali feldspar syen. which contains almost no plagioclase. Plagioclase may appear in syenitic rocks in addition to perthitic alkali feldspar also as a separate phase. Dark mica biotite and amphibole hornblende are usual mafic constituents. Alkali syenites may contain somewhat unusual minerals like aegirine, arfvedsonite, etc. Common accessory minerals are zircon, apatite, sphene, magnetite, and ilmenite.

The approximate extrusive (volcanic) equivalent of syenite (more precisely alkali syen. which is rich in sodium) is trachyte. Phonolite is an approximate equivalent of foid syenite. I said “approximate” because the classification principles used to classify plutonic and volcanic rocks are in most cases different. While the classification of plutonic rocks is based on mineralogical composition, the same approach is not practical with fine-grained or even partially glassy volcanic rocks. They are classified according to their chemistry and the results are plotted on the TAS diagram.

Syenitic rocks are usually associated with other plutonic rocks. They form relatively small intrusive bodies or parts of larger intrusions. Most syenitic seem to be associated with extensional tectonic regime (rifting continents). At least some of them are believed to be fractionation products of alkali-rich basaltic magmas, but there are several different mechanisms responsible for the genesis of a syenitic magma. In this sense it is again similar to granite which is also defined solely by its mineralogical composition although there are very different possible ways how granitic magma can evolve. It is perhaps inevitable because minerals are something we can determine, unlike genesis which in many cases is still a mystery.

An immense number of terms describing many varieties have been used in the past: foyaite, canadite, durbachite, umptekite, nordmarkite (quartz syen.) and perthosite (alkali feldspar syen.) are perhaps the best known unofficial names of syenite varieties. Shonkinite is a dark-colored variety of foid syenite which generally contains more pyroxene than alkali feldspar. Larvikite is a famous decorative stone from Norway. It exhibits a characteristic schiller, but larvikite contains more plagioclase than alkali feldspar and is therefore monzonite. Many so-called syenites are actually monzonites. For some reason the term “monzonite” seem to be little known and rarely used.

The term “syenite” is actually an ancient one. There is a town named Syene (nowadays: Aswan) in Egypt. However, the rocks near that town after which this rock got its name are actually hornblende-bearing granites according to modern classification principles.

Quartz alkali feldspar syenite rock sample — Quartz alkali feldspar syenite from Estonia. Width of sample 8 cm.

*A sample from Lilla Söderby, Sweden. Width of sample 10 cm.*

*Alkali feldspar syenite (also known as umptekite) from Sweden. Width of sample 12 cm.*

*Nepheline monzosyenite from Sweden consisting of albite, nepheline, and mafic minerals. Such rocks rich in sodium feldspar albite are sometimes referred to as canadite. Width of sample 17 cm.*

Conglomerate

June 12, 2025August 16, 2012 by Siim

Conglomerate is a sedimentary rock formed by the lithification of rounded or sub-rounded gravel (grains larger than 2 mm in diameter). Conglomerate is strongly related to sandstone. It is actually a type of sandstone, although it may not be technically correct to say so. Conglomerate is composed of clasts larger than 2 mm (sand is composed of grains smaller than 2 mm).

Quartzite conglomerate — An outcrop of quartzite conglomerate. The Bergen Arcs, Norway. Width of view 1.5 m.

Conglomerates are differentiated from sedimentary breccias which are composed of angular clasts. Sometimes it may be difficult to say whether the grains are angular or rounded enough. It is therefore unavoidable that one geologist’s conglomerate will be another’s breccia. I generally prefer to name these rocks conglomerates if there is at least some rounding apparent. Breccia in my opinion is composed of really angular, wedge-like clasts with a strong variation in size. Breccias often show clear signs of shattering which is a result of a sudden and energetic event. Conglomerates tend to be composed somewhat more evenly sized clasts. Conglomerates may be clast-supported (clasts are in contact) or matrix-supported (clasts are separated from one another by fine-grained matrix that binds the clasts together).

There are many different ways to make a conglomerate. Conglomerate deposit may be an ancient riverbed or a coastline. Conglomerates form at the base of mountain ranges or even as a glacial deposit. Tillite is a lithified till (poorly sorted glacial debris) which may be called a matrix-supported conglomerate if there are enough granules, pebbles, cobbles, and boulders present. At least 30% of the rock should be composed of clasts larger than 2 mm in diameter in order to call that rock a true conglomerate.

Conglomerate from the Ordovician near Bergen in Norway. The clasts are made of quartzite and are slightly flattened due to metamorphism.

http://picasaweb.google.com/107509377372007544953/2015#6222506286111321554
An outcrop of conglomerate in Cyprus consisting of rounded gabbro clasts from the Troodos ophiolite.
http://picasaweb.google.com/107509377372007544953/2015#6222506284972796994
The same outcrop near Kouklia in Cyprus.
http://picasaweb.google.com/107509377372007544953/2015#6222506304666598834
Weakly consolidated river deposits in Cyprus, north of Paphos.
http://picasaweb.google.com/107509377372007544953/2015#6222506286312355618
Fine-grained conglomerate pebble with quartz clasts and dolomite cement from the northwestern coast of Estonia. Red grain in the middle is almandine garnet.
http://picasaweb.google.com/107509377372007544953/2015#6222506331673485682
A boulder of conglomerate on a lithified lahar (volcanic mudflow) at the coast near Portraine in Ireland.
http://picasaweb.google.com/107509377372007544953/2015#6222506336319050930
A close-up of the same boulder.
http://picasaweb.google.com/107509377372007544953/2015#6222506338465327746
A conglomerate boulder in the Spanish Pyrenees consisting of gray limestone and red sandstone clasts.
http://picasaweb.google.com/107509377372007544953/2015#6222506367473228162
Conglomerate is a widespread rock type in northern Spain. These deposits formed as a result of the orogeny (rapidly rising mountains were eroded rapidly as well).
http://picasaweb.google.com/107509377372007544953/2015#6222506368882312546
A boulder of conglomerate in southern Ireland (Ballydowane Cove).
http://picasaweb.google.com/107509377372007544953/2015#6222506370082584018
Vertical layers of alternating sandstone and conglomerate at Bunmahon Head, Ireland. These layers were originally horizontal.
http://picasaweb.google.com/107509377372007544953/2015#6222506387764654290
More sandstone and conglomerate at Bunmahon Head. It is easy to see that original up direction was to the left.
https://picasaweb.google.com/107509377372007544953/2015#6222506386819093746
Conglomerate deposits often give rise to peculiar landforms like this tower in northern Spain.
Coastal cliff of Saint Lucia composed of conglomerate and volcanoclastic rocks
Conglomerate beneath volcanoclastic rocks. Western Coast of Saint Lucia.
http://picasaweb.google.com/107509377372007544953/Conglomerate#5777285276379481682
More landforms composed of conglomerate in Spain. Conglomerate is easily erodable. Once there is a crevice formed, it tends to widen which results in many tower-like rock formations.
http://picasaweb.google.com/107509377372007544953/Conglomerate#5777285254744189058
A closer view of these conglomeratic cliffs.

Gneiss

June 11, 2025August 15, 2012 by Siim

Gneiss is a very widespread rock type, especially in the lower parts of the continental crust, but it is also a common rock on the surface in some places (Scandinavia, Canada, and other shield areas where crystalline rocks are not covered by a layer of sedimentary rocks).

A sample from Karelia, Russia. This specimen has a composition of an ordinary granite: pink K-feldspar, gray quartz, and black biotite. Width of sample 11 cm.

The term “gneiss” was brought to scientific usage from the German language. It was originally a mining term, meaning a country rock in the Ore Mountains (Erzgebirge) which contained metalliferous veins. The term (gneist) was first recorded in print by Agricola (Georg Bauer) in his famous posthumously published book De Re Metallica which remained the most important mineralogy and mining textbook for the next two centuries. The book was published in 1556¹.

Most of the mineral grains of gneissose rocks are visible to the naked eye. Banding in this rock is a result of mineral segregation into separate, typically light- and dark-colored layers. Light-colored layer is usually composed of feldspars and quartz. Most important dark minerals are hornblende and biotite. Individual bands are usually 1-10 mm in thickness. Layers larger than that imply that partial melting or the introduction of new material have probably taken place. Such rocks are called migmatites. It is often difficult to distinguish it from migmatite because there is a gradational transition from one to another. Hence, terms like “migmatized gneiss” are commonly used. It is not well understood how the segregation takes place, but it must be the result of extreme pressure and shear stress deep in the crust.

The protolith of gneiss may be an igneous rock, in this case it is called an orthogneiss. It forms probably because of shear in vicous granitic magma. Paragneiss is a variety with a sedimentary protolith. Even in the latter case, gneissic banding has nothing to do with original layering of sedimentary rocks. These original features are completely obliterated by the metamorphic processes involved in the formation of this rock type.

Paragneiss in most cases is thought to be the end product of metamorphism of a pelitic (clay-rich) sedimentary rock (shale, argillite, claystone, etc.) that metamorphosed first into slate, then became phyllite, schist, and finally gneiss. Still deeper burial or more intense heating may result in migmatization and finally complete melting of gneiss.

Despite being clearly oriented, this rock is not considered to be foliated because it is not fissile along the layering. So, when hammered, gneiss behaves like a uniform homogenous rock. In this sense it is similar to igneous rocks like granite and gabbro and not similar to related metamorphic rocks like schist and phyllite which are foliated.

It is important to note that gneiss is a rock type that is defined by its oriented texture, rather by its mineralogy or chemical composition. Hence, qualifying terms are often added to the rock name: amphibolite gneiss or hornblende gneiss, for example. The term “gneiss” without any additional information is commonly imagined to be compositionally similar to granite (K-feldspar, quartz, biotite).

Gneiss is a product of regional metamorphism. This is a type of metamorphism which is associated with mountain building. Gneisses form deep below the forming mountain ranges and are exhumed many millions of years later when the mountains get carried away by the erosion.

The cores of the continental landmasses are typically composed of such grayish gneisses. This very old (from the Archaean) sample is from Karelia, Russia. Width of sample 16 cm.

An augen gneiss from Estonia (glacial erratic from the Finnish Bedrock). Width of sample 30 cm.

A sample from an unknown location, possibly from Karelia.

A contact between gneiss and pegmatite. Nyelv, Norway.

A contact between gneiss and granitic pegmatite. Trollstigen, Norway.

A sample of migmatitic gneiss. Nyelv, Finnmark, Norway. Width of sample 30 cm.

Biotite gneiss. Evje, Norway. Width of sample 14 cm. It would be logical to assume that dark biotite-rich bands represent metamorphosed muddy layer in a sandy sediment but this is not necessarily the case. In lower grade metamorphic rocks the original fabric of the protolith is indeed often recognizably preserved, but higher grade rocks like gneiss show compositional banding which does not need to represent the original banding of the protolith. Furthermore, metamorphic differentiation can create compositional layers where none previously existed².

Gneiss is widely used as a dimension stone. It has a nice combination of hardness and durability with beautiful texture.

Wall made of gneiss blocks in Sweden. Even the rocks that do not seem to have a banded appearance do have it when looked from a different direction.

References

1. Tomkeieff, S. I. (1983). Dictionary of Petrology. John Wiley & Sons.
2. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.

Laterite

June 11, 2025August 8, 2012 by Siim

Laterite is a reddish weathering product of basalt. At least this is what it is in India where this rock or a soil type was first defined by a Scottish scientist Francis Buchanan-Hamilton in 1807. However, not all laterites are enriched in iron and sometimes they are not even reddish. Some lateritic rocks (bauxite) are mined because of their high aluminum content. Iron-rich variety is mostly used as a construction stone, especially in Asia.

*A sample from India. Laterite is very common in India due to climatic reasons. Width of sample 9 cm.*

http://picasaweb.google.com/107509377372007544953/Coll#5774367096818871522
An outcrop of laterite in Northern Ireland near the Giant’s Causeway.

Laterite is a residual material. This is what is left of common silicate rocks if we remove much of silica, alkali, and alkaline earth metals. It is mostly composed of iron, aluminum, titanium, and manganese oxides because these are the least soluble components of the rocks undergoing a type of chemical weathering known as laterization or lateritization. Iron-rich variety consists of hematite and goethite. These minerals give reddish color to the soil/rock and sometimes such deposits are mined for their iron and nickel content (in Cuba and New Caledonia). Aluminous laterite (bauxite) is mostly composed of aluminum hydroxides gibbsite, diaspore, and boehmite. Not all bauxites are laterites, though. Some bauxite deposits are associated with limestones and karst phenomena. Lateritic bauxite is a weathering product of aluminous silicate rocks (granite and similar).

Laterite is foremostly a type of earthy soil, but it hardens into solid rock in air and is therefore suitable as a construction material. Lateritic soil almost lacks fertility and is generally not suitable for agriculture.

Certain conditions are needed for the laterite deposits to form. Modern examples are found in climatic regions which are characterized by warm air temperature, abundant rainfall, and dry periods. Lateritic soils are common in savannas, but not in the rainforests and jungles (where clay is dominant in soil) which lack dry period. Modern deposits are also unknown in temperate regions where weathering largely means the formation of clay minerals instead of lateritic soils.

http://picasaweb.google.com/107509377372007544953/Coll#5774370944422185650
A close-up of a lateritic sample from Northern Ireland. The width of the sample is 8 cm.

Bauxite rock sample — Bauxite is an aluminum-rich variety. Width of sample is 8 cm.

Sulfur, gypsum, and hydrocarbons

June 11, 2025July 17, 2012 by Siim

Sulfur is usually associated with volcanoes. It is indeed frequently present around fumaroles which indicate recent or active volcanism but most of it is actually concentrated in salt domes which are buoyantly rising salt diapirs in the crust. These salt domes are made of marine evaporite deposits. How can sulfur be there?

These domes are mostly composed of salt and other evaporite minerals, including calcite, anhydrite, and gypsum. When the salt diapir rises it will be in contact with meteoric ground water which will preferentially dissolve and carry away salt (NaCl) as the most soluble mineral there. Hence, the concentration of less soluble evaporites rises in the upper part of the salt dome (cap rock). The upper part of the cap is usually composed of calcite, below this is gypsum and further down it changes to anhydrite (unhydrated gypsum).

Salt domes are very interesting features economically but not so much because of their salt content. Salt used to be really expensive commodity centuries ago but these times have passed. It is crude oil that is often associated with salt domes that interests us so much. Oil and other hydrocarbons are lightweight and tend to migrate upward. Oil reservoirs form only if their upward movement is somehow restricted. Common structural traps for oil are salt domes, more precisely the area directly around them. Why is this important in the context of this post? Because we need (or certain bacteria needs) hydrocarbons as an energy source. They feed on it and undertake certain chemical reactions which in simplified form look like this:

CaSO₄ (anhydrite) + CH₄ (methane or other hydrocarbons) + bacteria = H₂S (hydrogen sulfide) + CaCO₃ + H₂O.

Waste product of the bacterial metabolism hydrogen sulfide will react with oxygen to form elemental sulfur:

2H₂S + O₂ = 2S + 2H₂O

Here is a beautiful rock sample which is a result of these reactions:

http://picasaweb.google.com/107509377372007544953/Rocks#5766202940193127506
Sulfur and gypsum — a sample of a salt dome cap rock from Germany. Width of sample 11 cm.

So it seems that an odd combination of sulfur, gypsum, and crude oil is actually pretty common. If one finds sulfur in a cap rock of a salt dome, it may be an indication that something valuable is down there. However, sulfur itself is useful mineral resource as well and may be mined by injecting superheated water into the cap rock which mobilizes sulfur which can be then pumped out and used to make sulfuric acid.

Pumice

June 6, 2025July 10, 2012 by Siim

Pumice is a lava froth. This rock type is well known because of its lightness. It usually floats in water because of extreme vesiculation. It is usually light-colored and in most cases corresponds compositionally to rhyolite or dacite. There is a reason for that. These silica-rich rocks are highly polymerized (silicon tetrahedra are linked to each other to form a three-dimensional network which impedes the internal flow of lava). Basaltic lava, on the other hand, contains much less silica and flows more easily.

Pumice samples — Samples from Sanorini (left) and Tenerife (right). Pumice is typically light-colored and very vesicular.

This rock is a trap to volcanic gases that tried to escape from magma but were unable to do it because they simply could not break out and formed many gas-filled vesicules instead. Trapped gases occupy much larger volume than they did when dissolved in magma. Hence, it is perhaps needless to say that it forms as a result of explosive volcanic eruptions.

Explosive volcanoes that produce this rock type are predominantly located at the continental margins above subduction zones (The Pacific Ring of Fire, for example). This environment is favorable for this type of volcanism because lava above subduction zones is rich in volatiles (gases expand when magma ascends) and it is also enriched in material incorporated from the continental crust (siliceous mahma is more viscous).

Pumice from Santorini — This piece of rock from Santorini was thrown out of a volcano approximately 3600 years ago during the Minoan eruption. The eruption was one of the most powerful in historic times, 30 cubic kilometers of rocks were ejected from the volcano¹. The width of the specimen is 40 mm.

*Phonolitic pumice from Tenerife. Width of sample is 6 cm.*

http://picasaweb.google.com/107509377372007544953/Tenerife#5832686321337998642
A layer of pumice between darker layers of scoria in Tenerife. Pumice layer is a product of violent Plinian eruption farther away. Scoriaceous mafic dark-colored lapilli were ejected from nearby vents (Strombolian eruptions).

*Pyroclastic rocks in Santorini. White is pumice. Larger black and brown volcanic rocks are andesite blocks.*

http://picasaweb.google.com/107509377372007544953/Tenerife#5835477260570033682
Granadilla pumice in Tenerife is a lapillistone composed of phonolitic pumice.

Coulée — *Glassy coulée in the background and a hill made of frothy lapilli in the lower part of the picture. Tenerife.*

Scoria (from Tenerife; on the left) and pumice (from Santorini) are both vesicular but pumice is usually much lighter (in color and weight) and contains more small holes.

References

1. Schmincke, Hans-Ulrich (2005). Volcanism. Springer.

Greisen

January 11, 2016July 3, 2012 by Siim

Greisen is a hydrothermally metamorphosed granitic rock. It is composed mostly of light-colored mica (muscovite, lepidolite, zinnwaldite) and quartz.

http://picasaweb.google.com/107509377372007544953/Beil#5760980632227326242
Greisen with cassiterite from Namibia. Width of sample 10 cm.

It is a type of endoskarn. It means that these rocks in most cases formed inside the granitic pluton itself that provided heat and hydrothermal fluids to initiate the greisenization. Therefore, granite seems to have metamorphosed itself to greisen by late hot magmatic fluids that contained many chemical elements incompatible in common minerals forming granitic intrusions. This is the reason why greisen is usually a natural concentrate of somewhat unusual minerals. Some of them may be economically interesting. These minerals are cassiterite (tin ore), fluorite (fluorine), rutile (titanium), lepidolite (lithium), tourmaline (contains boron but used mostly as a semi-precious gemstone), wolframite (tungsten), topaz, etc.

It should not be a surprise that the term “greisen” was originally a mining term. It originates from Germany (the Ore Mountains) and meant a rock that contained tin ore (cassiterite) with quartz, mica, and little or no feldspar.

In most cases this rock is a metamorphosed S-type granite (‘S’ refers to sedimentary protolith). Such granites typically have a relatively high content of incompatible elements and therefore often give rise to pegmatites too. It is usually coarse-grained and gray in color with a glittering appearance because of high mica content. The fluids that cause greisenization usually follow cracks and fissures in the granitic pluton (and sometimes in adjacent country rocks also), but sometimes these cracks are so closely spaced that almost the whole granite body is turned into greisen.

Epidosite

June 12, 2025June 9, 2012 by Siim

Epidosite is a metamorphic rock consisting of epidote and quartz. This rock type is a product of hydrothermal metamorphism. Its parent rock was a mafic igneous rock.

It is not an uncommon rock in certain conditions. I have written about massive pyrite and umber, which are characteristic rocks of complete ophiolites (like the Troodos Ophiolite in Cyprus). These rocks are enriched in metals (copper, zinc, manganese). Epidosite, on the other hand, is very much depleted in these elements, although they are present in mafic dikes (parent rock of epidosite).

It seems logical to assume that seawater that circulates in the oceanic crust near the spreading zones heats up and alters the dikes by turning them into epidosite. By doing that it carries away not only metals but also sodium and magnesium and leaves behind rocks enriched in calcium and silicon. Hence, epidosite is an important piece of the puzzle, which helps us understand where this material is coming from that is carried to the seafloor by the hydrothermal vents called black smokers.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737566216193442338
Sheeted dikes in Cyprus, which were sometimes more or less greenish in color. It is mineral epidote that gives them this bright green tone.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737565985927334242
Some dikes are almost apple-green, which indicates that they are composed mostly of epidote and quartz. Original mafic minerals are completely replaced.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737566013550253314
Weathered slope of an epidositic roadcut.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5752041155700155106
A sample from Cyprus. Width of sample 10 cm.

Harzburgite

June 12, 2025June 4, 2012 by Siim

Harzburgite is an igneous plutonic rock. It is composed of orthopyroxene and olivine, it is one of ultramafic rocks and belongs into the peridotite group. These are rocks that are abundant in the mantle but scarce on the surface.

http://picasaweb.google.com/107509377372007544953/Diagrams#5750234259867272898
Harzburgite has a strict definition which is best explained graphically. Ol, Opx, and Cpx represent olivine, orthopyroxene, and clinopyroxene respectively on this ternary plot. So, harzburgite may contain only small amounts of clinopyroxene and at least 40% (but no more than 90) of it is olivine¹.

Fresh rock is dark green, but its components pyroxene and especially olivine are easily hydrothermally altered or weathered. Therefore, harzburgite visible on the surface often has yellowish or reddish brown color.

Harzburgite is formed in the mantle when part of the peridotitic rock melts. This melt has basaltic composition and moves upward (melt is less dense than solid rock).

Clinopyroxene starts melting before, so as the rock partially melts, the composition of the peridotite becomes more and more depleted in clinopyroxene and relatively enriched in orthopyroxene (lherzolite becomes harzburgite).

Dull green is olivine, orthopyroxene is dark brown. Tappeluft, Norway. Width of sample 10 cm.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737569693773138802
A sample from the Troodos Mountains in Cyprus (ophiolite complex).
http://picasaweb.google.com/107509377372007544953/Cyprus2#5737569988660181714
Harzburgite from the Troodos Mountains. Note how orthopyroxene crystals stand out by reflecting light much better than the rest, which is mostly composed of olivine.

References

1. 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.

Gabbro pegmatite

June 13, 2025May 24, 2012 by Siim

Pegmatites are predominantly granitic in composition but not necessarily. The definition of a pegmatite says nothing about the composition. They are simply very coarse-grained igneous rocks.

It is generally well known that the slower the rate of cooling the coarser the rocks will be. Hence, pegmatites should be rocks that cooled especially slowly? Actually, just the opposite is true. Pegmatites most likely cooled relatively fast because they are usually found at the edges of batholiths (cooler host rocks are nearer). It is the low viscosity of magma that makes them coarse-grained. Such a magma contains lots of volatile compounds (mostly water) that break the links between silica tetrahedrons and thereby make the magma less polymerized which enables large crystals to grow.

Pegmatites are mostly granitic because granitic magma contains much more water than magmas with different composition. It is especially true if the granitic magma formed by the melting of former metasedimentary rocks that contain lots of hydrous minerals. gabbroic magma usually contains much less water because of obvious reasons — its source rocks (peridotite mostly) contains very small amount of volatile compounds.

However, sometimes, for some reasons, some parts of gabbroic intrusions do solidify as pegmatites. I saw a nice example of a gabbroic pegmatite in Cyprus. These rocks represent the lowest parts of the oceanic crust which host gabbroic batholiths that feed the sheeted dike complex above. Through sheeted dikes magma moves upward and if it successfully makes its way to the seafloor, solidifies there as a pillow lava (gallery of pillows from Cyprus).

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737570035563110994
Gabbroic pegmatite at the Loumata River Valley.