Schist is a strongly foliated medium-grade metamorphic rock. It is characterized by an abundance of platy or elongated minerals (micas, chlorite, talc, graphite, amphiboles) in a preferred orientation. Varieties of this rock type share similarities in appearance (schistosity) but may be highly variable in composition. Individual mineral grains are discernible by the naked eye. This property sets it apart from slate. Schist is one of the most widespread rock types in the continental crust.

Some schists contain no platy minerals like sheet silicates or graphite, but in this case these rocks have to display a linear fabric (elongated minerals in sub-parallel orientation). Amphibole-bearing rocks with a lineated fabric belong to this group.

Schistosity is a type of foliation, characterised by the preferred orientation of elongated or platy mineral grains (which are abundant in schistose rocks). Schistosity is a result of pressure in the crust which forces the grains to align perpendicular to the force applied. This force may be compressive (in mountain ranges) or simply caused by the weight of the overlying rocks. Schist can form only if the compressed rock contains lots of elongated or platy grains. Growth of mica crystals during the course of metamorphism makes the schistosity more evident.

Metamorphic reactions between minerals upon increased burial will lead to the loss of schistosity because feldspar increases in abundance as micas become unstable. This process will lead to the formation of high-grade metamorphic rock gneiss (and gneissose fabric which can be described as a poorly developed schistosity). Sometimes schist is imagined to contain at least 50% of elongated minerals¹, but in many cases the distinction between these rocks is just based on the qualitative assessment of a geologist describing the rocks in the field.

Schistose rocks are fissile, they have a tendency to split along sub-parallel planes (sometimes described as s-surfaces). This also sets it apart from gneissose rocks that have a lineated fabric but do not possess a fissile character. Fissile character gave this rock type its name — the Greek word skhistos means ‘split’, from the base of skhizein ‘cleave’. The rock name was introduced into the French language (schiste) in the late 18th century³.

The differentiation between schist and lower-grade metamorphic rocks slate and phyllite is also somewhat problematic. There is no easy way to quantitatively decide when one ends and another begins. The individual mineral grains in slate are not visible to the naked eye while they are clearly visible in schist. Phyllite is between them in metamorphic grade. Its constituent platy mineral grains are large enough to impart a silky sheen to the cleavage surfaces of the rock.

The family of schistose rocks is compositionally very diverse but most of them are derivatives of former mudstones metamorphosed to various aluminous schists (metapelites). Another major part of the family are rocks with a mafic igneous protolith. These include greenschists and blueschists. Green color is given to the former mostly by a chlorite group minerals. The latter contains bluish amphibole glaucophane. Sedimentary rocks that were rich in organic matter metamorphose to graphitic schists.

The name of a particular schistose rock depends on the dominant minerals present — muscovite-garnet-staurolite schist, for example. Several less known names have been given to a specific varieties: staurotile (contains staurolite porphyroblasts), prasinite (metamorphosed mafic rock with epidote, chlorite and hornblende in equal proportions), sismondinite (chloritoid is the dominant mineral phase).

Aluminous varieties often contain large crystals in a finer matrix. These crystals formed as the metamorphism progressed and they can convert to each-other as conditions change. Such large and often euhedral crystals are known as porphyroblasts. Common minerals that form porphyroblasts are garnet, staurolite, kyanite and andalusite. Porphyroblasts somewhat resemble phenocrysts in igneous rocks — both are larger crystals in a fine(r) matrix but the mineralogy is distinctly different. Quartz is a common phenocryst in igneous rocks, but it never occurs as a porphyroblast in metamorphic rocks. Feldspar, micas, olivine, pyroxenes and amphiboles are all common phenocrysts but uncommon as porphyroblasts².

Foliation surfaces are commonly wavy which reflects the presence and growth of porphyroblasts. Microscopically, schists commonly show a crenulation fabric which indicates the presence of older foliation that may represent an earlier episode of deformation⁴.

Common minerals in schistose rocks indicate that these rocks formed at low- to intermediate grade conditions (subgreenschist, greenschist, blueschist, and amphibolite facies). This roughly corresponds to temperatures in the range of 300-600 °C and pressures from several to several tens of kilometers. This means that not only composition but the formation conditions too are highly variable.

Schist is rich in flaky and soft sheet silicate minerals which makes it structurally weaker than gneiss or granite. That is the reason why this rock type is used less frequently as a building stone. However, some varieties have an attractive appearance which makes them useful as a facing or decorative stone. Schist may be worth mining if it contains useful minerals in large concentration. Common minerals extracted from schistose metamorphic rocks are garnet, kyanite, talc and graphite.

References

1. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.
2. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.
3. Schist. English Dictionary. Oxford University Press.
4. Van der Pluijm, B. A. (2007). Schist. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 16. 103.

Marble is a metamorphic rock consisting predominantly of calcite or dolomite. It is a metamorphosed carbonate rock (limestone or dolomite rock).

Marble is a sparkling and typically granoblastic rock. Granoblastic rocks have uniformly sized, equidimensional, anhedral grains produced by recrystallization in a solid state (metamorphism). Grain boundaries are usually straight or slightly curved and triple junctions between the adjacent grains define angles of about 120°².

The term “marble” in geology is restricted to true metamorphic rocks in which the carbonate minerals have recrystallized. This process generally increases the average grain size, which gives marble its sparkling and attractive appearance. The sparkle is the reason why we call it that way — Greek marmaros, sparkling. In commerce, coarse-grained sedimentary carbonate rocks that take polish (grainstone, travertine) and sometimes even alabaster (gypsum) are also treated as marble¹.

Pure calcitic limestone generally yields phaneritic (visibly crystalline) rock. Dolomitic rocks may metamorphose to dolomarble (dolomite marble) or calcitic marble if impure. Younger rocks are usually calcitic. Dolomarbles are more frequent in older (precambrian) terranes.

Older marbles are often dolomitic. This sample comes from a Paleoproterozoic terrane. Ahola, Finland. Width of sample 8 cm.

Impurities in carbonate protoliths (mud, sand, organic matter) considerably complicate the metamorphic reactions. Reaction between calcite and silica (sand) yields wollastonite and liberates carbon dioxide:

CaCO₃ + SiO₂ → CaSiO₃ + CO₂ (Although in real rocks the sequence of reactions is more complicated. Wollastonite is not among the first minerals to form during the marmorization process.)

Similar reaction with dolomite rock yields calcitic marble with Mg-rich olivine (forsterite):

2CaMg(CO₃)₂ + SiO₂ → Mg₂SiO₄ + 2CaCO₃ + 2CO₂

Forsterite with calcite in a metamorphosed dolomite rock. Width of sample 9 cm.

Thermal metamorphism of dolomite rocks around contact aureoles produces calcite with periclase (MgO) which is later hydrated to brucite (Mg(OH)₂):

CaMg(CO₃)₂ → CaCO₃ + MgO + 2CO₂

Metamorphism of impure dolomite rock may (in addition to forsterite) also yield tremolite, diopside, talc, serpentine, monticellite, merwinite, humite, and anthophyllite¹. Ophicalcite is a variety that contains serpentinized forsterite or diopside.

If the calcitic protolith contained clay, Al-rich phases like Ca-garnets (grossular, andradite) and several other minerals like scapolite, melilite, and epidote may form. In dolomitic mudstones, spinel and amphiboles may form in addition to the aforementioned minerals. Iron-bearing phases in protoliths metamorphose to magnetite.

Conglomeratic variety, which is composed of white dolomite, reddish calcite, and green mica fuchsite. Fauske, Norway. Width of sample 20 cm.

Rocks closely related to marbles are skarns. They form when silicate fluids from hot igneous intrusions react with carbonate country rocks. The result is a mineralogically complex and very diverse mixture of calc-silicate minerals with calcite. Skarns are often mined because they contain valuable ore minerals. Skarns are too rich in silicate minerals to be called marble and sometimes they contain no carbonate minerals at all (used up in metamorphic reactions).

Skarn sample consisting of pink calcite, black actinolite, and green diopside. The carbonate protolith was almost certainly dolomitic because both actinolite and diopside are magnesium-bearing minerals. Tapuli, Sweden. Width of sample 10 cm.

Organic matter in limestone metamorphoses to mineral graphite. Graphite gives gray or speckled appearance to the rock, but metamorphic reactions of carbonaceous material with water-rich fluid may liberate carbon dioxide and methane, leaving the rock snow-white:

2C + 2H₂O → CH₄ + CO₂

This happens around magmatic intrusions (water-rich fluid is of magmatic origin), because the devolatilization of marble produces CO₂-rich fluid².

An outcrop in Karelia, Russia. White mineral is calcite, dark-colored streaks are composed of graphite.

Marble may be snow-white either if the protolith was very pure or if the rock has been bleached by a water-rich fluid. Fauske, Norway. Width of sample 14 cm.

This rock sample is not marble, it is quartzite (metamorphosed sandstone). Quartzite and marble may be very similar in appearance, but marble is a soft rock, which is easily scratched with a needle. And it also effervesces in dilute hydrochloric acid while quartzite does not react. Dolomarble is more resistant to acids, but it too will effervesce if powdered (scratched with a needle first). Hinnøya, Norway. Width of sample 9 cm.

The coarsening of average grain size with metamorphism is a process known as the Ostwald ripening. This is a spontaneous thermodynamically-driven process that occurs because molecules on the surface of a grain are energetically not as stable as the ones inside. This also explains why small soap bubbles merge into larger ones. The major difference here is that the coarsening of marble takes millions of years. Protoliths may be rich in fossils as limestones often are, but the recrystallization (also known as marmorization) completely obliterates any hint of it.

A coarse-grained calcitic variety from Leivset, Norway. Width of sample 9 cm.

A block of metaconglomerate consisting of white heavily stretched dolomarble clasts in a pink calcitic matrix with a significant amounts of silicate minerals, especially sheet silicates. Løvgavlen quarry, Norway.

Carbonatite is a relatively rare igneous rock that compositionally resembles marble, although there are significant differences in minor element compositions. The genesis of carbonatite and marble are completely different because the first is a true magmatic rock. This carbonatite sample is composed of calcite with green fluor-apatite. Siilinjärvi, Finland. Width of sample 18 cm.

A sample of carbonatite, which may be easily misidentified as graphite-bearing marble. The sample is from Alnö Island in Sweden. Width of sample 13 cm.

Carbonatite may look like a very coarse-grained marble variety. White mineral is calcite just as it is usually in marble. But carbonatites are rich in rare earth elements (>500 ppm) and they also contain much more strontium (>700 ppm) barium (>250 ppm) and vanadium (>20 ppm)¹. Alnö, Sweden. Width of sample 8 cm.

Albitite is a hydrothermal metasomatic rock that may superficially resemble marble, but it is composed of silicate feldspar-group mineral albite. Kiruna, Sweden. Width of sample 11 cm.

Marble is a metamorphic rock. However, mostly because of marketing reasons, some coarse-grained limestones (grainstones) and dolomites are also sold as marble varieties. The sample is from Estonia. Width of sample 10 cm.

Marble is extensively used as a dimension stone and it has been favored by architects since the antiquity. Marble, because it is composed of calcite, is a relatively soft material, which makes it very useful for sculptors. David by Michelangelo, one of the most famous sculptures of all time, is made of marble from Carrara. Lots of historical landmarks are made of it. Marble continues to be an important building material. Both pure white and impure varieties with decorative textures (flow textures develop easily in calcitic marble even at low temperatures) and colors are valuable. The latter are often imitated in modern background patterns. It has one major weakness. It is less durable than silicate rocks like granite and gabbro because of acids dissolved in rain that slowly destroy it. This is why marble is best used indoors where it is protected from the elements.

Marble is also quarried as a source of lime in areas where sedimentary carbonate rocks are lacking. Lime is an artificial material (CaO or Ca(OH)₂ in hydrated form) made by heating calcium carbonate at about 1000°C in a lime kiln. Lime is used mostly in the cement industry, but it has many other industrial uses as well. Crushed rock may be used to combat acidification of lakes, for example, or to raise the pH of soils.

References

1. Bowes, D. R. (1990). Marble. In: The Encyclopedia of Igneous and Metamorphic Petrology (Ed. Bowes, D. R.). Springer. 479–480.
2. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.
3. Bell, K. (2004). Carbonatite. In: Encyclopedia of Geology, Five Volume Set. Academic Press. 217-233.

Shale is a lithified mud — a sedimentary rock composed mostly of clay- and silt-sized grains. There are several ways to define shale. Some definitions are rather narrow. Glossary of Geology published by the American Geological Institute defines shale as a laminated, indurated rock with >67% clay-sized minerals¹. This definition clearly discriminates between shale and mudstone. The latter is a similar rock but without notable lamination. It also separates shale from siltstone, which is a mudstone in which the silt predominates over clay.

Shale is a fine-grained sedimentary rock that typically shows fine lamination. Finnmark, Norway. Width of sample 9 cm.

Sometimes, however, these rocks are treated as one big family of related and extremely widespread rocks which are collectively referred to as shale, mudrock, or mudstone. These rocks are definitely the most common sedimentary rocks in the crust. It has been estimated that more than half² of all the sedimentary rocks are various types of mudstones. They are followed by carbonate rocks and sandstones.

Shale is an economically important rock. It may be mined as a fossil fuel (oil shale), but even more importantly, it is a source rock of crude oil and natural gas. Shale is also the rock from which we are extracting hydrocarbons via the use of hydraulic fracturing (fracking).

Shale as a typical sedimentary rock is clearly layered and it may be folded by a later orogenic event. Shale outcrop from the northern Norway. Hammer for scale.

Mud covering a dry riverbed in La Palma, the Canary Islands. The main ingredient of shale is clay which is here on the way from the disintegrated rock higher in the mountains to the sea where it will be finally deposited. Rivers do the hard job of carrying all the mud to the ocean.

A shale outcrop in Scotland. Hammer for scale.

Pebbles of shale on the coast in Estonia. Shale is relatively easy to identify. It tends to produce flat pebbles with a dark-colored dull surface.

Shale and mudstones

A brief overview of rock names used to describe mudstones or rocks derived from them:

Mudstones rich in coarser silt tend to be lighter in color and do not show fine lamination typical to proper shale. This siltstone from the Spanish Pyrenees is part of a turbidite. Width of sample 12 cm.

Diamictite is poorly sorted rock with a muddy matric. This diamictite from northern Norway is of glaciogenic origin (tillite) from the Varangian glaciation. Width of sample 12 cm.

Turbidite is a sedimentary rock unit containing many such siltstone-mudstone pairs deposited on the seafloor during the same episode of gravity-driven subaqueous avalanche of mud. Turbidity sequence is typically composed of many alternating layers of sil and clay. The silt settles before clay which is why there are at least two distinctive layers deposited during the same event. The samples are from a single outcrop in Spain, but they were not next to each other there. The width of the samples is about 20 cm.

A shale (turbidite) outcrop in Loughshinny, Ireland.

Composition

Shale is so widespread because its main constituents (clay minerals) are very common at the surface. These minerals form as a result of chemical weathering — disintegration of rocks in wet/moist conditions. The minerals that yield clay are various silicates which predominate in the igneous and metamorphic rocks. The most important clay minerals are kaolinite, smectite (montmorillonite) and illite. The first two are common in younger shales. Illite tends to dominate in older (Paleozoic) shales because burial leads to the illitization process which converts smectite to illite.

Mud is a mixture of water, clay and silt (sand). Therefore mudstones also contain various amounts of silt (grain size 2-63 micrometers) and sand in addition to clay minerals. If silt dominates, the rock is usually named siltstone. Silt is mostly composed of mineral quartz, but it may also contain feldspar group minerals and other rock-formers, including heavy minerals.

Important constituents in mudstones may be carbonate or siliceous grains. Both are usually biogenic in origin. Muddy sediments containing lots of these constituents are named marl and sarl, respectively (marlstone, sarlstone if lithified).

Pyrite is a common mineral in mudstones that formed in reducing conditions. Note the greenish color which also is an indication that free oxygen was not available during the diagenesis. Elba, Italy. Width of sample 22 cm. TUG 1608-6763.

Black shale containing euhedral cubes of pyrite and veins of quartz. Width of sample 8 cm. TUG 1608-2799.

A shale with white calcite veins. These veins are post-depositional (formed in the rock later). Loughshinny, Ireland. Width of sample 10 cm.

Organic matter

A very important component of many shales is carbonaceous material. This is organic matter usually occurring in the rocks as kerogen (a mixture of organic compounds with high molecular weight). Although kerogen does not form more than about 1% of all the shales, the vast majority of kerogen is in mudstones. Shales that are rich in organic matter (>5%) are known as black shales. Black color is given to these rocks by organic matter. Organic matter should be decomposed in normal conditions by bacteria, but high productivity, rapid deposition and burial or lack of oxygen may preserve it. Pyrite is a common sulfide mineral in black shales. Organic matter and pyrite occur together in the same rock because both need oxygen-free conditions for their formation.

Some shales that are especially rich in organic matter are known as oil shales. They yield hydrocarbons on distillation. Oil shale may be used as a fossil fuel, although it is relatively “dirty” fuel because it usually contains lots of unwanted (not burning) minerals. And because of the aforementioned pyrite that causes environmental damage after decomposing to sulfuric acid at the surface.

Black shale is a variety of shale containing lots of organic matter that gives it a black color. These rocks are rich in pyrite and several metals like vanadium, uranium, etc. They have been mined in the past as a source of uranium. Black shale in Estonia.

An outcrop of black shale in Estonia.

Formation of shale

Clay minerals that were formed by the disintegration of silicate minerals are usually carried away from their formation place by a running water. They will be settled when fluid turbulence caused by currents and waves is no longer able to counteract the force of gravity. Clay minerals are small enough to be carried in the suspension for a long time. They will be settled after forming larger aggregates either because of flocculation or because of biological activity (filter-feeding organisms that excrete fecal pellets containing mud).

Most clay minerals ultimately make it to the ocean where they are finally deposited on the shelf and continental slope. These water-rich sediments on the gentle continental slope are gravitationally unstable. Some trigger mechanisms like an earthquake, tsunami or simply the weight of the overlying sediments may unleash huge and rapidly moving sediment-laden density currents moving down the slope. These flows are known as turbidity currents and the sediment so formed as turbidite. Turbidite is often composed of alternating silt- and clay-rich layers which form because silt tends to settle more rapidly and before the clay, while clay-rich layers form after that and are thicker in more distal parts of the turbidite sequence. Many such layers may follow each other, forming a thick marine sedimentary unit.

Deposited mudstone contains disoriented clay aggregates which creates lots of pore space that is filled with water. As more sediments accumulate, the weight of the overlying sediments causes compaction — clay aggregates take preferred orientation perpendicular to the stress direction, pore space is reduced, and water is pressed out of the rocks. As the temperature and pressure increase, changes in mineralogy will commence. This is not metamorphism, though. These changes take place at relatively shallow depths and moderate temperatures and the process is called diagenesis. There is, of course, no sharp boundary between diagenesis and metamorphism. In many cases it may be next to impossible to tell for sure whether the particulate rock is still sedimentary or already metamorphic. Pelitic rocks in hand sample are usually considered to be metamorphic when they demonstrate clear slaty cleavage and have a more reflective surface due to larger mica flakes grown at the expense of former clay minerals.

Heavily folded turbidite at Loughshinny, Ireland.

Green color indicates reducing conditions of formation. Finnmark, Norway. Width of sample 19 cm.

Sole markings are common features (casts) on the lower surfaces of shale layers. They can be used to show the way up and paleocurrent directions.

Folded shale outcrop. Finnmark, Norway.

Diagenesis and hydrocarbons

The process of illitization (smectite is transformed to illite) is a major change that takes place in mudstones during the diagenesis. Illitization consumes potassium (provided usually by detrital K-feldspar) and liberates iron, magnesium and calcium, which can be used by the other forming minerals like chlorite and calcite. The temperature range of illitization is about 50-100°C³. Kaolinite content also decreases with increased burial depth. Kaolinite forms in hot and humid climate. The drier temperate climate tends to favor smectite. The reason is that lots of precipitation washes soluble ions out of the rock, while drier climate does not accomplish this task so effectively. Kaolinite is favored in humid climate because it contains only aluminum in addition to silica and water. Aluminum is highly residual while the constituents of smectite (magnesium and calcium, in addition to aluminum and iron) get carried away more easily.

Another major and economically very important process that takes place during diagenesis (sometimes this stage is referred to as catagenesis) is the maturation of kerogen into hydrocarbons. Kerogen is a waxy substance trapped in the rock, but it will mature into lighter hydrocarbons that are able to move out of the shale and migrate upwards. This process can take place at temperatures between about 50-150°C⁴ (oil window). This corresponds usually to 2-4 kilometers of burial depth. Lighter hydrocarbons liberated during the processes (known as catalytic and thermal cracking) are now free to migrate upwards. They can form exploitable oil and gas reservoirs if stopped by some sort of structural trap which may be an anticline or a fault boundary. The rock layer that stops the upward movement is in many cases another layer of shale because compacted shale is a tough barrier for liquids and gas. Shale can also form an aquiclude between water-bearing layers for the same reason — it does not allow water to flow easily through the rock (has low permeability).

This is also the reason why some of the formed hydrocarbons are not able to migrate out of the source rocks. This resource is still at least partly available to us if we drill holes and inject pressurized water into the rock which will cause it to fracture. This method is known as hydraulic fracturing (fracking). Cracks formed will be kept open by the sand-grains injected with the water and hydrocarbons trapped in the rocks will become recoverable. Fracturing actually is a common process in the crust. Mineral veins and dikes are cracks in the crust opened and sealed by a highly pressurized fluid or magma.

An oil shale (variery kukersite) from Estonia continues to be used as a fossil fuel and raw material for the shale oil industry. The rock is very rich in fossils (bryozoans, trilobites, brachiopods). Kukersite is weakly laminated.

Kerogen-rich shale from Russia. Width of sample 10 cm.

Pictures of related rocks

These are slate slabs. Although shale also demonstrates fissility, it does not break into such thin sheets of hard rock and it is clearly duller in appearance.

This is a muddy limestone in which light-colored carbonate-rich layers are alternating with siliciclastic (muddy) layers. Biri, Oppland, Norway. Width of sample 9 cm.

Sandstone also does not need to be pure quartz. It often contains appreciable amounts of clay which may be converted to mica and chlorite during the diagenesis associated with burial. Width of sample 18 cm.

A metamorphosed siliciclastic sedimentary rock now composed of metamorphosed sandstone (quartzite) with a layer of metamorphosed mud (slate).

Metapelite is a metamorphic rock which has a mudstone protolith. Staurolite schist is a metapelite. There is a cross-twinned staurolite porphyroblast in the foreground. Tohmajärvi, Finland. Width of sample 19 cm.

Slate is a metamorphosed shale. It has a slaty cleavage (tendency to break into thin sheets of rocks).

Mica schist is a metapelite — a metamorphosed clay-rich sedimentary rock. Red crystals are almandine garnet porphyroblasts. Narvik, Norway. Width of sample 14 cm.

A metamorphosed mudrock clearly containing both clay- and quartz-rich material. Width of sample 14 cm.

References

1. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.
2. Davis, Joseph R. (2007). Shale. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 16. 383-386.
3. Nesse, William D. (2011). Introduction to Mineralogy, 2nd Edition. Oxford University Press.
4. Robb, L. (2005). Introduction to Ore-Forming Processes. Blackwell Science Ltd.

Diamictite is a poorly sorted or non-sorted terrigenous non-calcareous sedimentary rock that contains variously sized clasts from clay to boulders in a muddy matrix. Diamicton (or diamict) is a non-lithified diamictite (sediment). The definition above was proposed by Flint et al. in 1960¹ and it was meant to be and still is purely descriptive without any genetic connotations.

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This rock is sedimentary, it contains larger clasts in a fine-grained matrix-supported groundmass. The material is terrigenous and non-calcareous. It clearly fulfils every criteria of diamictite and it almost certainly is of glaciogenic origin. It is a tillite from the Varangian glaciation. Finnmark, Norway.

There is a need for a descriptive rock definition in this case because the rocks that superficially look alike may form in different ways. In many cases diamictite is a glaciogenic rock tillite. Sometimes diamictite is erroneously considered to be a synonym of tillite. But it may also be a lithified lahar (volcanic mudflow), volcanic flank collapse breccia, it may form underwater as a part of a turbidite flow, it may be composed of dropstones in marine sediments, etc. These are all lumped together under an umbrella term diamictite.

That, however, creates a problem because modern geology strives to explain how rocks and sediments form(ed). It is not enough if we simply describe them. From that point of view “diamictite” does not fit well into contemporary science. Every time someone uses the term, the question of genesis immediately arises. “Diamictite” has its role in the field, though. It continues to be useful as a preliminary term to describe rocks with a certain appearance.

The majority of rocks named so seem to be glaciogenic. Glaciers carry all sorts of material from clay to large rocks scraped from the ground as the ice advances. There are no mechanisms to sort them according to size. As ice melts, all this non-sorted debris is left behind as a sediment known as till. Once till is buried and lithified, it becomes sedimentary rock tillite, which may be described by scientists as diamictite if they are not certain about its origin.

Sometimes there are large boulders in fine marine sedimentary rocks. These rocks may be dropstones that fell vertically through the water column and were released by melting icebergs or perhaps even ejected from volcanoes as volcanic bombs. The term tillite clearly does not apply in this case. Ancient lithified mudflows are another way to produce rocks that can be described as a matrix-supported (clasts are generally isolated from each other by a muddy matrix) mixture of variously sized material. “Diamictite” that is free of any genetic connotations is a handy way to name these rocks before we know more about them.

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Varangian diamictite (tillite) with a large boulder. Finnmark, Norway.

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Lithified tillite near the Vatnajökull glacier in Iceland. Width of sample 30 cm.

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Glacial dropstones in a fine-grained marine sedimentary rock from Morocco. Clasts are over 10 cm in diameter.

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An outcrop of diamictite in Morocco with glacial dropstones.

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A sample from a lithified Carboniferous lahar deposit from Ireland. Width of sample 9 cm.

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Volcanic debris avalanche that forms when gravitationally unstable flank of a steep volcanic edifice partially collapses is a non-sorted mixture of variously sized clasts in a muddy matrix. This outcrop is in St. Lucia (the Lesser Antilles).

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Diamictite (tillite) from Norway that funnily resembles dead fish. Varanger Peninsula. Width of sample 40 cm.

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A sample of diamictite carrying clasts of igneous and metamorphic rocks. Varanger Peninsula, Norway. Width of sample 16 cm.

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Diamictite or a matrix-supported breccia from Tenerife. This mixture is located in the Las Cañadas caldera and probably is a result of the formation process of the caldera. Width of view 1.7 meters.

References

1. Flint, R. F. (1960). Diamictite, a substitute term for symmictite. Geological Society of America. Bulletin, volume 71, page 1809.

Pyroxenite is an ultramafic plutonic igneous rock. Ultramafic means that more than 90% of the rock is composed of magnesium- and iron-rich minerals like pyroxenes, amphiboles, and olivine. In pyroxenite the dominant mafic mineral is a pyroxene. Pyroxenite may contain up to 40% olivine. More than that means that the rock is peridotite. If felsic minerals like feldspars constitute more than 10% of the rock, it is melanocratic gabbro. Hornblendite is a similar igneous rock, but it is composed of amphiboles instead of pyroxenes.

Pyroxenite is uncommon at the surface because it occurs predominantly deep in the crust or in the mantle and it is susceptible to both metasomatic alteration and weathering. It usually occurs together in the same igneous complex with peridotite. Pyroxenite mostly forms by accumulation of pyroxene crystals in the mafic-ultramafic igneous intrusions. It forms variously shaped igneous bodies like sills, layers, dikes, etc.

Pyroxenite is a common rock in the lower part of the oceanic crust, where it is associated with depleted peridotite. “Depleted” means that peridotite has gone through partial melting to yield basaltic melt, which forms the upper part of the oceanic crust (sheeted dikes and pillow lavas). Depleted peridotite is rich in orthopyroxene (harzburgite) and associated pyroxenites are usually either orthopyroxenites or olivine orthopyroxenites. Pyroxenite may occur as xenoliths in volcanic rocks.

Metamorphic rocks with a pyroxenite composition should be named differently. These rocks are usually high-grade granulites (granofelses). The rock name pyroxenite should be reserved to true igneous rocks to avoid unwanted confusion and ambiguity.

Pyroxenite and peridotite may host economically important metals like chromium, nickel, platinum, iridium, osmium, etc. Serpentine and talc are mined from metamorphosed ultramafic rocks.

http://picasaweb.google.com/107509377372007544953/2015#6207651330236112226
A classification diagram of ultramafic rocks. Pyroxenite is yellow, peridotite white. Pyroxenite is divided into sub-types: orthopyroxenite (>90% orthopyroxene), clinopyroxenite (>90% clinopyroxene), websterite (both clino- and orthopyroxene in significant amount (>10%) but less than 10% olivine), olivine orthopyroxenite (orthopyroxene, less than 10% clinopyroxene, 10-40% olivine), olivine clinopyroxenite (clinopyroxene, less than 10% orthopyroxene, 10-40% olivine), olivine websterite (less than 40% olivine, both clino- and orthopyroxene more than 10%). Only olivine and pyroxenes are included in these calculations. All other constituent minerals are neglected only hornblende is included if it forms a significant part of the rock¹.

http://picasaweb.google.com/107509377372007544953/Rocks#5873740852688104978
Augite is a common (monoclinic) member of the pyroxene group that occurs frequently in pyroxenites. Width of sample 35 mm.

http://picasaweb.google.com/107509377372007544953/2015#6196128022954185378
Orthopyroxenes are also very common in pyroxenites. They may be recognizable by brown sub-metallic luster (crystals in the lower left), but in many cases orthopyroxenes and clinopyroxenes are indistinguishable in hand sample. Kemi, Finland (host rock of chromite). Width of sample 12 cm.

http://picasaweb.google.com/107509377372007544953/2015#6191004279438416274
Ultramafic rocks are usually dark-colored. They may be yellowish or greenish when altered or weathered. An outcrop of peridotite-pyroxenite. Hullvann, Norway.

http://picasaweb.google.com/107509377372007544953/Tenerife#5846710708669175778
Xenolith of pyroxenite in trachytic host rock. Width of the xenolith from La Palma is 7 cm.

http://picasaweb.google.com/107509377372007544953/2015#6196127170749211522
Olivine orthopyroxenite. Olivine is a common constituent in pyroxenite. The boundary between pyroxenite and peridotite is arbitrarily set at 40% olivine. Tappeluft, Norway. Width of sample 17 cm.

http://picasaweb.google.com/107509377372007544953/2015#6207676614699659602
Pyroxenite pebble on the southern coast of La Palma, Canary Islands. Width of sample 6 cm.

http://picasaweb.google.com/107509377372007544953/2015#6196127851132538578
Olivine orthopyroxenite. Tappeluft, Norway. Width of sample 16 cm.

http://picasaweb.google.com/107509377372007544953/2015#6190951161905456610
Pyroxenite does not need to be black or dull-colored. Sometimes it may be colorful when it contains garnet and pyroxene is bright green chromian diopside. Pyroxenite often forms cumulate layers in ultramafic intrusions. Yellow mineral is olivine and the rock is peridotite. Lower pyroxenite layer is about 5 cm thick. Åheim, Norway.

http://picasaweb.google.com/107509377372007544953/Rocks#5842261460204530978
Chromite is a common accessory mineral in ultramafic rocks, including pyroxenite. These chromite crystals come from pegmatitic pyroxenite from the Rustenburg mine, Western Bushveld, South Africa. Width of view 5 mm.

http://picasaweb.google.com/107509377372007544953/Peridotite#5779439247917903922
Pyroxenitic rocks get easily altered (metamorphosed) to serpentinite. Troodos ophiolite, Cyprus. Width of sample 11 cm.

http://picasaweb.google.com/107509377372007544953/2015#6196127849628250690
This rock is composed of pyroxene group mineral hedenbergite with serpentine. Is it partly serpentinized pyroxenite? No, it is not. Both serpentine and pyroxene here are part of a skarn. This is a contact-metasomatic rock that forms when carbonate rocks react with hot fluids from igneous intrusions. Serpentine as a Mg-rich mineral suggests that the carbonate country rock was probably dolomite. Tapuli, Sweden. Width of sample 13 cm.

http://picasaweb.google.com/107509377372007544953/2015#6207676612323331522
Pyroxene (mostly orthopyroxene here, see brown reflections from the cleavage surfaces) is a common mineral in high-grade metamorphic rocks of granulite facies (a red mineral is garnet). Pyroxene is common in these rocks because it is anhydrous. Sometimes such rocks are named pyroxenites. This is what they are purely compositionally but this practice is not recommended. It goes against the logic of rock classifications that aim, if possible, to sort rocks into different types by their genesis, not by their look or composition. There are some reasonable exceptions, but naming metamorphic rocks according to igneous nomenclature is anything but reasonable. This rock is garnet-orthopyroxene granofels or granulite although the latter term is problematic too because of associations with metamorphic facies of the same name. Holsnøy, Norway. Width of sample 13 cm.

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.

Carbonatite is an igneous rock composed of at least 50% carbonate minerals¹. Most carbonatite occurrences are intrusive rocks but some are volcanic. This is a deviation from a common practice in geology. Gabbro and basalt, granite and rhyolite, and many other such pairs are compositionally equivalent but named differently because the former is intrusive and the latter volcanic. Why is there an exception for carbonatites? Probably for two reasons.

Firstly, carbonatites are exotic rocks. Their composition is truly unusual. The vast majority of igneous rocks are silicic. Carbonate igneous rocks are grouped together because they stand out. Another reason may be the relative scarcity of these rocks. Carbonatites were once considered to be very rare oddities. This is not anymore the case. We have discovered more than 500 occurrences worldwide from different tectonic settings⁶. Carbonatites even occur on oceanic islands. But they continue to be somewhat enigmatic rocks because their formation mechanisms are still a matter of debate and it even took some considerable time until scientists were certain that carbonatites are really true igneous rocks. Evidence supporting this claim is overwhelmingly strong. We have seen modern flowing carbonatitic lava (Ol Doinyo Lengai volcano in Tanzania), have found Pele’s tears, chilled margins, dikes, plutons, etc.

Classification

The dominant carbonate mineral in most carbonatites is calcite. These carbonatites are named calcite-carbonatites. If plutonic, calcitic carbonatite is named sövite (sovite) and alvikite when volcanic. Dolomite-carbonatites (also known as beforsite) and ferrocarbonatites (ankerite or siderite is the Fe-bearing carbonate mineral) are less abundant types of carbonatites and natrocarbonatite is very rare – Ol Doinyo Lengai volcano is erupting natrocarbonatitic lava. This lava is composed of minerals nyerereite and gregoryite — very rare sodium-, potassium-, and calcium-bearing carbonate minerals. Igneous rocks that contain less than 50% (but more than 10%) of carbonate minerals should be named carbonatitic ijolite, nephelinite or whatever the dominant rock type happens to be. Igneous rocks with less than 10% carbonate minerals are carbonate- or calcite-bearing rocks.

Another way to classify carbonatites is to use the bulk chemical composition of the rock. Calciocarbonatite, magnesiocarbonatite and ferrocarbonatite are the three most common types of carbonatites according to this scheme. Calciocarbonatite has to be at least 80% pure CaO (in a ternary plot of Ca-Mg-Fe). If the rock contains more than 20% magnesium (MgO) and iron (with manganese) (Feo + Fe₂O₃ + MnO) then it is either magnesiocarbonatite or ferrocarbonatite (depending on which of them is more abundant). Silicocarbonatite is a carbonatite that is rich in silicate minerals (SiO₂ > 20%). Chemical classification is used if the composition is very complex or if the rock is too fine-grained for the mineralogical composition to be accurately determined⁵.

Composition

Major minerals of carbonatitic igneous rocks are various carbonates (calcite, dolomite, ankerite, siderite, bastnäsite, nyerereite, gregoryite, burbankite, parisite).

Other minerals that frequently occur in carbonatite are phosphates apatite and monazite; halides fluorite, sylvite, and halite; oxides pyrochlore, magnetite, perovskite, baddeleyite, hematite, ilmenite; sulfides pyrite, pyrrhotite, galena, and sphalerite; and silicates olivine, micas (phlogopite, vermiculite), pyroxenes (diopside, aegirine-augite), and amphiboles.

Average chemical composition of major (in weight percents) and minor elements Li-Gd (in parts per million) of four major carbonatite types⁶. Only these minor elements are shown which exceed 100 ppm in at least one carbonatite type.

Natrocarbonatite is markedly different from the other types and is usually considered to be a very special case rarely encountered in nature. Other carbonatite types are geochemically more similar to each other.

http://picasaweb.google.com/107509377372007544953/2015#6190953270634074706
Carbonatite may strongly resemble marble, but chemical analysis will easily solve the question. Carbonatites are all rich in light rare earth elements (>500 ppm). They also have elevated concentration of strontium (>700 ppm), barium (>250 ppm) and vanadium (>20 ppm)⁶. Calcite-carbonatite (sövite) from Alnö Island, Sweden. Width of sample 8 cm.

Occurrence

Carbonatite occurrences are small, but they are widely distributed. Regions that stand out with many carbonatite locations are Fennoscandia, Quebec (and other provinces of Canada), Eastern Africa, Southern Africa (almost half of all carbonatite occurrences are in Africa), India, etc. Oceanic islands with carbonatite occurrences are Fuerteventura (the Canary Islands) and Santiago (Cape Verde). Carbonatites have been found from all continents, including Antarctica. They were once considered to be a result of continental rifting. Now we know that only about half of them are indeed associated with rifting-related magmatic processes. Others are related to large faults and magmatic upwellings.

Carbonatite, although volumetrically insignificant, is important rock type scientifically. There are perhaps several different mechanisms that can lead to the formation of these rocks, but one of the most seriously considered mechanisms is a very low degree (<0.01%) of partial melting of parent rocks. Carbonatite occurrences are sensitive indicators of thermal instabilities in the mantle. They may mark the locations of deep mantle plumes, astenospheric upwellings, crustal delamination, etc⁶. Carbonatites also provide valuable information about the composition of the mantle because these rocks almost certainly originate from below the crust.

It is interesting that almost all carbonatite bodies are within or found next to silicic rocks with unusually alkaline chemical composition. Carbonatite body together with associated alkaline rocks is known as carbonatite complex. Rocks that are commonly associated with carbonatites are nephelinite, phonolite, nepheline syenite, ijolite, melilitite, pyroxenite, syenite, lamprophyres, etc. These are mostly silicic rocks that are unusually rich in alkali metals and relatively poor in silica. They do not contain quartz because most of the available silica is consumed by feldspars and some (if not most) by feldspathoids (nepheline dominantly) because even feldspars have too high silica to alkali metals ratio for these magmas. The association of highly alkaline magmas and carbonatites that repeats itself over and over in many localities means that the mechanism behind their origin has to enable the formation of both rock types.

http://picasaweb.google.com/107509377372007544953/2015#6190953261483521282
Carbonatites seem to be associated with alkaline igneous rocks. The association of carbonatite with such rocks is known as carbonatite complex. This rock sample is ijolite. It is composed of alkali pyroxene (aegirine-augite or aegirine) and nepheline (red). Alnö island, Sweden. Width of sample 5 cm.

Viscosity

The main physical property that separates carbonatitic magma from more common silicic magma is viscosity. Silicic magmas are highly viscous and the more silica they contain, the more viscous they get. Carbonatitic magma, however, is an ionic liquid. There is no polymerization between silica tetrahedra because they are almost absent. Carbonatitic magma may therefore travel in the crust very rapidly (tens of meters per second). The flow of carbonatitic lava is also very rapid and can easily engulf people trying to run away from it. Something like that could not happen with common silicic lava.

Field relations

In terms of field relations carbonatites are pretty usual magmatic rocks. They occur in plutons, dikes, sills, cone sheets and sometimes as volcanic rocks (both pyroclastic tuffs and lavas exist). Most of them are plutonic. About 40 are extrusive and nine of them lavas⁶.

http://picasaweb.google.com/107509377372007544953/2015#6196128164402845010
Calcite-carbonatite from Alnö, Sweden. Alnö carbonatites occur in linear dikes and they are found together with highly alkaline rocks (ijolite). Width of sample 13 cm.

Ages

Carbonatite magmatism is not a new phenomenon. Oldest dated carbonatite rocks are from the Archaean. Tupertalik (3.0 Ga) in Greenland is the oldest. The youngest are modern lavas from the Ol Doinyo Lengai. Carbonatite magmatism seems to become more intense towards recent times. It may be a real trend which reflects the much lower geothermal gradient now which may be needed for a very low degree of partial melting. On the other hand, large part of carbonatite occurrences (especially extrusive carbonatites) may have been lost because they are not among the most easily preserved rock types.

Uses

Carbonatite is the principal source of niobium which comes from the mineral pyrochlore (mined in Brazil and Canada — Araxá, Tapira, and St. Honoré mines). Carbonatites also contain lots or rare earth elements. They can be extracted from primary magmatic minerals or from minerals deposited from hydrothermal fluids. The most important REE-bearing minerals are bastnäsite, monazite, britholite, burbankite, parisite, and synchesite. Carbonatites are also important source of phosphates because apatite is a very common primary magmatic mineral in carbonatites. Notable apatite mines are located in Kola peninsula (Russia) and Siilinjärvi (Finland). Phosphates may be very abundant in carbonatites which explains why more money is made from this commodity than from any other mineral found in carbonatite. Carbonatite may be also mined as a source of fluorine (mineral fluorite deposited from hydrothermal solutions). Some carbonatites contain appreciable amount of thorium and uranium (minerals monazite and thorite). Titanium may be extracted from anatase and perovskite and carbonatite from Uzbekistan is known to contain very small diamonds. This discovery may be more important scientifically than economically because diamonds need very high pressure to form which supports the theory that carbonatites originate from the mantle and were subjected to considerable compressive forces. Vermiculite is also mined from carbonatitic rocks in Brazil (Ipanema).

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Rödbergite is a dolomite-calcite-carbonatite from the Fen complex that is reddish due to finely dispersed hematite, goethite and martite (hematite pseudomorphs after magnetite). The rock has been mined in the past (1652-1927) as an iron ore. In addition to iron the rock has elevated concentration of thorium and rare earth elements². Rödbergite seems to be a hydrothermally altered magnetite- and pyrite-bearing ankerite-ferrocarbonatite (rauhaugite) found nearby. It is estimated that about one million tons of ore were mined and current reserves are of the same order of magnitude³. This sample although clearly rusty in appearance is relatively poor in iron-bearing minerals. Massive and easily accessible iron ore has been largely removed during the centuries of mining which reached over hundred meters in depth. Rødberg, the Fen Complex, Norway. Width of sample 13 cm.
http://picasaweb.google.com/107509377372007544953/2015#6203917543890201858
An outcrop of rödbergite. The Fen Complex, Norway.
http://picasaweb.google.com/107509377372007544953/2015#6196126689122422210
Sövite from Siilinjärvi, Finland. Green mineral is fluor-apatite. Width of sample 18 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196126733935221778
Siilinjärvi carbonatite is mined as a source of phosphate (green fluor-apatite). Dark spot is Mg-biotite phlogopite. Siilinjärvi carbonatite is among the oldest (from the Archaean) known carbonatites in the world. Width of sample 19 cm. By the way, this sample resembles hedgehog for some. Find out that the association goes beyond mere visual resemblence: Hedgehog rock.
http://picasaweb.google.com/107509377372007544953/2015#6196126731385903378
Siilinjärvi carbonatite is largely a mixture of carbonatite-glimmerite (biotitite). The latter is an ultramafic rock composed of biotite (variety phlogopite in this case with alkali amphiboles richterite and riebeckite⁴). Width of sample 11 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196126720564367122
Glimmerite with green fluor-apatite crystals and calcite. This sample is not carbonatite because carbonate minerals clearly constitute less than half of the rock. It should be named carbonatitic biotitite⁵ (or carbonatitic glimmerite). Siilinjärvi, Finland. Width of sample 9 cm.

Origin

The question of how is such an unusual magma generated has been an intriguing problem for a long time and is still waiting for a satisfying explanation. There are three major hypotheses. Carbonatitic magma may be a direct partial melt of mantle rocks. In this case the source rock has to melt very slightly (<0.01%). Carbonatite may form as an immiscible blob of liquid. In this case alkaline and carbonatitic magma are like oil and water that do not form homogenous solutions. And finally carbonatitic magma may be generated as a result of extreme crystal fractionation. It seems likely that all three may be part of the explanation and even within one complex the formation mechanism may involve several of the aforementioned mechanisms. For example: it began with partial melting deep in the mantle. The melt migrated upward and separated into alkaline and carbonatitic melts and possibly had some already formed crystals removed from the melt through crystal fractionation. The exact mechanism is almost certainly unique to every specific carbonatite complex and it seems likely that carbonatites form in more than one way.

The source of the carbonatitic magma may be primary or recycled crustal rocks (subducted carbonate rocks). The current consensus based mostly on isotopic data seems to support the version that carbonatitic magma is primary (carbon dioxide-bearing mineral phases were present in the mantle since the formation of the Earth).

Fenitization

Hydrothermal fluids from carbonatitic and alkaline magma infiltrate the country rocks surrounding the complex and alter their composition. This metamorphic process is known as fenitization and the rock so formed as fenite. Minerals formed through this process involve sodic pyroxenes and amphiboles, feldspar, feldspathoids and calcite. Fenitization within the mantle may play an important role in the generation of several other exotic igneous rocks like kimberlites and lamproites.

http://picasaweb.google.com/107509377372007544953/2015#6196128222820392514
Country rocks surrounding the carbonatite complex are commonly metasomatically altered by the hot fluids from the cooling complex. Such rocks are known as fenite. This sample is composed of carbonatitic vein (dark-colored) and metasomatically altered migmatitic gneiss (now fenite) which is composed of alkali pyroxenes, alkali amphiboles, calcite, nepheline, alkali feldspar, titanite, fluorite, etc. Alnö, Sweden. Width of sample 10 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190951076526659554
Impure carbonatite with abundant alkaline silicate minerals (silicocarbonatite or fenitic carbonatite). Alnö, Sweden. Width of sample 11 cm.

References

1. Bell, Keith, ed. (1989). Carbonatites — Genesis and Evolution.
2. Bergstøl, Sveinung & Svinndal, Sverre. (1960). The carbonatite and per-alkaline rocks of the Fen area.
3. Andersen, Tom. (1983). lron ores in the Fen central complex, Telemark (S. Norway): Petrography, chemical evolution and conditions of equilibrium.
4. Puustinen, Kauko. (1971). Geology of the Siilinjärvi carbonatite complex, Eastern Finland.
5. 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.
6. Bell, K. (2004). Carbonatite. In: Encyclopedia of Geology, Five Volume Set. Academic Press. 217-233

Diorite is a plutonic igneous rock with intermediate composition between mafic and felsic rocks. It is visibly crystalline and usually has a granular texture (composed of roughly equally sized crystals) although the appearance may vary widely. Its volcanic (fine-grained) analogue is andesite.

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Diorite (red) on the QAPF diagram. Diorite in the wider sense (yellow) includes adjacent fields of quartz diorite, quartz monzodiorite, monzodiorite, foid-bearing monzodiorite (foid is a shorter way to say feldspathoid), foid-bearing diorite, foid monzodiorite, and foid diorite.

Diorite has a strict definition based on the QAPF diagram — more than 90% of feldspar is plagioclase, quartz content is lower than 5% and plagioclase contains more sodium than calcium. The latter distinguishes it from gabbro.

Main minerals are plagioclase (oligoclase, andesine) and hornblende. Plagioclase usually dominates over hornblende and other mafic minerals. It frequently contains smaller amounts of pyroxene (usually augite but also orthopyroxene), biotite, quartz, olivine, and magnetite. Accessory minerals include apatite, sphene (titanite), ilmenite, zircon, etc.

Similar rocks are gabbro (pyroxene-plagioclase rock, contains more mafic minerals and plagioclase is calcic), monzodiorite (contains more alkali feldspar) and quartz diorite (contains 5-20% quartz). Granodiorite and tonalite are granitoids that contain more than 20% quartz.

Average chemical composition, determined by 872 chemical analyses of dioritic rocks¹ (numbers are mass percents, recalculated volatile-free to total 100%):

SiO₂ — 58.34
TiO₂ — 0.96
Al₂O₃ — 16.92
Fe₂O₃ — 2.54
FeO — 4.99
MnO — 0.12
MgO — 3.77
CaO — 6.68
Na₂O — 3.59
K₂O — 1.79
P₂O₅ — 0.29

Diorite is usually associated with subduction zones. It may be a product of partial melting of mafic protolith, but it may also form as a result of mixing between mafic melt and felsic country rocks. It occurs in both continental and oceanic crust and is not a rare rock type, but it is significantly less common than granite and other granitoids. Diorite may form larger plutons, but it often forms smaller magma bodies like sills, dikes and stocks.

It is used as a dimension and building stone, but not very widely because of limited supply. Diorite is usually known by the name black granite (it is not the only rock type named so) although geologically they are different rocks.

Diorite may not be easy to distinguish from similar rocks. Amphibolite is a metamorphic rock with roughly the same composition. Leucocratic gabbro may look alike and also all kinds of altered mafic rocks may create lots of confusion. See the images below.

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Typical diorite has roughly equally sized black (mostly hornblende) and white (sodic plagioclase) crystals. Plagioclase usually dominates over mafic minerals. Width of sample 8 cm.
http://picasaweb.google.com/107509377372007544953/2015#6202054919611749762
Diorite inclusion in granodiorite. Sierra Nevada, California. Length of the inclusion 10 cm.
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Diorite with tonalite pegmatite. Hannukainen, Finland. Width of sample 9 cm.
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Sample from Hannukainen, Finland. Width of sample 8 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196128075221668946
Uralite porphyrite is compositionally similar rock. It is an altered andesite (volcanic analogue of diorite and hence has much finer grain size). Uralite is a name given to secondary hornblende (or actinolite) phenocrysts. Ylinen Savijärvi, Finland. Width of sample 9 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196127571937556130
This sample has some resemblance, but contains too much quartz. It is quartz diorite. Narvik, Norway. Width of sample 8 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196128031282680498
This rock sample contains even more quartz (gray mineral) which makes it granitoid (tonalite). Kaatiala, Finland. Width of sample 10 cm.

http://picasaweb.google.com/107509377372007544953/2015#6190952948549058306
Leucocratic norite (gabbroic rock where the mafic mineral is orthopyroxene) may be similar too. Rogaland, Norway. Width of sample 11 cm.

References

1. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.