Hawaiite is a volcanic rock that resembles basalt. It is generally somewhat lighter gray in color than basalt which usually is either black or dark gray. Hawaiite often contains olivine phenocrysts.
http://picasaweb.google.com/107509377372007544953/Hawaii#5869710172808014610 Hawaiite from Hawaii (Mauna Kea lava flow in NW part of the island few kilometers from Hapuna Bay). Width of the sample is 12 cm.
This rock type was originally defined by an American geologist Joseph Iddings in 1913 as an olivine basalt with andesine as the normative plagioclase1. This definition makes it different from ordinary basalt which has more calcic plagioclase in its composition. Sodic plagioclase tends to be lighter in color which explains why it is also lighter in color.
Modern definition of this rock type is based on the chemical composition as is the case with most other volcanic rock types. It is a sodic trachybasalt with Na2O – 2.0 ≥ K2O2. This definition may be somewhat cumbersome and alienating to many but it makes it possible to define the rock type precisely and unequivocally. However, in my experience, this term is relatively rarely used. “Trachybasalt” is preferred although it also seems to find use in scientific papers only.
The term “hawaiite” is obviously from Hawaii and the samples below are also from the Big Island of Hawai’i. These samples of the rock type shown below are from its type locality. Hawaiite in Hawaii typically erupts during late-stage volcanic activity3 which is fairly typical. Late-stage volcanic activity tends to be more alkaline (magma contains more sodium and potassium) not only in Hawaii but in other hot spot volcanic islands as well. Typical example of alkaline volcanism can be found in the Canary Islands.
http://picasaweb.google.com/107509377372007544953/Hawaii#5869706604519190354 Classification of volcanic rocks based on TAS diagram. Trachybasalt field annotated in yellow. Hawaiite is a sodic trachybasalt. Trachybasalts that are not hawaiite have no specific names. They are simply potassic trachybasalts.
http://picasaweb.google.com/107509377372007544953/Hawaii#5869716124905668482 A sample (10 cm across) resting on Hawaiian soil. Green mineral is olivine.
Bog iron is a type of iron ore composed of mostly mineral goethite and other iron-bearing minerals with impurities like clay or plant debris.
http://picasaweb.google.com/107509377372007544953/Rocks#5854471542847046882 Bog iron is typically rust-colored and composed of limonite. It is a mixture of iron-bearing minerals, most important of them is goethite. Width of sample from Estonia is 14 cm.
It used to be a very important ore of iron before better alternatives like banded iron formations were discovered. It has no industrial significance anymore but not because its iron content is low. Bog iron may in many cases be quite rich in iron. The main problem is that it forms only a thin layer of rust-colored and porous layer at the bottom of bogs. It simply is not economical to search for such thin and laterally widespread deposits.
Bog iron is associated with bogs because bog water contains very little oxygen and can therefore hold iron. Oxygen would quickly react with iron and precipitate it as a rusty ferric iron (with an oxidation number of +3) which is insoluble in water. Iron with an oxidation number of +2 (ferrous iron) is soluble and can be held in solution by bogs. Bog iron forms if something provides oxygen. It can be oxygenated ground water, for example1.
This mineral resource is associated with northern latitudes where peat bogs are widespread. Such areas are Scandinavia in Europe, and extensive areas in northern Russia and Canada.
http://picasaweb.google.com/107509377372007544953/Rocks#5854471532647116290 Bog iron is not massive. It contains lots of pores and its density is lower than one would assume from an iron ore. Width of sample from Estonia is 12 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5854471542965091826 Width of sample from Russia is 13 cm.
Quartzite is a metamorphic rock consisting largely or entirely of quartz1. In the vast majority of cases, it is a metamorphosed sandstone.
http://picasaweb.google.com/107509377372007544953/Rocks#5850032429587221954 Pure quartzite is grayish rock with only one dominating mineral — quartz. Width of sample 14 cm.
The transition from sandstone to quartzite is gradational. There is little mineralogical change. Quartz, the main constituent of sandstone, is not altered to other minerals if the sandstone is relatively pure, but it is recrystallized during the metamorphism. Original grainy look of sandstone is lost to a variable degree. Some quartzites are still similar to sandstones, only more strongly held together, while others are completely recrystallized so that all the features like fossils or original texture are obliterated and grain boundaries have disappeared. It is difficult to say where the boundary between these rocks exactly is. Rock sample that may be quartzite for one geologist might be strongly cemented sandstone for another.
Another common mineral in sandstone is iron oxide hematite. It gives reddish color to sandstones and it too is not altered during metamorphism. Most other minerals, however, can not tolerate high temperature and pressure as well. Hence, impure sandstones may give rise to quartzites that contain minerals like mica, sillimanite, kyanite (metamorphosed clay minerals), pyroxenes and amphiboles like diopside and tremolite (metamorphosed dolomite plus silica from sandstone), and even wollastonite (metamorphosed pure calcite cement with silica added) and many others. Quartz-rich muddy sediments usually metamorphose to schist because there is too much aluminum-rich clay minerals. Quartzite usually is the result of metamorphism if the source material is relatively pure sandstone.
Quartzite is a common metamorphic rock because its protolith sandstone is very widespread. Quartzite outcrops can be found in many mountain ranges all over the world. It usually is associated with current or former mountain ranges because mountain-building is the process that is responsible for the deep burial and associated metamorphism that transforms sandstone to quartzite. Quartzite is a very hard rock. It tolerates weathering well because its main constituent quartz is very resistant to both physical and chemical disintegration. These properties make quartzite a useful building stone which is used to build walls, stair steps and floors. It is used in road construction and as a railway ballast.
You may encounter terms like orthoquartzite and metaquartzite. The former is actually a very pure unmetamorphosed quartz-rich sandstone which is more commonly known as quartz arenite. Metaquartzite is more or less synonymous with quartzite. These terms are old-fashioned and little used nowadays2.
Pure quartzite from Telemark, Norway. Width of sample 9 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5850032472223480994 It is very tough and durable rock that contains no pores and unlike many other metamorphic rocks it is not fissile or foliated. Width of sample 11 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5850032467430058482 Some relict bedding may be preserved. Width of sample 11 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5850032432404882018 Quartzite may resemble marble at first. You may try to scratch the surface with a needle. If you can do it, then it is probably marble. Another and even better test is to use acid. Calcite, the main constituent of most marbles, reacts vigorously with dilute hydrochloric acid but there is no reaction with quartz. Width of sample 13 cm.
These slightly flattened clasts in conglomerate are made of quartzite. The rock formed in the Ordovician and it was once a riverbed. The outcrop is near Bergen in Norway.
Feldspar-rich quartzitic rock – metamorphosed arkose. Aust-Agder, Norway. Width of sample 19 cm.
Quartzite from Estonia from the Proterozoic (it formed roughly 1.4 billion years ago). Width of sample 4 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5850058499748028770 This dry cascading riverbed in Norway is composed of quartzite.
http://picasaweb.google.com/107509377372007544953/Rocks#5850032427024934130 Red color is usually given to the rock by iron oxide hematite. Width of sample 9 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5850032480050062514 It is a vein of quartz with pyrite cutting through quartzite. Width of sample 11 cm.
http://picasaweb.google.com/107509377372007544953/Sandstone#5781342252999777362 The sample from Finland is either strongly cemented sandstone or weakly metamorphosed quartzite. It is old (1.4 Ga) and a part of crystalline basement. Hence, it is usually considered to be a metamorphic rock. Width of the sample 4 cm.
http://picasaweb.google.com/107509377372007544953/Syenite#5830113476261131490 This wall in Western Ireland (Connemara) is built of local quartzite blocks.
http://picasaweb.google.com/107509377372007544953/Syenite#5830113478740527506 Wall in Santorini, Greece. This material is local as well. Santorini is not entirely volcanic island. The highest point of the island is a non-volcanic mountaintop of the Cyclades range.
Bauxite is an aluminum-rich sedimentary rock. It is the principal ore of aluminum. Aluminum in bauxite is hosted by aluminum hydroxide minerals, mostly gibbsite4. The major impurities are iron oxides and hydroxides (which give reddish color to most bauxites) and clay minerals. Bauxite is a weathering product of aluminum-bearing rocks (usually igneous rocks).
Bauxite is either pale pink, orange, or reddish rock, which often contains rounded concretionary masses of aluminum hydroxides and iron (hydr)oxides. These are known as pisoliths. They resemble ooids, but are generally less regular in shape, although their genesis is similar — there is a nucleus around which mineral matter started to accumulate. Width of sample 8 cm.
Bauxite is a result of intense leaching in a hot and humid climate with alternating wet and dry seasons and good downward drainage. The climate promotes vegetation that provides organic acids, which help to dissolve rocks in percolating water that carries more soluble components away, leaving only aluminum and also often iron as the least mobile common ions behind. This process is known as lateritization. Hence, bauxite is an aluminum-rich laterite. The accumulation of aluminum-rich residuum, as opposed to one enriched in iron, is a function of higher rainfall, but also lower average temperature (around 22 °C rather than 28 °C)6.
However, there are actually two types of bauxites. The genesis of only one of them can be adequately explained by the lateritization process. Lateritization usually means that aluminum-bearing igneous rocks (like granite) are slowly weathering to clay minerals, which are then altered to aluminum hydroxides by continuous leaching of silica. The other type is associated with limestones. This type of bauxite is known as karst bauxite because it accumulates in karst solution depressions within limestone plateaux. These depressions act as sediment traps where either eolian or fluvial aluminum-bearing sediments can accumulate to be later altered to bauxite. This material is bentonitic clay (weathered volcanic ash) in Jamaica, for example6. The chemical process of bauxite formation is similar in both cases (clay is leached to aluminum hydroxides) but the source material and the general geological situation of the two types are different.
The chemical reactions that constitute the lateritization process5:
2KAlSi3O8 + 2H+ + 9H2O = Al2Si2O5(OH)4 + 2K+ + 4H4SiO4 (K-feldspar dissolves in acidic groundwater to form clay (kaolinite) plus potassium and silicic acid dissolved in water and carried away.)
Then the process continues to take silicon from kaolinite and turn it to aluminum hydroxide:
Al2Si2O5(OH)4 + 5H2O = 2Al(OH)3 + 2H4SiO4
Bauxite deposits in southern Europe, Jamaica, Haiti, and Turkey are associated with karst. Extensive bauxite deposits in Guinea (largest in the world), Australia, Brazil, India, Surinam, and Indonesia are lateritic deposits. Lateritic bauxites constitute more than three-fourths of the world’s bauxite resources. The largest producer is Australia, which provides roughly one third (125 million metric tons annually) of the world’s total output1.
Bauxite is usually considered to be a rock type, but considerable amount of poorly consolidated material is often also named that way. Bauxitic or lateritic soil is scientifically known as oxisol2. Sometimes “bauxite” has been used as a mineralogical term. In this case it refers to mixed aluminum hydroxide minerals of uncertain identity, analogous to the terms “limonite” and “wad” for iron and manganese hydroxides, respectively3.
Bauxite deposits form close to the surface. Hence, all the mines in the world are open pit. About 8-14 tons of bauxite is needed to produce one ton of aluminum1. So the ore of aluminum needs to be fairly rich in this metal to be worth mining. Aluminum is extracted solely from bauxite, but that does not mean that there are no alternatives. Aluminum is the most widespread metal in the crust. That is why we can be so picky and choose only the best possible ore. Only two chemical elements (silicon and oxygen) are more abundant in the crust (here is a longer post about the composition of the Earth’s crust). Aluminum could be extracted from clay minerals or even feldspars. It is not currently being done because there are adequate deposits of bauxite and these deposits are mostly located in more or less normal countries which are not likely to play ridiculous political power games. One of the largest consumers of aluminum is the USA which has no metallurgical-grade bauxite mines. But this does not seem to cause much worry because of the reasons mentioned above.
A sample of bauxite.A bauxite sample from Hungary. Width of sample 9 cm.Bauxite from Greece. Width of sample 12 cm.Bauxite. Width of sample 14 cm.
Anthracite is a type of coal with a highest rank. Coal rank measures the carbon content which is in correlation with calorific value and metamorphic grade. It has a very high carbon (over 90%) and low volatile content (below 5%)1.
Anthracite is a shiny black rock. Width of sample from Ukraine is 8 cm.
Anthracite is the most desirable type of coal because it contains more energy than other types and it is also the most environmentally friendly of them because of purity. Unfortunately, only approximately 1% of all the coal is anthracite. So we also burn bituminous coal, lignite (brown coal), and even peat which is the source material of coal.
Anthracite, unlike other coal types which are sedimentary rocks, is a metamorphic rock. It was buried so deep that no plant remains have survived. I don’t want to say that coal contains easily recognizable plant remains but they do contain bands with fancy names like vitrain, clarain, etc. that represent different materials. Such bands are missing in anthracite which is more uniform and has a semimetallic luster.
Difference between anthracite and coals with a lower rank is obvious not only because of luster but also because it does not soil fingers. It has a smooth shiny black surface that may resemble obsidian but the material it not as dense and lacks the look of a glass.
Total world production of anthracite was 606 million metric tons in 2011 and it is rising fast. It was only 445 million tons just five years earlier. China is by far the largest producer, followed by Ukraine and Russia2.
http://picasaweb.google.com/107509377372007544953/Rocks#5846265491708838386 Bituminous coal has lower rank and calorific value but it is more common than anthracite. The sample is from the Donets Basin, Ukraine. Width of sample is 8 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5846265489370250434 Lignite or brown coal is even poorer than bituminous coal but it is still extensively mined. Width of the sample from Germany is 6 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5846265495055458738 Peat is the source material of all the other coal types shown above. It accumulates in wetlands and is composed of plant remains. It has a fairly low calorific value but it is still used as a fuel in many parts of the world, especially when pressed into briquettes. The sample from Estonia is 8 cm in width.
Diatomaceous earth or diatomite is a light-colored sedimentary rock composed chiefly of siliceous shells (frustules) of diatoms. Diatomaceous earth is a soft and friable rock. It leaves hands dusty if touched and has a fragile feel as if it has a delicate and light-weight internal structure. This feeling is not misleading. Diatomite is composed of many unicellular algae with a hollow opaline test. Diatomite often floats in water just like pumice, at least initially before the pores are filled with water. The porous nature gives it many useful properties. Diatomaceous earth is widely used in industry for many different applications.
http://picasaweb.google.com/107509377372007544953/Rocks#5843782176702339570 Pure diatomite is white as this sample from Armenia. Its width is 7 cm. Diatomite is friable and fragile very light-weight rock.
Diatomaceous earth is usually known as diatomite to geologists but for the general public diatomaceous earth is much more frequently used term. Geologically speaking, “diatomaceous earth” (often abbreviated DE or D.E.) is more appropriate term for unconsolidated sediment and diatomite for a consolidated rock. People who have experience with diatomaceous earth probably know it as a white powder, not as a rock type. However, this powder is directly manufactured from a naturally occurring rock.
There are more terms than diatomite and diatomaceous earth when referring to this material. German word kieselgur (or kieselguhr) has been used since the 19th century because first industrial scale mining of diatomaceous earth took place in Germany. This diatomite was used by Alfred Nobel as an absorbent and stabilizer for nitroglycerine. His invention which made the use of nitroglycerine much less hazardous is known as dynamite. It was the beginning of a large-scale diatomite use for industrial purposes although the material itself was known well before. Diatomite was used in antiquity by the Greeks as an abrasive and in making lightweight building brick and blocks. Blocks of diatomite were used for the dome of the Hagia Sophia church in Constantinople (now Istanbul in Turkey). This architectural wonder (its diameter is 30 meters) was much easier to construct with light-weight building blocks of diatomite1.
Another term that may be encountered is tripolite. Originally it referred to diatomaceous earth deposits near Tripoli (capital of Libya) but in many cases the term has been applied to deposits far away from Libya. Hence, tripolite is just a synonym of diatomite. An impure (up to 30% clay) Danish variety of diatomite is called moler1.
Diatomite deposits do exist in many places and are extensively mined nowadays because of its highly useful properties. The main producing countries are USA and China1.
Diatomaceous earth is composed of diatom frustules. Diatoms are a large group of (mostly) unicellular algae. They live in both oceans and lakes and are one of the most common types of phytoplankton. Majority of diatoms use sunlight to photosynthesize, so they are producers in the food chain. Their frustules are almost bilaterally symmetrical, hence the name (Greek diatomos means ‘cut in two’). They are not entirely symmetrical because one half of their frustule valves is slightly smaller than the other to fit inside the edge of it. Frustules are made of amorphous opaline silica (SiO2·nH2O). The majority of frustules are between 5 and 200 micrometers in diameter3.
The most important thing for geology is the fact that they build their frustules out of silica that is in solution in sea or lake water. This silica comes from weathered silicate rocks which are very abundant in the crust. Diatoms are not the only ones who use silica. Some sponges for example secrete silica to make internal spicules that look like three-armed Mercedes stars. However, these spicules are never abundant enough to provide material for a distinct sediment and rock type. Another matter is with radiolarians (amoeboid zooplankton) who also use silica. Radiolarian and diatom shells fall to the bottom of the water body after its inhabitants die and form there radiolarian (radiolarite as a rock type) and diatomaceous ooze, respectively.
Diatomite deposits are mostly geologically young. Diatoms themselves have become abundant only in the past 50 Ma2. Since the Miocene (it began 23 million years ago) diatoms have been major producers of lacustrine (related to lakes) sediments3. Most commercially exploited diatomaceous earth deposits are lacustrine as well although the largest producing deposit that outcrops near Lompoc in California is a marine deposit of Miocene age1.
http://picasaweb.google.com/107509377372007544953/Rocks#5844824539927221186 Impure diatomite from Georgia (country in the Caucasus). Common impurities in diatomaceous earth are clay minerals and iron oxides. Width of sample is 10 cm.
Diatoms can be really abundant if the supply of nutrients and dissolved silica are both high. Diatom blooms are often seasonal which can lead to varved sediments at the bottom of lakes. This is the case at some areas of Lake Malawi in Africa where diatom-rich layers form in dry windy season when turbulent mixing brings nutrients up to the surface but runoff from the land is at a minimum. Layers rich in silicate clastic material and organic debris form during rainy season3.
Most commercially exploited deposits of diatomaceous earth are lacustrine but diatoms are even more abundant or at least cover much larger areas of seafloor, especially at high latitudes. Most pelagic oozes are foraminiferal (47%). Siliceous ooze covers 15%. It is mostly diatomaceous ooze around Antarctica. Diatoms are also abundant in the extreme Northern Pacific north of the Aleutians. Radiolarians are common in equatorial waters. The rest of the abyssal plain (38%) is covered with abyssal brown clay. Sedimentation rate of diatomaceous ooze is 2-10 mm in thousand years4.
Siliceous oozes are a source material of siliceous rocks. Not only diatomite and radiolarite but also very hard cryptocrystalline siliceous rock known as chert or flint. Chert is of course very different material. It is dense and very hard without any pores at all but the material it is made of must be buried siliceous ooze. This is another reason why diatomaceous earth deposits tend to be young — older material is buried and compacted to form chert.
http://picasaweb.google.com/107509377372007544953/Rocks#5844824537583990146 Diatomite from Russia. Width of sample is 12 cm.
Diatomaceous earth is used as a filter in wine and beer industry among many others. It is added to toothpaste because of its mildly abrasive properties. Diatomite is used as an absorbent in dynamite as mentioned earlier. It is a filler in rubber and plastics. It is also used in cat litter because it absorbs moisture. Porosity makes it a good thermal insulator. Diatomaceous earth has also find use as a natural insecticide. This list is not exhaustive. Some clever chaps are even selling it as a food additive that is supposed to do a lot of amazing things to our health. Whether it is effective or not is mostly a matter of belief in my opinion.
Igneous rocks are rocks that are formed from melted rocks1. Igneous rocks are one of the three main classes of rocks. The others being sedimentary and metamorphic rocks.
http://picasaweb.google.com/107509377372007544953/Rocks#5842150396292384898 Igneous rocks from left to right: gabbro, andesite, pegmatite, basalt, pumice, porphyry, obsidian, granite, and tuff.
Molten rock material below the surface is called magma. Magma that has reached the surface is lava. Igneous rocks that solidified within the crust are plutonic (or intrusive) rocks. Those igneous rocks that solidified at or very near to the surface are volcanic rocks. If volcanic rocks form as a result of explosive volcanic eruptions, fragmental rocks known as pyroclastic rocks form. Non-explosive or less explosive eruptions result in lava flows, which form lava rocks when solidified.
Classification
Igneous rocks are mostly classified on the basis of their composition (either mineralogical or chemical), but there are a number of exceptions where classification is based on texture. Plutonic rocks (coarse-grained igneous rocks that solidified in the crust) are usually classified according to their mineralogical composition. Most important minerals upon which these classification schemes are based on are quartz, feldspars, pyroxene, olivine, hornblende, and feldspathoids.
The classification of most plutonic rocks is based on the QAPF diagram. Mafic and ultramafic plutonic rocks have their own classification diagrams because quartz and feldspars (cornerstones of the QAPF classification) are missing in these rocks. Pegmatite is a notable exception. Most pegmatites have a granitic composition, but the defining parameter is the grain size: pegmatites are exceptionally coarse-grained plutonic rocks.
Volcanic lava rocks are classified according to their chemistry because they are too fine-grained to make mineralogical classification practical and they often contain glass (rock material without crystalline structure) which further complicates the situation. Volcanic rock types like basalt, andesite, and phonolite have a strictly defined boundaries on the TAS diagram which is used to classify volcanic rocks.
Fragmental volcanic rocks are known as pyroclastic rocks. They are composed of material (ash, lapilli, bombs, blocks) that were thrown out of volcanic vents during violent explosive eruptions. They have their own classification principles which are based on the average grain size and relative proportions of the constituents.
Plutonic rocks
http://picasaweb.google.com/107509377372007544953/Chert#5808891582074096306 QAPF diagram for plutonic rocks is a double ternary plot. Q, A, P, and F correspond to quartz, alkali feldspar, plagioclase, and feldspathoids, respectively. Note that quartz and feldspathoids are mutually exclusive. One rock sample is not allowed to contain both of them. This is no arbitrary rule. These minerals indeed do not co-exist in one rock sample for chemical reasons. Feldspathoid-bearing rocks are relatively rare. Rocks that contain abundant quartz and feldspar are much more common. The most well-known example is granite.
http://picasaweb.google.com/107509377372007544953/Tenerife#5841862866360779090 Monzonite is a plutonic rock that is somewhat similar to granite, but it contains less quartz and more plagioclase than granite does (monzonite is in the middle on the QAPF diagram). Plutonic rocks are coarse-grained or at least visibly crystalline because they solidified deep underground where crystallization process was slow. The sample is from La Palma, Canary Islands. La Palma is a volcanic island, but some plutonic rocks like monzonite and gabbro crop out in small erosional windows. Width of sample is 6 cm.
Volcanic rocks
TAS diagram for volcanic rocks. This is purely chemical classification. It can be applied if the chemical composition of a rock sample is known. However, rock types shown above also have characteristic mineral assemblages and typical appearances which helps to give them provisional field names. These often turn out to be not entirely correct, but basalt is usually easily distinguishable from rhyolite, for example. As a general rule, rock types on the left tend to be darker and those on the right lighter in both color and weight.Rhyolite porphyry from Scotland with K-feldspar and quartz phenocrysts. Rhyolite is a volcanic rock with a granitic composition. Volcanic rocks are fine-grained (individual crystals are not visible to the unaided eye), but they frequently contain larger crystals (phenocrysts) embedded in fine-grained groundmass. These phenocrysts formed before tha magma was expelled from the volcanic vent. Width of sample 8 cm.
Pyroclastic rocks
Classification of pyroclastic rocks resembles classification principles of clastic sedimentary rocks. Ash is a volcanic analogue of sand. Lapilli corresponds to gravel and blocks and bombs are volcanic equivalents of boulders. The most important pyroclastic rock type is tuff (lithified volcanic ash, analogous to sandstone). These three diagrams are based on classification principles defined by the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks2.
Rocks that do not fit neatly into the system
Obsidian is composed almost entirely of glass. Hence, it has no mineralogical composition. Chemical composition can be determined, but the most striking and obvious feature of obsidian is its glassy nature. All such rocks regardless of their chemistry are therefore called obsidian or volcanic glass. Obsidian is usually lava rock, but some pyroclasts also have a glassy texture. There are glassy sand grains from Hawaii on the picture above. These small pieces of obsidian possess a basaltic composition. In majority of cases obsidian has a felsic composition. Width of view is 20 mm.Diabase is another example of a rock type that dares to challenge our classification schemes. It is compositionally equal to basalt and gabbro but texturally somewhere between them. Such rocks mostly formed in the crust but at relatively shallow levels close to the surface. Diabase and other such rocks are sometimes called hypabyssal rocks although this term seems to be somewhat old-fashioned nowadays. White crystals are plagioclase phenocrysts. The rock sample is from La Palma, Canary Islands. Width of sample is 5 cm.Scoria is a highly vesicular volcanic rock. It usually has a basaltic composition, but this is not a defining parameter. It just has to be very porous. Tenerife, Canary Islands. Width of sample is 7 cm.Carbonatite is a real stranger in the igneous world. It can be volcanic or plutonic, but the most striking thing is its composition. Unlike other igneous rocks which are mostly composed of silicate minerals, this one is compositionally close to limestone and marble, hence the name which refers to its carbonate composition. Such exotic igneous rocks have their very own classification principles. They are better kept separately from the rest or we risk to ruin our illusion that we understand something. Carbonatites are indeed still mysterious rocks. They are uncommon, but still occur frequently enough that ignoring is not an option. Carbonatite is not the only exotic igneous rock type. Such exotic rocks like lamproites, kimberlites, lamprophyres, etc. are for various reasons treated separately just like carbonatites. However, they are all volumetrically insignificant. They can be safely ignored if one just wishes to obtain a general overview of igneous rocks. The sample is from Germany.
Composition of igneous rocks
The range of chemical compositions of igneous rocks reflects the average bulk composition of the crust. The most important chemical elements are oxygen and silicon. Common igneous rocks comprise 40…77% of silica (SiO2). Other important oxides are alumina (Al2O3), magnesia (MgO), lime (CaO), soda (Na2O), and potash (K2O).
Average chemical composition of granitic and basaltic rocks based on 2485 and 3594 analysed rock samples, respectively3:
Rock type
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
Granite
71.84
0.31
14.43
1.22
1.65
0.05
0.72
1.85
3.71
4.10
0.12
Basalt
49.97
1.87
15.99
3.85
7.24
0.20
6.84
9.62
2.96
1.12
0.35
Numbers given in the table above are weight percents. Granite contains much more silicon and potassium than basalt does. Basalt is richer in iron, titanium and magnesium.
Real igneous rocks do not generally contain these chemical compounds (potash, lime, etc.). This is just a historical way to express the chemical composition of igneous rocks. We still do it that way although it has little logical justification anymore. In reality, igneous rocks are largely composed of minerals. Silica indeed occurs as a mineral quartz, but there are no magnesia or potash. Common Mg-bearing minerals in mafic igneous rocks are olivine and pyroxenes. Most important potassium-bearing minerals are K-feldspar and micas.
The chemical composition of rocks determines its mineralogical composition because the chemical elements are the building blocks of minerals. Granite is largely composed of quartz and K-feldspar because it contains so much silicon and potassium. Basalt contains lots of iron and magnesium which are used to make pyroxenes and olivine. Basalt contains much more calcium than granite does. Hence, dominant feldspar in basalt is Ca-bearing plagioclase, not K-bearing K-feldspar.
Igneous rocks also contain a number of less important or volumetrically less significant minerals which use up chemical components that are present in smaller quantities. Common accessory minerals in igneous rocks are apatite, titanite, magnetite, ilmenite, zircon, tourmaline, beryl, epidote, rutile, allanite, garnet, chromite, spinel, monazite, etc4.
Common igneous minerals
Quartz (W), muscovite mica (N), plagioclase (O), and microcline (S) are all very common minerals in igneous rocks. These crystals are unusually large for a normal igneous rock (each one roughly 10 cm across). They are from a pegmatitic granite. Evje, Norway. Many other examples below are also from pegmatitic rocks. I use them because their constituent minerals are large enough to be seen easily.
http://picasaweb.google.com/107509377372007544953/Pegmatite#5783233096591935202 This rock sample demonstrates that pegmatite does not need to be granitic in composition. Here is an example of pegmatitic gabbro which is composed of light-colored plagioclase and dark-colored pyroxene augite. Both plagioclase and pyroxenes are very common minerals in the crust. They are especially common in basaltic rocks.
http://picasaweb.google.com/107509377372007544953/Tenerife#5841862778873146786 Olivine is an important green mineral in mafic and ultramafic igneous rocks. Here is a sample of picrite from La Palma, Canary Islands. Width of sample is 5 cm.
Micas are important minerals in many igneous rocks, especially in those that have a felsic composition. They are easy to identify because of their characteristic appearance. Two of the most common mica minerals are brown biotite and greenish gray muscovite.Feldspathoid-bearing igneous rocks occupy the lower triangle of the QAPF diagram. The most important feldspathoid mineral is nepheline. Rock sample above is nepheline syenite (foyaite). Gray is alkali feldspar, nepheline is darker gray. Width of sample is 16 cm.
Igneous rocks in the field
http://picasaweb.google.com/107509377372007544953/Rocks#5842275035533497362 Igneous rock are formed from melted rocks. Hence, we first have to melt something. Here is an example of this process which is known as anatexis — partial melting of pre-existing rocks. Rocks that melt do so partially because their constituent minerals have different melting temperatures. Hence, source rocks and molten matrials extracted from them have different compositions. In this case you can see a light-colored melt and a dark-colored metamorphic residue. Picture taken in Western Norway. Rock that has formed that way is known as migmatite. Width of view is over one meter.
http://picasaweb.google.com/107509377372007544953/Ireland#5754567703961574658 Intrusive rock granite on the western coast of Ireland. I have also written about the biogenic white sand covering the beach there.
http://picasaweb.google.com/107509377372007544953/Rocks#5842255277221274978 Boulder on a beach on the NE coast of the Gulf of Finland in Karelia. This boulder comes from the Vyborg batholith that is largely composed of rapakivi granite.
Igneous rocks may form beautiful columns like these columns of the Giant’s Causeway on the northern coast of Northern Ireland. Columns form as cooling magma or lava contracts.
http://picasaweb.google.com/107509377372007544953/Rocks#5787194349045543586 Volcanic vents are fed with molten material by tabular sheets of magma which are known as dikes. Here is an outcrop in Cyprus that is entirely composed of sheeted dikes — one inside another. Such a sheeted dike complex is a common feature of the oceanic crust. Cyprus is one of the best locations in the world where rocks that once formed the oceanic crust are nicely exposed on the surface. Dikes are composed of basalt and diabase.
http://picasaweb.google.com/107509377372007544953/Tenerife#5828535171367209522 Pillow lava forms underwater. The outer crust of the lava is quenched in contact with cold seawater. The interior still fills with molten material to form a pillow-like morphology. Fresh seafloor is covered with such pillows. This outcrop of pillow basalt is in La Palma. It formed when the island was still only a seamount beneath the waves. It is exposed because of uplift and deep erosional scar known as the Caldera de Taburiente.
http://picasaweb.google.com/107509377372007544953/Tenerife#5820695530161357554 Igneous rocks that solidified on the surface often form lava flows. An edge of a lava flow in La Palma that looks like a huge pile of rocks.
Sometimes lava flows have a smooth continuous surface. Such lava is called pahoehoe.
http://picasaweb.google.com/107509377372007544953/Tenerife#5835569151026130866 Pyroclastic density current is the most deadly expression of volcanism. These rocks in Tenerife were deposited by a fiery cloud of ash, rocks, and gas that cascaded down the flanks of a volcano and incinerated everything in its path. Width of view is 0.8 meters.
http://picasaweb.google.com/107509377372007544953/Tenerife#5832686321337998642 Sequence of pyroclastic rocks 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). Width of view is 12 meters.
http://picasaweb.google.com/107509377372007544953/Tenerife#5840801031484486946 Large pyroclastic rocks (>64 mm) are known as bombs and blocks. Volcanic bomb resting on smaller lapilli-sized pyroclasts on the crest of Cumbre Vieja in La Palma. Width of sample is 15 cm (longer dimension).
Mineral resources associated with igneous rocks
Igneous rocks are an important sources of many metals and diamonds and they make an excellent ornamental and building stone.
Felsic coarse-grained pegmatites often contain interesting minerals which contain lots of relatively rare chemical elements. Tin, fluorine, tungsten, zinc, thorium, lithium, beryllium, etc. may be extracted from felsic pegmatites. Ultramafic igneous rocks may be rich in chromium, titanium, iron, vanadium, and nickel5.
Magmatic hydrothermal fluids can carry substantial amount of metals like copper and gold which can be transported away from their source rocks and deposited within other rock types. Gold in quartz veins comes from magma. It is associated with chlorine which makes it possible to dissolve gold in water and carry it away from its source. Reaction between magmatic fluids and granitic rocks results in greisens which can carry substantial amount of chemical elements like tin and fluorine. Reactions between magmatic fluids and carbonate rocks give rise to many interesting calc-silicate minerals. These rocks are known as skarns and they also frequently contain valuable metal-bearing minerals.
http://picasaweb.google.com/107509377372007544953/Rocks#5787194233322780322 Igneous rock diabase is a popular tombstone material. Width of sample is 25 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5842261460204530978 Chromite crystals picked out of a chromite concentrate from the Rustenburg mine, Western Bushveld, South Africa. Width of view is 5 mm.
http://picasaweb.google.com/107509377372007544953/Beil#5749477433183348242 Kimberlite is a rare, but economically very important igneous rock because it contains diamonds. Kimberlite fills pipe-like structures (diatremes) in the crust. These structures were created by violent gas-charged eruptions. Small and easily erodable tuff rings form as a result of this, but most of the kimberlitic magma never sees daylight. We have to dig deep to get the diamonds. The sample is from Kimberley (“blue ground”), South Africa. Width of sample is 11 cm.
Volcanic bomb is a pyroclast which was semi-molten (viscous) while ejected from a volcanic vent and is therefore shaped while in flight. Volcanic bombs are larger than 64 mm in diameter1. Smaller pyroclasts are called lapilli. Pyroclasts (>64 mm) that were already solid when ejected are called blocks. Blocks are typically more angular because unlike bombs they are not aerodynamically shaped by airflow while in flight.
http://picasaweb.google.com/107509377372007544953/Tenerife#5840801030853835650 An aerodynamically shaped bomb ejected from the Cumbre Vieja, La Palma, Canary Islands. Width of sample is 12 cm.
The shape of volcanic bombs varies greatly. They are usually somewhat elongated. Sometimes ribbon bombs form when long strands of lava are flung from the vent. The trailing edge of bombs is often elongated because the outermost plastic lava is pushed backwards by the rush of passing air. If the bomb spins, lemon-shaped spindle or dicus bombs form2.
Volcanic bombs are mostly produced by Strombolian and Vulcanian eruptions. These eruption types usually produce basaltic lava. Hence, volcanic bombs commonly possess a basaltic or similar mafic composition.
Volcanic bombs are heavy and often fly at high speed. It obviously makes them very hazardous to people around the active vents. However, bombs do not travel very far. The most violent Vulcanian-type eruptions have produced ejection velocities 200-400 m/s which have thrown bombs almost 5 km from the vent3. Bombs may travel even farther, but in this case they continued their journey by rolling and bouncing downwards. Each bounce rounds them by knocking off corners. At the base of the slope they are smooth lumps which are known as cannon-ball bombs.
http://picasaweb.google.com/107509377372007544953/Tenerife#5840801031484486946 Another bomb with a smooth vesicular surface and slightly elongated shape from the Cumbre Vieja. Width of sample is 15 cm (longer dimension).
http://picasaweb.google.com/107509377372007544953/Tenerife#5840801032641129554 And another one from La Palma resting on smaller lapilli. These three bombs were found close to their vents more than 2000 meters above sea level.
http://picasaweb.google.com/107509377372007544953/Tenerife#5840801072109386466 Smooth cannon-ball bomb on the flank of Cumbre Vieja. This bomb is far away from its vent (about 600-700 meters lower). It managed to travel so far because it is large (1.2 meters in diameter) and because the flanks of La Palma are unusually steep.
http://picasaweb.google.com/107509377372007544953/Tenerife#5840801111095276482 The interior of a sample from a scoria cone in Tenerife, Canary Islands. The trailing edge of the bomb is to the left. The vesicles are stretched which is a common phenomenon and a result of aerodynamic shaping. Width of view is 38 cm.
http://picasaweb.google.com/107509377372007544953/Tenerife#5840801096439651282 The exterior of the bomb shown in the previous picture. You can see that many smaller pyroclasts (lapilli) are attached to the bomb which demonstrates that the bomb was viscous while it landed on the ground.
http://picasaweb.google.com/107509377372007544953/Tenerife#5840801135539156658 Volcanic bomb from Tenerife with a vesicular scoriaceous core. Many volcanic bombs have a core which may in some cases have a different lithology. Even non-volcanic rocks are found to form cores of volcanic bombs.
http://picasaweb.google.com/107509377372007544953/Tenerife#5840801133042876594 Interior of a fragment of a ribbon bomb from Tenerife. Stretched vesicles are parallel to the long axis of the bomb. Width of view is 20 cm.
http://picasaweb.google.com/107509377372007544953/Tenerife#5840801148173696082 Exterior of the bomb shown above.
http://picasaweb.google.com/107509377372007544953/Hawaii#5868866983042219042 A bread-crust bomb resting near the summit of Mauna Kea, Hawai’i at an altitude of 4100 meters. This type of bomb forms when viscous, gas-rich lava is ejected from the volcanic vent. The exterior of the bomb solidifies quickly while the soft interior continues to expand because of gases that exsolve from the lump of lava. Just like in baked loaf, the internal expansion causes the brittle outer crust to crack2. Mauna Kea, Hawai’i. Width of the bomb is 26 cm.
References
1. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute. 2. Francis, P. & Oppenheimer, C. (2003). Volcanoes, 2nd Edition. Oxford University Press. 3. Morrissey, Meghan M. (1999). Vulcanian Eruptions. In: Encyclopedia of Volcanoes (Ed. Sigurdsson, H.). Academic Press. 463-475.
Scoria is a highly vesiculated lava or tephra1. It is usually dark-colored and has a mafic composition. It is difficult to say for sure whether it is a rock type or not. It is tempting to say no. Scoriaceous rocks are simply dark-colored volcanic rocks with lots of variously sized and usually smooth-sided holes (vesicules) in it. It seems that “scoriaceous” simply refers to their porous texture.
A sample from a cinder cone in Tenerife, Canary Islands. Width of sample 7 cm.
However, the same is true with pumice. Pumice is usually felsic, but no strict chemical definition exists. Pumice can be rhyolitic, dacitic, phonolitic, etc. It just needs to be very porous and light-weight lava froth and yet it is still considered to be a rock type.
These rocks have several similarities. They are both volcanic rocks (often pyroclastic) and they contain vesicules. Vesicules in pumice are usually smaller and more irregularly shaped. Pumice is a very lightweight material that usually floats in water. Scoria is lightweight also, but it sinks in water. Its vesicles can be much larger than vesicles in pumice are. It is often glassy just as pumice. Fresh rock may be distinctly pitch black and shiny. Older material is duller and brown or even reddish due to oxidized iron. It contains much more iron (which gives black color to fresh rock) than average pumice which contains more alkali metals (potassium, sodium).
Vesicules within scoria (and in pumice also) form when volcanic gases are released from magma. They are released because of decreasing pressure as magma moves upward (solubility of gases in liquids is dependent on pressure — higher pressure means better solubility). Mafic magma is less viscous than felsic magma. That’s why gas bubbles can move more freely and join each other and form larger vesicules. Bubbles that form in felsic magma can not move as well. Hence, there are much more pores in pumice than in scoria and they are smaller.
Both lava flows (especially their upper part) and pyroclastic fragments can be scoriaceous, but it is usually the latter that the term “scoria” is associated with. It even seems that sometimes the terms “lapilli” and “scoria” are used interchangeably. Small usually monogenetic volcanic cones are known as scoria (or cinder) cones. It is easy to make an obvious conclusion that this fragmental material these cones are mostly made of is scoria. This conclusion is not entirely mistaken. It is scoria indeed, but not because it is fragmental. It is scoria because it is vesicular.
Scoria as a pyroclastic material (tephra) usually has a size of lapilli (2-64 mm) which is larger than volcanic ash and smaller than volcanic blocks and bombs.
A sample of scoria from Etna, Sicily.
Small scoriaceous lapilli from Cumbre Vieja, La Palma, Canary Islands. The samples from Tenerife and Etna (above) look like classic scoria, but these small lapilli here are actually more typical. They are not very vesicular and vesicules are not large, but this is how scoriaceous lapilli usually look like. The two samples above are not rare or unusual, but still they are specifically chosen in the field out of many candidates to demonstrate the very vesicular nature of “true scoria”. Width of view 6 cm.
Pumice is in many ways similar rock, but it is generally lighter in both weight and color. Santorini, Greece. Width of view 4 cm.
Scoria cone on top of the Hawaii Island more than 4000 meters above sea level. Scoria cone is a small monogenetic volcano that is usually situated on the flanks of a bigger volcano.
Field of scoriaceous lapilli in La Palma.
Pieces of scoria (from Tenerife) and pumice (from Santorini) on the same picture.
Tuff is a volcanic rock consisting of consolidated volcanic ash. It is a product of explosive volcanic eruptions. It can be considered to be a volcanic analogue of sandstone. Both sand grains and volcanic ash grains have a diameter between 1/16…2 mm. However, the term is often used for many pyroclastic rocks (pyroclasts are fragments thrown out of volcanoes during explosive eruptions — ash, lapilli, bombs, blocks) that contain fragments a lot larger (or smaller) than 2 mm (1/16 mm). Sometimes the term is used even for consolidated material that has undergone limited posteruption reworking1. This is not only that the general public is using the term too loosely, but geologists too use terms like “lapilli tuff” and “tuff breccia” that contain lots of larger pyroclastic material.
Classification of polymodal (consisting of differently sized pyroclasts) pyroclastic rocks. This rock in the strict scientific usage is on the lower right corner with ash tuff (mean diameter of pyroclasts less than 1/16 mm which is analogous to siltstone). At least 75% of the pyroclastic material of these rocks has a mean diameter less than 2 mm. Lapillistone is mostly composed of lapilli (more than 75%), pyroclastic breccia (agglomerate) are mostly composed of volcanic blocks (bombs). Tuff breccia and lapilli tuff are more mixed polymodal pyroclastic rocks. Figure drawn by the author, data (classification principles) taken from Le Maitre, 20052.
Pyroclastic material deposited from pyroclastic density currents (pyroclastic flows) is known as ignimbrite. Ignimbrite can be considered to be a special type of tuff (it is sometimes known as welded tuff although not all ignimbrites are welded).
Thin layer of light-colored tuff between layers of lapilli tuff. Tenerife.Close-up of the thin tuffaceous layer shown above. It is mostly composed of consolidated volcanic ash but contains also significant amounts of fine-grained lapilli (pyroclastic analogue of gravel). Width of view 13 cm.Crystal tuff from La Palma. Width of sample is 5 cm.A sample from Armenia. Width of sample is 13 cm.
http://picasaweb.google.com/107509377372007544953/Tenerife#5835477262148313186 Lapillistone from Tenerife. It is composed of lapilli-sized pumiceous pyroclasts with very little ash between them. Width of view 55 cm.
Phreatomagmatic tuff breccia from Germany. Phreatomagmatic eruption is an explosive volcanic eruption caused by interaction of ground water and magma3. Width of view is 12 cm.Tuff is basically a volcanic sedimentary rock. As many sedimentary rocks, it is also often layered. The Massif Central, France.Hydrothermally altered tuff (it could be reworked material) in Tenerife. Gray card for scale (longer edge 9 cm).Mafic scoria below (probably lapilli tuff). On top of it is a thin layer of pumiceous lapillistone and then tuff composed of fine pumice shards. On top of it is again a layer of pumiceous lapilli tuff. Tenerife. Width of view 80 cm.Lapillistone, tuff, and unwelded ignimbrite. Tenerife. Width of view 1.8 meters.
http://picasaweb.google.com/107509377372007544953/Tenerife#5836571987685513522 An outcrop of unwelded ignimbrite in Tenerife.
http://picasaweb.google.com/107509377372007544953/Tenerife#5830757878227475474 An outcrop of welded ignimbrite. Southern coast of Gran Canaria.
http://picasaweb.google.com/107509377372007544953/Tenerife#5836571997853181474 Close-up of Tenerife ignimbrite. It contains pseudovesicules that were formerly filled with pyroclasts of phonolitic pumice. Width of view is 40 cm.
http://picasaweb.google.com/107509377372007544953/Tenerife#5835569151026130866 Ignimbritic lithic breccia (tuff breccia) in Tenerife. Width of view 0.8 meters.
http://picasaweb.google.com/107509377372007544953/Tenerife#5832686321337998642 Pyroclastic layers of scoria and pumice (lapilli tuff) 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). Width of view is 12 meters.
http://picasaweb.google.com/107509377372007544953/Tenerife#5830757815158545458 Ignimbrite with flattened pumice lapilli (fiamme) from Gran Canaria. Pyroclasts deposited by pyroclastic flow were so hot after deposition that they fused together and started to flow as a very viscous liquid. Width of view 12 cm.
A hand sample of ignimbrite from Gran Canaria with prominent fiamme (flattened pumice lapilli). Width of view 9 cm.
http://picasaweb.google.com/107509377372007544953/Tenerife#5829978158983537218 Block-and-ash flow deposit is a type of ignimbrite that forms as a result of non-explosive collapse and fragmentation of volcanic domes that provides angular volcanic blocks to the flow. It is a tuff breccia according to the classification diagram above. Blocks are about 10-25 cm in width.
Tuff is a popular building stone where it is available because it is relatively easy to cut and shape. Gran Canaria.And finally a picture of tufa which has nothing to do with the topic of this post. There is just a linguisting similarity between the rock names. Tufa is actually just a type of soft and porous chemically precipitated limestone, usually associated with springs. Width of the sample from Estonia is 13 cm. Green lithic fragments are pieces of glauconitic sandstone.
This rock has been used as a building material since ancient times. It is relatively soft and therefore easy to work with. The famous statues on the island of Rapa Nui (Easter Island) are carved from this material.
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