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.

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.

References

1. Barth, T. F. W. (2007). Quartzite. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 14. 722.
2. Pettijohn, F. J., Potter, P. E. & Siever, R. (1973). Sand and Sandstone. Springer.

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 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.

References

1. Murray, Haydn H. (2007). Bauxite. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 2. 654.
2. Stow, D. A. V. (2005). Sedimentary Rocks in the Field: A Color Guide. Academic Press.
3. Nesse, William D. (2011). Introduction to Mineralogy, 2nd Edition. Oxford University Press.
4. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.
5. Hale, Martin (1999). Ore Deposits. In: Encyclopedia of Geochemistry (Encyclopedia of Earth Sciences Series) (Ed. Marshall, Clare P. & Fairbridge, Rhodes W.). Springer. 447-453.
6. Robb, L. (2005). Introduction to Ore-Forming Processes. Blackwell Science Ltd.

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 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.

References

1. Stow, D. A. V. (2005). Sedimentary Rocks in the Field: A Color Guide. Academic Press.
2. US Energy Information Administration

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.

References

1. Dolley, Thomas P. (2000). Diatomite. US Geological Survey Minerals Yearbook. PDF-document.
2. Wignall, P. B. (2005). Anoxic Environments. In: Encyclopedia of Geology, Five Volume Set. Academic Press. 495-501
3. Talbot, M. R. (2005). Lake Processes and Deposits. In: Encyclopedia of Geology, Five Volume Set. Academic Press. 550-561
4. Piper, D. J. W. (2005). Deep Water Processes and Deposits. In: Encyclopedia of Geology, Five Volume Set. Academic Press. 641-649

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

Pyroclastic rocks

Rocks that do not fit neatly into the system

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.

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.

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.

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.

References

1. Raymond, Loren A. (2007). Igneous rocks. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 9. 14-20.
2. 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.
3. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.
4. Nesse, William D. (2011). Introduction to Mineralogy, 2nd Edition. Oxford University Press.
5. Robb, L. (2005). Introduction to Ore-Forming Processes. Blackwell Science Ltd.