Dolomite or dolostone is a carbonate sedimentary rock containing more than 50% by weight of the mineral dolomite. Dolomite rock may contain calcite, but the content of calcite should not exceed 10% of the calcite-dolomite pair’s content³.

There is a considerable amount of confusion over the name of this rock. The problem is that dolomite is both a mineral and a rock type. Hence, many geologists prefer to name the rock a dolostone (term proposed in 1948). This usage, however, has met stiff resistance. The main reason for that seems to be the fact that the rock type got its name well before the mineral and should therefore be given a precedence. Dolomite as a rock type was first described in 1791 by a French geologist Déodat de Dolomieu who investigated samples from the Italian Alps².

Dolomite rock has been called “dolomites” (in plural), especially in the literature published before 1990, but the term “dolomites” should be used only for different types of dolomite⁴. Glossary of Geology, published by the American Geological Institute, does not recommend “dolostone” and even says that this term is obsolete³. Hence, I decided to avoid “dolostone” here and named the article “dolomite rock” to differentiate it from the mineral.

http://picasaweb.google.com/107509377372007544953/Rocks#5788747495099595634
Dolomite rock from Saaremaa, Estonia. The width of the sample is 10 cm.

Three types of dolomite rock can be distinguished. The most common type of dolomite rock is a former limestone that was dolomitized. These dolomite rocks are often referred to as secondary dolomites, especially in the older literature. Dolomitization means that calcium carbonate (minerals aragonite or calcite — the main constituent of limestone) were replaced by calcium magnesium carbonate (mineral dolomite) through the action of magnesium-bearing water percolating the limestone or limy mud. Dolomite may precipitate out of aqueous solutions (sandstones with a dolomitic cement) and some dolomitic rocks are so-called primary dolomites. These were formed in lagoons where dolomite directly precipitates out of saline seawater, but such dolomites are much rarer than previously thought⁴. Primary dolomite deposition is known only from few cases of Holocene (last 12,000 years) age. It is possible that primary dolomites were somewhat more widespread in the past, but this hypothesis is difficult to prove or reject because of later diagenetic (processes affecting sediment after deposition) overprinting of original material². However, it seems very hard to believe that primary dolomite deposition was once the dominant way how dolomitic rocks were formed because laboratory experiments have shown that dolomite does not precipitate from aqueous solutions at the atmospheric conditions (pressure 1 atm, temperature below 60 °C)⁴.

Dolomite rocks are important oil reservoir rocks because average dolomite has usually higher porosity than limestone. This difference is generally believed to be the result of dolomite being denser mineral than calcite. Note that this is not because magnesium (atomic number 12) is heavier than calcium (atomic number 20). Despite the opposite being true: calcium is heavier than magnesium, dolomite (2.85) is still denser than calcite (2.71). Hence, the atoms in the lattice of dolomite have to be more tightly packed and consequently one mole of dolomite takes less room than one mole of calcite. If original limestone gets replaced by dolomite rock, pore space is therefore expected to increase. However, this explanation (for the higher porosity of dolomite rock) has been challenged. According to a paper published in the Special Publication of the Geological Society of London, the explanation is that limestones lose porosity through compaction and cementation, whereas dolostones resist compaction and retain much of their porosity⁵.

Dolomite rock is a very common sedimentary rock, especially older carbonate rocks (formed before the Mesozoic) tend to be dolomites whereas younger carbonates are predominantly various limestones. Dolomite is usually very similar to limestone and the two are often indistinguishable in the field. Geologists usually carry small bottles of dilute hydrochloric acid to test carbonate rocks. Limestone (calcium carbonate) is strongly effervescent in acid, but dolomite reacts very weakly. Another way to differentiate between them is to use alizarin red S which turns calcite bright red, but does not affect dolomite.

Dolomite rock may contain significant amount of silicate minerals (quartz, clay minerals), sulfides (especially pyrite), and evaporites (or their replacements). Dolomites may contain fossils, but they are usually poorly preserved because of diagenetic overprinting. Most fossils of original limestones become obliterated during the dolomitization.

Dolomite in Morocco. Originally horizontal bedding is inclined because of compression caused by a mountain building. Pen for scale.

http://picasaweb.google.com/107509377372007544953/Rocks#5788747554259659666
Conglomerate (quartz gravel with one almandine grain in the middle) with a dolomitic cement. The width of the sample from Estonia is 6 cm.

http://picasaweb.google.com/107509377372007544953/Rocks#5788747579737966338
Ripples in silty dolomite rock from the Devonian of Estonia.
http://picasaweb.google.com/107509377372007544953/California02#5872323536224039042
Famous sailing rocks of Racetrack Playa are mostly composed of dolomite.
http://picasaweb.google.com/107509377372007544953/2015#6196127987356077426
Dolomitized stromatolite. The original layered structure of stromatolite is still evident. Tornio, Finland. Width of sample 18 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5788747633644352562
Dolomite from Estonia with weathering rinds. Dolomite easily obtains brown color on outcrop (it helps to differentiate dolomite from limestone) because iron may easily substitute for magnesium in the dolomite structure (dolomite becomes ankeritic and obtains brown color because of iron).
http://picasaweb.google.com/107509377372007544953/2015#6190952069931770786
Tempestite layer in dolomite from northern Norway (Trollholmsund). Tempestite is a layer of breccia caused by a strong storm that disturbed the already formed sediments which subsequently rapidly redeposited as a mixture of differently sized clasts. Width of view 30 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5788747658322686706
Dolomites are mountains in the Alps which are made of dolomite rock. These mountains are also named after Déodat de Dolomieu.

References

1. Baker, Paul A. (2007). Dolomite rock. In: McGraw Hill Encyclopedia of Science & Technology. 10th Edition. McGraw-Hill. Volume 5. 654-655.
2. Machel, Hans G. (2004). Dolomites. In: Encyclopedia of Geology, Five Volume Set. Academic Press. 654-655
3. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.
4. Machel, Hans G. (2003). Dolomites and dolomitization. In: Encyclopedia of Sediments & Sedimentary Rocks (Ed. Middleton, V.). Springer. 234-241.
5. Lucia, F. Jerry (2004). Origin and petrophysics of dolostone pore space Geological Society, London, Special Publications DOI: 10.1144/GSL.SP.2004.235.01.06

Arkose is a feldspar-rich sandstone. It is commonly coarse-grained and usually either pink or gray (depending on the color of feldspar).

Definition

Quartz is usually the dominant mineral in arkose, but feldspar makes up a significant part of the rock (in some cases feldspar may exceed quartz in content). There is no universal agreement, but often 25% of feldspar is set as a requisite for calling sandstone an arkose¹.

This is actually more complicated because graywacke (muddy sandstone) may also contain more than 25% feldspar. However, it is not an arkose for the majority of geologists. In most cases, sandstone that is called an arkose, is coarse-grained enough that feldspar grains (which are usually rather angular) can be easily seen with the unaided eye. That has lead to a field description according to which any sandstone that contains appreciable amounts of feldspar in named an arkose. Feldspar is usually pink in color and is therefore easily seen. Such rocks may contain much less than 25% feldspar.

There is no natural discontinuity in the abundance of feldspar in sandstones, but 25% is probably chosen because sandstones that contain that much feldspar are quite distinctive. Sometimes rocks that contain appreciable amount of feldspar, but clearly less than 25% (10…20) are named subarkose. Arkosic sandstones generally contain small amounts of matrix (small dust grains filling the pore space between larger sand grains). Hence, common arkose is an arenite (“clean” sandstone). If arkose is “dirty” (contains more than 15% of muddy component), it is named feldspathic (gray)wacke or arkosic wacke. Arkose has also been defined as a sandstone that contains more than 25% labile constituents of which feldspar forms more than half. According to this definition, arkose might contain as little as 12.5% feldspar².

Arkose may be associated with reddish sandstones (redbeds), but they should not be confused. Most redbeds are not rich in feldspar. It is a mineral hematite that covers sand grains as a fine pigment and gives a reddish color to redbeds. Some volcanic sands (pyroclastic sediments) may resemble it because they frequently contain reddish fragments (volcanic glass, weathered scoria, fragments of tuff).

http://picasaweb.google.com/107509377372007544953/Sandstone#5787980547557069570
Torridonian sandstone (below) is a precambrian arkose from Scotland.

Metamorphosed arkose or feldspar-rich quartzite. Aust-Agder, Norway. Width of sample 19 cm.
http://picasaweb.google.com/107509377372007544953/2015#6197322360557740066
Heavily metamorphosed and recrystallized arkose. Green mineral is epidote. Salhus, the Bergen Arcs, Norway. Width of view 20 cm.

Composition and texture

Quartz and K-feldspar (usually microcline) are the most important minerals in arkose. Plagioclase is clearly less important than K-feldspar and only in rare cases is plagioclase the dominant feldspar in this rock type.

Why is this the case? Probably because arkosic rocks mostly tend to occur in tectonic settings (continental rifts) which favor the emplacement of K-rich granites instead of Na- and Ca-rich granodiorites and tonalites. Plagioclase feldspar is also clearly less resistant to weathering than K-feldspar, so its chances to survive as a sand grain are not that good. Swauk arkose in Washington State (USA) is an example of arkose sandstone where plagioclase is the dominant feldspar. It is presumably derived from a quartz dioritic source².

Other notable minerals are micas, amphiboles, rock fragments, clay minerals, heavy minerals, etc. Micas (both biotite, muscovite and also chloritized biotite) tend to be larger than other grains and lie parallel to the bedding. Clay minerals (mostly kaolinite) presumably come from the weathering of K-feldspar. This rock may contain amphiboles if the source material (granite) contained it, but generally mafic minerals are either absent or make up only a small part of the rock. Conglomeratic zones within arkose sequence is a pretty common phenomena. Sand grains making up arkose are usually either angular or poorly rounded. This indicates that in the majority of cases the transportation route has not been very long although there are exceptions. It is not correct to say that arkose is always deposited near its source area. The Mississippi sands near Cairo (Illinois, USA) contain about 25% feldspar but the source area is 1700 km upstream. Ripple marks are not very common in arkose and bedding features are generally thick and not very well-developed, although some units may show strong cross-bedding².

Occurrence and origin

It is not a rare type of sandstone (it forms up to 15% of all sandstones), but it needs special conditions to form and is therefore not very widespread. The defining property (large feldspar content) needs a good explanation because feldspar (although more common than quartz in the rocks) is much less resistant to weathering and is therefore usually destroyed (converted to clay) even before the quartz grains are liberated from the feldspar-rich rocks.

Hence, it was long believed that arkosic sandstones form in climatic regions which do not support weathering because they are either too dry or too cold. It definitely is the case in some instances because chemical weathering (which is the most important form of weathering) is heavily dependant on the availability of water and to a lesser extent warm temperature which accelerates chemical reactions. However, it is obvious that many arkose formations had no such conditions during their formation. Nowadays it is believed that arkose forms where there is a crystalline source material rich in feldspar (mostly granite or gneiss) and a high relief with a consequent rapid erosion which gives no time to break up feldspar. This is the case with granitic batholiths which are surrounded by down-faulted blocks (grabens) which collect the eroded clastic detritus rich in feldspar. So it seems that relief plays much more important role in the formation process than adverse climatic conditions do. Some older precambrian samples may resemble granite (because of secondary overgrowths of quartz and feldspar) so much that it might be difficult to tell them apart².

In some cases arkosic sand may be the product of in situ weathering of granite. Such a loose material is known as grus and it may cover granite plutons as a thin weathered veneer of sediments which does not need high relief to form. These granite washes are mostly marine (requires transgression over the granitic terrain) and therefore form a genetically distinct type of arkose because the majority of such sandstones are clearly subaerial. Grus is generally composed of more angular feldspar and the content of feldspar is also usually higher than it is in transported arkose².

Etymology

The term “arkose” was first used by a French geologist Alexandre Brongniart in 1826 who applied this term to some feldspathic sandstones in the Auvergne region of France².

http://picasaweb.google.com/107509377372007544953/Sand#5787716056121113634
A feldspar-rich sand sample from Saint Pierre and Miquelon (French island near the coast of Newfoundland). It could become arkose after lithification. The width of the view is 10 mm.

References

1. Siever, R. (2007). Arkose. In: McGraw Hill Encyclopedia of Science & Technology. 10th Edition. McGraw-Hill. Volume 2. 190-191.
2. Pettijohn, F. J., Potter, P. E. & Siever, R. (1973). Sand and Sandstone. Springer.

Shungite is a carbon-rich metamorphic rock. It is largely composed of carbon, up to 98% in some cases, but interesting fact is that this carbon is not mineral graphite. Shungite is believed to be a metamorphosed oil shale, although there are other possible explanations. The most famous locality where large deposit is found is in Karelia, near Shun’ga village (hence the name of the rock type).

Karelian shungite is very old (2.0 Ga) which makes it one of the earliest known occurrences of oil shale, albeit metamorphosed. It has been a favorite example of an abiogenic petroleum source rock but there is no doubt anymore that shungite has a biogenic origin, most likely algal or bacterial¹.

The chemical composition of shungite is similar to anthracite (metamorphosed coal) but their genesis is different. Oil shale is a former accumulation of marine microorganisms (bacteria, algae), but coal formed from organic matter deposited in terrestrial environments.

The rock is black in color, but it is not greasy and dirty as one would expect a carbon-rich rock to be. This is because it does not contain graphite. Carbon is almost structureless (amorphous or nanocrystalline) and it forms coal-like seams. It is believed to be one of the oldest known occurrences of petroleum generation. There is no crude oil in Karelia at the moment, but shungite is believed to have been both the kerogen-rich source rock of crude oil and also petroleum itself that have migrated (both vertically and horizontally) away from the source rocks.

http://picasaweb.google.com/107509377372007544953/Rocks#5786232496728987410
Shungite from Karelia. Width of sample 7 cm.

Shungite was for some time known to contain fullerenes (carbon nanotubes and spheres) but studies conducted later have failed to confirm this. It has gathered fame as a natural medicine for a long time. It is true that shungite has anti-bacterial properties but it is highly unlikely that it has all the powers attributed to it (kills and devours anything that harms people and concentrates and restores all that is good). The fact that this rock is old does not give it any magical power. After all, two billion years is nothing very special in geology. Many gneissose rocks are much older than that, unfortunately they tend to be largely neglected and only have a value as a railroad ballast.

References

1. Melezhik, V. A., Fallick, A. E., Filippov, M. M., & Larsen, O. (1999). Karelian shungite—an indication of 2.0-Ga-old metamorphosed oil-shale and generation of petroleum: geology, lithology and geochemistry Earth-Science Reviews, 47 (1–2), 1-40 DOI: 10.1016/S0012-8252(99)00027-6

Amphibolite is a common metamorphic rock. It is made of amphiboles (usually hornblende) and plagioclase. Most samples have a relatively simple composition: hornblende + plagioclase. Garnet (almandine), pyroxene, biotite, titanite, magnetite, epidote, chlorite, and quartz are also frequent constituents.

Amphibolite is a common rock of the metamorphic facies with the same name. However, not all amphibolites were formed within the pressure-temperature limits of the amphibolite facies and by no means are all rocks of this metamorphic facies amphibolites. So one should be careful when comparing amphibolitic rocks with the metamorphic facies that carries the same name. The temperature range of formation is usually 400…500 °C¹. Amphibolites are usually either black or dark green, depending on the color of the dominant amphibole. These rocks are usually coarse-grained enough for the individual mineral grains to be seen with the unaided eye.

http://picasaweb.google.com/107509377372007544953/2015#6196127317767358722
Garnet is a common mineral in amphibolites. The sample above contains small reddish garnet crystals. Senja, Norway. Width of sample 11 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196127308718613442
A contact between amphibolite (on the right) and tonalite pegmatite. Dark mineral in the lower left is biotite. Other minerals in tonalite are plagioclase and quartz. Senja, Norway. Width of sample 10 cm.

Amphibolite is a metamorphosed mafic igneous rock (basalt, gabbro) although it is usually difficult to determine the protolith because original features are often obliterated. Basalt is composed of pyroxene + plagioclase. In order to make amphibolite out of basalt, we need heat and pressure to initiate chemical reactions and also need to add water because amphiboles, unlike pyroxenes, are hydrous minerals. According to some sources, impure carbonate rocks (rich in clay content) may also metamorphose to amphibolitic rock. Prexifes para- and ortho- were used in the past to denote sedimentary and igneous protoliths, respectively. However, the term “para-amphibolite” is outdated and generally not used anymore. It is hard to think of an impurity in carbonate rocks where you can end up with amphibole and plagioclase irrespective of the type of metamorphism.

Amphibolite either lacks or has weakly developed foliation. Schistose rocks with a similar composition are hornblende schists. High grade dark-colored hornblende-bearing rock is hornblende gneiss.

It is a common rock type in mountain ranges and shield areas. It is a very common product of regional metamorphism which is associated with mountain building episodes (such rocks are often lineated or foliated because mountain building is a dynamic event that involves motion). Granitic intrusions (batholiths) are often surrounded by amphibolitic metamorphic rocks. Amphibolites are often associated with other metamorphic rocks like quartzite, schist, marble, gneiss. These rocks represent different protoliths that were metamorphosed during the same mountain building episode. Stripes of metamorphic rocks like these are often next to one another in geological maps.

The term “amphibolite” was invented by a French geologist Alexandre Brongniart, but he used it in a different manner. For Brongniart, every rock where amphiboles formed majority was amphibolite. That includes also melanocratic variety of igneous rock diorite and ultramafic rock hornblendite. Modern usage dates from a German geologist Harry Rosenbusch.

Amphibolite rarely hosts economic minerals, but the rock type is widely used as a construction material in road building (aggregate) and as a railroad ballast.

http://picasaweb.google.com/107509377372007544953/2015#6190952677684298834
A sample from Arendal, Norway. Width of sample 9 cm.

http://picasaweb.google.com/107509377372007544953/2015#6190952758380395938
Amphibolite is shiny because hornblende has a strong luster. Arendal, Norway. Width of sample 10 cm.

References

1. De Vore, George (2003). Amphibolite. In: Encyclopedia of Sediments & Sedimentary Rocks (Ed. Middleton, V.). Springer. 593-594.

Monzonite is a plutonic igneous rock intermediate in composition between syenite and diorite. Monzonite contains less quartz and more plagioclase than granite. Latite or trachyandesite are the approximate volcanic equivalents of monzonite.

http://picasaweb.google.com/107509377372007544953/Monzonite#5784665633670973394
Monzonite has a strict definition, which is based on the QAPF diagram. In this diagram, monzonitic rocks occupy a central position. They have roughly equal amounts of alkali feldspar (A) and plagioclase (P), and little or no quartz (Q). Small amounts of feldspathoids may be present in foid-bearing monzonites.

It is a relatively uncommon rock type. It usually does not form its own plutons. Monzonitic magma most likely forms only a part of a generally more acidic (granitic) intrusions. Although monzonite itself is not a particularly well-known or widespread rock type, but it has given part of its name (monzo-) as a prefix to several other varieties of plutonic rocks (monzogranite, monzogabbro, foid monzosyenite, etc.).

This prefix means that there are significant amount of both alkali and plagioclase feldspars. Monzonite itself got its name after Monzoni in Northern Italy. Its godfather was a German geologist Leopold von Buch.

It may not be very widespread rock type, but the usage of the term “monzonite” is scarcer still. Many “syenite” and “granite” samples are actually monzonites. It may be partly due to the different traditions in Europe and North America which can create confusion. Quartz monzon. in America may be granite (monzogranite) or adamellite in Europe. “Adamellite” itself is unfortunately also ambiguous term. So the situation is really complicated and in my opinion it should be better for everyone if we all follow one classification scheme (shown below), although it definitely is not without its own problems.

Most important minerals in monzonitic rocks are definitely feldspars. Small amounts of quartz and feldspathoids may occur. Most important mafic minerals are biotite, augite, and hornblende. Sphene (titanite) and apatite are common accessory minerals. Feldspar is often perthitic (plagiopclase in alkali feldspar) or antiperthitic (alkali feldspar in plagioclase), which makes it harder to visually estimate whether the rock sample is monzonite or not.

Windsorite, ukrainite, masanite, sörkedalite, larvikite, vallevarite, amherstite, and kjelsasite are all varieties found in specific locations.

http://picasaweb.google.com/107509377372007544953/Monzonite#5784665620929985074
A sample from France. Approximately equal amounts of alkali and plagioclase feldspar, very little quartz, and a subordinate amount of mafic minerals. Width of sample 17 cm. TUG 1608-2630.

Larvikite is a variety from Norway. It is extensively used as a dimension stone because of the beautiful schiller effect. Larvik, Norway. Width of sample 17 cm.

http://picasaweb.google.com/107509377372007544953/Monzonite#5784665546127142946
Polished surface of a larvikite sample from Norway. Plagioclase is antiperthitic (contains alkali feldspar). Width of sample 7 cm.

Tombstone in Norway made of iridescent monzonite (larvikite).

http://picasaweb.google.com/107509377372007544953/Tenerife#5841862866360779090
A sample from La Palma, Canary Islands. Width of sample 6 cm.

Pegmatite is an exceptionally coarse-grained plutonic igneous rock. Most pegmatites have a mineralogical composition of granite but composition has no defining importance here. Pegmatites may have any imaginative magmatic composition and they are actually known to contain a large number of unusual minerals.

Pegmatite is a very coarse-grained igneous rock. Simple pegmatites are composed of large crystals of ordinary minerals. Here is a sample of alkali feldspar granite pegmatite from Northern Norway which is composed of alkali feldspar (pink), quartz (gray) and biotite (black). Width of view 50 cm.

The main constituents of pegmatites are usually at least several centimeters in diameter or more. The average grain size for all occurrences is approximately 10 cm. Pegmatites may contain huge crystals of mica, beryl, tourmaline, etc. which may be several meters across. Largest spodumene crystal found was 15 meters long¹. Pegmatites have an extreme variation in grain-size. Largest magmatic crystals found so far are many meters in length. Most pegmatites have a fairly simple composition: K-feldspar (either orthoclase or microcline) + quartz + some other minerals. Complex pegmatites commonly contain tourmaline, lepidolite, topaz, cassiterite, fluorite, beryl, etc. Pegmatites are not rare rocks, but their overall volume is small. They form small marginal parts of large magma intrusions known as batholiths. They form as a late-stage magmatic fluid starts to crystallize. This fluid is rich in water, other volatiles, and chemical elements incompatible in main magmatic minerals.

This is the reason why pegmatites are so coarse-grained and why they contain so much unusual minerals. They are coarse-grained because of high volatile content which makes the magma less viscous and therefore enhances mineral growth (chemical elements are free to move to look for and join a suitable and already existing crystal). Unusual minerals form because the fluid is enriched in exotic chemical elements like lithium, boron, beryllium, rare earth elements, etc. These elements are forced to form their own mineral phases because they are rejected by major rock-forming minerals like quartz, feldspar, and others.

This wealth of minerals makes pegmatites often valuable as a mineral resource. Pegmatites may be mined because of their high content of feldspars, clay (if weathered), mica, or many metal-bearing minerals. Pegmatite is also a source of gems like beryl, tourmaline, zircon, etc.

Most pegmatites are granites with or without exotic minerals, but mafic pegmatites (gabbro, diorite) are known as well. Silica undersaturated (without quartz) magmatic rocks may be also pegmatitic.

The term “pegmatite” was first used by a French mineralogist René Haüy, but he used this term as a synonym of graphic granite. Contemporary meaning was given to the rock type in 1845 by an Austrian mineralogist Wilhelm Heidinger. It may be somewhat surprising, but to this day sometimes geologists confuse these terms. In fact, it is quite easy to be confused because graphic granite, or more precisely, K-feldspar with cuneiform intergrowths of quartz, are common in pegmatitic rocks.

Common minerals in a granitic pegmatite: quartz (left), muscovite (up), plagioclase with oligoclase composition (right), and microcline. These crystals (each one about 10 cm across) come from a pegmatitic rock. Evje, Norway.

Tourmaline pegmatite. Tourmaline is black, white is plagioclase, gray is quartz. Haapaluoma, Finland. Width of sample 15 cm.

Pegmatite with tourmaline, feldspar, and quartz. The width of the sample from Catalonia is 7 cm.

Spessartine (Mn-garnet), sodic plagioclase, and muscovite in pegmatite. Width of sample 10 cm.

Gabbroic pegmatite from Cyprus. White is plagioclase, black is pyroxene.

Vein of coarse-grained granite in a host rock of fine-grained granite. This decorative rock is used as a tombstone in Baton Rouge, Louisiana, USA. Width of view 25 cm.

The term “pegmatite” was first used by a French mineralogist René Haüy for such rocks that we nowadays know as graphic granites. Evje, Norway. Width of sample 9 cm.

Pegmatite in which the main ingredients are biotite mica (black), microcline (pink), oligoclase (white), and quartz (gray). The rock has a monzogranitic composition. Evje, Norway.

Cleavelandite is a lamellar variety of almost pure albite. It forms as a late-stage mineral in pegmatites by replacing other minerals. Width of sample from Ontario is 5 cm.

Pegmatite that contains two common minerals (biotite and garnet). But very unusual is that this is all there seems to be. No feldspars or quartz. Garnet crystals have well-developed crystal faces. Width of sample 13 cm. Senja, Norway.

Tonalite pegmatite. Black is biotite, white is plagioclase, gray is quartz. Trælen, Senja, Norway. Width of sample 17 cm.

Pegmatite with plagioclase, epidote, and muscovite. Møre og Romsdal, Norway.

Large plagioclase crystal from a pegmatitic rock demonstrating polysynthetic twinning (parallel grooves) characteristic to plagioclase feldspars. Evje, Norway.

Large feldspar crystal with greenish amazonite. Aust-Agder, Norway. Width of view 13 cm.

Pegmatite with graphic granite and plagioclase. Evje, Norway.

A contact between gneiss (on the right) and granitic pegmatite. Trollstigen, Norway.

References

1. Jahns, Richard H. (2007). Pegmatite. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 13. 124-126.

Sandstone is a consolidated sand. It is a very widespread and well-known sedimentary rock. It should be no surprise because sandstones make up 10…20% of all sedimentary rocks and sedimentary rocks are by far the most common rocks at the surface (see more interesting numbers pertaining to sand in a post brain games with sand grains).

Sandstone is very often visibly layered. The width of the sample from Scotland is 7 cm.

Sandstone is composed of sand-sized (0.0625…2 mm) mineral grains, rock fragments, or pieces of fossils which are held together by a mineral cement. It grades into siltstone, shale or mudstone (grains less than 0.0625 mm in diameter) and conglomerate (or breccia if the clasts are angular) if the average grain-size exceeds 2 mm¹.

Sandstone and other clastic sedimentary rocks differ from the igneous rocks in possessing a framework of grains which only touch each other but are not in a continuous contact. Consequently, sandstone contains a network of pores which are at least partly filled with a mineral cement. However, sandstone does not need to contain open pores, they may be, and often are, completely filled with a cementing material. The definition of sandstone is based on the size of the framework grains. No reference is made to the genesis.

Whether composition has any significanse is a tougher question to answer. It is generally not important whether it is composed of mineral grains or lithic fragments and what is the origin and proportion of these particles. Sandstone may even contain biogenic grains (shells, coralline algae, etc), but a rock that contains more than 50% of sand-sized carbonate grains is usually named calcarenite which is a type of limestone.

Naming of varieties

Sandstones are widespread and compositionally variable rocks which gives rise to a myriad of more narrowly defined varieties:

Sandstone or related rock type	Description
Arenite	A general term for all sandstones.
Arkose	A feldspar-rich (>25%) variety.
Calcarenite	A limestone variety composed of sand-sized non-terrestrial carbonate grains. Sandstone which is composed of terrestrial limestone fragments is calclithite.
Calclithite	A variety of terrigenous sandstone consisting of carbonate grains (>50%) from disintegrated limestones.
Flagstone	A sandstone that is readily split into thin flags suitable for paving. Fissility is given to the rock by mica.
Graywacke	A term that has been defined in several ways which has caused confusion and ultimately lead to its use only as a field term. Graywacke is generally imagined to be dark-colored, coarse-grained, lithic, well-indurated, and immature sandstone.
Greensand	A sandstone or sand that contains lots of green clay mineral glauconite.
Grit	A coarse-grained variety with angular grains. The term has also been applied to fine-grained gritty rocks in the past.
Orthoquartzite	A relatively pure light-colored quartz-sandstone. The term has been applied to well-indurated (quartzitic) rocks in the past, but nowadays it seems to involve all pure sandstones regardless of how friable they are.
Psammite	A synonym of sandstone and arenite.
Quartz arenite	An almost pure sandstone. Quartz content is above 90-95%.
Quartzite	A metamorphosed sandstone. The term has been applied to hard sedimentary sandstones.

Composition

Sandstones are composed of mineral grains or rock fragments that were once part of another rock. Therefore, it seems logical to assume that all rock-forming minerals have a chance to become sandstone constituents. Theoretically it is true, but in reality minerals differ greatly in their ability to resist weathering.

Many rock-forming minerals are simply dissolved during the transport as sand grains. Pyroxenes and amphiboles are very abundant minerals in certain dark-colored igneous and metamorphic rocks but they are relatively rare in sandstone. Feldspars are even more widespread and also more resistant to weathering. Hence, feldspars are quite common in sandstone although significantly reduced in quantity. Variety that contains more than 25% feldspar is named arkose. Quartz, on the other hand, is a common rock-forming mineral (although not as widespread as feldspars) and it is almost insoluble in water and physically very hard. This is why quartz is so abundant in sand. Some sandstones (quartz arenite) are almost exclusively composed of quartz grains. Micas are common minerals in rocks and form a significant part of certain micaceous sandstones.

The framework grains in most samples are either mineral grains (composed of only one mineral) or rock fragments (mineral aggregates of one or several minerals). It is the lithology of the source area that decides which ones will dominate. Granite, gneiss, and other coarse-grained crystalline rocks yield mostly mineral grains, but fine-grained rocks like basalt and shale can contribute mostly lithic fragments. Lithic and muddy varieties (graywacke) tend to be darker in color than white or reddish “cleaner” quartzose sandstones.

In addition to framework grains, sandstones also consist much smaller silt- or clay-sized clasts known collectively as matrix and a mineral matter between the grains that holds them together. This is known as cement. The cement is usually either carbonate (calcite and dolomite are very common) or silica (chemically precipitated material identical in composition to quartz grains). Small amounts of iron oxides are very common also. These oxides are mostly all what is left of unstable iron-bearing minerals like aforementioned amphiboles and pyroxenes.

Major sandstone constituents like quartz, feldspar, calcite, and iron oxides are usually accompanied by small amounts of other minerals known as heavy minerals. Important heavy minerals are magnetite, garnet, ilmenite, epidote, and zircon. Overview of minerals often found in sand is here: sand minerals.

Formation

Sandstone forms when sand layers are buried under sediments. Ground water that moves through the sand layers carries dissolved mineralized matter which precipitates over time to bind individual sand grains into solid rock. The most common binding agents are quartz, calcite, and iron oxides.

Sand dune in Sahara (Morocco). Sand dunes are in constant motion as you can see here the sand grains blowing off the crest of the dune. But as soon as environmental conditions change enough for the dune to lose its mobility, sand grains will be slowly cemented together by minerals precipitating out from groundwater and the formation of sandstone begins.

Structures

Sandstone structures are easily visible to the naked eye and their study is usually possible only in outcrops. Their scale is simply too large to be studied microscopically. Sedimentary rocks are usually layered and sandstone is no exception. Individual layers are made visible mostly by the variation in grain size. Layers are often easily noticeable because they may be differently colored. This is often also the result of a grain-size variation because water flows more easily in coarser layers and leaves behind more iron oxides. However, layers may also differ in original mineral content right after the deposition.

Common structure is graded bedding which means that the grain-size gets gradually smaller, usually from coarse at the bottom to the finer sand at the top. This indicates that the current that carried sand grains gradually lost its velocity. Cross-stratification or cross-bedding is a structure where parts of the rock sequence are deposited at an angle to the main sequence. Cross-bedding suggests that the sand form as a whole was slowly moving down-current. There are many types of cross-stratification which are not easy to differentiate because outcrops generally show us only a two-dimensional snapshot of the whole sandy dataset.

Color

The color of sandstone is highly variable. The most common mineral in most sandstones is quartz which is colorless if pure. Hence, pure quartzose sandstone tends to be light-colored (picture below about quartz arenite). However, these sand grains are often covered with very fine-grained hematitic pigment which gives variable shade of reddish color to the rock. The cement is usually responsible for the color of sandstone although the main coloring agent may sometimes make up less than 1% of the rocks volume. This is a common situation with redbeds to which vivid red color is given by a small amounts of iron oxide (mostly hematite). Sandstones that contain lots of rock fragments (lithic sandstones) are often dark-colored. Such sandstones are known as graywacke although this term is a bit old-fashioned nowadays.

Types

Sandstone is a granular rock, but usually it is assumed that these grains are mostly composed of silicate minerals. Clastic rock which is composed of carbonate shells is considered to be a special type of limestone (calcarenite or coquina), not sandstone or conglomerate.

Uses

Sandstone is a rock type which has many uses. Strongly cemented rocks are used as a building material all over the world where the material is readily available. Sandstone is often used in construction.

Crushed sandstone (as sand) is a common filling material in road construction and sand is a principal component of concrete. Pure quartz sand is a source of silica which is used to make glass, carborundum, and semiconductors. Some strong rocks with sharp grains (gritstone) are good for grinding.

Special types of crushed sandstones are used in agriculture as a soil conditioners (lime sand) or fertilizer (glauconite sand). Chemical industry uses sandstone because it is very resistant to most acids (however, this is true if the sand is really almost pure quartz sand). Because the rock is porous, it is by far the most important reservoir rock of ground water and hydrocarbons (crude oil and natural gas).

This rock is also a very valuable material for geologists because it is abundant, resistant to diagenesis, and contains lots of information to reconstruct the Earth’s geologic history.

References

1. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.

Peridotite is a dark-colored igneous rock consisting mostly of olivine and pyroxene. It is an important rock type because the Earth’s mantle is predominantly composed of it.

http://picasaweb.google.com/107509377372007544953/Peridotite#5779457195004254610
A classification diagram of ultramafic rocks. Note that neither peridotite nor pyroxenite are single rock types. They are further divided into subtypes. There are four subtypes: dunite, harzburgite, lherzolite, and wehrlite. How to read this diagram? It is a typical ternary plot which are often used in geology. Ol, Opx, and Cpx represent olivine, orthopyroxene, and clinopyroxene respectively. Dunite is a type of peridotite that is almost monomineralic (more than 90% of it is olivine). Wehrlite contains almost no orthopyroxene but is rich in clinopyroxene and olivine. Harzburgite is rich in olivine and orthopyroxene but there can be only up to 5% clinopyroxene. Lherzolite is the least pretentious of them — it is a mixture of two pyroxenes and olivine¹.

This rock is relatively rare at the surface and is often altered by hydrothermal metamorphism or weathering because its constituent minerals are unstable at the weathering environment.

Olivine, the principal component of peridotite, makes up more than 40% of the pair olivine + pyroxene (or amphibole). A rock type with less olivine is named pyroxenite (or hornblendite if there are more amphiboles than pyroxenes). Peridotite is a plutonic rock, it is mostly composed of visible mineral grains. Other notable minerals that are often present are chromite, garnet, and plagioclase. It is an ultramafic rock (mafic minerals make up more than 90% of the rocks composition).

Peridotite is often massive (homogeneous) just like other plutonic rocks, but it can be also layered. Layered peridotite is a cumulate rock formed by crystal settling from magma. Layered ultramafic typically occur at the base of gabbroic intrusions. The Troodos ophiolite in Cyprus is a well-known locality where such rocks can be seen at the surface. Massive samples are often brought to the surface as xenoliths inside basalt.

It is the source rock of basalt. Basaltic magma forms when peridotite is partially melted. Basalt and peridotite differ in composition because rocks are mixtures of minerals, but each mineral has its own melting temperature.

Some minerals start melting earlier and form basaltic magma which migrates upward because of lower density. Rest of the rock that did not melt have also different composition from the source rock, it is enriched in minerals with a higher melting temperature. This is the mechanism how different types of peridotite came to be. Lherzolite and wehrlite are sometimes referred to as a fertile mantle. These types of peridotite yield basaltic magma when they melt partially. What is left after the basaltic component is removed is called a depleted mantle. As a rock type it is harzburgite or in extreme cases dunite.

The main components of this rock, olivine and pyroxene, are unstable minerals in the weathering environment. By the time these rocks reach the surface they are often heavily altered by a hydrothermal metamorphism or weathering. The rocks that are called peridotites are therefore often heavily metamorphosed and should be called serpentinite instead of peridotite which it once was.

This rock itself is usually not a notable mineral resource (see the exception from Norway below), but valuable stuff may be associated with it. Chromite is a principal ore of chromium. Talc is found in some metamorphosed peridotitic rocks. Serpentinite is used as a decorative stone because of its interesting texture. Nickel and platinum are usually associated with ultramafic host rocks.

Dunite xenolith in basaltic lava from Hawaii. Width of sample 8 cm.

http://picasaweb.google.com/107509377372007544953/Peridotite#5779439380397705650
A sample from the Troodos ophiolite in Cyprus with a weathering rinds. Unaltered rock is in the middle. Width of sample 11 cm.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737569693773138802
Harzburgite from the Troodos ophiolite. This is example of a depleted mantle.

http://picasaweb.google.com/107509377372007544953/Peridotite#5779505387689222818
Layered harzburgite from the Troodos ophiolite.

http://picasaweb.google.com/107509377372007544953/Peridotite#5779439247917903922
Serpentinite sample from the Troodos ophiolite. It is a hydrothermally altered peridotitic rock.

http://picasaweb.google.com/107509377372007544953/2015#6190951168417918930
Main minerals are pyrope, chromian diopside and olivine. Åheim, Norway. Width of view 25 cm.

http://picasaweb.google.com/107509377372007544953/2015#6190951161905456610
Pyroxenite layers in a layered peridotite intrusion. Lower pyroxenite layer is about 5 cm thick. Åheim, Norway.

http://picasaweb.google.com/107509377372007544953/2015#6190953178323592850
Another sample from Norway showing pyrope and diopside phenocrysts in a finer groundmass which is mostly composed of olivine. Width of sample 11 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.