It is much harder than I thought. I am no expert in marine biology but I would really like to know what my sand samples contain. I have turned to several biologists and paleontologists but so far I have received very little useful insight.
Well, now I am turning to You. If you happen to be a marine biologist or know someone who could help me, please let them know of me or my blog. I have some nice material and I am willing to share it here but pictures without explanations are not much worth.
Here are some biogenic fragments picked from a fine-grained sand sample collected in Zakynthos, Greece. I know most of them are foraminifera but there is some other stuff too.
Biogenic grains from a sand sample collected in Zakynthos, Greece. The width of the view is 7 mm.
Sand is full of wonders. There are lots of really nice and unusually large forams in a sand sample from Cyprus.
Forams (foraminifera) are amoeboid protists. They are benthic or planktonic sea creatures (they are not animals) who build calcareous (mostly) tests (or shells) which become biogenic sand grains after the owner of the test dies.
Tests of foraminifera are usually less than 1 mm in diameter but these are almost 2 mm. I am not sure about the genus. It could be Amphisorus but Sorites and Marginopora are possibilities as well.
Take a look at the star sand also. These are forams as well and look amazing.
Forams picked from the sand sample collected in Paralimni, Cyprus. The width of the view is 20 mm.
Obsidian is a massive volcanic glass. The term ‘massive’ has several (although related) meanings in geology but here it means that the rock (obsidian is a rock type, not a mineral) is homogenous. It lacks layering, cleavage, foliation, phenocrysts, etc. It is just a piece of volcanic glass without further conditions. In the majority of cases obsidian solidified subaerially (on land). Volcanic glass that formed underwater has alternative names like tachylite and hyaloclastite.
http://picasaweb.google.com/107509377372007544953/Rocks#5848238745712314674 Typical obsidian is either black or slightly reddish and often demonstrates beautiful conchoidal fracture. Width of sample 11 cm.
So the volcanic glass and obsidian are not synonyms although in many cases you can freely use both terms. It is definitely not wrong to use ‘volcanic glass’ instead of ‘obsidian’ but you should be careful the other way around — volcanic glass is not always obsidian.
Volcanic glass is an igneous rock that is composed of largely uncrystallized magmatic material. Most of it is not crystallized because the crystals had two difficult problems which restricted their growth. The first one is time. Large crystals need a lot of time to grow. There is very little of it when viscous magma is pushed out of a volcano and cools rapidly. I already gave a subtle hint what the second problem might be. It is the viscosity of magma/lava. If the magmatic body is very thick or viscous, the crystals have a really hard time forming because they simply don’t have new material coming in if almost nothing is able to move inside the magma body. The result is that everything solidifies just randomly as glass.
So obsidian forms from viscous magma only? Usually yes, but not always. Most obsidians are rhyolitic in composition. This lava is the thickest because it has the highest silica content. Why is that important? Because silica causes magma to polymerize. There are countless bridges (chemical bonds) between oxygen anions of silica (SiO2) which is the reason why this magma is so hard to move. If the magma contains lots of metals (cations) then it is less viscous because these cations break the framework structure of silica. I think this point is important and worth paying attention to because lots of people seem to think that rhyolitic magma is more viscous than basaltic magma only because its temperature is usually lower.
Vesicular basaltic volcanic glass from Punalu’u Beach, Hawaii. Width of view 20 mm.
http://picasaweb.google.com/107509377372007544953/California02#5876808909850640834 Obsidian with gray layers of rhyolitic pumice in Long Valley Caldera in California. Pumice and rhyolite have very similar composition but they differ in texture — pumice is highly vesicular while obsidian is massive.
Is some volcanic glass still basaltic in composition? Yes, but in this case the cooling has been really rapid. This is the case if basaltic lava is flowing into the water. There are some nice black sand beaches in Hawai’i that are composed of fragments of volcanic glass with basaltic composition.
How to identify small obsidian fragments like sand grains? In most cases it is not too hard to do by optical examination only. Obsidian is usually black although reddish varieties are pretty common also. Obsidian has a strong luster and conchoidal fracture. This means that fracture surface is smoothly curving (like a seashell).
Obsidian is usually black. This color is caused by minute inclusions and tiny crystals in the glass. Red color is caused by the same stuff that gives red color to weathered basalt, desert sand and K-feldspar. It is mineral hematite (iron oxide).
http://picasaweb.google.com/107509377372007544953/Rocks#5848481665432908098 Obsidian is usually dark and its surface is shiny.
Obsidian is not stable in the weathering environment but it does not mean that it can not last millions of years. Obsidian on the Moon may be billions of years old because the Moon is dry. The same applies here on the Earth as well. In dry areas obsidian can last pretty long. However, obsidian formations older than the Cenozoic (it began 65 million years ago) are unknown.
Here are two pictures of obsidian grains picked from coarse-grained sand samples.
Obsidian from the Yellowstone National Park. Width of view 35 mm.
Obsidian from NE California (The Cascades volcanic range). Width of view 22 mm.
Magnetite (Fe3O4) is a common iron oxide mineral. It is a member of the spinel group. These are minerals that share the same structure but differ in chemical composition. Other notable members of the group are chromite and spinel. Magnetite is among the two major sources of iron. The other important iron-bearing mineral is hematite.
http://picasaweb.google.com/107509377372007544953/Chert#5807632822805960610 Crystals of magnetite are opaque with slightly bluish black color. Width of view 25 mm.
Composition
More precise way to express the chemical composition is to differentiate between di- and trivalent iron: Fe2+Fe23+O4. However, this is the ideal end-member composition. Real crystals found in nature almost always contain variable amount of Al, Cr, Mn3+, and Ti4+ substituting for Fe3+ and Ca, Mn2+, Mg replacing Fe2+. Titaniferous variety is named titanomagnetite. The term has been applied somewhat loosely, but it is best to restrict it to those varieties where the ulvöspinel phase can be demonstrated by X-ray analysis1. Ulvöspinel is an end-member of the spinel group with the following composition: Fe22+TiO4.
The composition expressed as Fe3O4 may cause some confusion. Oxygen has an oxidation state of -2, and iron usually have oxidation states of +2 or +3 (ferrous and ferric iron, respectively). To form a crystal, these oxidation states must balance or cancel each other out but 4 × -2 = -8 which is not balancing 6 (2 × 3) or 9 (3 × 3). Is there an error in the formula?
Not really. To overcome this problem it is useful to treat it as a mixture of two iron oxides with oxidation states of +3 and +2 respectively (Fe2O3 and FeO) which are combined in a certain way and form magnetite crystals. It is important to understant that magnetite is not a mixture in the strict sense. It is a crystalline solid in which different iron atoms are chemically combined with oxygen atoms.
Magnetite (grayish black) showing typical octahedral crystal shapes. Yellow mineral is chalcopyrite. Width of view 30 mm. Skarn-related polymetallic ore deposit. Hannukainen, Finland.
Properties
The most striking property of magnetite is very strong ferrimagnetism. It makes the mineral easily identifiable because it is strongly attracted to a hand magnet. Ferrimagnetism is caused by opposing, but unequal magnetic moments within the crystals which results in permanent and spontaneous magnetization of the material.
The presence of di- and trivalent iron in the crystal lattice is the reason why magnetite is so strongly magnetic. Divalent (+2) and trivalent (+3) iron have unequal magnetic moments that are not balancing each other. Magnetite is the most magnetic mineral.
High iron content gives magnetite its opaqueness and black color. Spinel which shares the same structure is variably colored and transparent because it contains magnesium and aluminum instead or iron.
Magnetite is dense (specific gravity 5.20) mineral. This is considerably above common silicate minerals (usually 2.5–3.5) which is why rocks containing appreciable amount of magnetite feel heavy in hand sample. Hardness is about 6 on the Mohs scale. Magnetite has no cleavage but parting may be distinct. Crystals are brittle and fracture is uneven.
This is how magnetitic sand aligns itself in the presence of a strong external magnetic field. There is a neodymium magnet placed beneath the sample. Crystals from Talofofo Beach, Guam, USA. Width of view 10 mm.
Occurrence
Magnetite is a very common (but usually accessory) mineral in igneous and metamorphic rocks. It occurs in a wide variety of igneous rocks as small octahedral or anhedral grains. It may form larger segregations in contact-metasomatized carbonate rocks (skarns) where it is associated with calcite and calc-silicate minerals like diopside, andradite, actinolite, tremolite, etc.
Massive variety may also occur in some mafic layered intrusions. It may form in regionally metamorphosed rocks where it forms at the expense of iron hydroxides (goethite, limonite) and oxides (hematite).
It is the main iron-bearing mineral in the oldest Algoma-type banded iron formations where it is associated with chert.
Magnetite is among the most common minerals in heavy mineral fraction of sand. Its grains in sand are generally much smaller than lighter mineral grains because of different settling velocity. Most magnetite grains in sand are rounded but some show characteristic octahedral morphology. It is never elongated because of cubic (isometric) crystal system.
Magnetite is common in sand because it is abundant in many rock types and it is also moderately resistant to weathering. In some places beach sand may be so concentrated in magnetite that it could be used used as an iron ore. In New Zealand a sand deposit called Ironsand is used to make steel.
Magnetite is altered in the weathering environment to hematite, goethite or other iron oxides and hydroxides. Martite is a pseudomorphous hematite after magnetite.
Magnetite is a common heavy mineral in sand. This rock is a metamorphosed sand deposit which seems to be very rich in heavy minerals magnetite (black) and garnet (red). Varanger Peninsula, Northern Norway. Width of sample 18 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196127722114477058 Actinolite (green) with magnetite and calcite. Kiruna, Sweden. Width of sample 8 cm. Magnetite crystals forming black stripes in light-colored sand. It is one of the most common constituents of heavy minerals in sand. White Park Bay, Northern Ireland.
http://picasaweb.google.com/107509377372007544953/2015#6196126801209467186 Magnetite with amphibole group mineral tremolite in skarn. Skarn is a contact-metasomatic rock. It forms when hot silicic magma comes to contact with carbonate country rocks (dolomite, limestone, marble). The result is unusual assemblage of calc-silicate minerals like tremolite, diopside, andradite, wollastonite, etc. These rocks also frequently contain ore minerals because late-magmatic fluids are usually enriched in incompatible chemical elements that have no place in the crystal structure of common magmatic minerals. Skarn was originally a miners term for the gangue minerals (calc-silicates) surrounding the ore veins. Width of sample 8 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196127795531026770 Magnetite in skarn. Gangue minerals are serpentine and talc. These minerals hint that there must be a major source of magnesium. These rocks indeed formed when magma intruded and reacted with dolomitic (Mg-Ca-carbonate) marble. Tapuli, Sweden. Width of sample 11 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196127827237245602 Skarn sample with magnetite, diopside (Ca-Mg-pyroxene), and calcite. Tapuli, Sweden. Width of sample 12 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196127988439098066 Magnetite is a common hydrothermal mineral that occurs in quartz veins with other ore minerals. This sample also contains quartz (white), pyrite, and chalcopyrite. Hannukainen, Finland. Width of sample 11 cm.
Uses
Magnetite is a major source of iron. Banded iron formations are precambrian metasedimentary rocks where the iron-bearing phase is usually either magnetite or hematite. Very rich magnetitic iron ore is in Kiruna (northern Sweden) although the formation details are not clear (it is not banded iron formation). Skarn-related iron ores are also mined although they tend to be less voluminous. Iron may be also extracted from placer deposits (heavy mineral sand).
It is industrially used as a feedstock in the manufacture of other iron-bearing materials. Magnetite has been used to make high density concrete to nuclear reactors. It is also used as a black pigment2.
Naturally magnetized magnetite is called lodestone. Normally it is only attracted to hand magnet, but magnetite itself does not attract objects mader of iron. Lodestone is different because it does that too and it readily aligns itself along the magnetic lines of the Earth. This makes lodestone useful in navigation as a natural magnetic compass. It is not entirely clear why some magnetites are naturally magnetized, but lodestones contain inclusions of maghemite (spinel group mineral) and one theory associates it with magnetic fields surrounding lightning bolts. This could explain why lodestones are found close to the surface, not from deep iron mines.
Magnetite crystals have been found in the brains of several species, including humans. It has been hypothesized that birds could make use of it to navigate, but it is not clear what benefits can they provide to humans.
http://picasaweb.google.com/107509377372007544953/2015#6196127759275533122 Massive chunk of iron ore which is composed of almost pure magnetite. Iron ore from Kiruna is world-famous as a very rich high-grade ore. The sample feels very heavy when compared to usual silicate rocks. Kiruna, Sweden. Width of sample 13 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190951051055692018 Magnetite with jasper and hematite. These minerals come from the hydrothermally altered oceanic crust. Hot newly formed oceanic crust at the mid-ocean ridge is full of cracks which allow seawater to intrude the crust. Water heats up when circulating within the rocks and leaches metals out of the basaltic crust. Metals are precipitated when this very hot and metal-rich water enters the ocean again through black smokers. These metal deposits are known as SedEx-type (sedimentary exhalative) ore deposits. Løkken ophiolite, Norway. Width of sample 13 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196126915729581058 Algoma-type banded iron formation (BIF) from the Archaean. Magnetite is the principal iron-bearing ore mineral in these very old iron ores. Banded iron formation is the main source of iron although the majority of these deposits are from the Proterozoic. Bjørnevatn, Norway. Width of sample 17 cm.
http://picasaweb.google.com/107509377372007544953/Chert#5807632624753639538 Superior-type banded iron formation from Kryvyi Rih, Ukraine. Superior-type BIFs are the main source of iron. Iron-bearing mineral in these rocks is usually either hematite or magnetite. Width of sample 10 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196126996112193458 Magnetite in quartz. Bjørnevatn, Norway. The original banding of BIF is disturbed by the metamorphic processes. Width of sample 11 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196127716127610130 Iron ore from Kiruna. The main minerals are magnetite, calcite, actinolite, and apatite. Kiruna is the largest iron mine in Europe. Yet the formation details of these rocks are still poorly understood. Width of sample 14 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196127712765912274 Magnetite with feldspar. Kiruna, Sweden. Width of sample 16 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196127731778962802 Magnetite with calcite (white) and pyrite (iron sulfide). Kiruna, Sweden. Width of sample 14 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196127778630677378 Magnetite in syenite porphyry. Kiruna, Sweden. Width of sample 15 cm.
http://picasaweb.google.com/107509377372007544953/Chert#5807632919137273842 This is sand from the North Island of New Zealand. It is used as an iron ore. The black grains are titanomagnetite (total titanium content of the sample is 4 percent). Iron makes up 20 percent of the sample (XRF data). Yellow grains are silicate minerals. Width of view 10 mm.
http://picasaweb.google.com/107509377372007544953/2015#6190951521250257778 It is a major constituent in a heavy mineral fraction of sand. Lots of black minerals on this gold pan are magnetite grains. There is also gold (yellow spots). Tankavaara, Finland.
Foraminifera or more precisely their calcareous tests are very common components of many beach sands.
Lots of so-called coral sands are composed mostly (or in large part) of foram tests. These tests are usually worn out and barely recognizable. Still, some fresh examples of certain genuses look really spectacular. Probably the best example is a star sand from Ryukyu Islands, Japan.
This sand is actually composed of several foram species. Two of the most eye-catching genuses are Baculogypsina and Calcarina. Sometimes they are treated separately and called the Star sand and the Sun sand, respectively.
Below are pictures of selected foram tests picked from a sand sample collected in Hatoma Island (200 km east of Taiwan).
Foram species Baculogypsina sphaerulata. The width of the view is 15 mm.
Foram genus Calcarina. The width of the view is 20 mm.
Quartz grains in sand may be rounded or angular but they show their crystal faces extremely rarely. There are two good reasons for that.
Quartz is a tough mineral. It is resistant to both physical and chemical attack. But as millions of years pass they eventually obtain more and more rounded shape. Many quartz grains we encounter in beaches or desert dunes may be very old and their crystal faces, if they had them, are long gone.
But there is another and even better explanation. Most quartz grains never existed as beautiful crystals. Quartz is primarily an igneous mineral — it crystallizes out of magma. If the magma body cools, crystals start to form. Most of these crystals belong to the silicate minerals — they contain silicon and oxygen.
First silicates that start to crystallize take in addition to silicon and oxygen other elements such as iron, magnesium, and calcium into their crystal structure. If all these elements are removed from the melt and there is still silicon and oxygen available (it happens in most cases), then the rest crystallizes as pure silicon dioxide which is a mineral we call quartz. Therefore, quartz is among the last minerals to form in a cooling magma body and it has to take the space which is still available between the crystals that have already formed. This space is obviously irregular and provides no opportunity for the crystals to grow as they would like.
But sometimes quartz forms crystals with well developed faces. How is it possible? It can happen if the quartz crystals form in a vug — a crack or fracture in the rocks. It crystallizes out of water that circulates in such cracks. In this case there is nothing to stop quartz crystals to grow just like they want to as long as there is free room to do that.
Quartz from these vugs will be liberated as sand grains when the weathering reaches their host rocks. It should be obvious now that such quartz crystals are very rare in sand. There are some other environments where quartz crystals grow but none of them is a significant source. Below you can see one of these samples. These euhedral (with well developed crystal faces) quartz crystals grew in a gypsum deposit in New Mexico, USA. They are known as the “Pecos diamonds”.
Crystals of quartz (the Pecos diamonds) picked from a coarse-grained sand that comes from a gypsum deposit in New Mexico, USA. The width of the view is 15 mm.
Spodumene is a fascinating mineral. It is one of the pyroxenes. This is a family of minerals which give black color to basalt. Spodumene, however, is anything but black.
Industrial concentrate of spodumene crystals. The crystals are typically prismatic and they have clear vertical striation (runs parallel to the longer axis of crystals). The concentrate is from the Talison Lithium Mine in Australia. The mineral is extracted there from pegmatite which is uniquely rich in spodumene — about 50% of the rock is composed of this mineral.
I like this mineral as a good example that neither the color nor the chemical composition (which determine the color) are important in general silicate mineral classification. It is structure that matters. We take a look at the chemical composition only to separate one mineral (spodumene, diopside, jadeite, etc.) from another inside a mineral group, but not to distinguish one mineral group (pyroxene, amphibole, garnet, tourmaline, etc.) from another.
Why isn’t it black like most of its more abundant relatives? Because it contains no iron. And no magnesium. This mineral is a lithium pyroxene, its chemical composition is LiAlSi2O6. It forms long chains just like all other pyroxenes and has therefore prismatic habit.
Is it common in sand? No, not really. It is not even common in rocks. Still, there is one rock type in which it is not rare. This rock is very interesting too because it contains often rare minerals. It is pegmatite. Pegmatite is a coarse-grained igneous rock that crystallized deep inside the crust from the late magmatic liquid that contained lots of chemical elements that didn’t fit into the crystal structure of common minerals. Hence, pegmatites often contain rare minerals (spodumene, tourmaline, beryl, lepidolite, etc.) that contain rare elements (lithium, fluorine, boron, uranium, rare earths, tantalum, niobium, etc.). There is another reason why it isn’t a common mineral in sand. It is not stable in the weathering environment.
Spodumene, although not very common, can sometimes grow crystals with enormaous dimensions. Crystal more than 10 meters long and 65 tons in weight was found in Etta Mine, South Dakota, USA. Spodumene has several uses. It is an industrial mineral that is mined for its lithium content. That’s the stuff we need to make batteries for our computers. Fortunately, it is not the only source of lithium we have. Most of it is extracted from brines as lithium carbonate.
There is a beautiful abandoned quarry in Karelia with bright white walls and deep blue lake at the bottom.
This is Ruskeala marble quarry near the border of Finland. This area was part of Finland before the Second World War but then Russians thought that they don’t have enough land yet. Now we have to go to Russia to enjoy this breathtakingly beautiful place.
The marble quarried in Ruskeala has been used extensively in many famous buildings of Saint Petersburg. Some quarries are abandoned now because of dynamite use which cracked the rock and made it therefore unsuitable for monuments or decorative purposes.
Marble from Ruskeala quarry in Karelia, Russia. White mineral is calcite, dark stripes are composed of graphite. The diameter of the rock is 13 cm.
This marble is really old, it formed in the early Proterozoic. Rocks of that age and even significantly older ones from the Archaean are common in Karelia. This area is geologically the core of Europe around which all other parts like a puzzle pieces one after another later joined.
Sand is usually composed of mineral grains that come from disintegrated rocks. Rocks are mineral aggregates, they generally contain several distinct minerals. If we have a sand that is compositionally close to the rocks it comes from, then we call it immature. The opposite situation is a sand that is almost exclusively composed of quartz and other ultraresistant minerals. Such sand is mature or evolved because it has lost significant part of its former components.
Sand sample collected near Bridalveil Fall, Yosemite National Park, California. This is river sand. Its source rocks are very near. Sand is composed of plagioclase (white), quartz (transparent), K-feldspar (yellow, reddish), and biotite (black, some are brown and green due to weathering). These minerals put together into one rock give us aigneous rock granodiorite (similar to granite but contains more plagioclase than K-feldspar). The width of the view is 10 mm.
Sand composed of rock fragments (lithic sand) is definitely the most immature sand type in existence but I’m not focusing on that here. This post is devoted to immature sand that is largely composed of mineral grains.
Many minerals are unstable in the weathering environment. Therefore the immature sands can not be located far from their parent rocks. In addition to time, the most important variables that determine the rate of weathering are temperature and the availability of moisture. Hotter climate makes chemical reactions faster and water is needed to generate these reactions. Maybe you have heard that weathering is the disintegration process of rocks which is driven by temperature changes, frost-thaw cycles, etc. That is true to some extent but chemical weathering is by far the most important agent of weathering.
So, where do we have the best chances to see this type of sand? We need cold and/or dry climate plus short transport distance. Our chances are pretty good if we start looking for this type of sand in the riverbanks because the transport distance there is often very short. Sand grains on the beach are often evolved, especially in hot and humid climates. Beaches of Florida, for example, are predominantly composed of almost pure quartz (some contain significant amount of biogenic grains also, but this is not important here). Beaches of Canadian Arctic on the other hand are not mature at all. They generally contain lots of feldspar and other minerals in addition to quartz.
This is beach sand from the Canadian Arctic. Coronation Gulf, Nunavut. Sand is composed mostly of quartz (transparent), feldspars (red), and hornblende (black). Its source area could be a metamorphic terrane composed of granitic gneiss and amphibolite. The width of the view is 10 mm.
Gypsum is a relatively rare constituent of sand. An exception is a large dune field in New Mexico White Sands National Monument that is entirely composed of tabular gypsum grains.
Why is gypsum rare in sand? Because it is moderately soluble in water. Gypsum crystallizes out of concentrated solutions — it is an evaporite mineral. It can also quite easily go into solution again. Anything soluble is generally not going to last long in sand. Gypsum sands in New Mexico exist there because this state is not too famous for a wet climate, quite the contrary. The area also has no outlet to the sea which means that gypsum grains that are dissolved in rain water have no escape from the area and eventually may become sand grains again.
Ordinary sand grains made of quartz are the disintegration product of granite, sandstone, or other quartz-containing rocks. The crystals of quartz can be very old. Gypsum grains in the White Sands National Monument are different. They are not the product of disintegration of rocks. These grains are formed in the salty brines which get their high dissolved gypsum from the gypsum containing sedimentary rocks nearby.
White gypsum dunes of the White Sands National Monument in New Mexico, USA. Photo: davebluedevil/Wikimedia Commons.
Sand from the White Sands National Monument in New Mexico that is composed entirely of gypsum grains. Width of view 5 mm.
, 2nd Edition. Prentice Hall.
, 2nd Edition. Oxford University Press.
