Silica and sand

June 11, 2025November 8, 2011 by Siim

What is silica? How is it related to sand? What does it mean that silica sand is made of quartz? It is not difficult to answer these questions but unfortunately the websites I checked contained some erroneous information which needs to be sorted out.

Sandstone in Tabina quarry
Siliceous Devonian sandstone in Estonia.

Silica or silicon dioxide (SiO₂) is a chemical compound consisting of one silicon and two oxygen atoms. Quartz is a common mineral with the same chemical composition but quartz and silica are not synonyms. Specific minerals always have a definite crystal structure while chemical compounds have no such restriction — just like every piece of carbon is not a diamond. Quartz is made of silica but so are also cristobalite, tridymite and few other minerals (polymorphs of silica). They are collectively referred to as silica minerals.

Quartz is the most common sand-forming mineral. However, it is not the most common mineral in the crust. That honor goes to feldspars. If the particular sand deposit contains almost nothing but quartz, we often call it a silica sand. Such sand deposits are said to be mature because other rock-forming minerals are already broken down by the weathering process leaving only the super-resistant quartz as a residue. Silica sand is a mineral resource. It is mined mostly for glass-making. Another major use of sand is a concrete production but that does not need sand to be as pure.

Some beautiful beaches are made of silica sand. Beach of Siesta Key in Florida is especially famous for its white sand. Not all white sands are made of silica, though. White Sands National Monument in New Mexico is a dune field which is composed of sand made of gypsum. There are lots of light-colored beach sands around the world but many of them (especially in low latitudes) are made of small pieces of corals and other sea creatures. This sand is calcareous (composed of calcium carbonate) but some biogenic grains are siliceous as well. For example radiolarians (ameoboid protozoa) and diatoms (algae) have siliceous shells.

Sometimes sandstone is said to be siliceous. What does that mean? It may be a sandstone which is cemented by the silica minerals quartz or chalcedony (cryptocrystalline quartz) or it could be a sandstone which is composed predominantly of silica minerals (although sometimes feldspars are included). So you really need to dig deeper and ask critical questions if someone is talking about a siliceous sandstone as its meaning is not immediately obvious.

Siesta Key Beach
Beach sand in Siesta Key, Florida is almost pure silica sand (composed almost exclusively of quartz grains).

Every rock has a story

December 1, 2015November 7, 2011 by Siim

An archeologist studying the ruins of a medieval castle in Estonia discovered a rock like which he had never seen before. He was curious enough to pick it up to ask geologists what it might be. I was not the geologist he asked help from. It was my supervisor. I noticed that rock on his book shelf and immediately knew that I have a rock just like that in my collection as well. This rock is trachyte and it is from Western Germany (the Eifel volcanic field). Trachyte is a volcanic rock. It contains less silica than rhyolite and more alkali metals (sodium, potassium) than dacite and andesite. It is generally light-colored and porphyritic. Our trachyte contains phenocrystals of sanidine, plagioclase, biotite, and hornblende in a groundmass composed of K-feldspar, sodic amphiboles, and diopside.

Trachyte (this sample is from my collection, not from the castle ruins). Note the large sanidine phenocryst in the lower right part of the picture. The diameter of the sample is 8 cm.

This rock is more specifically from the mountain of Drachenfels in the Siebengebirge volcanic range. According to Wikipedia there is even a legend associated with this rock: “A German legend recounts that Siegfried – the hero of the Nibelungenlied – killed a dragon living in a cave in the mountain, then bathed in its blood to become invulnerable. Hence, the mountain is named the “Dragon’s Rock” — Drachenfels”.

But what is so strange about all this? The castle I am talking about is really old. There was an Estonian stronghold already before the conquest of the Teutonic Knights (13th century). The wall from which this particular rock was found is built definitely later but probably no later than the beginning of the 16th century, when the whole castle, which belonged to the Teutonic Orders local branch Livonian Order, was completed. The castle was destroyed during the Livonian War which took place during the second half of the 16th century. Our bigger neighbors for some reasons have had an especially nasty habit throughout the history to settle many of their differences on our soil. In this particular case the belligerents were Russia on one side and Sweden, Livonian Confederation, Poland, and Denmark-Norway on the other.

It is really surprising that rocks (at least one) were brought here from Western Germany (few thousand kilometers away) when we have abundant supply of our own rocks. It only reminds me something I already know: that every rock indeed has its own story and sometimes this story is not entirely natural.

The ruins of Viljandi Castle in Southern Estonia. The western wall of the convent building. This is the tallest still standing construction of the castle. Photo: Wikipedia.

Alone on the volcano

June 11, 2025November 5, 2011 by Siim

Santorini (Thira) is an island in the Mediterranean and possible location of ancient Atlantis. The island is strangely crescent shaped. Its inner edge is high and steep while the outer edge is low.

Santorini resembles a mountain which somehow lost its highest central part. That may sound like a fantasy but that’s exactly what geologist believe happened to the island some 3600 years ago. It was a result of an enormous volcanic eruption which produced more than 60 cubic kilometers of volcanic material. Partially emptied magma chamber collapsed (Wikipedia says that its top was blown off but this is just a popular misunderstanding of a caldera forming process) and the central part of the volcano vanished below the waves.

This volcanic eruption devastated the settlement of Akrotiri in Southern Santorini. This settlement is believed by many to be the location of Atlantis. It is, actually, the most probable location if you ask it from geologists. It makes absolutely no sense to place it in the middle of the Atlantic Ocean as it is often done. It simply can not be there.

Satellite photo of Santorini. Nea Kameni is the dark-colored island in the middle of the caldera. Photo: NASA.

The formation of the caldera 3600 years ago was not the first one there and it will not be the last one either. Santorini is volcanically active (last eruption in 1950) although the current volcanic activity is not concentrated in the main island. There is a small island named Nea Kameni (New Rock) in the middle of the caldera where the volcano is rebuilding itself once again.

I visited Santorini and Nea Kameni in the early spring of 2011. It was off-season. The number of tourists was small but I like it that way. If possible I’d like to visit geologically interesting areas without large crowds. Besides it is far too hot during the summer months. But such somewhat arrogant attitude has its drawbacks also. I really wanted to visit the volcano (Nea Kameni) but there were no boat trips available. We asked (I was there with my wife and 8 months old son) from several places but it seemed to be hopeless. It was really frustrating. That damn island was almost within reach if looked from the caldera rim but still so inaccessible. We still didn’t gave up and one man in the harbour agreed to take us there with his boat. It wasn’t cheap but I think it was worth it. There were only three of us and the volcano.

Maerl from Ireland

December 1, 2015November 4, 2011 by Siim

It seems to be a widespread misunderstanding that biogenic sand is a tropic phenomenon. It is true that corals prefer warm, clear, and shallow water but corals are not the only organisms that contribute to the formation of biogenic sand. Many so-called coral sands contain no coral reef fragments at all. One of them is a maerl which is not uncommon on the beaches of Ireland, Scotland, and Bretagne.

http://picasaweb.google.com/107509377372007544953/Chert#5807632592190233298
Fine-grained biogenic sand consisting of red algae, sponge spicules, sea urchin spines, clams, sea snails, and foraminifers that occur close to maerl deposits. Width of view 5 mm.

However, the term ‘maerl’ is probably not well known. Even the Glossary of Geology (1997 edition) doesn’t mention it. It is a sand or gravel (many maerl fragments are actually larger than 2 mm – the upper limit of a sand grain) that is predominantly composed of delicate aragonitic shells of a coralline red algae (Rhodophyta). Two of the most common species on the Ireland’s western coast are Phymatolithon calcareum and Lithothamnion corallioides.

The color of these algae is red or pink but the color vanishes when algae dies. Maerl sand is predominantly white. Red algae has important role in the coral reef ecosystem but they could successfully exist without the help of corals. Maerl is not composed of only red algae. It is interestingly versatile. There are lots of sponge spicules. These are clear siliceous (mostly) structural elements of sponges. They look very cool – just like the logo of Mercedes-Benz. There are also clams, foraminifers, snails, and sea urchins. Maerl is sometimes composed entirely of biogenic fragments. It could be fine- or coarse-grained.

Coralline red algae live in varying depths of water (less than 20 meters below the sea level in NE Atlantic) and get occasionally washed onshore with stronger waves. This sand is not without economic use. It has been used in agriculture (as a soil conditioner), cosmetics, bone surgery, water purification, and even as a food additive.

http://picasaweb.google.com/107509377372007544953/Chert#5807632589430685138
Coarse-grained maerl composed of red algae but also clams, sea snails, and quartz grains. Width of view 15 mm.

What is black sand

June 11, 2025November 3, 2011 by Siim

Black sand is sand that is black in color. It seems to be very simple. But what is behind this concept? How is this type of sand formed? What is it made of? There is no single and easy answer to these questions because there are a number of different dark sand grains that can form black sand and hence there are several different ways how black sand can form.

Black volcanic beach sand — Black sand on a volcanically active oceanic island. Puerto Naos, La Palma, Canary Islands.

The realm of black sands can be broadly divided into two parts, both of them having subdivisions. The most widespread type of black sand is composed of volcanic minerals and lava fragments. Such sands are especially common on the coasts of volcanic islands (Hawai’i, the Canary Islands, the Aleutians, etc.).

Black sand beaches are black because many volcanic minerals and rocks are dark-colored. Common rock types of volcanic islands are basalt (black when fresh), andesite (usually dark gray) and volcanic glass (often black in color). The minerals that give black color to these rocks are predominantly pyroxenes (mostly augite), amphiboles (mostly hornblende) and iron oxides (mostly magnetite). Such sands are heavier than ‘normal’ light-colored sands and become very hot on a sunny day. Dark color and heavyness are both caused by high iron content. Iron gives black color to most minerals because it absorbs light very well and it is also heavy.

Black sand on the Reykjanes Peninsula
Black volcanic sand on the Reykjanes Peninsula in Iceland.

Black sand on La Palma
Black sand on the western coast of La Palma, Canary Islands.

Basalt cobbles on the beach
Basalt is the most common source rock of black sand. Photo taken near the southern tip of La Palma.

Black volcanic sands may contain many non-black grains like green olivine crystals, reddish (usually because of weathering) volcanic rocks, light-colored quartz (when the source area is continental) and carbonate biogenic grains (coral sand). Most volcanic minerals are not very stable. They decompose pretty rapidly. These sands are said to be compositionally immature (mature sands are composed of quartz and other minerals very resistant to weathering). They also contain unusally high content of lithic (rock) fragments which have not broken up yet to form a sand composed of individual mineral grains.

Volcanic minerals in beach sand of Martinique.
Fine-grained volcanic beach sand from Martinique. Green prismatic mineral is augite. Black is magnetite. Width of view 7 mm.

Black sand composed of volcanic glass.
Black beach sand composed of volcanic glass. Punalu’u Beach in Hawai’i.

Another type of black sand occurs mostly in continental settings. It is heavy mineral sand. Heavy minerals are minerals which have a specific gravity above 2.9. There are almost all colors present among the heavy minerals but they seem to be dark compared to usually light-colored quartzose sand. Heavy mineral sands are usually composed of minerals that are relatively resistant to weathering. Such minerals are tourmaline, magnetite, garnet, rutile, ilmenite, zircon, epidote, staurolite, etc. Heavy minerals are in most cases disseminated among the light-colored (and usually much larger) quartz grains but in certain conditions they tend to accumulate.

You probably have seen dark stripes on a sandy beach which may even be mistaken for an oil pollution. These streaks are composed of tiny gems that were carried high on the beach either by big waves or streams but they successfully managed to avoid flowing back with the receding waves because of their above average density. The most common heavy minerals forming black sands are perhaps magnetite, garnet and epidote. They are widespread enough in the rocks and resist weathering moderately well. They are more resistant than typical minerals of volcanic black sands (olivine, pyroxene, hornblende) but not as resistant as rutile, tourmaline, and zircon. But the latter three never make up the bulk of rocks and therefore are rarely very concentrated in sand.

Heavy minerals in beach sand
Heavy minerals forming black stripes in light-colored sand. White Park Bay, Northern Ireland.

magnetite grains aligned in the external magnetic field
Magnetite grains in the presence of a strong external magnetic field. There is a neodymium magnet placed beneath the sample. Magnetite crystals are from Talofofo Beach, Guam, USA. Width of view 10 mm.

Word games with ignimbrite

June 11, 2025November 2, 2011 by Siim

Ignimbrite is a violent and often deadly rock type. Ignimbrite is a deposit of a pyroclastic flow (extremely hot mixture of gases and volcanic particles flowing down the flanks of a volcano). It is often welded (hence the synonym welded tuff) but not necessarily. According to the Glossary of Geology, all kinds of pyroclastic flow deposits are ignimbrites.

‘Ignimbrite’ is also a nice and intriguing word. It is a combination of latin words ignis (fire) and imber (rain). But the latter word could easily be replaced with nimbus (cloud) as well. I don’t know what is the deal actually. Often used synonym of a pyroclastic flow is nuée ardente (burning cloud in French). Maybe this is an accidental ambivalence? Or is it deliberate and therefore very clever find for this volcanic phenomenon? Or is the etymology of an ignimbrite misinterpreted and actually it was originally thought to be ignis+nimbus. There is even a fourth possibility that the words imber and nimbus are so closely related in Latin that basically these are two slight variations of the same word.

It would be nice if someone more familiar with Latin or the etymology of geological terms could clarify this issue.

Ignimbrite is a rock type – a deposit of a pyroclastic flow.

Ignimbrite rock sample — Welded ignimbrite with pumice fiamme from Gran Canaria. Width of sample 9 cm.

The most photographed rock

December 1, 2015November 1, 2011 by Siim

The Roque Cinchado is a rock formation in the Island of Tenerife (Canary Islands). It is one of a group (Roque del García) of erosional remnants in the vicinity of Pico del Teide volcano. It is worth to take a look and in this particular case I am definitely not the only one who thinks that way. I have heard that the Roque Cinchado is the most photographed rock in the whole world. This may be true but I am still sceptical. After all, aren’t several beaches claiming to have the whitest sand in the world and several manmade structures claim to be the most photographed buildings (Eiffel Tower, Taj Mahal).

http://picasaweb.google.com/107509377372007544953/Tenerife#5835129402231697250
The Roque Cinchado – volcano-sedimentary erosional remnant near the Pico del Teide, Tenerife.

What about Uluru in Australia or the Delicate Arch (Utah, USA) or the Devils Tower (Wyoming, USA)? I am not even talking about the Grand Canyon in Arizona but maybe it isn’t a rock and doesn’t qualify? Yes, reasonable people will positively confirm that the Grand Canyon is not A rock. It is a deep canyon with walls composed of different rock types. But can we say that the Cinchado is a rock? I’d rather say its more like an erosional remnant of a volcano-sedimentary sequence although it is probably overkill for most people. I really don’t know the answer to that intriguing question of most photographed rock but it is not that important for me anyway.

Important, somewhat puzzling and actually saddening is the fact that even as famous as the Cinchado is, there is very little geological information available in the web. How did it form? What are the layers made of? Almost nothing. You have to dig into research papers but most of them are not publicly available. Only thousands of different and equally useless photos of the Cinchado are available and now I am adding yet one more.

However, I try to excuse myself by adding few words about the geology as well. The Island of Tenerife is one huge volcanic edifice that is actually second in size after only the Island of Hawaii (Big Island). Current peak of the island is called the Pico del Teide (3718 meters above the sea level). It is situated in the Las Cañadas caldera which means that the volcano have somehow destroyed its former and probably even much higher peak and later built a new one. There have been much controversy and scientific dispute about the origin of the caldera. We know how calderas form in the majority of cases. Volcanic edifice collapses after enormous eruption which partially emptied the magma chamber beneath the volcano. But the Las Cañadas caldera seems to be different. We know now that there is a sedimentary sequence on the sea floor right next to the island reaching 50 km in length. The volcano, therefore, did not collapse vertically, it was an enormous landslide. Such landslides seem to be pretty common in this region. There are three major debris avalance deposits right next to Tenerife and many bordering other islands in the archipelago as well. By the way, one seems to wait its time in the island of La Palma nearby. Let’s hope it won’t happen anytime soon.

But what has it to do with the Roque del García? Well, this rock formation is simply all that somehow escaped the landslide. There were probably even two landslides at different times, one on both sides of the rocks. Common erosional agents like rain and windblown sand grains took over after the slump and gradually shaped the rocks the way we know them now. Eventually the Cinchado will be destroyed by the erosional forces that helped to create it but revolution always eats its children. Doesn’t it?

What about the composition of the Cinchado? It is composed mainly of pyroclastics, alluvial fan breccias, and conglomerates of volcanic material. The clasts are mostly phonolitic as are most of the lava flows there. There are also several phonolitic dikes crosscutting the sedimentary layers.

http://picasaweb.google.com/107509377372007544953/Tenerife#5835129403524547218
Volcanic breccia beneath the Roque Cinchado.

I visited the same place again several years later. Here is an overview with many pictures: Roques de Garcia.

Greensand and green sand

June 12, 2025October 30, 2011 by Siim

Greensand is a sand or sandstone which owes its unusual color to a mineral glauconite. Glauconite is mixed with other sand grains in all possible proportions. Glauconite grains are usually rounded and dark green in color. Glauconitic sandstones are marine in the majority of cases. Many greensand formations seem to have formed either during the Cambrian or Cretaceous Periods.

Baltic Klint near Paldiski — Baltic Klint in Estonia (near Paldiski, Pakri Peninsula). It is composed of limestone (topmost layer), glauconitic sandstone (greensand), kerogene containing shale (all Ordovician) and a phosphatic Cambrian sandstone.

Closeup of a greensand (glauconite sand) from France. Width of view 20 mm.

However, greensand is not the only green sand in existence. There are several green minerals and in certain cases they may be abundant enough to give green color to the sand. Most famous example is definitely olivine. Green sand beach near the southern tip of Hawaii Island (Papakolea Beach) is world-famous but greenish beach sands containing lesser amount of olivine are not uncommon in volcanically active areas.

Is it all? No, there are a number of other green minerals. Malachite, chlorite, epidote and serpentine are all responsible for the green color of some sand samples in specific locations. But these cases are really specific and spatially very confined.

Olivine — Green mineral is olivine. It is a common mineral in dark-colored igneous rocks like basalt. Papakolea, Hawaii. Width of view 20 mm.

Epidote sand from a mine in Nevada, USA.

This greenish sand is composed of almost pure olivine and is a result of weathering of dunitic rocks. Gusdal quarry, Norway.

Is Papakolea the only green beach — Papakolea beach in Hawaii.

Sand minerals

June 11, 2025October 29, 2011 by Siim

Sand is a mixture of different materials. You will find more in the post What is sand. Here is an overview of minerals which are the most common sand constituents. Most minerals may occur as sand grains somewhere. So do we have to cover thousands of minerals here? No, we really need to know less than 50 of them to have a reasonably good overview of all the likely possibilities. Other minerals are rare in sand or are found only in specific locations. Mineral identification is so much easier if you know what is the range of possibilities. The following list of minerals in sand is here to help you achieve just that.

Quartz

There is no other mineral that is as important in sand as quartz. It is really almost everywhere and forms the bulk of sand composition in most cases. Pure quartz is transparent but quartz can have almost any color. The grains are usually rounded and they may be covered by a very fine hematite pigment which gives them a rust-colored appearance. Why is quartz so common in sand? It is a widespread rock-forming mineral and it is also extremely resistant to weathering. Quartz has no cleavage. So we never see planar surfaces on fresh fractured grains. Rocks that contain lots of quartz are sandstone, quartzite, gneiss, granite, and many others.

Chalcedony

Chalcedony is composed of microcrystalline quartz and moganite (there is a slight structural difference between them). It is so fine-grained that individual crystals are impossible to see with a naked eye. Even light microscope is of little help. Chalcedony is formed by the crystallisation of silica gels. It is common cementing material in sedimentary rocks. Chert (rock type) is a fine granular microcrystalline quartz (may also contain moganite).

Sanidine

Sanidine is one of feldspars which are very important rock-forming minerals. Feldspars make up more than half of the composition of the crust. Sanidine itself is definitely not the most common among them. It occurs primarily in volcanic rocks (rhyolite, trachyte, phonolite). We have the best chances to encounter sanidine in volcanic sands of felsic composition. Sanidine is one of K-feldspars (Potassium-rich feldspars). Other common K-feldspars are microcline and orthoclase which are more frequently found in sand.

Orthoclase and microcline

Common K-feldspars but they are not as resistant to weathering as quartz. K-feldspars disintegrate to clay minerals. It may be quite difficult to differentiate feldspars from quartz but they generally appear to be more blocky. Feldspar grains may have planar cleavage surfaces and they often show signs of weathering. K-feldspars are commonly white, yellow or pink in color. Microcline generally forms deeper in the crust than orthoclase but it is pretty complicated task to differentiate one from the other. K-feldspars are the most important building blocks of granite.

Plagioclase

Plagioclase feldspars are definitely the most widespread feldspars overall but their resistance to weathering is not good. Plagioclase decays faster than K-feldspars. We have the best chances to see plagiocalse containing sand in volcanically active areas where fresh sand rich in volcanic minerals is abundant. Or in areas where the rocks are nearby and rich in this mineral (granodiorite, tonalite). Plagioclase crystals are often elongated. They are usually different shade of gray in color. Plagioclase is a common mineral in mafic igneous rocks (basalt, gabbro).

Muscovite and biotite

These minerals are two of the most common mica varieties. Micas are very easily recognizable because they occur as very thin and flexible flakes. Sometimes larger blocks of mica occur but they can be split into an almost endless number of ultrathin layers. Muscovite is generally colorless while biotite is brown or black. Both muscovite and biotite are common rock-forming minerals. They are most common in river sands. Wind-blown (eolian) sands generally contain much smaller amount of micas.

Glauconite

Glauconite is somewhat different from most other minerals discussed here because it is not formed from magma nor is it created during the metamorphism of existing rocks. It forms through the sedimentation process as rounded green pellets in marine sediments. Glauconite is the principal component of greensands in which it forms a mixture with quartz.

Clay minerals

Clay minerals themselves are not sand-forming minerals. They can not be because their size is not large enough to be considered sand. However, they are often present in sand as mud (when wet) or dust (when dry). Beach sands are generally free of dust and so are eolian deposits. Dust is present in many river and especially inland sand samples. Most clay minerals are the weathering product of feldspars but many other common minerals also have a destiny to become clay.

Pyroxene

Pyroxenes are important rock-forming minerals but they are unstable in the weathering environment. Therefore we can usually see them in immature sediments of volcanic origin. Pyroxenes give black color to basalt. Volcanic sands are black because of pyroxene and some other dark-colored minerals of magmatic origin. The most abundant member of pyroxene group is augite. Not all pyroxenes are black. Some are colorful or transparent but they occur rarely. Pyroxene grains are usually elongated.

Amphiboles

Amphiboles are elongated as well as pyroxenes and generally also black or greenish. They are closely related to pyroxenes. Amphiboles endure weathering somewhat more successfully and are therefore not as rare in sands. The most common amphibole is hornblende. Amphiboles tend to have a stronger luster than pyroxenes have. Amphiboles are common minerals in igneous (diorite) and metamorphic (amphibolite) rocks.

Pumpellyite

Pumpellyite probably occurs in sand more often than reported because it is usually misidentified as epidote (these two are actually related). Pumpellyite is formed in metamorphic rocks (mostly glaucophane schist) and hydrothermally altered mafic igneous rocks (basalt). Pumpellyite is usually green or bluish green in color.

Epidote

Epidote is not a single mineral. It is a group of related yellowish or greenish minerals but we treat them as one mineral because you would need pretty sophisticated analytical tools to identify them more precisely. Epidote is a metamorphic mineral. By saying that I mean that this mineral as a sand grain is a weathering product of metamorphic rocks.

Tourmaline

Tourmaline is formed in granitic pegmatites. It is a relatively rare mineral in rocks but pretty common in the heavy mineral fraction of most sand samples because of its extreme resistance to weathering. Tourmaline is usually black (variety named schörl) although it could be also brown (dravite) or colorful (elbaite). Tourmaline lacks cleavage. Although tourmaline appears to be black, it is actually translucent. If your microscope has an additional light source below the sample, you can use it to see that black grains become translucent and brown. It distinguishes tourmaline from opaque ilmenite which is also black and may superficially be very similar. However, this method is not too useful to differentiate tourmaline from pyroxenes and hornblendes. Tourmaline has a very strong pleochroism. This property is useful if you have an access to a polarizing microscope.

Olivine

Olivine is usually green and mostly present in volcanic sands. It is the least resistant to weathering among the common heavy minerals. Epidote may sometimes be misidentified as olivine. Weathered olivine may be brown, orange or yellowish.

Garnet

Garnet is very common heavy mineral (actually mineral group) in many sand samples. Most garnets are pink, orange or red. Garnet have isometric crystal structure which means that they are very rarely elongated. Garnets are formed in both metamorphic (schist, amphibolite, eclogite) and igneous (some granites, peridotite) rocks. When sand contains abundant garnet, it usually contains epidote and magnetite as well. Some garnets (pyrope) are useful index minerals when diamond-bearing kimberlite pipes are searched for. Staurolite grains (deeper red color) may be misidentified as garnets.

Sillimanite

Sillimanite is a metamorphic mineral. It occurs mostly in schist and gneiss. It may be present in lesser amount in some granites as an accessory mineral. As a sand grain it usually is accompanied by other metamorphic minerals like kyanite, staurolite, mica, and garnet. It is usually colorless or light brown in color. Sillimanite grains are mostly elongated.

Kyanite

Kyanite is closely related to sillimanite. These two together with andalusite have the same chemical composition but they are different structurally. Kyanite is blue or gray and bladed. It is also a metamorphic mineral like sillimanite.

Staurolite

Staurolite is a metamorphic mineral from medium-grade metamorphic pelitic (parent rocks contained lots of clay) rocks just like kyanite, sillimanite and garnet. They all contain large amount of aluminum. When these minerals are present in sand, you can say with a high degree of certainty that this sand is a weathering product of a metamorphic terrain. Staurolite is brown or reddish brown.

Titanite (sphene)

Titanite is not abundant anywhere but it is present in small amounts in many mostly plutonic (formed deep inside the crust, not volcanic) igneous and some metamorphic rocks. It is often associated with biotite and hornblende. It is not very abundant in sand composition but sometimes you can find characteristically wedge-shaped grains which may have varying color.

Topaz

Topaz is a rare but quite hard (number eight on the Mohs scale) mineral. Most topaz grains crystallized from magma. It is not an easy task to identify topaz. I am pretty sure that most topaz crystals are misidentified and overlooked as ordinary quartz. Topaz is usually colorless.

Zircon

Zircon is one of the most resistant minerals. Oldest zircon crystals are almost as old as the Earth itself. These crystals are the oldest Earth material we know. Meteoritic material is older but it is extraterrestrial in origin. Zircon crystals are generally very small and elongated. They are usually transparent and contain inclusions. It is easy to identify them when using high magnification and illumination from below the sample. Zircons are very resistant but as time goes by they tend to partly destroy themselves by internal radiation. They contain small amount of uranium which may replace zirconium in the crystal strucure. This fact makes zircon grains very valuable to geologists as a geological chronometers.

Apatite

Apatite is present in small quantities in many igneous and metamorphic rocks. It is also an important biomineral. Your teeth, for example, are largely made of this mineral. Apatite crystals are usually elongated and have colorless or pale shades of blueish, greenish or yellowish color.

Monazite

Monazite is an igneous and metamorphic mineral. Monazite grains are usually very small and not easily spotted. They are mostly pale yellow or colorless. Monazite is a valuable mineral resource. It is mined because it contains some rare and valuable chemical elements (cerium, lanthanum, thorium).

Xenotime

Xenotime is related to monazite. It is also valued because of its rare component (yttrium) and some other valuable elements which may replace yttrium in the crystal structure of xenotime. Xenotime is an igneous and metamorphic mineral just like most other minerals described here. It is resistant to weathering and occurs often in the heavy mineral fraction of sands although it is often misidentified as zircon. Xenotime is elongated and usually colorless, yellow or light brown.

Rutile

Rutile occurs in small amounts in igneous and metamorphic rocks. It is a common heavy mineral in sand because it is very resistant to weathering. Rutile is reddish brown and usually elongated. It is easier to spot when the sample is illuminated from below. Rutile is a titanium oxide. It is mined for its titanium content.

Anatase

Anatase has the same composition as rutile. It is also resistant to weathering and comes from igneous rocks or hydrothermally altered veins. Anatase is pretty similar to rutile. It is also mostly deep brown.

Cassiterite

Cassiterite is a tin oxide. Cassiterite comes from igneous rocks or hydrothermally altered veins. Cassiterite is usually brown but it is not easy to identify. Some grains may be twinned in a characteristic way (elbow twins). Cassiterite may be very similar to rutile.

Corundum

Corundum is a very hard mineral (number 9 in the Mohs scale). It is widespread but nowhere abundant. Corundum forms in igneous and metamorphic rocks. It is usually not accompanied by quartz because corundum forms in silica deficient conditions (quartz is silicon oxide – crystallization product of excessive silica). Corundum may be colored in many different shades. Blue corundum is also called sapphire. Red corundum is ruby. Corundum grain usually catches attention because of its unusual color.

Hematite

Hematite is actually very common component of many sands and sandstones but more often not as large sand-sized grains. It is mostly very fine-grained pigment on the surface of other grains. Hematite gives reddish rust-colored hue to many sandstones. Such hematite pigment is deposited from the groundwater moving in the pore space of sand deposits. But larger hematite grains also occur in the heavy mineral fraction of sand.

Ilmenite

Ilmenite is a widespread accessory mineral in many igneous and metamorphic rocks. It is opaque and has a metallic black color. It is weakly magnetic due to intergrown magnetite. Ilmenite grains tend to be somewhat tabular. Ilmenite is mined for its titanium content.

Magnetite

Magnetite is easy to identify because it is very magnetic. Magnetite grains are generally small and equant. Some grains may have well-developed octahedral crystal faces. Magnetite comes from igneous and metamorphic sources.

Chromite

Chromite has the same crystal structure as magnetite but it is only weakly magnetic. Chromite forms in ultramafic igneous rocks. Chromite is an ore of chromium.

I did not mention many minerals which you probably have seen as sand grains. Even my own collection contains many sand samples which are composed almost solely of minerals that are not even mentioned here. These samples are curiosities. I am pretty sure you can walk around the coastline of the entire world looking for a blue sodalite sand without finding it unless you visit specific mines or quarries. Some minerals, like aragonite and calcite are very common components of biogenic sand grains but they usually do not occur as individual sand-forming crystals. Gypsum forms beautiful white sand dunes in New Mexico but is generally not present in sand.