Almandine is one of the garnets. The most common of them. Its ideal composition is Fe3Al2(SiO4)3. Fe can be replaced with Mg or Mn. If there is Mg more than Fe, we call this garnet pyrope. If there is more Mn than Fe, then it is spessartine. There are more garnet varieties but they do not form continuous solid solutions with the three mentioned above.
Almandine is a common mineral in metamorphic rocks, mostly schists and gneiss. But it is a usual constituent of felsic igneous rocks too. Almandine is very common in sands heavy mineral fraction. It is so widespread because it is a major component of several rock types and moderately resistant to the weathering. It is among the most beautiful minerals one can see in sand. If you notice pink grain, it most likely is almandine. However, it may also have darker red color. The composition is not fixed and therefore there is no single identifiable color.
It is not too hard to distinguish garnet from other minerals because it is isotropic. You still need a polarizing microscope though. But there is really no sure and easy method to say what type of garnet it is. Color might be a good indicator. Spessartine tends to have more yellowish colors and pyrope is dark red or even purplish. However, color is often misleading indicator. So it probably is better to call these reddish grains garnets without attempting to specify. Things might get more complicated because there are some insiduous minerals that may be misidentified as garnet. These are staurolite and red chrome-bearing spinel.
http://picasaweb.google.com/107509377372007544953/Chert#5808422097271089698 Almandine is the most common of garnets. These crystals contain some magnesium which puts them part way into the pyrope group. That might also explain darker red color. Thanks to Bill Beiriger for these beautiful crystals. Width of the view is 10 mm.
You can find lots of pictures of almandine-bearing rocks in an overview article of the garnet group.
Heavy minerals are minerals with a density greater than 2.8…2.9 g/cm3. Why these numbers and what for is this range needed? Why not just one number? The most common minerals in most sand samples are quartz and feldspar. Calcite and dolomite are common cementing minerals in sandstones. All of these minerals have densities below the range shown above. Heavy minerals are usually volumetrically insignificant. However, there are a large number of heavy mineral species, each of them having their own story to tell. Therefore geologists often need heavy minerals to get as much information out of the studied rock as possible.
Heavy minerals in beach sand from Sri Lanka. Most intensely colored grains are spinel crystals. Width of view 20 mm.
http://picasaweb.google.com/107509377372007544953/2015#6190951519258082962 Heavy minerals including gold on a gold pan. This concentrate is panned from river sediments in Lapland, Finland.
It is very uncomfortable to do if only one grain out of one hundred or less is what we are looking for. We therefore seek methods to somehow separate heavy minerals from the bulk of the sand. Obvious way to do that is to use some heavy liquids which have a density greater than that of quartz (2.65 g/cm3) but lighter than that of most minerals.
Several liquids with slightly different densities have been used. That’s the reason why there is range instead of fixed value. The liquid used to separate heavy minerals from the rest is usually bromoform (its density is 2.89 g/cm3). It is liquid at room temperature and feels abnormally heavy as one is usually not used to liquids as dense as bromoform. Its high density is a result of three bromine atoms in its chemical formula (CHBr3). This liquid has a nasty downside. It is poisonous and has a disgusting odor. I guess it is actually a good thing because you don’t want to be exposed to the vapors of it. There are some more recently developed alternatives though which do not have such an adverse health effects, most notably polytungstate liquids.
Heavy minerals are useful to study the provenance of a sand or sandstone. “Provenance” is a fancy term geologists love to use when they talk about the place where the sand grain broke out of its parent rock and began its journey as a sediment particle. How do we study provenance? We take a look at the heavy mineral fraction and make sure what is its composition. Let’s say it contains garnet, staurolite, and kyanite. What can we say about that? I think we can reasonably safely assume that this sand is a weathering product of a metamorphic terrane because this mineral assemblage is very typical to metamorphic rocks. If there are lots of augite, magnetite, and olivine, then it probably comes from an igneous source.
http://picasaweb.google.com/107509377372007544953/Rocks#5877446908198751970 Heavy minerals are sorted out by a running water near the coastline at Pfeiffer Beach, California.
http://picasaweb.google.com/107509377372007544953/Rocks#5878256035179681490 A closer look at the Pfeiffer Beach sand. Main heavy mineral species are garnet, epidote, zircon, magnetite, spinel, staurolite, etc. Width of view 8 mm.
It is a very broad approach. Unfortunately more detailed studies into the provenance often give debatable results because there are so many factors that can alter the composition of the heavy mineral suite. These are mostly weathering, burial diagenesis, hydrodynamic sorting, and mechanical abrasion during the transport.
Many diamond-bearing kimberlite pipes are discovered by studying heavy mineral fraction of a sand. We need to look for pyrope (which is heavy mineral) for example. This is a rare Mg-bearing garnet that is associated with diamonds in kimberlite pipes. Its presence in the river sand may give us a hint that a kimberlite pipe may be nearby. To hunt down its location, we have to go upstream and take many samples until pyrope and some other index minerals found in abundance in kimberlite pipes suddenly disappear. Heavy minerals have other applications in forensics, oil and gas industry, etc.
Heavy minerals sometimes get naturally concentrated as a heavy mineral sand and there were, of course, no bromoform involved. It was moving water either in a stream or beach that did the job. Sometimes the sand is so concentrated in heavy minerals that it has a real economic value as an ore. Sand collectors also love these black sand deposits. Such heavy mineral concentrates are called placers.
Gold panning is an activity used to separate gold flakes and nuggets from these placers. However, gold is not the only mineral that is mined from placers. These minerals are also cassiterite (tin ore), ilmenite (titanium), magnetite (iron), rutile (titanium), monazite (rare earths), chromite (chromium), zircon (zirconium), etc. Australia is particularly well-known heavy mineral source, but heavy mineral deposits occur in many places.
Some common and not so common heavy minerals in sand and some of their properties:
Mineral
Density
Stability in weathering
Stability in diagenesis
Provenance
Anatase
3.82…3.97
High
High
Felsic igneous rocks, hydrothermal veins, alteration product of titanite or ilmenite.
Amphiboles are elongated and generally dark-colored silicate minerals. Amphiboles are both structurally and compositionally similar to pyroxenes (other large group of silicate minerals). Both amphiboles and pyroxenes are very important rock forming minerals. However, their presence in sand is generally much smaller than one would assume. Amphiboles weather pretty easily (although not as rapidly as pyroxenes).
This is amphibole arfvedsonite from Greenland (Ilimaussaq intrusion). Second grain in the upper row demonstrates strong luster and third grain shows typical cleavage striations. The width of the view is 15 mm.
The composition of amphiboles is quite complicated. It is complicated because of numerous possible replacements of ions in several different sites in the crystal structure.
Most common amphibole is hornblende. This mineral has no definite composition either. It is therefore also divided into several minerals. However, these subdivisions are not important here because the identification of them is impossible without pretty sophisticated and very expensive analytical tools.
Amphibolite is a very common rock where amphiboles as the name suggests are key ingredients. Amphiboles in amphibolite are mostly hornblende. White mineral is plagioclase. Senja, Norway. Width of sample 9 cm.
How to identify amphiboles? They are generally black or dark green. Sand grains made of amphibole are usually elongated. They may have vertical striation (sign of a cleavage). There are actually two cleavage planes but the chance to see both of them when dealing with sand grains is not too high. Amphiboles usually have strong luster which distincts them from pyroxenes which are usually duller black or green.
Mineral besides pyroxenes that could easily get misidentified as amphibole is black tourmaline (schorl) and black spinel. They are not nearly as common in rocks but as sand grains they are much more resistant. Tourmaline has very strong pleochroism and it is often brownish. Spinel grains very often have smooth spots with intense luster that resemble volcanic glass. Amphiboles have no such feature. Spinel and tourmaline are also often rounded because usually they have been sand components for a long time. Amphiboles, however, tend to be more often prismatic. They are generally younger because they simply won’t last as long.
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.
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.
Ilmenite is an iron titanium oxide. It is the principal ore of titanium. It is black (or dark gray) and has a metallic luster. It is usually weakly magnetic. The mineral itself is actually not magnetic, but it is often intergrown with magnetite, which very strongly responds to the magnetic force. You can use a hand-held magnet to test sand grains. If they are only lazily turning themselves but not jumping vigorously towards the magnet, then they are most likely ilmenite grains, not magnetite.
Ilmenite in anorthositic host rock. Blåfjell mine, Rogaland, Norway. The sample is from an abandoned mine, but not far from Blåfjell is a working Tellenes mine (picture below). Width of sample 13 cm.
Another aspect that separates it from magnetite is the crystal structure. Magnetite is an isometric mineral, it forms double pyramids (octahedrons) just like diamond. These octahedrons are often worn off in sand, but in this case magnetite is rounded and still roughly isometric. Ilm. is at least theoretically different, its grains tend to be tabular. These minerals are both opaque minerals.
The crystal structure of is identical to hematite. It is compositionally pretty close to hematite as well. The only difference between them is that one Fe atom (in hematite) is replaced by titanium atom (in ilmenite). The chemical formula is FeTiO3. This is ideal formula which often is pretty close to the reality, but some of the iron may be replaced with magnesium and manganese.
http://picasaweb.google.com/107509377372007544953/Coll#5851075733977440210 Beach sand from India containing lots of ilmenite. Brown grain in the lower left is leucoxene. Light blue elongated grain is kyanite. Transparent crystals are quartz grains. The width of the view is 5 mm. Ilmenite crystal should ideally look like this.
Grains are often altered to leucoxene. Leucoxene is a mixture of several oxide minerals, it is not a mineral itself. It looks like a very fine-grained light-colored or brownish coating on ilmenite grains.
Ilmenite is found in igneous and metamorphic rocks although it very rarely forms a major component of a rock. If that happens (in iron- and titanium-rich magmatic cumulates) then it is usually mined for its titanium content. Titanium extracted from ilm. in the form of titanium dioxide is used as a white pigment in paints and sunscreens among other things. Millions of tonnes of ilmenite are mined every year, but the majority of it comes from sand, not from the hard rocks. It is resistant to weathering and therefore common in sand. It is usually accompanied by magnetite and possibly by a small amount of rutile and zircon as well (they are even more resistant to weathering).
http://picasaweb.google.com/107509377372007544953/Coll#5851075735049738098 Tellenes mine in Norway. It is fun to think that white paint and sunscreen come from this black hole. Image: Mikenorton/Wikipedia.
http://picasaweb.google.com/107509377372007544953/Coll#5851075703239069858 Ilmenite and leucoxene grains. Some grains demonstrate half-completed leucoxenisation process (few examples are annotated). The width of the view is 3.8 mm.
http://picasaweb.google.com/107509377372007544953/Coll#5851075705332924162 Beach sand containing ilmenite (black), leucoxene, quartz, almandine, and zircon. Calvert Cliffs State Park, Soloman Islands, Maryland.
Staurolite is a common mineral in medium-grade aluminous metamorphic rocks. The composition is very rich in aluminum: Fe2Al9O6[(Si,Al)O4]4(OH)2. This indicates that the protolith had to be a clay-rich sedimentary rock. It is a hydrous mineral which makes it unstable at very high pressures.
Schist with a large twinned staurolite porphyroblast. Tohmajärvi, Finland. Width of sample 19 cm.
Staurolite is an orthosilicate: ratio of (Si,Al):O is 1:4 and the silica tetrahedra are isolated from each other. Structurally similar silicate minerals are kyanite and garnet which tend to occur often together with staurolite as they are all Al-rich minerals formed during metamorphism. Other common metamorphic minerals that often occur with them are kyanite, muscovite, biotite, chloritoid, and cordierite.
Crystals are elongated and dark brown. Edges of crystals may be lighter and even orange in color if the light is able to penetrate the crystal. Twinning is very common (crystals crossed with 90 or 60 degree angles between them) which are very characteristic and therefore useful in identification.
Dark, elongated staur. crystal in a metamorphic rock schist with garnet, kyanite and muscovite. Width of sample 7 cm.Outcrop of staurolite schist. Tohmajärvi, Finland.
This mineral is resistant to weathering and is therefore very common constituent in sand. It is part of the heavy mineral fraction, its density is 3.7…3.8 g/cm3. It may be easily mistaken for garnet because small grains are often not elongated and they are signifiantly lighter in color than large crystals in rocks. Staurolite as a sand grain is orange, garnet is usually pink (spessartine (Mn-rich garnet) may resemble staurolite in color, though). Polarizing microscope may be helpful to tell them apart. Garnet is an isometric mineral which will be dark in crossed polars while staurolite has interference colors.
Blue circles mark garnet and green circles staur. grains. Sand sample is from the Mediterranean Coast of France (Rayol-Canadel-sur-Mer). Other grains are quartz, K-feldspar, and plagioclase. Width of view 10 mm.
Staurolite is mostly used as an abrasive (hardness 7-7.5) in sandblasting applications. So in this regard it is also very similar to garnet. Twinned crystals have a commercial value as they resemble crosses which can be used as earrings or pendants.
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.
, 2nd Edition. Prentice Hall.
, 2nd Edition. Oxford University Press.
