Glauconite is a green-colored mineral. It is structurally similar to micas and is sometimes considered to be one of the mica minerals. Despite that it appears usually in rounded ill-formed platelets which are not at all similar to typical flakes of common micas like muscovite and biotite. However, it still possesses perfect cleavage like other micas, we just don’t see it. Sometimes it is included in the clay mineral group. Whatever the case, glauconite is definitely a sheet silicate. These confusing nuances only show that our classification principles are imperfect and we shouldn’t take them too seriously. They are there only to guide us.

Glauconite is usually a component of sandstones. It occurs in sand-sized granules in marine sandstones. If abundant, it gives distinct green color to sandstones that are called greenbeds or greensands. It may also occur in carbonate rocks. My home country Estonia has both glauconitic sandstone and glauconitic limestone layer (limestone on top of the sandstone).

Glauconite may be abundant component of beach sand if greenbeds are exposed in a coastal cliff. The chemical formula of this mineral is very complex. This is typical to clay minerals. They are all structurally and compositionally variable and exceedingly difficult to understand. That’s probably why we still know surprisingly little about this fairly common mineral. Glauconite forms mostly at the expense of another and more common sheet silicate biotite.

Glauconite is pretty distinctive with its bluish-green and rounded pellets, but not all green granules in sand are glauconitic. Chlorite may form similar grains. Glauconite may occur together with pyrite. They both need reducing conditions to form which is probably caused by the presence of decaying organic matter which consumes all free oxygen.

Baltic Klint near Paldiski (Pakri Peninsula) in Estonia. The klint section is composed of limestone (topmost layer), glauconitic sandstone (greensand), kerogene containing alum shale (all Ordovician) and a partly phosphatic Cambrian (mostly) sandstone with layers of shale. This klint section is about 25 meters high.

Antozonite is a dark-colored variety of fluorite.

It is dark-colored (dark violet or black) because the crystal structure of fluorite (CaF₂) is partly damaged by alpha-radiation. I don’t know what caused the radiation in this case but it had to come from the fluorite itself because alpha particles (two neutrons and two protons combined) have very low penetration depth, they can’t even go through paper. So, the fluorite had to be radioactive in the first place.

Cerium (the most common rare earth metal) can replace Ca in the crystal structure of fluorite and it has many radioactive isotopes but these tend to decay by electron emission (beta radiation). The sample shown below is from Wölsendorf, Bavaria. This is the type locality of antozonite, it was first described there.

Interesting aspect associated with this mineral variety is that it emits strong odor when crushed. It is also called stink-fluss (stink-flow). Unfortunately, I haven’t tested it because the sample pictured below doesn’t belong to me. The odor according to several web-based sources is fluorine gas but I am really not sure. Fluorine is a highly reactive element. It is a bit hard to believe that free fluorine just sits there in the middle of the crystal and waits to be liberated. But, as I said, I don’t know. Maybe it is even true but I am pretty sure that as soon as it is liberated it will react away. Which makes me think that the result might be hydrofluoric acid (HF) if it reacts with atmospheric water and all of a sudden I feel that I really don’t want to crush the rock anymore. Hydrofluoric acid is an especially nasty compound with which geologists have to work sometimes but it is really poisonous.

Antozonite was originally most likely pretty ordinary fluorite (with some radioactive components) that precipitated out of hydrothermal solutions. On the picture below you see one big and black (but slightly violet) and some smaller fluorite veins in a host rock that seems to be granite (it is composed of gray quartz and pink K-feldspar).

Talc is a common mineral in some metamorphic rocks. It is a sheet silicate which means that it is structurally similar to micas and clay minerals. This mineral is always an alteration product, it forms at the expense of other preexisting minerals. The alteration reactions that produce talc are metamorphic — they took place at elevated temperatures and pressures. This is in contrast to chlorite, somewhat similar and often accompanying green-colored sheet silicate mineral group, that may form through metamorphism but also as a weathering product of other minerals.

To form it, magnesium and water are needed in addition to silica. The chemical composition is: Mg₃Si₄O₁₀(OH)₂. There are not too many choices available which contain lots of magnesium. These are ultramafic igneous rocks and dolomitic carbonates. OH-group in the chemical formula is a hint that the alteration that produces talc is most likely hydrothermal.

Indeed, this is the case. Talc is a hydrothermal mineral formed often at the expense of serpentine which is also an alteration product of olivine and pyroxenes (common components of ultramafic rocks). Talc may form as dolomite metamorphoses, but in this case the dolomite needs to be impure. Otherwise, the lack of silica will be limiting the growth of talc. It also seems to be a mineral that easily gets pushed aside by other minerals. It forms only when the conditions are just right. If we have Al, Ca or K in the precursor rocks, the formation of chlorite, tremolite or phlogopite (respectively) would be favored. Tremolite is a Ca-bearing amphibole group mineral, phlogopite is a Mg-rich variety of biotite.

Talc is usually light green in color. The most common talc-bearing rocks are talc schist and soapstone. The former is schistose as its name implies and the latter is massive and indeed soapy — its surface is somewhat slippery which is the result of very low hardness. The flakes start slipping in relation to each other if you apply pressure to them with your bare hands, this is what causes the soapy feel.

Talc is so soft that you can scratch it with a fingernail and easily cut with a knife. It is the softest mineral on the Mohs hardness scale with a hardness of 1. However, do not expect that you can easily cut any sample of soapstone (also named steatite) like a bread because they are composed of intergrown crystals and they often contain other minerals as well.

The softness and waxy feel are useful properties that make the identification of the mineral quite easy. Pyrophyllite is similar mineral, it may be very hard to distinguish these two, but pyrophyllite occurs in aluminum-rich rocks (metamorphosed claystones) and it isn’t as widespread.

Talc occurs in sand usually as lithic soapstone or schist fragments, but not for very long because it tends to alter to chlorite.

Talc schist with chlorite from Finland (The Jormua ophiolitic complex). The width of the sample is 10 cm.

The minerals associated with talc in the photo above are fairly typical. The rock was originally probably ultramafic igneous rock. The rock turned into serpentinite and later serpentinite metamorphosed to talc and magnesite. Chlorite could be the last mineral to form at the expense of talc. This is just a guess. In reality things might have been somewhat different and they definitely were much more complex. But the sample illustrates what might happen to ultramafic igneous rocks and it also demonstrates the fact that metamorphic reactions are complex and there are many steps that need to be made to turn peridotite or similar rock into this chlorite-rich greenstone.

Quartz is among the most widespread minerals, constituting 12% of the crust by volume. Quartz is so common because it is composed of oxygen and silicon, which are by far the most important chemical elements in the crust. It contains no rare chemical elements which could limit its growth. Hence, we see it often in igneous rocks, especially those igneous rocks that contain lots of silicon (granite, rhyolite, pegmatite, granodiorite, etc.). The most important igneous rocks that contain little or no quartz are mafic/ultramafic (gabbro, basalt, peridotite) and feldspathoid-bearing rocks (nepheline syenite, for example, but such rocks are relatively rare).

http://picasaweb.google.com/107509377372007544953/Rocks#5805071014597717554
Quartz crystals (Herkimer diamonds) found from cavities in dolomite rock. New York State, USA. Width of view 10 mm.

http://picasaweb.google.com/107509377372007544953/Rocks#5842989770630082594
This mineral has no cleavage. Hence, it has a smooth conchoidal fracture and easily recognizable luster. Width of sample is 11 cm.

It resists weathering very well and it is also physically hard. Thus, we very often see it in sediments. It is the most important sand-forming mineral. It is more voluminous in sand than all other minerals combined. Continental sand is mostly composed of this mineral. Volcanic islands in the middle of oceans (Hawaii, The Canary Islands) are one of the few places where it is not among the sand-forming minerals because the generally basaltic volcanic material usually does not contains it.

It is also widespread in metamorphic rocks because in relatively pure form it is not afraid of alteration, high temperature, and high pressure. sandstone becomes quartzite in the course of metamorphism. This is in stark contrast to pelitic (clay-rich) rocks which go through very complicated series of mineral reactions.

Quartz is also the most important hydrothermal mineral, filling cracks in the crust with many other and often economically important minerals.

Quartz in sand is usually white or transparent and very often rounded because its grains might be very old, sometimes even billions of years. It has no own specific color because it lacks chromophore elements. It could have almost any color, depending on impurities. Amethyst, for example, is a violet variety that contains a small amount of iron. It is often easy to identify in rocks because it lacks cleavage (no planar broken surfaces) and has specific greasy-looking luster. Note that crystal faces are different, they have vitreous (glassy) luster.

This mineral in hydrothermal veins is usually massive and white in color. It is called milky quartz. The white color is given by numerous minute fluid inclusions. Width of sample 9 cm. The sample is from Morocco (Anti-Atlas).

http://picasaweb.google.com/107509377372007544953/Coll#5823755327463125186
St. Peter Sandstone from the Ordovician Period is composed of almost pure and well-rounded quartz grains. Sand from this formation is widely used for fracking purposes. Width of view 20 mm.

http://picasaweb.google.com/107509377372007544953/Rocks#5847099160896443730
Morion is a black variety of smoky quartz. This morion crystal is 12 cm in length and shows typical fracture of quartz.

Blue color in quartz is usually caused by fine rutile crystals. Bergen, Norway. Width of sample 6 cm.
It is almost the sole component of Siesta Key beach sand in Florida.
Quartz (gray) forming an intergrowth in alkali feldspar crystal in a pegmetitic igneous rock (graphic granite). The sample is from Southern Norway. Width of view 14 cm.

Pyrope is a garnet just like almandine and several others.

Easiest way to see what makes the difference between garnet minerals is to take a look at the table below:

Mineral	Composition	Group
Pyrope	Mg3Al2(SiO4)3	Pyralspite
Almandine	Fe3Al2(SiO4)3	Pyralspite
Spessartine	Mn3Al2(SiO4)3	Pyralspite

Pyrope is a magnesium garnet, almandine is an iron garnet, and spessartine is a manganese garnet. Note that most natural minerals are mixtures of the three (dominantly mixtures between pyrope and almandine or almandine and spessartine). Absolutely pure endmembers don’t exist in nature anyway but solid solutions in garnets are really common. Actually, pure endmember is really rare. Most commonly up to 70% of the sites in crystal lattice that are shared between Fe, Mg, and Mn are occupied by Mg in pyrope. The composition of garnet may be expressed the following way: Py62 Al35 Sp3. Mineral with such a composition is called pyrope. To make things more complicated, other chemical elements may also enter the crystal structure. Chromium, for example, is especially common.

http://picasaweb.google.com/107509377372007544953/2015#6190951168417918930
Another sample of peridotite with lots of purple pyrope. Åheim, Norway. Width of view 25 cm.
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Pyrope demonstrating its hardness and durability as it stands out while olivine and pyroxene are not nearly as resistant to weathering. Width of sample 11 cm. Åheim, Norway.

These three minerals are collectively called pyralspites (pyrope + almandine + spessartine). We need this term because there are more garnets than these three mentioned above.The rest of the garnet group minerals are somewhat different. They are called ugradites (uvarovite + grossular + andradite). There are no continuous solid solution between the two groups. Therefore, I will not pay attention to the ugrandite group now. There are even more garnet varieties possible but these are really rare and have no geological significance.

All pyralspite minerals are shade of red — light pink, yellowish red, red, purple, very dark red (almost black) are the most common possibilities. The color depends on the grain size. They all tend to be lighter in sand and darker as large grains (this is universal among minerals). But the color also depends on the composition. Pyrope is the darkest of the three. It is usually dark red, purple, or almost black.

Unfortunately, color only is not a reliable guide. Perhaps more useful is to know from what type of rocks are these minerals coming from. Almandine is the most common of the three and it comes mostly from metamorphic rocks (garnet schist, garnet amphibolite). Spessartine comes from igneous rocks like granite and pegmatite. Pyrope comes from ultramafic rocks (peridotite, pyroxenite, kimberlite, and serpentinites derived from them) or ultrahigh pressure metamorphic rock eclogite.

Pyrope is a semi-precious gemstone partly because of its beautiful dark red color and partly because it is much rarer than almandine. It is rare, of course, on the upper part of the crust. The rocks containing pyrope are not rare at all deep below.

Pyrope is one of the index minerals in diamond prospecting. It does occur together with diamond in kimberlite pipes. It is not a typical kimberlite mineral but kimberlite pipes often contain pyrope-bearing blocks as xenoliths. These diamond containing pipes are small and hard to find. The grains found in river sand might indicate that ultramafic rocks could be somewhere near. Prospectors just need to go upstream and take samples until they find no pyrope anymore. Then, if they are lucky, diamond bearing kimberlite pipe could be nearby.

Zircon is a Zr-bearing silicate mineral (ZrSiO₄). It is very interesting and useful mineral in several ways. It is unique mineral because it is mined for several reasons. It is a principal source of zirconium and hafnium (it always contains small amount of Hf which replaces Zr) and contains yet smaller amounts of rare earth elements too. But it is also an industrial mineral. Its useful properties are extremely high melting temperature and strong resistance to chemical attack.

http://picasaweb.google.com/107509377372007544953/Coll#5822980311210985986
Zircon sand (concentrate) from South Africa. Width of view 12 mm.

Zircon is very important mineral to geologists. Small amount of zirconium is replaced with uranium which is a radioactive element. We know the half-life of uranium and can therefore calculate the age of zircon crystals. But this is not all. It is used in geochemical studies to determine magmatic sources (mantle vs crustal) and composition of magma it crystallized from among several other applications.

Zircon is relatively little known despite being so important. Perhaps because it is rare? Actually not, it is a very common mineral. Zircon is originally igneous mineral but it is hard and very resistant to alteration and weathering. So it occurs in metamorphic and sedimentary rocks as well. The problem with it is the size of the crystals — they are very small, almost invisible. Large crystals are valued gemstones which are named “hyacinth”.

Why is it so small? Common magmatic minerals like feldspars, pyroxenes, etc. want to know nothing about zirconium (chemical element). So large cation simply will not fit into their crystal structure. Poor zirconium finds no one to play with and has to form its very own silicate mineral which we know as zircon. The crystallization starts more or less simultaneously in very many spots but zirconium is not particularly common chemical element. There is simply not enough material to grow large crystals. Hence, we find zircon often but the crystals are really microscopic and often entirely surrounded by other minerals which have later grown around already formed crystals.

Zircon grains are the oldest terrestrial material we have found so far. Oldest ones are 4.40 billion years old which is pretty good result (The Earth itself is about 4.54 billion years old).

Zircon is a very common mineral in sand. These sand grains are small but they are still often easily spotted. Euhedral grains (tetragonal prisms with dipyramidal terminations) are especially characteristic but all zircon grains tend to have dark band parallel to the outer edge (because of high refraction index). This is seen under the microscope if the grains are illuminated from below the sample (light goes through the grains).

http://picasaweb.google.com/107509377372007544953/Chert#5808419184807447314
Samples of zircon concentrates from India (on the left), USA, South Africa, and Australia. Note that there are slight color differences but generally zircons are light-colored or colorless. There are 4 square centimeters of sand on every square-shaped photo.
http://picasaweb.google.com/107509377372007544953/Chert#5808419188485673810
Zircon concentrate from South Africa. Grains are either rounded and slightly elongated or form clearly elongated teragonal prisms with dipyramidal terminations. The width of the view is only 3.1 mm.
http://picasaweb.google.com/107509377372007544953/Chert#5808419191899047730
Zircon crystals (brown) surrounded by biotite (black) in a pegmatite from the Seiland Island, Norway. The length of the largest crystal is 14 mm.

Monazite is a phosphate mineral containing rare earth metals (Ce,La,Th)PO₄. These rare earths or lanthanides can substitute each other in the crystal structure. There are more lanthanides that can enter the lattice, these three are the most common.

This mineral mostly crystallizes out of magma (granite, syenite, pegmatite, carbonatite), but it occurs in several metamorphic rocks as well. It is resistant to weathering and is therefore common mineral in sand. But its grains are usually very small and therefore difficult to spot and identify. Why are they small? The reason is the chemical composition. Lanthanides are chemical elements that are required for the monazite to form. These elements are not wanted by more common minerals because lanthanides do not fit into their crystal structure. Hence, there is little competition and the crystallization centers of monazite form in many places. However, these elements are still pretty rare and there simply is not enough material for the large crystals to form.

Its grains in sand are usually yellow, reddish, or brown. They are mostly rounded and slightly elongated.

Monazite is an important mineral resource, it is one of two minerals that are mined for their rare earth content, the other being bastnäsite which is even more important as a source of lanthanides but not as common in sand. It is mined from sand, mostly beach sand.

http://picasaweb.google.com/107509377372007544953/Rocks#5805070984208524530
Monazite concentrate from North Carolina, USA. Width of view 3 mm.

Mica is a group of minerals. Mica is a sheet silicate and the sheets are easy to separate from each other because there are weak chemical bonds holding them together. This gives them perfect cleavage — the most characteristic feature of mica minerals. The term “mica” is, however, not easy to define. If we try to do it structurally then we should include illite and glauconite but the former is usually considererd to be a clay mineral and the latter is mostly not considered to be mica because it tends to occur as rounded pellets, not as thin sheets.

Mica flakes are elastic, this separates them from chlorite (another sheet silicate) for example. There are almost 30 mica minerals but only four are important (abundant in geological record).

These four are muscovite, paragonite, biotite, and lepidolite. Phlogopite and sericite are sometimes included but phlogopite is just a variety of biotite and sericite is a very fine-grained muscovite.

Mica minerals are really important, they make up 5% of the Earth’s crust. They occur in many different rock types.

Muscovite is a common mineral in felsic igneous rocks (mostly plutonic). It could also be an alteration product of other silicates, mostly feldspars (in this case it is usually called sericite).

Paragonite is similar to muscovite and is most likely more common than usually believed because it is often misidentified as muscovite. It is found in low-grade metamorphic rocks.

Biotite is a very common mineral just like muscovite. It occurs in igneous rocks as well but forms before muscovite (crystallization temperature range is higher), contains iron and magnesium, and is therefore much darker-colored than muscovite.

Lepidolite contains lithium. It is pink in color and occurs mostly in pegmatites with other minerals (like beryl, spodumene, topaz, and tourmaline) that need unusual chemical elements in their crystal structure. It may be sometimes hard to distinguish from muscovite.

I have also written about zinnwaldite which definitely belongs to the mica group but is not a valid mineral species anymore.

Mica minerals are moderately resistant in the weathering environment and are therefore frequently present in sand and sandstone. The most micaceous sand seems to be river sand, followed by beach sand. Eolian (wind-blown) sand usually contains no or very little mica.

Leucoxene is an alteration product of titanium-bearing minerals. Leucoxene is not a mineral because it lacks defined crystal structure and its chemical composition is far too variable to be expressed as a chemical formula. It is an alteration product of minerals like ilmenite, rutile, and titanite (sphene).

Some ilmenite grains demonstrate half-completed leucoxenisation process (few examples are annotated). The width of the view is 3.8 mm.

Other minerals often present are pseudorutile, anatase, hematite, and goethite¹. The color of the material is variable. It may be light gray, brown, yellow, orange, reddish, etc. It looks earthy because it is a mixture and therefore never forms beautiful crystals. It is an economic mineral mined for its titanium content. It is usually mined together with ilmenite.

There are black ilmenite and colorful leucoxene grains on the picture below. The width of the view is less than 4 mm, so the grains are really small and indistinguishabe to the naked eye. The picture shows leucoxene grains in different shades of color and some grains demonstrate the leucoxenisation process being halfway completed.

Beach sand containing ilmenite (black), leucoxene (yellow spots on ilmenite grains), quartz, almandine, and zircon. Calvert Cliffs State Park, Soloman Islands, Maryland.

References

1. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.

Rutile is a common mineral in sand and one of the most important sources of titanium. The other important titanium-bearing mineral is ilmenite. It has very simple chemical composition (TiO₂). So it is an oxide like ilmenite and magnetite. Crystals are usually elongated and typically deep reddish brown although that color is best seen only in very small crystals. Larger grains are almost opaque and have a metallic luster. Smaller grains have intense adamantine luster because of extremely high refractive index. This index measures how much light bends (or its velocity slows down) when it enters the crystal.

http://picasaweb.google.com/107509377372007544953/Rocks#5805071052583082386
Concentrate of rutile from Pooncarie, New South Wales, Australia. The few transparent grains are zircon crystals. Width of view 4.2 mm.

It is quite stable in the weathering environment and may therefore occur in sand as single crystals or it could be a part of a cryptocrystalline aggregate of several titanium and iron oxides and hydroxides. Such an aggregate is called leucoxene. Leucoxene forms as a weathering product of ilmenite. Hence, rutile as a main component of leucoxene and ilmenite are often mined together as an ore of titanium. Rutile is sometimes concentrated enough to form a placer deposit. Most of this mineral is mined from such placers. It also occurs sometimes as fine needles in quartz or mica crystals.

Rutile occurs in many igneous (mostly plutonic) and metamorphic rocks (especially amphibolite, eclogite, and metamorphosed limestones) but usually as small crystals. Big crystals may grow in pegmatites or hydrothermal veins with quartz and apatite. Rutile grains in sand are also small but their deep red color is pretty distinctive. It is noticed best if the light source is below the microscope slide.

It has two polymorphs (same composition but different structure) – anatase and brookite, which have similar properties but are not so widespread.

The grains on the picture below are all smaller than 125 micrometers. I sieved the sand to remove the larger grains because grains as small as these need high magnification which makes the depth of focus very shallow. So the grains need to be almost uniform in size and the surface as planar as possible. Why did I chose smaller grains instead of larger ones? Because only the small grains or the edges of the bigger ones show the characteristic red color. Grains larger than a quarter of a millimeter are usually practically opaque.

Width of sample 5 cm.

Fine rutile is responsible for blue color in quartz crystals. Bergen, Norway. Width of sample 6 cm.