Serpentine is a mineral group that contains chrysotile, antigorite, and lizardite. A rock type that is composed mostly of serpentine group minerals is serpentinite. I chose to leave out chrysotile for now because this mineral has distinctly different appearance and properties from the other two. It has a fibrous habit and is the most widely used asbestos mineral. It definitely deserves a separate article. Here I will continue with antigorite and lizardite which are not fibrous and are therefore much better suited to become sand grains.

Serpentine (Mg₃Si₂O₅(OH)₄) is a metamorphic mineral group. It was made at an expense of mafic and especially ultramafic igneous rocks like peridotite and pyroxenite. Such rocks are common in the mantle but quite rare at the surface. When they do crop out, they are often altered by hydrothermal fluids. Hence, we often see meta-peridotite instead of real peridotite. The original mineralogy of the rocks is significantly changed. Rocks that are mostly composed of these minerals are called serpentinites. It often contains all three mineral varieties — fibrous chrysotile veins often alternating with massive antigorite/lizardite. Serpentine may also form in contact metamorphosed carbonate rocks.

Antigorite and lizardite may have a very similar appearance. Thus, there is no reliable method to say which is which in a hand sample. Distinguishing between the two requires more sophisticated approach but even X-ray diffraction may give ambiguous results because they are structurally similar minerals and they tend to occur together.

Antigorite and lizardite are mostly green and they look greasy or waxy. Both light and dark green colors are possible. Mineral grains are darker when they contain more magnetite inclusions. It is easy to make sure that they mostly do contain lots of tiny magnetite crystals because serpentine grains are usually highly susceptible to magnetic field although the mineral itself is not magnetic at all.

Why do they contain magnetite inclusions? To understand that we first have to take a look at the chemical composition of pyroxenes and olivine which are the source material of serpentine. They both usually contain iron but serpentine contains very little iron (its chemical formula contains none but small amount of Mg ions may be replaced with Fe). Most of the iron ions of the precursor minerals are not incorporated into the crystal structure of serpentine, so they tend to form separate iron oxides, most frequently magnetite and hematite.

Antigorite and lizardite may form sand grains but they have only a local importance near the rocks containing these minerals. Serpentine usually alters to chlorite. Good place to look for serpentine are ophiolite sequences. Ophiolite is a piece of oceanic lithosphere that is tectonically pushed on top of the continental crust. The grains below are from a beach sample collected in Corse, France. An ophiolite sequence is exposed there.

Antigorite grains picked from a sand sample collected in Northern Corse near Albo. The darker the grains, the more magnetic they are. The wedge-shaped grain on the left that is significantly darker in the upper corner is an especially good example. It flips its darker side towards the hand magnet if the magnet is slowly moved towards the grain. The width of the view is 17 mm.

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Serpentine minerals occur in skarns. These are metasomatic rocks (metamorphism with extensive change in the bulk chemical composition) that formed as silicic magma reacted with carbonate rocks. Skarns often contain metallic ore minerals and unusual Ca-bearing silicates. Serpentine is widespread here because the host rock is dolomitic marble that provided lots of magnesium. Gray mineral is magnetite (iron oxide). Tapuli, Sweden. Width of sample 11 cm.
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Skarn sample consisting of serpentine and hedenbergite (iron-rich Ca-clinopyroxene). Tapuli, Sweden. Width of sample 13 cm.
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Skarn with magnetite (gray), serpentine (yellowish green), talc (dark green and micaceous), and calcite (white) as the dominating minerals. Tapuli, Sweden. Width of sample 9 cm.

Sand sample from the Thassos Island in Greece is so rich in interesting minerals that it would be a shame to put it back to the drawer so soon. I used it before to illustrate the article of kyanite. I mentioned there that this sand contains epidote among many other interesting minerals.

Epidote is a very common mineral in sand. Green mineral found in sand is often epidote. However, epidote is definitely not the only one among green minerals. It could also be olivine, pyroxene, glauconite, chlorite, or pumpellyite among others that may be confused with epidote.

I will leave longer overview about this interesting mineral for another day. I have noticed that many sand samples from Greece seem to be especially rich in epidote. The grains on the picture are very small and the photo is not particularly rich in details but what I like about the picture is the color of the grains. This very well represents the dirty or pistachio green color that is so characteristic to epidote. Glauconite and chlorite are darker and fresh olivine tends to be “cleaner” green which makes identification easier.

Epidote has one good cleavage that runs parallel to the longer edges and controls the shape of the grains by making them elongated and the edges of the grains often relatively straight even in worn-out sand grains.

Sodalite is a mineral and also a mineral group. The latter contains minerals sodalite, nosean, and haüyne. Lazurite also belongs to this group, but it can be treated as a variety of haüyne. Sometimes lapis lazuli is erroneously also included. It is a beautiful blue gem but it is not a mineral. It is a rock type that contains lots of lazurite.

Sodalite grains from Namibia. Width of view 15 mm.

All sodalite group minerals are feldspathoids. What does it mean? These are minerals that somewhat resemble feldspars — both structurally and chemically and they have a similar role in igneous rocks. But they contain less silicon in relation to other ions in the crystal structure. Feldspathoids form in silicon deficient igneous rocks. Silicon deficiency means that there is not enough silicon for feldspars, let alone for quartz. Therefore, you will not find quartz together with feldspathoids in igneous rocks. Hence, no hope to find sodalite in granite for example. However, there is no restriction for feldspars to occur in the same rocks with feldspathoids. If there is silicon deficiency, some of the magma will crystallize as feldspathoids instead. Hence, they take the place which normally is reserved for feldspars.

Silicon deficient magma is not rare but then it usually contains lots of iron and magnesium. Such magma is called mafic or ultramafic. The latter also contains no quartz and the former may contain only very small amount of it. However, that is not enough for the feldspathoid minerals. Let’s take a look at the chemical composition of sodalite: Na₈Al₆Si₆O₂₄Cl₂. It is clear that in addition to silicon deficiency one more important condition must be fulfilled — sodalite needs lots of sodium (Na). The combination of low silicon and high sodium is pretty rare and so are rocks that contain sodalite or other feldspathoid minerals.

Now it should become clear that we are dealing with minerals that are not easy to find. However, if the right conditions exist for the minerals to form, then they may form a significant portion of the rocks composition. They often are the most important minerals in the rocks that contain them, just like feldspar is usually the most important mineral in granite. Feldspathoid-bearing igneous rocks are usually called foid-bearing. This is officially accepted and agreed upon simplification.

Sodalite is the most common mineral of the sodalite group but not among feldspathoids. That honour goes to nepheline which often occurs together with it. Sodalite is usually blue but other colors (green, yellow, pink, gray, colorless) are possible also. This mineral group is usually identified by its color. They are also optically isotropic. Hence, polarizing microscope is useful to confirm the identification. Sodalite, nosean, and haüyne are not easily distinguished from each other. More advanced methods like X-ray diffraction or chemical analysis is needed for that but it may be helpful to know that plutonic rocks (foid-bearing syenite for example) usually contain sodalite. Nosean and haüyne are normally restricted to volcanic rocks like phonolite and alkali basalt¹.

In addition to igneous rocks, sodalite group minerals also occur in contact metamorphosed carbonates. Lapis lazuli is such a rock. It usually contains calcite and pyroxene in addition to feldspathoids.

This mineral is not a common sand constituent but may locally comprise a significant amount of some sand samples which are the disintegration products of foid-bearing rocks exposed nearby. Blue sand is highly sought after among sand collectors. It is usually sodalite that gives blue color to such sand.

References

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

Kyanite (Al₂SiO₅) is a common mineral in aluminum-rich (pelitic) metamorphic rocks. Kyanite commonly forms large crystals (porphyroblasts) which usually have a distinct blue color. Its crystals are typically elongated and rectangular or with step-like features which is a typical sign of good cleavage in several directions. In addition to cleavage there is also a good basal parting. The lines running almost perpendicular (85°) to the longer edges are parting cracks. Parting is somewhat similar to cleavage but not every crystal has it and they are created by external stress.

Kyanite has an alternative name also — disthene. Both of these names have a meaning relating to the properties of the mineral. “Kyanos” is blue in Greek and disthene (also from Greek) means that its hardness varies. It is considerably weaker along the crystal (5 on Mohs scale like apatite) and stronger across it (7 like quartz).

Kyanite is typically light blue, but not always. It may be white, yellow, or gray. But even black, pink, and green colors are possible. Hence it is not very wise to identify kyanite by its color, although it is a common perception that kyanite is a blue mineral. Beautiful blue crystals may be used as gemstones.

The mineral has several polymorphs. These are minerals that have the same composition, but differ in the crystal structure. These are sillimanite and andalusite. Their composition is quite simple: Al₂SiO₅ and one may change to another as the metamorphism progresses.

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It is usually easy to identify because of elongated tabular crystals that are usually light blue in color. Kapteeninautio, Finland. Width of sample 15 cm.
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Large sample of kyanite crystals with quartz. Kapteeninautio, Finland. Width of sample 28 cm.

Typical order is andalusite → kyanite → sillimanite, but not always because kyanite tolerates very high pressure but not too high temperature. So it may avoid the sillimanite phase if the temperature is not rising as fast as the pressure does. That may happen in subduction zones, for example. Kyanite is sometimes found in eclogites which are metamorphic rocks of very high pressure. The crystals on the picture below also come from eclogite.

Kyanite is almost always a metamorphic mineral (it is sometimes also found in pegmatites, kimberlites and veins). The rocks containing it were most likely once muddy seabed or something like that. Eclogite, however, is usually metamorphosed basalt or similar rocks. Rocks that most typically contain this mineral are schist and gneiss. It is one of the index minerals of metamorphism. It means that kyanite is used to roughly estimate the conditions (temperature, pressure) that prevailed when these minerals formed. It often occurs together with staurolite, sillimanite, garnet, andalusite, and other metamorphic minerals. It is quite common constituent in sand because its resistance to weathering is good and it is a common constituent of several rock types.

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Kyanite crystal picked from a beach sand of Thassos Island in Greece. Width of view 12 mm.
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Crystals shown above are picked from this very interestingly versatile beach sand. Other notable minerals it contains are quartz, feldspar, epidote, staurolite, and almandine. Can you spot a grain of eclogite in the middle? Yes, the one with green pyroxene and red garnet. Metalia beach, Thassos Island, Greece. Width of view 9 mm.

Chromite is a black chromium-bearing mineral, FeCr₂O₄. It is a principal ore of chromium (Cr). Chromite is a member of the spinel group. These minerals share the same structure but differ in composition. Al and Mg in the lattice of spinel are replaced by Cr and Fe, respectively.

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Crystals picked out of a chromite concentrate from the Rustenburg mine, Western Bushveld, South Africa. Width of view is 5 mm.

The appearance of chromite may be similar to magnetite (which also belongs to the spinel group but has slightly different structure). Both are opaque and occur in octahedral crystals (double pyramids with eight crystal faces). However, both minerals occur more often as anhedral or granular masses. It isn’t hard to tell them apart because chromite may be only weakly magnetic while magnetite shows very strong affinity to magnets.

Pure chromite is rare. It usually contains magnesium which replaces iron. Other elements like trivalent Fe, Zn, Mn, and Al may be present as well. Chromite that contains more Mg than Fe is named magnesiochromite. There is continuous solid solution between the two and most commonly natural crystals are somewhere in the middle between the two endmembers but more on the side of magnesiochromite. Hence, most chromites are probably not chromites sensu stricto.

This mineral is relatively resistant to weathering. It may be important constituent of sand but not everywhere. It is found in mafic and ultramafic rocks like pyroxenite and peridotite (including dunite). These rocks are widespread deep below in the mantle, but they are not common rocks near the surface. Best chances to find it in sand is near ophiolites. These are assemblages of rocks which once formed ocean floor but are now tectonically pushed on top of the continental landmasses. It is also found in some meteorites.

Chromite in igneous rocks may form bands or layers which are very important mineral resources. These layers are the result of a crystal settling in a magma chamber. Such deposits are for example Bushveld complex in South Africa and the Great Dike in Zimbabwe. It is among the first minerals to crystallize out of magma. Important chromite mining countries are Kazakhstan, South Africa, Zimbabwe, and India. Chrome is needed for a variety of purposes, but it is mostly used to make stainless steel.

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Serpentinized ore from The Zhob Valley, Pakistan. Green is serpentine, black is chromite. This is a cumulate rock associated with ultramafic rocks. Width of sample is 6 cm.

You can take a much closer look into similar rock from the Stillwater complex in Montana. Here are thin section images and gigapans by Ron Schott.

Spinel is both a mineral and a mineral group. As a mineral it has the following chemical composition: MgAl₂O₄. As a mineral group it shares the same structure but varies in chemical composition. Even magnetite (Fe²⁺Fe³⁺₂O₄) and chromite (Fe²⁺Cr₂O₄) belong to this group.

http://picasaweb.google.com/107509377372007544953/Rocks#5805071090057377074
Grains picked from a beach sand sample collected in Hambantota, Sri Lanka. The width of the view is 10 mm.

The group is divided into three series: spinel series (trivalent ion is aluminum), magnetite series (trivalent ion is iron), and chromite (trivalent ion is chromium) series.

Spinel series contains several end-member minerals of which spinel (sensu stricto) is the most common. Natural crystals are usually not pure endmembers. There are extensive replacements between different metal ions taking place which means that the minerals of the group are mostly solid solutions and that is the reason for such a variable color we encounter in the whole group, series, and the mineral itself.

The coloring agents of the series are Cr (red), Fe²⁺ (blue, green, black), Fe³⁺ (brown), Zn (bluish green), etc.

All minerals with spinel structure are optically isotropic and they have no cleavage. These are useful characteristics that help to identify the mineral. The mineral is often strongly colored. Red variety may be easily mistaken for garnet (both are isotropic as well) but spinel has usually deeper and darker red color. Grains in sand generally have matte surface with glossy patches.

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Grains picked from a beach sand sample collected in Hambantota, Sri Lanka. Width of view 10 mm.

Spinel (sensu stricto) is mostly a metamorphic high-temperature mineral. It occurs in metamorphosed calcareous sediments (both regional and contact metamorphism) and argillaceous (rich in clay) high-temperature metasediments. It also occurs in igneous rocks and sometimes in hydrothermal veins too.

It is a physically hard mineral (harder than quartz) and is quite resistant to chemical weathering as well. Hence it is not uncommon in sand. Black minerals with matte surface and glossy patches are often spinel grains although their isotropism may be hard to determine because they are usually almost opaque. Tourmaline may be similar but is usually brownish, has often elongated and striated crystals, and has very strong pleochroism. It has no pleochroism and is always ‘extinct’ in crossed polarizers. You will need a polarizing microscope to test these properties (isotropism and pleochroism).

There is only one question. I would like to know how many different minerals can you find in the image below? Can you name some? They are sand grains picked from a beach sand sample collected in Sri Lanka — the land of gems.

Answer

I know that photo only is usually not enough to identify a mineral. But in this case it should be possible. There is only one mineral. This mineral is well-known for its variable colors. Deep red, almost transparent, gray, blue, bluish green, brown, black. These are the colors that characterize this mineral which was mined in Sri Lanka many hundreds of years ago and continues to this day (by hand digging). No, it isn’t sapphire (corundum) which is the most important gemstone mined there. But this mineral is valued as a gemstone as well. It is spinel.

Cassiterite is a mineral composed of tin and oxygen (SnO₂). It is the most important ore of tin. Pure cassiterite is light-colored mineral, but in real world it contains some iron, which makes the crystals often look almost opaque. The grains in sand are usually light to dark brown. Crystals are commonly twinned and it is a very helpful characteristic in mineral identification. Its twins are called elbow twins because two crystals are joined like the upper arm and forearm. Beautifully preserved specimens do not last in sand. Hence, we have not much hope finding perfect examples of twinned crystals. Take a look at the scheme below to find out how to spot elbow twins when studying sand grains.

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This is how easily identifiable cassiterite sand grain may look like. Two crystals are joined like upper and forearm forming a characteristic notch (visor tin) between them (pointed by an arrow). There is no difference between the “upper arm” and “forearm”.

Cassiterite has strong luster which is usually described as adamantine or submetallic. Such luster is common to minerals with high refractive index. It is unusually heavy (density is ~7 grams per cubic centimeter) for an apparently nonmetallic mineral. There are several minerals that may look similar. Rutile (titanium oxide) for example may look alike, but it usually has deeper reddish color. Even garnet may sometimes look very similar to it.

http://picasaweb.google.com/107509377372007544953/Rocks#5805070887504369634
Cassiterite grains handpicked from a beach sand sample of La Turballe, France (The Bay of Biscay). How many elbow twins can you find? Width of view 7 mm.
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Sand sample from La Turballe. Width of view 20 mm.

It is usually an igneous mineral crystallizing out of a felsic magma. Most tin ores are associated with granitic rocks which host cassiterite as an accessory mineral. It is a frequent component of hydrothermal veins and greisens also.

It is not abundant in rocks because tin is not among the commonest elements of the crust. But cassiterite is resistant to weathering and therefore pretty common (although rarely abundant) mineral in sand (stream tin).

Cornwall in England was once famous mining area, but there is no tin mining taking place anymore. Most important cassiterite sources are Malaysia, DR Congo, Indonesia, Bolivia, and Nigeria.

Dolomite is a common rock-forming mineral. It is the primary constituent of dolomite rock (dolostone) and dolomarble. It is similar to the most common carbonate mineral calcite. These two share some similarities in structure and are very closely related chemically. Calcite is chemically CaCO₃ (calcium carbonate), dolomite is CaMg(CO₃)₂ (calcium-magnesium carbonate). It means that half of the calcium in the structure of calcite is replaced by the magnesium in dolomite. However, there is no solid solution between the two because calcium and magnesium ions have different size and hence can not replace each other directly in the crystal structure.

Dolostone is the most important rock type that is composed mostly of it. Dolostone was probably originally limestone which was altered by Mg-bearing water circulating in the rocks. Dolomite does not precipitate out of aqueous solutions like calcite does (in normal circumstances). Carbonate rock is dolostone if at least 50 percent of it is dolomite.

This rock type is very interesting because we still do not understand very well how it formed. Dolostone is also economically important rock type because it contains lots of pores (this is the result of dolomitization) and act therefore as an oil reservoir. Dolomite forms large part of some hydrothermal deposits where beautiful crystals may be found. It is also found in calcareous metamorphic rocks and sometimes even as a primary mineral in igneous rocks.

This mineral very often forms rhombohedral crystals. The same habit is characteristic to calcite as well, but calcite is more commonly anhedral. This is good to know because it is one of the most important characteristics that helps to identify the mineral. It is a carbonate mineral and hence reacts with dilute hydrochloric acid like calcite and aragonite. However, the reaction with cold acid is not nearly as vigorous. Small sand grain composed of calcium carbonate takes generally 30 seconds or more to dissolve in dilute HCl (10%). Dolomite needs 10 minutes or even more. However, this acid test may still sometimes yield false results. Another way to distinguish between the two is to use organic dye alizarin red S which turns calcite bright red, but does not affect dolomite.

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Dolomitic sand grain with characteristic rhombohedral shape measuring approximately 1.5 mm.

The same crystal, but here it is covered with acid. The bubbles contain carbon dioxide that escaped when the crystal structure broke up.

Using hydrochloric acid may not be a foolproof method to differentiate between dolomite and calcite, but it is tremendously useful in field studies to quickly determine whether the cement of clastic rocks is carbonate or not and it is also very useful in the study of sand samples. I often pick yellowish grains out of sand if I am not sure whether it is a badly worn out biogenic grain or feldspar for example. I just need to put one grain into a drop of acid on a glass slide and the answer is right there.

How common is it in sand? Not really common. It is a common cementing mineral in sandstone, but dolom. as a sand grain will not last very long. Dolomitic sand grains may form important part of some lithic sands which are the disintegration products of carbonate rocks nearby. The grain in the picture above is from Namibia. It is part of a very interesting blue colored sand composed mostly of sodalite.

Dolomite in anorthositic granulite host rock. Bergen, Norway. Width of sample 13 cm.

This mountain range in Northern Italy is largely composed of dolomite rock and is appropriately called the Dolomites.

Goethite is a common weathering product of iron-bearing minerals like magnetite, pyrite and siderite. It is an oxyhydroxide or iron — FeO(OH). The most characteristic feature of this mineral is brown to yellowish color. It may resemble hematite (Fe₂O₃), but hematite is either gray or reddish. It changes to hematite on dehydration. Powdered hematite is reddish brown, but goethite is yellowish brown. Both of these minerals are good paint pigments. Goethite is the main constituent of yellow ochre.

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Oolite consisting of goethitic ooids from Germany. Width of sample 12 cm.

It is a common iron-bearing mineral in soil. It is usually formed under oxidizing conditions. It precipitates directly from marine and meteoric waters and is the main mineral phase containing precipitated iron in these environments. It may form mixtures with other similar minerals which may alter the color. Limonite is a mixture of several iron oxides and hydroxides, goethite usually being one of the most important constituents. It may be mined as a mineral resource. Bog iron is mostly composed of it. Goethite is the principal component in sedimentary iron ore in the Lorraine Basin of France¹.

The mineral is named after the German poet Johann Wolfgang von Goethe who was also interested in mineralogy.

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Sand sample from Martha’s Vineyard, Massachusetts, USA. Brown rounded pellets are goethite grains. Width of view 10 mm.
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Bog iron is mostly composed of it. Width of sample from Estonia is 14 cm.

References

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