Minerals

Epidote is a common pistachio-green silicate mineral. This light-green color helps to distinguish it from chlorite, which may occur with it but is usually darker green. However, epidote may have various shades of green color, larger crystals are almost black.

Epidote is a mineral, but there are other structurally similar minerals which together are known as the epidote group. Hence, it has both wider and narrower meanings. Other minerals of the group are clinozoisite, zoisite, allanite and piemontite. Epid. is the most common of these minerals and forms a solid solution series with clinozoisite. Clinozoisite can be described as epidote without or with little iron content. Zoisite shares the same composition with clinozoisite but has a different crystal structure. Zoisite is not as common as clinozoisite. Allanite contains rare earth elements and piemontite is a relatively rare form of reddish variety that contains manganese.

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Sand-sized crystals demonstrate that it forms elongated crystals. Width of view 38 mm. Nevada, USA.
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Hydrothermal vein in a granitic rock. There is a crack in the middle which allowed the hydrothermal fluids to flow and metamorphose the rock. The assemblage of epidote with pink K-feldspar and quartz is also known as unakite. Width of sample 11 cm. Arendal, Norway.

This mineral occurs mostly in various metamorphic rocks (greenschist and amphibolite facies), but it may also crystallize directly from felsic magma. In metamorphic rocks it forms at the depth of 5…25 km and temperature between 300…650°C. It also occurs in blueschist facies rocks and in metamorphosed carbonates. It is widespread in metamorphic rocks. Epidote is a very common hydrothermal alteration mineral. This alteration process, if it happens with feldspars, is known as epidotization. Saussuritization is a hydrothermal alteration of plagioclase feldspar which yields epidote. It is usually found in mafic igneous rocks.

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Epidotic alteration on a gneiss boulder. Varanger Peninsula, northern Norway. Width of sample 15 cm.

It is pretty resistant to weathering and is therefore common mineral in sediments. The heavy mineral fraction of sands very typically contains green epidote grains with black magnetite and pink almandine garnet. However, there could be many different green-colored minerals in sand which seriously complicates the identification. Green minerals may also be olivine, pyroxene, chlorite, pumpellyite, etc.

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Epidosite is a rock type that contains lots of epidote. The specimen from Cyprus (Troodos ophiolite) is about 10 cm in width.

Sand-sized grains from a mine in Nevada, USA.

Natrolite is a zeolite group mineral. Zeolites are minerals that are structurally somewhat similar to feldspars (they all have three-dimensional framework of silica tetrahedra) but their structure is much more open. Zeolites were once considered to be of minor importance because most zeolites are very fine-grained but X-ray studies in the last 50 years have revealed that zeolites are actually very common minerals in some sedimentary and metamorphic environments and form the largest single group of silicate minerals.

Natrolite in limburgite from Kaiserstuhl, Germany. The width of the crystal aggregate is 21 mm.

Zeolite group contains over 80 minerals. In addition to that there are more than 600 synthetic minerals which we haven’t found in nature. Obviously, there must be a good reason to make so many synthetic minerals and there is. The open structure of zeolites makes them ideal molecular sieves (they trap certain molecules which fit into the void). These voids are normally filled with water but water can be easily driven out by heating the material. Hence, zeolites can be used several times as a desiccant for example. Zeolites can be tailor-made for specific purposes like removing heavy metals from mine waste or cleaning up radioactively contaminated areas. Some zeolites are even useful in medicine as a blood-clotting agent (useful in battles or disaster areas). One zeolite mineral, erionite, may pose some risk to human health. Long term exposure to erionite fibers may cause similar lung pathologies as asbestos minerals do.

Most zeolites are too fine-grained to be identified without X-ray analysis but there are exceptions. Natrolite is probably the best-known zeolite because it forms beautiful white radiating crystals in the vesicles of mafic volcanic rocks. Other zeolite minerals that sometimes form large crystals are heulandite, chabazite, analcime, thomsonite, and stilbite. Analcime is sometimes treated separately from other zeolites because it may be considered to be one of the feldspathoids. Zeolites are usually either white or pastel shades of other colors.

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Natrolite in an alkaline volcanic rock. The sample demonstrates both the outer form and the internal structure of the aggregate of natrolite. The sample is from the Czech Republic. Width of view 8 cm.

Natrolite (white) and prehnite (green). Prehnite is not a zeolite group mineral. Width of sample 12 cm.

Vesuvianite is a calcium-bearing silicate mineral. It occurs typically in impure metamorphosed carbonate rocks (silicate marble) and skarns.

It is usually green, brown, or yellow and also known by the name idocrase. It is usually associated with other calc-silicate minerals like diopside, garnets, wollastonite, etc. It may occur in alkaline igneous rocks (nepheline syenite) and in metamorphosed mafic dikes, which also contain serpentine, garnet (grossular), diopside, and epidote. Garnet and vesuvianite-bearing metamorphosed mafic igneous rocks are called rodingites.

The color of the mineral is mostly controlled by the amount of oxidation state of iron and titanium. Cyprine is a bluish Cu-bearing variety. Californite is an olive green variety, known also as American jade or California jade because of jade-like appearance.

Vesuvianite may strongly resemble grossular (garnet group mineral). Both of these minerals occur in silicate marbles.

Vesuvianite is a relatively common mineral, but only in cerain rocks, which have a rather restricted occurrence. Overall, it is an uncommon mineral species and considered to be one of the so-called rock-forming minerals only because it is a noteworthy phase in some rocks.

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Crystal (6 cm) with garnet in a skarn from Kristiansand, Norway. TUG 1608-4632

Andradite is a member of the garnet group minerals.

There are two groups of garnets: pyralspites (Al-garnets) and ugrandites (Ca-garnets). Andradite is a member of the latter.

Here they are, the garnet group minerals:

Andradite occurs mostly in metamorphosed carbonate rocks which are known as skarns. These rocks have a variable mineralogy. Grossular (another Ca-garnet) may also occur in skarns, although not as frequently as andradite.

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Andradite (brown) in a calc-silicate rock skarn with green diopside and white wollastonite. Willsboro, New York State, USA. Width of view 5 cm. TUG 1608-4877.

It forms when carbonates react with magmatic fluids that carry iron. The chemical reaction that is responsible for the formation of this mineral looks something like this:

3CaCO₃ (calcite) + Fe₂O₃ (hematite) + 3SiO₂ (quartz) → Ca₃Fe₂(SiO₄)₃ (andradite) + 3CO₂ (carbon dioxide)

This needs not to go exactly like that and the environment is definitely much more complex. Hence, we have several other minerals (magnetite and pyroxenes, for example) forming at the same time.

It may sometimes form during the metamorphism of Ca-bearing (mafic) igneous rocks, especially varieties topazolite and demantoid form that way. So, it can be found in greenschists and serpentinites which are typical metamorphics formed at the expense of mafic igneous rocks.

There is even a third alternative. Sometimes dark-colored Ti-bearing varieties (melanite, schorlomite) may be found in rare alkaline and silica-deficient igneous rocks like nepheline syenite, phonolite, etc.

Andradite has an array of alternative names like colophonite, demantoid, topazolite, melanite, etc. (there are indeed many more of them). These usually have narrower meaning, but mineralogically they are all the same mineral. “Colophonite”, unfortunately, is a term that is sometimes used to describe vesuvianite also.

Corundum is a mineral with a very simple chemical composition: Al₂O₃. Aluminum and oxygen are both abundant chemical elements in the crust. Therefore, it should be a really common mineral? Actually, this is not true. Yes, it is a rock-forming mineral but its occurrence in rocks is surprisingly low when considering its simple composition. What’s the matter? The problem for corundum is that there are very strong competitors for aluminum. There is usually not enough aluminum in the magma that could be left over after feldspars have taken out all they needed. And there is an entire array of aluminum-loving minerals in metamorphic rocks.

So, life isn’t easy for poor corundum. Its a price that often has to be paid when you choose not to specialize. But on the other hand, sometimes here and there opportunities emerge. Hence, it may occur in a number of different rock types, both metamorphic (mica schist, hornfels, metamorphosed bauxite, metamorphosed carbonates) and igneous (syenite pegmatite, nepheline syenite, and many others).

Corundum is a pretty common mineral in sand because it is very hard and it resists weathering well (doesn’t react with acids). It is especially well known for its hardness. Its a member of Mohs hardness scale (number nine, just below diamond). For this reason it is frequently used as an abrasive although usually synthetic version is used for this purpose.

Pure corundum has no color but natural crystals are usually yellow, blue, red, white, gray, or green. Corundum has alternative names because it is also an important gemstone. Red variety is ruby and most other colors are known as sapphire. The color is given to the crystals by impurities. Chromium is responsible for the red color of ruby. Blue is caused by iron and titanium. Yellow sapphire contains both di- and trivalent iron.

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Detrital ruby grains from Songea, Southern Tanzania. The width of the view is 21 mm.

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Artificial corundum is commonly almost without impurities and has therefore no color. The width of the view is 4 mm.

I mostly write about minerals that are abundant and occur in many places. The mineral I am writing about today is nothing like that. Charoite was first properly described in 1978 and it is so far known from only one locality. The Murun Massif in eastern Siberia is a syenitic magma intrusion. There are metamorphic aureoles commonly surrounding such intrusions which may contain unusual minerals.

In our particular case the rocks surrounding the intrusion are limestones. So it is a mineral that got lots of calcium from limestone, and Potassium + silicon from syenitic magma. We can say that charoite is yet another calc-silicate mineral and the rock itself is another version of skarn.

This mineral is a hydrous chain silicate, but it does not belong to the amphibole group. The mineral that is commonly associated with it is another rare chain silicate canasite.

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Charoite (purple) with canasite (light-colored). Murun Massif, Sakha Republic, Russia. Width of sample 8 cm. TUG 1608-4301.

Pure quartz is a transparent mineral. Quartz is usually almost pure SiO₂ but even very small amounts of other elements or mineral inclusions can seriously alter the color. One of the strongest coloring agents is mineral hematite. Very fine-grained hematite pigment often covers sand grains in a sandstone. But hematite may also form inclusions inside the crystals.

There are many names given to the varieties of quartz. One of them is “eisenkiesel” which is not nearly as well known as jasper, agate, onyx, etc. This is red quartz which is strongly colored because of hematite inclusions. Sometimes quartz crystals that contain other iron-bearing oxides are named the same way. This term obviously has its origin in the German language but the beautiful crystals I want to demonstrate today come from Spain. They were known as Jacinto de Compostela quartz crystals already centuries ago. These crystals grow in a gypsum-bearing marl deposit which makes them somewhat similar to the Pecos Diamonds from New Mexico that grew in a gypsum deposit.

Zinnwaldite is one of the mica minerals.

It is not a mineral anymore because The International Mineralogical Association that attempts to take care of the mineralogical nomenclature decided that there is no need for this mineral name. Possibly because it is compositionally like a middleman between siderophyllite and polylithionite. I am not sure whether the name has been discredited completely or is it ok to use the term for the compositions between the two mentioned before.

However, the sample shown below deserve to be called that way no matter what because they are from Zinnwald/Cinovec at the German/Czech border in The Ore Mountains which obviously gave name to that mineral.

Zinn means ‘tin’ in German. That does not mean that zinnwaldite contains tin but the mineral often associated with it does and is the principal ore of tin. This mineral is cassiterite.

It is compositionally similar to pink mica lepidolite (they both contain lithium and fluorine) and resembles biotite in appearance. Zinnwaldite is darker than lepidolite because it contains iron. Iron always makes the color of minerals darker. Minerals that contain lots of iron are often black (common pyroxenes and amphiboles like augite and hornblende, for example).

Zinnwaldite occurs in granitic pegmatites and greisens (often with cassiterite). It is also frequently associated with fluorite, lepidolite, spodumene, topaz, beryl, tourmaline, and monazite.

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Zinnwaldite from Zinnwald. Width of sample 7 cm.

Beryl is a mineral that usually occurs in granitic pegmatites. It is rare enough and occurs often as beautiful crystals. So, it should come as no surprise that it is a valued gemstone. It is most commonly named by gemologists either emerald (deep green), aquamarine (light blue) or morganite (pink).

Beryl (Al₂Be₃Si₆O₁₈) is a silicate mineral like most other crust-forming minerals. What makes it special is a chemical element beryllium. There are about 3…6 ppm (parts per million) of Be in the crust. Beryl is the most important of Be-bearing minerals. There are actually only few noteworthy minerals that contain this element. Therefore, beryl is not only important as a gemstone but also as a source of Be.

Beryllium ion is so small (its atomic number is 4) that most minerals can not find use for it. So, it gets concentrated in residual magmatic fluids and becomes part of a special mineral tailored for it. It is beryl which is therefore most commonly found in pegmatites and hydrothermal veins.

The color of this mineral is variable. Soft hues of blue, green and yellow occur most commonly. Crystals have a vitreous luster. Apatite crystals may be similar. They also occur in hexagonal prismatic crystals and have a variable color but apatite is significantly softer (hardnesses 5 and almost 8 for apatite and beryl, respectively, in the Mohs scale). This mineral is commonly associated with quartz, K-feldspar, micas, tourmaline, and other minerals that may occur in granitic pegmatites. Sometimes it occurs in granite or syenite (this rock is similar to granite but contains very little quartz or not at all). Some metamorphic rocks may also contain it. Emerald, which is the most valued type of beryl, occurs usually in metamorphosed carbonate rocks.

Fluorite is a mineral that mostly occurs in hydrothermal veins. The chemical composition is very simple: CaF₂. It is not very common mineral because fluorine (F) makes up only 0.06% of the mass of the crust and there are other minerals also that want fluorine into their crystal structure. Most notable of them are tourmaline, amphiboles, micas, and apatite, but all of them can replace fluorine with something else. This is something that fluorite can not do — it needs fluorine to build up the crystal structure. The composition is usually almost pure CaF₂, but some minor substitutions of Ca with Sr, Y, or Ce are possible.

However, it is fairly common in certain environments. Fluorine tends to concentrate in late magmatic fluids. Therefore, it usually forms as a component of pegmatites, greisens, and hydrothermal veins with lots of other minerals with unusual composition. Fluorite may sometimes occur in granite and syenite or even in sandstone as a cementing agent (although rarely). Fluorite also may occur in clastic sediments but it isn’t very common detrital mineral.

It was chosen by a German mineralogist Friedrich Mohs in 1812 to be one of the minerals in his hardness scale of minerals (it is number four, harder than calcite but softer than apatite).

This mineral does not have its own color. Almost any color tone is possible, but it is usually violet, blue, green, yellow, or colorless.

The color may be variable but some tones are very characteristic (as shown by the sand grains or the crystal druse below) and help to identify the mineral. Crystals are common, they form beautiful cubes. Anhedral (without well-developed crystal faces) fluorite may resemble quartz but fluorite is significantly softer mineral. Fluorite is the principal ore of fluorine.

Even almost black version of this mineral exists, it is named antozonite. The crystal structure of antozonite is partly damaged due to alpha radiation which comes from the mineral itself (fluorite can be radioactive if some of the Ca is substituted by radioactive elements). Antozonite emits strong odor when crushed. It is also called stink-fluss (stink-flow).