|Minas Gerais in Brazil (size: 9.5 cm × 4.0 cm × 3.1 cm)|
|General and classification|
|chemical formula||XY 3 Z 6 (T 6 O 18 ) (BO 3 ) 3 V 3 W
X = (Na, Ca, K, □), Y = (Fe 2+ , Mg, Mn 2+ , Al, Li, Fe 3+ , Cr 3+ ), Z = (Al, Fe 3+ , Mg, Cr 3+ ), T = (Si, Al, B 3+ ), B = (B 3+ ), V = ((OH), O), W = ((OH), F, O)
(and possibly department)
|see single minerals|
|Crystal class ; symbol||ditrigonal-pyramidal; 3 m|
|Twinning||rarely twins after the prismatic surfaces|
|Mohs hardness||7 to 7.5|
|Density (g / cm 3 )||2.82 to 3.32|
|Cleavage||none, but often vertical discharge C|
|Break ; Tenacity||shell-like|
|colour||see single minerals|
|transparency||transparent to opaque|
|Pleochroism||sometimes very strong depending on the mineral|
|Special features||Crystals show piezoelectric , pyroelectric effect and strong pleochroism|
XY 3 Z 6 (T 6 O 18 ) (BO 3 ) 3 V 3 W
- X = ( Na , Ca , K , □)
- Y = ( Fe 2+ , Mg , Mn 2+ , Al , Li , Fe 3+ , Cr 3+ )
- Z = (Al, Fe 3+ , Mg, Cr 3+ )
- T = ( Si , Al, B 3+ )
- B = (B 3+ )
- V = (( OH ), O )
- W = ((OH), F , O)
X, Y, Z, T and V in the formula stand for certain lattice positions and can be occupied by the elements given in brackets or, in the case of V and W, by a hydroxide ion . The elements specified within round brackets can also represent each other in the formula ( substitution , diadochie), but are always in the same proportion to the other components of the mineral. In contrast, B stands exclusively for boron in the 3+ oxidation state. The symbol □ stands for a space in the crystal lattice.
Tourmaline has a hardness of 7 to 7.5 and a white streak color . The color itself is extremely variable and can even vary along the long axis of a single crystal. Often there are blue, green, red, pink, brown or black variants. Light-colored crystals with a dark tip are called Mohrenkopf tourmalines and red crystals with a green shell are often called watermelon tourmalines. The thin cross-sections that can sometimes be seen through supposedly black tourmalines, which can have a pattern comparable to that of agate, are particularly beautiful .
The so-called pleochroism can often be observed on tourmalines , which means that a crystal has different colors depending on the direction in which it is viewed. Green tourmalines viewed from the side often look darker and darker brown if you look further and further from the tip. Another special color would be an almost always very dark bluish-red shade, which first turns into a pure blue and then into an almost pure black when the crystal is turned.
The pleochroism goes hand in hand with the property of tourmalines to absorb complementary polarized light; cut thin disks can act as polarization filters .
Another special feature of tourmaline is the piezoelectric and pyroelectric effect that occurs on its crystals : mechanical stress through pressure or torsion or a change in temperature causes opposing crystal ends to be electrically charged in opposite directions.
Etymology and history
The name comes from the Sinhalese word thuramali (තුරමලි) or thoramalli (තෝරමල්ලි), which was generally used for colorful gemstones found in Sri Lanka . For Europe the name was first documented in writing in 1707 or in 1711 under the name Chrysolithus Turmale . Larger quantities were only exported from Sri Lanka to Europe from the middle of the 18th century. At that time the Dutch East India Company practically had a trade monopoly in tourmalines.
Because of its pyroelectric properties , it was also referred to as the Aschetrekker (ash puller) in the Dutch-speaking world . Folklore reports that the Dutch made use of the pyroelectric properties by using tourmalines to remove the ash residue from their meerschaum pipes.
Fe tourmaline with the name Schörl occurs most frequently . The first more precise description of Schörl with the name "schürl" and its occurrence in the Saxon Ore Mountains was made in 1562 by Johannes Mathesius .
The etymological investigation of the geographical term Zschorlau , a municipality in Saxony with the former names Schorl, Schorel, Zschorl, and the name "Schörl" for a mineral of the tourmaline group led Andreas Ertl (2006) to suspect a common word stem (after Immisch (1866) Zschorlau comes from the Sorbian zorlo or žórło, source ). Zinnstein (cassiterite), often associated with Schörl (black tourmaline rich in Fe 2+ ), was found and mined near Zschorlau . The following names were still in use until around 1600: "Schurel", "Schörle" and "Schurl". In the 18th century, the name "Schörl" caught on in the German-speaking world and is still used today. In the 18th century the terms “shorl” and “shirl” were introduced into the Anglo-Saxon language area, and in the 19th century also the terms “common schorl”, “schörl”, “schorl” and “iron tourmaline”.
The name Dravit was first used by Gustav Tschermak , Professor of Mineralogy and Petrography at the University of Vienna, in his "Textbook of Mineralogy" published in 1884 for tourmaline rich in magnesium and sodium, the occurrence of which was near the village of Unterdrauburg in Carinthia, i.e. in the "Drave region “, The area along the Drava (Latin: Dravus ), the Austro-Hungarian monarchy. Today the tourmaline site belongs to Slovenia as a type locality for dravite near the municipality of Dravograd - the site is located at Dobrova pri Dravogradu.
The chemical composition, which Tschermak specifies for Dravit in 1884, corresponds to the approximate formula NaMg 3 (Al, Mg) 6 B 3 Si 6 O 27 (OH), which except for the OH content corresponds well with today's final link formula for Dravit, NaMg 3 Al 6 B 3 Si 6 O 27 (OH) 4 or NaMg 3 Al 6 (BO 3 ) 3 (Si 6 O 18 ) (OH) 4 .
A lithium tourmaline ( elbaite ) was one of three minerals from Utö in Sweden , in which the new alkali element lithium was determined by Johan August Arfwedson in 1818 . The Italian island of Elba was one of the first sites from which colored and colorless tourmalines were extensively analyzed chemically. In 1850, Karl Friedrich Rammelsberg described fluorine in tourmaline for the first time . In 1870 he proved that all tourmaline varieties have chemically bound water.
In 1889, Scharizer suspected a substitution of the hydroxide ion in red lithium tourmaline from Schüttenhofen in today's Czech Republic. Wladimir Iwanowitsch Wernadski published the name "Elbaite" for lithium, sodium and aluminum-rich tourmaline from Elba with the simplified formula (Li, Na) HAl 6 B 2 Si 4 O 21 in 1914 . It is very likely that the type material for Elbaite comes from Fonte del Prete, San Piero in Campo, on the island of Elba.
In 1933 Winchell published an updated formula for Elbaite, H 8 Na 2 Li 3 Al 3 B 6 Al 12 Si 12 O 62 , which today is generally written with the notation Na (Li 1.5 Al 1.5 ) Al 6 (BO 3 ) 3 [Si 6 O 18 ] (OH) 3 (OH) is used.
The tourmaline supergroup is divided into primary groups and secondary subgroups. The occupation of the X position with alkali ions (Na, K), calcium or vacancies is the criterion for the three primary tourmaline groups:
- Alkali group: (Na + + K + )> Ca 2+ and (Na + + K + )> □
- Calcium group: Ca 2+ > (Na + + K + ) and Ca 2+ > □
- X-vacancy group: □> (Na + + K + ) and □> Ca 2+
The occupation schemes and coupled substitutions in positions Y, Z, V and W provide the criteria for the other subgroups of the primary tourmaline groups.
The individual minerals of the tourmaline groups are given below with the chemical composition of their end members:
Alkali subgroup 1
- Chromium dravite: Na Mg 3 Cr 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 OH (IMA1982-055)
- Dravite: Na Mg 3 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 OH (Tschermak, 1884)
- Fluorine dravite: Na Mg 3 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 F (IMA2009-089)
- Fluor-Schörl: Na Fe 2+ 3 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 F (IMA2010-067)
- Fluorosilaisite: Na Mn 2+ 3 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 F (IMA2012-044)
- Oxy-Vanadium-Dravite: Na V 3 (V 4 Mg 2 ) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (IMA1999-050)
- Schörl: Na Fe 2+ 3 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 OH (Matthesius, 1524)
- Tsilaisite: Na Mn 2+ 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 OH (IMA2011-047)
- Vanadium-Oxy-Chromium-Dravite: Na V 3 (Cr 4 Mg 2 ) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (IMA2012-034)
- Potassium dravite: K Mg 3 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 OH (hypothetical)
Alkali subgroup 2
- Elbaite: Na (Li 1.5 Al 1.5 ) Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 OH (Vernadsky, 1913)
- Fluorine Elbaite: Na (Li 1.5 Al 1.5 ) Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 F (IMA2011-071)
Alkali subgroup 3
- Chromo-Alumino-Povondrait: Na Cr 3 (Al 4 Mg 2 ) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (IMA2013-089)
- Oxy-Chromium-Dravite: Na Cr 3 (Cr 4 Mg 2 ) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (IMA2011-097)
- Oxy-Dravite: Na (Al 2 Mg) (Al 5 Mg) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (IMA2012-004a)
- Oxy-Schörl: Na (Fe 2+ 2 Al) Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 O (IMA2011-011)
- Povondrait: Na Fe 3+ 3 (Fe 3+ 4 Mg 2 ) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (IMA1990-E)
- Vanadio-Oxy-Dravite: Na V 3 (Al 4 Mg 2 ) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (IMA2012-074)
- Potassium povondrait: K Fe 3+ 3 Fe 3+ 4 Mg 2 Si 6 O 18 (BO 3 ) 3 (OH) 3 O (hypothetical)
Alkali subgroup 4
- Darrellhenryite: Na LiAl 2 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (IMA2012-026)
Alkali subgroup 5
- Fluorine Buergerite: Na Fe 3+ 3 Al 6 (Si 6 O 18 ) (BO 3 ) 3 O 3 F (IMA1965-005)
- Olenite: Na Al 3 Al 6 (Si 6 O 18 ) (BO 3 ) 3 O 3 OH (IMA1985-006)
- Buergerite: Na Fe 3+ 3 Al 6 (Si 6 O 18 ) (BO 3 ) 3 O 3 OH (hypothetical)
- Fluoro-olenite: Na Al 3 Al 6 (Si 6 O 18 ) (BO 3 ) 3 O 3 F (hypothetical)
Alkali subgroup 6
- Na-Al-Al-Al-Tourmaline: Na Al 3 Al 6 (Al 3 Si 3 O 18 ) (BO 3 ) 3 (OH) 3 OH (hypothetical)
- Na-Al-Al-B-tourmaline: Na Al 3 Al 6 (B 3 Si 3 O 18 ) (BO 3 ) 3 (OH) 3 OH (hypothetical)
- Fluorine-Na-Al-Al-Al-Tourmaline: Na Al 3 Al 6 (Al 3 Si 3 O 18 ) (BO 3 ) 3 (OH) 3 F (hypothetical)
- Fluorine-Na-Al-Al-B-tourmaline: Na Al 3 Al 6 (B 3 Si 3 O 18 ) (BO 3 ) 3 (OH) 3 F (hypothetical)
- Maruyamaite: K (MgAl 2 ) (MgAl 5 ) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (IMA2013-123)
Calcium subgroup 1
- Adachiite: Ca Fe 2+ 3 Al 6 (Si 5 AlO 18 ) (BO 3 ) 3 (OH) 3 OH (IMA2012-101)
- Fluor-uvite: Ca Mg 2+ 3 (MgAl 5 ) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 F (Kunitz, 1930)
- Feruvit: Ca Fe 2+ 3 (MgAl 5 ) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 OH (IMA1987-057)
- Uvite: Ca Mg 2+ 3 (MgAl 5 ) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 OH (IMA2000-030a)
- Fluorine feruvite: Ca Fe 2+ 3 (MgAl 5 ) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 F (hypothetical)
Calcium subgroup 2
- Fluorine liddicoatite: Ca (Li 2 Al) Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 F (IMA1976-041)
- Liddicoatite: Ca Li 2 Al Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 OH (hypothetical)
Calcium subgroup 3
- Ca-Mg-O-Tourmaline: Ca Mg 2+ 3 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (hypothetical)
- Ca-Fe-O-Tourmaline: Ca Fe 2+ 3 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (hypothetical)
Calcium subgroup 4
- Ca-Li-O-Tourmaline: Ca Li 1.5 Al 1.5 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (hypothetical)
Space subgroup 1
- Foitite: □ (Fe 2+ 2 Al) Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 OH (IMA1992-034)
- Magnesio-Foitit: □ (Mg 2 Al) Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 OH (IMA1998-037)
Space subgroup 2
- Rossmanite: □ (LiAl 2 ) Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 OH (IMA1996-018)
Space subgroup 3
- □ -Mg-O-Tourmaline: □ MgAl 2 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (hypothetical)
- □ -Fe-O-Tourmaline: □ FeAl 2 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (hypothetical)
Space subgroup 4
- □ -Li-O-Tourmaline: □ Li 0.5 Al 2.5 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 O (hypothetical)
- Luinaite- (OH) (monoclinic): (Na, □) (Fe 2+ , Mg) 3 Al 6 (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 OH (IMA2009-046)
This space group is not centrosymmetric, it has no center of symmetry . The 3-fold c-axis, which in tourmalines is parallel to the longitudinal direction of the mostly prismatic crystals, is polar, i. H. Properties of the crystals differ in the direction and opposite direction of the axis. Morphologically, this is expressed in different surface characteristics at the upper and lower end of the polar axis. Furthermore, the lack of a center of symmetry allows pyro- and piezoelectric behavior, for which tourmalines are known.
The cations on the T position (Si 4+ , Al 3+ , B 3+ ) are connected by four oxygen atoms in such a way that the oxygen atoms lie on the corners of a tetrahedron with the cation in its center. Tourmalines are ring silicates . Their TO 4 tetrahedra are connected via two corners to neighboring TO 4 tetrahedra to form unbranched 6-membered rings with the composition [Si 6 O 18 ] −12 .
The cations on the B position (boron) are surrounded by three oxygen atoms. All atoms in the ion [BO 3 ] −3 lie in one plane. The oxygen atoms are at the corners of a triangle with the boron cation in the middle.
The cations in the X positions are surrounded by nine to ten oxygen atoms. The oxygen atoms lie on the corners of a trigonal antiprism, in the center of which are the cations that are one to two times charged.
The mostly divalent cations in the Y position are surrounded by six octahedral oxygen atoms. The oxygen atoms lie on the corners of an octahedron with the cation in the middle. Three of these octahedra are connected to one another via common edges to form trimers.
The mostly trivalent cations (Al, ...) on the Z position are also surrounded by six octahedral oxygen atoms.
The 6-silicate rings, M 2+ octahedral trimers (Y position) and trigonal antiprism of the X position are stacked on top of one another in the direction of the polar z axis. The free tetrahedron tips of the silicate rings all point against the z-axis and are connected to corners of the M 2+ octahedral trimers. The cations in the X position are centered above the silicate rings and connect them to the M 2+ octahedron trimer above . The M 3+ octahedra in the Z position are linked to form chains in the direction of the z axis via common edges and connect adjacent stacks of X, Y and tetrahedral positions.
The planar BO 3 anions lie in the ab plane and link the X coordination polyhedra with Z octahedra.
Few minerals have such a great variability in their color as tourmalines and numerous names have been coined for their color variations.
- Achroit : colorless tourmalines, mostly elbaite or rossmanite
- Aphricite : dark gray Schörl
- Brazilian chrysolite , Ceylon chrysolite : yellow-green tourmaline
- Brazilian emerald , emeralite : green, transparent tourmaline
- Brazilian peridot , Ceylon peridot : honey-yellow to green tourmaline
- Brazilian ruby , Siberian ruby : red, transparent tourmaline
- Brazilian sapphire : blue, transparent tourmaline
- Canary Tourmaline : light yellow tourmaline
- Cat's Eye Tourmaline : Tourmaline with Chatoyance in different colors
- Chameleonite , deuterolite : tourmaline with a color that changes depending on the lighting ( alexandrite effect ), probably dravite containing chrome
- Cromolite : green tourmaline
- Indigolite : blue tourmaline, probably Elbaite-Schörl mixed crystals
- Iochroit : purple tourmaline
- Mohrenkopf tourmaline : light tourmaline with a black tip
- Paraibaít : blue-green Elbaite containing Cu
- Rubellite : pink to red tourmaline, probably elbaite
- Siberite : purple rubellite
- Verdelite : green tourmaline, probably Elbait-Schörl mixed crystal
- Watermelon tourmaline : colored tourmaline with a pink core and green edge, mostly elbaite
Education and Locations
Tourmaline is found in the form of prismatic crystals in granitic pegmatites , but also in metamorphic rocks such as gneiss , the composition of which has been changed by boron-containing hydrothermal solutions .
Particularly beautiful specimens are used as gem stones , such as the rubellite , a red variant of tourmaline. The best-known example is probably the Bundesliga championship trophy, which is filled with a total of 21 tourmalines. The DFB Cup is also equipped with tourmalines.
Due to its effect as a polarization filter, cut tourmaline disks were already used in photography in the 19th century to suppress disturbing reflections. Polarization filters made of tourmaline, in addition to those made of calcite and herapathite, also found their way into microscopy early on , from which polarization microscopes were developed. Due to its special electrical properties, tourmaline is also used in electronics .
Tourmaline cut as a cabochon in different colors
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