Tourmaline

X = (Ca, Na, K, []), Y = (Mg, Li, Al, Mn, Fe 2 , Fe 3 , V, Cr, Ti, Cu, []), Z = (Al, Mg, Cr, V, Fe 3 , Ti), D = (Si, Al, B, Be)

The tourmaline consists of a mixed series in the trigonal crystal system crystallizing ring silicates with complex chemical composition:

XY3Z6 [ (BO3 ) 3T6O18 (OH, F, O ) 4]

X, Y and Z are represented here by an arbitrary element of the group indicated in brackets, the brackets (OH, F ) means that the hydroxide and fluoride ions can relate to each other in any mixing ratio, but always in the same relation to the other constituents of the mineral are (substitution). The symbol [ ] stands for a blank space in the crystal lattice.

Tourmaline has a hardness of 7 to 7.5 and a white stroke color. The paint itself is extremely variable and can even vary along the longitudinal axis of a single crystal. Frequently blue, green, red, pink, brown or black variants occur. A single crystal can often have multiple colors. Bright crystals with dark tip are Mohrenkopf tourmalines and red crystals with a green shell are often called watermelon tourmaline. Particularly beautiful are sometimes to be seen thin cross sections through supposedly black tourmalines which may have a comparable with the agate drawing with a variety of colors.

At tourmalines are often seen the so-called pleochroism, which means that a crystal has depending on the viewing direction different colors. From the side looked green tourmalines are often always look darker brown, if you ever look at from the tip. Other tourmalines are getting darker as you look from the tip. A particular color would still almost always a very dark bluish- red hue, which only transforms into a pure blue and then in an almost pure black when rotating the crystal.

The pleochroism is accompanied by the characteristic of the tourmalines to absorb complementary polarized light, polished thin disks can act as a polarizing filter.

Another special feature of the tourmaline is occurring in crystals piezoelectric and pyroelectric effect: The former states that mechanical stress due to pressure or torsion causes opposing crystal ends electrically charged opposite to the latter, that this charge is also caused by temperature changes.

Etymology and history

The name comes from the Sinhalese word thuramali ( තුරමලි ) or thoramalli ( තෝරමල්ලි ), which is commonly used for colored gemstones that were found in Sri Lanka. For Europe, the name was first occupied in 1707 and 1711 writing under the name Chrysolithus Turmale. Larger quantities were exported to Europe until the mid-18th century by Sri Lanka. At this time the Dutch East India Company had practically a monopoly of trade for tourmalines. Because of its pyroelectric properties he was called in the Dutch language area as Aschetrekker (ash puller ). Folkloristic is reported that the Dutch, the pyroelectric properties took advantage by removing the ashes from their meerschaum pipes with tourmalines.

Schörl

Most commonly, the Fe - tourmaline occurs with the name schorl. The first detailed description of schorl with the name " schürl " and its occurrence in the Saxon Erzgebirge was in 1562 by Johannes Mathesius ( 1504-1565 ).

The etymological study of the geographical term Zschorlau (municipality in Saxony with the original name schorl ) and the name " schorl " for a mineral of the tourmaline group can be a common root word suspect, who was already in use before 1400 AD. Near Zschorlau was cassiterite ( cassiterite ), frequently associated with schorl (black Fe2 - rich tourmaline) found and dismantled. Until about 1600 were the following names in use: " Schurel ", " Schoerle " and also " Schurl ". In the 18th century, then sat down in the German language, the name " schorl " by which even today is still used. In the 18th century, the terms " shorl " and " shirl " in the 19th century, the terms "common schorl ", " tourmaline ", " schorl " and "iron tourmaline " were introduced in the Anglo- Saxon world.

Dravite

The name dravite was first described by Gustav Tschermak (1836-1927, Professor of Mineralogy and Petrology, University of Vienna ) in 1884 published his " Textbook of Mineralogy " used for Mg-rich ( and Na - rich ) tourmaline, whose presence near the village Unterdrauburg lay in Carinthia, ie in the " Drava area," the area along the Drava (Latin: Dravus ), the Austro-Hungarian monarchy. Today, the tourmaline locality belongs ( type locality for dravite ) near the village of Dravograd ( the reference is located at Dobrova pri Dravogradu ) for the Republic of Slovenia. The chemical composition, the Tschermak 1884 specifies for Dravite corresponds to the approximate formula NaMg3 (Al, Mg) 6B3Si6O27 (OH ), up to the OH content well with today's Endgliedformel for dravite, NaMg3Al6B3Si6O27 (OH ) 4, respectively NaMg3Al6 ( BO3 ) 3 ( Si6O18 ) (OH ) 4, match.

Elbait

A lithium- tourmaline ( elbaite ) was one of three minerals from Uto (Sweden), in which the new alkali element lithium ( Li) in 1818 by Johan August Arfwedson was determined. The Italian island of Elba was one of the first localities, were extensively chemically analyzed by the colored and colorless tourmalines. In 1850, Karl Friedrich Rammelsberg described fluorine for the first time in tourmaline. In 1870 he proved that all tourmaline varieties possess chemically bound water.

Scharizer suspected in 1889 in red Li- tourmaline from Schüttenhofen (now Sušice, Czech Republic), a substitution of (OH ). Vladimir Ivanovich Vernadsky published the name " elbaite " for Li-, Na - and Al-rich tourmaline from Elba, Italy, with the simplified formula (Li, Na) HAl6B2Si4O21 in 1914. Very probably comes the type material for elbaite from Fonte del Prete, San Piero in Campo, Campo nell'Elba, Elba Island, Italy.

Winchell published in 1933 an updated formula for elbaite, H8Na2Li3Al3B6Al12Si12O62, which now generally with the notation Na ( Li1.5Al1.5 ) Al6 (BO3 ) 3 [ Si6O18 ] (OH ) 3 ( OH) will be used.

Individual minerals and varieties

The individual minerals of the tourmaline ( with its varieties ) are given below with their chemical composition:

• Buergerit NaFe33 Al6 [ F | O3 | (BO3 ) 3 | Si6O18 ]
• Chromdravit NaMg3 (Cr, Fe 3 ) 6 [( OH) 4 | (BO3 ) 3 | Si6O18 ]
• Dravite NaMg3Al6 [( OH) 4 | (BO3 ) 3 | Si6O18 ]
• Elbait Na ( Li, Al) 3Al6 [ (OH, F) 4 | (BO3 ) 3 | Si6O18 ] Achroite, colorless
• Indicolite, Blue
• Rubellite, pink to red
• Siberit, red-violet to blue-violet
• Tsilaisit, dark yellow ( rare)
• Verdelite, green

Education and Locations

Tourmaline is found in the form of prismatic crystals in granitic pegmatites, but also in metamorphic rocks such as gneiss, which were modified by boron-containing hydrothermal solutions in their composition.

Use

Particularly beautiful specimens found in jewelry use, such as the rubellite, a red variety of tourmaline. The best known example is likely the championship trophy of the Bundesliga be, which is staffed with a total of 21 tourmalines. Also, the DFB Cup is equipped with tourmalines.

Due to its action as a polarization filter cut Tourmaline slices were used in the 19th century in photography, in order to suppress spurious reflections shine. Early found polarization filter of tourmaline ( besides those of calcite and herapathite ) also input into the microscopy, resulting polarization microscopes were developed. Because of the special electrical properties tourmaline is also used in electronics.

Gallery

Multi-colored elbaite - green zone = verdelite until the red zone = pink rubellite

Elbaite, variety Rubellite

Verdelithfarbener Elbait, grown in quartz from Brazil - rough

Schorl, grown in quartz from Brazil

10 -millimeter Uvitkristalle on magnesite from Brumado / Bahia, Brazil

Verdelite the so-called Emerald -cut

Ground tourmaline cabochon in different colors as