The Tourmaline Group - a brief overview

Tourmalines are highly appreciated gems that form elongated crystals that exist in every color, from colorless over blue, red, green to black. Here we tell you something about the chemistry and properties of this mineral group, which includes Elbaite, Liddicoatite, Schörl, Dravite, Foitite, Uvite, Buergerite and many more.

The name of the mineral is beliefed to come from the ancient Cingalese word "Turmali" describing a mixture of gem rough. Tourmalines can come in every possible color - from colorless to black. Chemically, the tourmalines belong to the class of complex boro-silicate minerals. They have very complicated crystallographic properties. However, one may think of tourmalines as a special kind of glass. In fact, boro-silicate glass, which is thermally and mechanically very robust, is used in the kitchen for cooking or in chemical laboratories.

Tourmalines from long, often pencil-like, hexagonal crystals. This comes from the fact that during crystal-growth the molecules preferentially align along one axis. There are actually quite a number of different varieties of tourmaline (see above). However, the variety is not neccessarily related to its color. For instance, the varieties Elbaite and Liddicoatith can be found in every color. This is due to the fact that the color is caused by trace amounts of chemical elements ("impurities"). For example, traces of mangan (Mn) often cause a pink to red color, chromium (Cr) - for instance - is a source of green color, such as iron with charge 2+ (Fe^II). The exact compisition of the solution or melt, in which the crystals grow, varies quite often. This gives rise to different colors within the same crystal. This change can be best seen when a crystal is sliced up and polished. The picutre on the left shows a crystal cut alon the c-axis (long axis), the picutre to the right shows a typical tourmaline slices. It is cut perpendicular to the long axis. The colors remind one of a water melone (red core, green outer rim). Therefore, tourmalines with this color pattern are called Water-Melone-Tourmaline.



Tourmalines have early been recognized to have special properties. For example, people noticed that one can remove ashes from pipes after smoking, when the turmaline was rubbed before. In Holland this lead to the name "Aschentrekker" (ash-drawer). This property is a consequence of the chemical elements found in tourmaline and the way these elements are arranged. In this case the properties are due to the interplay of the charge of the elements building the elementary cell in the crystall and the geometry in which they are arranged. The property of the "Aschentrekker", namely that changes in temperature can charge the crystal, is actually of technical importance. Slices of tourmaline are used as temperature sensores at temperatures of several hundrets of degree Celcius. This effect is called pyro-electricity. You can also charge a tourmaline by pressurizing it - an effect called piezo-electricity.

However, tourmaline has more intersting properties. One such property is called pleochroism. This means that the mineral has different optical properties depending on the direction from which you look at the crystal. Material that display pleochroism are birefringet (double refraction of light). You can actually see this best when you look at a cut turmaline and look at from different views. You should be able to see that the color changes at least in intensity. In case of the tourmaline, which forms elongated, often pencil-like crystals, one color is seen when you look along the main (long or c-axis) of the crystal. The other color is seen from an angle perpendicular to the long crystal-axis. Another mineral being very well known for its pleochroism is Tanzanite, which actually displays three different colors.


Elbaite
Na[Li_1.5Al_1.5]Al₆Si₆O₁₈(BO₃)₃(OH)₃(OH)


This type of tourmaline is named after the Italian island Elba. It is the most popular of all tourmalines and can form crystals of spectacular size and color. Today, important Elbaite specimen come from Afghanistan, Pakistan, Namibia, Madagaskar, the U.S.A, and Brazil. The most important find ever came from the Jonas-Mine, Itatiaia, Brazil. There, on Good Freiday 1978 a pocket with Cranberry-red crystals up to 1 m and more than 100 kg ... certainly a millenium find!


Liddicoatite
Ca[Li₂Al]Al₆Si₆O₁₈(BO₃)₃(OH)₃(F)

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Liddicoatite is the calcium-analogue of Elaite. A very important site for Liddicoatite is the west-african island Madagascar.


Schörl
NaFe(II)₃Al₆Si₆O₁₈(BO₃)₃(OH)₃(OH)


Schörl is the most abundant of all tourmalines. It is actually a component of many rocks. Its dominant color of Schörl is black. However, black is not black! If one cut slices through black Tourmalines virtually all colors can be found. But these colors are so intense that the crystal does not permit any light to pass through it - it is not translucent any more. Therefore, in order to see any color in Schörl, the slices are are only a few micro-meters (one millionth part of a meter) thick.


Dravite
NaMg₃Al₆Si₆O₁₈(BO₃)₃(OH)₃(OH)


Dravit is a Magnesium-tourmaline named after the river "Drau" that originated in Carynthia (Austria). It can occur in many colors, but brown is the most important color. Dravite may form rather large crystals.


Uvite
CaMg₃[Al₅Mg]Si₆O₁₈(BO₃)₃(OH)₃(F)


This is the Ca-analouge of Dravit (comparable to Liddicoatite and Elbaite). It can be brown, green, red, or black in color. Its name comes from a procinec in Sri Lanka.




Book Tip and Reference:

extra Lapis English No. 3: Tourmaline, Christian Wiese Publishers, Munich (also available in German)
Highly recommended reading! Lot of introductory, historic as well as advanced information.

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