Heating Experiment of Copper-Bearing Tourmaline and Some Interesting Features Print
Recently, the Gem Testing Laboratory of Gem and Jewelry Institute of Thailand (GIT-GTL) has conducted a heat-treatment experiment on some copper bearing tourmalines known in the trade as ‘Paraiba-type’ tourmaline.  The aims of this experiment are to understand the formation of the electric-blue-color appeared after heating, and to find out if there is any evidence of heating for this type of stones that can use as identification criteria in the lab. In addition, our study is also intends to clarify the argument over the cause of red color appeared in the growth tubes and along fractures inside the stones found in the trade. Those red materials have recently been suspicious of an evidence to indicate the diffusion treatment of the so-called ‘Paraiba-type’ tourmaline from Mozambique, similar to those found in the red andesine/labradorite. The raw materials used in our experiment were samples of purplish blue tourmaline containing copper and manganese as the important trace elements. The stones were simply heat-treated in an electric furnace without any chemical additive used in the experiment. The heating was carried out in step at 150, 400, 600 and 800°C in both reducing and oxidizing conditions for the period of two-hours-soaking time in each step.

The heating results reveal that most samples show very slightly change in color when heated below 600°C; however as the heating was progressed  up to 600°C in  reducing and oxidizing conditions, the color of most samples turned to bright green or greenish blue resemble the typical color for ‘Paraiba-type’ tourmaline (Figs. 1 and 2)



Fig. 1 :  A tourmaline rough before (left) and after reduction heating at 600°C (right).  Scale bar is mm.



Fig. 2 :  Another tourmaline sample before  (left) and after oxidation heating at 600°C  (right). Scale bar is mm.

    The UV-Vis-NIR spectra of samples heated in reducing condition clearly reveal the decrease of absorption intensity in the blue-green region from 450-550 nm and peaked at 525 nm (related to Mn3+) while the intensities of very board absorption bands of copper (Cu 2+) peaked at 694  and  940 nm (Rossman,1991) still remain largely the same (Fig. 3).




Fig. 3 : UV-Vis-NIR spectra of the tourmaline sample in Fig.1 measured before and  
after reduction heating at 600°C.  (Y axis: Absorbance in arbitrary unit, X axis: Wavelength in nm,)


    The internal features commonly found in untreated samples are long growth tubes occasionally filled with yellowish brown secondary minerals and fluid fingerprints (Fig. 4).  After heating up to 600°C, the secondary minerals in those tubes turned into a brownish red material, particularly those that were heat-treated in oxidizing condition and located closer to the surface of the stones (Fig. 2). Other features found after the heat-treatment are tension cracks developed along growth tubes and fluid fingerprints (Fig. 5) similar to those features found in Nigerian tourmalines (Abduriyim, et al, 2006).


Fig. 4 : Intact long growth tubes and fluid fingerprints in an unheated sample.

Fig. 5 : Large tension discs developed along fluid fingerprints and growth tubes after heating at 600°C


    In order to understand the effect of heat-treatment on those secondary mineral better, a few samples were further heated up to 800°C in an oxidizing condition. After heating to such high temperature the originally yellowish brown secondary minerals appeared to have more intense brownish red color (Figs. 6, 7 and 8) as compared to those heated at 600°C. An even more interesting aspect of this heating experiment is that the red coloration is mostly intense around the end of growth tubes where they reach the surface of the stone, and obviously the red coloration of the growth tube is gradual lightening or decreasing from surface toward inside of the stones (Fig. 8).  The red coloration also appears along fractures that reach to the surface (Fig. 9) . The UV-Vis-NIR spectra still display fairly similar patterns to those heated at 600 °C (Fig, 10).  The brownish red materials formed after heating (Fig, 8) was identified as hematite based on Laser Raman Spectroscopy. These evidences suggest that the oxidation heating is the main cause of the appearance of red Fe-bearing material in the growth tubes and along open fractures (Figure 8).    
 



Fig. 6 : Tourmaline sample before (left) and after oxidation heating  at 800°C (right). Scale bar is mm



Fig. 7 : another tourmaline sample before (left) and after oxidation heating  at 800°C (right). Scale bar is mm



Fig. 8 : Magnification of heated samples in Figs. 6 and 7 showing brownish red features of secondary materials in
growth tubes after oxidation heating  at 800°C. The brownish red coloration appears mostly intense around the end
of growthtubes where they reach the surface and the red coloration gradually decreases inwards the stones. 



Fig. 9 : Magnification of heated samples in Figs. 6 and 7 showing the red coloration also appearing along the open fractures




Fig 10 : Representative UV-Vis-NIR spectra of  tourmaline sample measured before and 
after oxidation heating  at 800°C.  (Y axis: Absorbance, X axis in arbitrary unit:Wavelength in nm)
 

Lastly we would like to end this short communication with some important facts about the tourmaline as follows. It is commonly known in the trade that the color of tourmaline can be enhanced by heat treatment and/or irradiation. Among those, the most well-known ones are the ‘Paraiba-type’ tourmaline and rubellite which majority of them being sold in the market are the treated ones. As for the ‘Paraiba’ tourmaline it is named as a green to blue variety of elbaite tourmaline in which its color  is due to the presence of manganese and copper as important trace elements in its structure. The name ‘Paraiba’ derived from its original location in Brazil. Apart from Brazil, there are also some other localities, such as Nigeria and Mozambique, where there have been reported as being the sources of copper-bearing tourmaline. In fact, Mozambique, in particular, has become the world largest supplier of copper-bearing tourmaline in last few years. Most ‘Paraiba-type’ tourmalines from this source are declared to be heat-treated stones, especially those having purplish blue shade. However the criteria used for the identification of heat and/or irradiation treatments in most laboratories are still not conclusive enough to positively distinguish the treated from the untreated ones yet.



References:
Abduriyim, A., Kitawaki,H., Furuya,M., and Schwartz,D., 2006, Paraiba” –type Copper-Bearing Tourmaline from Brazil, Nigerid, and Mozambique:
                    Chemical   Fingerprinting by LA-ICP-MS
, Gems & Gemology, Vol. 42, No.1, p.4-21.
Rossman, G.R., Fritsch E. and Shigley, J.E., 1991 Origin of color in cuprian elbaite from Sao Jose de Batalha,
                     Paraiba, Brazil
., American Mineralogist, Vol. 76, p.1479- 484.
 
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