Friday, October 17, 2014

Single Crystal Growth of Tourmaline

Tourmaline is called the rainbow gemstone because it can be found in all colours of the rainbow, or light spectrum. The most common variety is black and opaque and called schorl, which is the sodium iron (divalent) end member of the group. The name is an old one dating back at least to 1400, it being derived from a village in Saxony, Germany, where it is found with cassiterite in a tin mine. In Australia black tourmaline prisms are commonly found with quartz and sometimes cassiterite in granite pegmatites . It wasn't until the 18th century that it was realized the coloured tourmalines and schorl were the same mineral structurally but differed only in chemical composition.

Dravite is a brown Tourmaline, being a sodium magnesium rich end member of ideal composition
NaMg3.(Al,Mg)6.(BO3)3.(Si6O18).(OH,F)4 It is apparent that the mineral has essential volatile components,viz., OH and B, or water and boron, so in order to synthesize the mineral it is necessary to use the hydrothermal method. Tourmaline readily grows from an oxide mix under hydrothermal conditions of 500 to 600 degrees C and and water pressures to 1000 bars. The crystals are minute prismatic and coloured according to the transition metal ions contained in the starting mixture (1).

To grow a single crystal of tourmaline you can control the growth on a seed plate by separating the oxide components i.e., using lumps of quartz and alumina etc under isothermal conditions, or by using the temperature gradient method with natural tourmaline as a nutrient. Subsequent to my studies on synthesizing tourmaline from oxide mixes at a later date when I was teaching at Victoria University, Wellington, New Zealand, I did play around for awhile trying to grow tourmaline single crystals. Recently,(45 years later), I have been ratting around in a cigar box of mineral specimens and vials of seed plates and, lo and behold, I found a vial labelled synthetic tourmaline. This coincided with finding my little digital microscope which I hooked up to the computer and proceeded to examine these seed plates from a experiment done ages ago.

I used the temperature gradient method with natural tourmaline as nutrient to try and grow tourmaline on two seed
plates, one being a basal plane plate and the other a pyramid plate. I have forgotten the details but I still have the results in a wee vial. The experiment would have lasted maybe 3 days at ca 600 degrees C and using 35% fill, with mineralizer forgotten. The results are interesting and are depicted in the photographs.

The first foto shows new black growth on one side of the basal seed plate and hardly any growth on the other side,
which is unexpected. The thickness of the new growth is no more than 1 mm and appears jet black, like schorl. On thinner parts and using a strong light the colour is a deep blue. It is to be remembered that the optic axis of tourmaline has very strong absorption of light, which is the o-ray, when viewed perpendicular to the basal plane. Also, tourmaline has the curious property of being hemimorphic, which means that one end of the c-axis is different from the other, i.e, the faces differ and so too does the growth rate. The seed plate shows striations paralleling the c-axis like natural tourmaline. The other seed plate cut at an angle to the c-axis has thinner new growth which appears a nice blue colour. The blue colour is due to iron probably derived from corrosion of the steel pressure vessels used.

Of course, it would take a lot more experimentation to perfect a good method to grow large single crystals of tourmaline, which may be of use in jewellery or for electronic applications. Now-a-days you can buy hydrothermally grown quartz as amethyst and citrine, as faceted gems of 20 carats and costing only $20. How it is possible to do this at a profit is beyond my comprehension. However, I offer this information on the matter in hope that it will be helpful for some student with a thesis on crystal growing. See also Reference (2) of a Russian experiments to grow tourmaline, found on doing a Google search. Also there were two Japanese and one German relevant references. Note that this is real tourmaline and not the "synthetic tourmaline" sold as cubic zirconia or spinel. Allano.

(1) "Synthetic tourmalines containing elements of the first transition series" Journal of Crystal Growth, 1967,1,238-244, by A. M.Taylor and B. C. Terell
(2) "Growth of tourmaline single crystals containing transition metal elements in hydrothermal solutions" Journal of Crystal Growth, 2011, Vol 318, 904-907 by Tatiana Setkova, Yury Shapovalov and Valdimir Balitsky