Monday, December 22, 2014

The Red Beryl Mystery


Natural red beryls are very rare and found in only one place in the world, that being in Utah, USA. Tiny specimens described as "Red Emeralds" were discovered in 1904 in fissures in a rhyolite lava flow in the Thomas Range (1). Later, in 1958 gem quality crystals were found in the Wah Wah mountains of Beaver Co., Utah. Since then small scale mining has been undertaken intermittently and supplied the gem market with cut and rough material, albeit of small size, much less than 1 carat size being the norm, and usually heavily included.

In 1971 the red crystals were identified for the first time by the Gemological Institute of America (GIA) and pronounced to be a red variety of beryl colored by traces of manganese (Mn), which also causes the red and pink color of the manganese minerals rhodochrosite and rhodonite, although in the beryl the manganese is present as a trace element with MnO less than 1.0 %. Analyses by others report MnO ranging between 0.18% to 0.82% and FeO ranging 1% to 3%.(2) The green color of proper emeralds is due to trace amounts of chromium or vanadium or both elements.

My interest in red beryl stems from reporting on the gem potential of synthetic cobalt beryl to the Gemological Society of G.B. in 1967, having grown in the laboratory some single crystals doped with cobalt and faceted a few of small size, about 0.5 carats (2). They had strong dichroism, purple to orange, and R.I average 1.565. The e-ray that was purplish red and showed a typical cobalt absorption spectrum with three dominant bands at about 545, 560 and 580 nm and appeared red through the Chelsea Filter. The o-ray, which was a pale orange in color, seemed devoid of the cobalt bands and so appeared green through the Chelsea Filter. This was observed using a measuring gemological spectroscope, which overall was a peculiar result. At that time (1967), I thought maybe in the future somebody or company will produce "red beryls" for the gemstone market, but you are not likely to make your fortune in doing so.

Recently I had reason to do a search on "cobalt beryl" and was surprised to find that even in the interlude to 2001 there have been recorded over 33 articles on the subject of red beryls and related doping of synthetic beryls with cobalt. What have I been missing out on?

From a gemological aspect, the most important and impressive is the GIA article "Hydrothermal Synthetic Red Beryl from the Institute of Crystallography, Moscow (3). The GIA scientists procured for study faceted red beryls up to 3.85 carat size and crystals up to 65 grams. The Russian method of growing beryl hydrothermally is a great advance over what we used in the 1965 by the Linde Company in USA. It was really an engineering problem. The Linde emeralds were grown in tiny platinum lined vessels of about 30 ml capacity. The engineering problem was how to scale up when the PT conditions were 600 degrees C and 500 to 1000 bars water pressure? The Russians solved the problem and I see that these red beryls were grown in stainless steel pressure vessels of 200 to 800 ml capacity; i.e., you need a reliable seal that doesn't leak after a week or more. The Russians get top marks for growing excellent crystals of red beryl and emeralds.

However, it is curious to note that the article title is "Hydrothermal Synthetic Red Beryl .... " and it is compared to the natural red beryl from Utah, whereas in fact it is a different "animal", being a cobalt beryl. The text says the crystals were grown with both manganese and cobalt to produce the red color. The analyses presented of the synthetic crystals gives consistently CoO ca 0.30% and MnO O.01 to 0.18%. Cobalt enters the structure mostly as Co2+ in the octahedral Al-site.

The appreciable ferrous iron present (FeO 1.32 to 1.62%) could originate from corrosion of the steel pressure vessel, or from the use of natural beryl nutrient, however it has minimal effect on the the color due to lack of strong absorption bands in the visible spectrum. On the other hand, analyses of natural red beryl do not contain cobalt and rely on manganese (MnO 0.18 to 0.82%) for the red color (chemical analysis does not specify whether Mn is 2+ or 3+).

A major supplier of synthetic crystals including hydrothermally grown beryls is Tairus Created Gems, which is a joint venture between the Russian Academy of Sciences (Siberian Branch) and Tairus (Thailand) Co. Ltd of Bangkok. Tairus grow for sale Red Beryl containing Co2+ and Co3+ and a Purple Beryl containing Mn3+, Co2+ and Co3+.

Data compiled at Cal Tech on beryl spectra (4)shows that manganese-containing beryls are pink when the manganese is present as Mn2+ (morganite) and red when Mn3+ is present (Utah naturals). Spectra is given for a synthetic pink beryl containing manganese. The absorption spectrum for natural red beryl, the e-ray gives a broad intense band from 450 to 600 nm, centered at 560 nm due to Mn3+. The o-ray is similar with the broad band centered at 545 nm. No cobalt bands are visible since there is no cobalt present.

I don't know how prevalent cobalt is in hydrothermally grown red beryls (great mystery), but if cobalt is normally added to the hydrothermal brew, then the detection of the cobalt absorption spectrum in the crystal makes for an easy way of telling the synthetic from the natural gemstone, which may be tricky in small gems. Maybe cobalt is added to make the crystal more red, than would occur with Mn2+ alone, rather than Mn3+ which seems to color the natural Utah crystals. The mystery prevails.

Some References
(1) "Red Emerald History", www.redemerald.com/history.html
(2) "Synthetic Cobalt Beryl" by A.M. Taylor, (1967) Journal of Gemmology, Vol. 10, No 8, pp.258-261
(3) "Hydrothermal Synthetic Red Beryl from the Institute of Crystallography, Moscow" by James E. Shigley et al., Spring 2001, Gems and Gemology pp.42-55.
(4) "Color in the Beryl Group; Beryl Visible Spectra (generally 350 - 2500 nm)" http://minerals.caltech.edu/FILES/Visible/BERYL/