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",
(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)"

Friday, December 5, 2014

Mexican Obsidian ... it makes the sharpest of knives!

The Pacific "Rim of Fire" is a region of volcanoes that are periodically erupting lavas and spewing ash into the atmosphere. When these lava flows cool quickly they freeze into a volcanic glass that is called obsidian. The lava may flow into the sea, or a lake, and thus cool quickly. Obsidians tend to be fairly high silica (70% +)of rhyolitic composition. Volcanic regions having occurrences of obsidian have been sought after by ancient man and used to fabricate arrowheads and knives. Obsidian makes the sharpest of knives and even today may be used by surgeons.

Snowflake obsidian depicted here is a favorite lapidary material for making pendants and bead jewelry. The lava has not cooled quickly enough to form all glass, but has allowed white cristobalite spherulites, a variety of silica, to nucleate and grow, and now appear within a glassy matrix.

A fortune teller's crystal ball?? No,a ball of silver sheen obsidian which is probably as good for telling fortunes. The sheen is said to be caused by aligned microscopic gas bubbles. The iridescent rainbow sheen obsidian opposite has been shown to be caused by inclusions of magnetite nanoparticles.

Mexico City is a good place to see and buy a good range of obsidian. The nearby ancient city of the Aztecs, Tiotihuacan, famous for its pyramids of the Sun and Moon, was once the center for obsidian workshops with dozens of shops producing obsidian knives, arrowheads, swords and ornaments. Now-a-days on tour you can visit some local museums or buy replica artifacts and obsidian specimens from the many stall holders.

The most famous location of old Aztec obsidian quarries is at Sierra de la Navajas (Hill of Knives) near Pachuca, not far away.

Another popular form of obsidian is that known as Apache Tears, which is found plentifully in Arizona. They are residual nodules of obsidian found within perlite rock which forms on weathering of the obsidian. These transparent tear-drop forms of obsidian are tumbled polished to make attractive pendants and other jewelry.

Sunday, November 23, 2014

Amber from Chiapas State, Mexico

A stroll around the Zocalo in Mexico City, if you are observant, will reveal stalls selling cheap amber jewellery and trinkets. The amber comes from occurrences in the mountains of Chiapas State near the border with Guatemala. Much of it is pressed amber showing embedded fossilized insects, scorpions, and even peso coins, the latter should make you suspicious of their authenticity.

To learn more about Mexican amber you have to go and stay awhile in San Cristobal de las Casas, a southern city of population about 100,000 and located up in the mountains at an altitude of 2100 meters. From Mexico City it is a journey of 1085 kms and takes 19 hours by bus. There are several interesting places to visit on the way, like Puebla and Oaxaca. San Cristobal is a junction for travelers going to or from the Yucatan and Guatemala. It is a haven for backpackers, hippies and tourists with its cheap accommodation and plentiful supply of Indian handicrafts.

In San Cristobal one must visit the Museo del Ambar de Chiapas which is housed in the Ex Convento de la Merced to see a huge collection of local amber and learn the history of its exploitation. The ancient Mayans of southern Mexico used and traded amber. The present day mines are near Simojovel and Totolapa in Chiapas State, some 80 kms north of the city. It occurs in a grey, micaceous sandstone of late Oligocene to early Miocene Age (20 to 30 million years ago) with a capping of lignite. It originates from the resin of the tree "Guapinol" and is associated with fossil brachiopods, gasteropods and molluscs.

Saturday, November 8, 2014

Antigua, Guatemala, is the jade center for Central America

The old capital of Guatemala, Antigua, is a beautiful Spanish colonial style town located in the Western Highlands. Tourists are attracted there by the thousands. It is also the center of the jade industry for there are valuable deposits of the rare jadeite-type of jade in the hinterland.

Jadeite is a pyroxene mineral, of ideal composition NaAlSi2O6, but with colouring metal ions replacing the Al. Usually it is green in colour, but can be white, mauve, emerald green etc. It is very tough, hardness 6 1/2 to 7 on Moh's scale, density 3.3, and microcrystalline like a quartzite. It will scratch nephrite jade, just. Roaming around town you will find street stalls offering cheap jade jewellery which is attractive for the tourist. Often it is a jadeite-albite rock having white patches of the feldspar, but looks quite nice. You have to go to lapidary/jewellery shops to see the valuable stuff, and pass the gun-toting armed guard on the way in.

The up-market jade jewellery is a combination of the best quality jade with diamonds, ruby and sapphire, emeralds and pearls so you need plenty of spare cash. A specialty is the reproduction of museum artifacts and death masks, the latter selling for a few thousand dollars. The carvings, and death masks which are replicas from mummies, are much sought after by well-heeled tourists from the US and Europe.

Click on foto to enlarge somewhat.

More details on Gautemalan jade are given in my Bootsnall article Jade at Antigua

Saturday, November 1, 2014

Chrysoberyl; an elusive gemstone in Australia

This is a self cut chrysoberyl of 2.25 carats and size 8 x 5 mm made from Brazilian rough alluvial material. I wonder why chrysoberyl is (apparently) not found in Australia? Maybe it does occur in our gem gravels but is not recognized or is mistaken for something else?

Chrysoberyl is typically a honey yellow to pale
greenish colour and sometimes is a cat's eye variety. Composition is BeAl2O4, a beryllium aluminate, so it would occur with other beryllium minerals like beryl and phenacite in granite terrains .It is very hard (Moh 81/2) and has an SG of 3.7. My cut stone has an RI of 1.74 - 1.75 (DR ca 0.01) which is close to that of sapphire(1.76 - 1.77) with which it could be confused. Chrysoberyl is found in the gem gravels of Brazil, Sri Lanka and Zimbabwe and..... why not Australia?

A mineral collector has pointed me in the direction of as a source of information on the matter. This is a huge database to do with minerals which has developed over the past 13 years. Yes, there are many recorded occurrences of chrysoberyl, particularly from pegmatites of the Harts Range, Northern Territory. These are small (to 1 inch)twinned crystals, specimen material, not facet grade. However, clear water worn pebbles of chrysoberyl are listed for the Weld River, Blue Tier district of Tasmania and look promising as gemstones. Gemstone hunters of the region should carefully study their haul of heavies from alluvial prospecting activities. The heavies on one's sieve would include gemstones with a Specific Gravity over 3.0, such as diamond, zircon, topaz, garnet, sapphire, ruby, spinel, tourmaline and maybe CHRYSOBERYL. Be alert and curious, and you might find one!

Friday, October 24, 2014

GEM NEWS: Argyle Pink Diamonds Get International Advertizing Push

Have you noticed on the Internet almost everywhere there is an advert for Argyle pink diamonds? This is part of Rio Tinto's push into the global market. The Australian diamond industry recorded a growth of 24% in 2013 and further growth in 2014 (Ibisworld). Biggest demand is from the US, followed by China and India.

The Rio Tinto's Argyle Mine in Western Australia produces more than 90% of the world's supply of pink diamonds. They are cut and polished in India and mostly sold in Antwerp.

Several companies specialize in selling pink Argyle diamonds, James Allen and Leibish Co being notable. Check what they have on hand for that special gift at Leibish Co.

Monday, October 20, 2014

Topaz crystals from Brazil

Everybody has their pet crystals which are part of the family, like one's dog or cat. I don't have any animals in my flat so I make do with crystals, minerals and fossils. Let me introduce my three favourite topazes of which two were bought at a mineral dealer's shop in Belo Horizonte in Minas Gerais, Brazil.

(1) The largest crystal weighs 873 grams and is flawless and colourless. In the foto it appears to have some colour but this is due to some residual iron oxide coating. It sits on its perfect basal cleavage and measures 2 x 4 inches and height 3 inches. It has a frosted water-worn surface of the original tetragonal prism and pyramid faces, which are terminated by the brilliant basal cleavage planes. You can see through it like binoculars. Each morning I use it as a weight to exercise my arm muscles.

(2) The smaller euhedral topaz weighing 149 grams exhibits the tetragonal prisms, sets of domes and pyramids and the basal pinacoid. Its size is 2 x 1 inches and 2 inches tall. It sits on its basal cleavage plane. It is a nice specimen and is a "handler crystal".

(3) Not to be forgotten is my favourite "handler topaz". Size is about 1 1/2 inches and it is a flawless angular
specimen except where the two basal cleavages are present either side.

Well there you are. How do I know that these crystals are topaz? (and not quartz, say). The perfect cleavage present is a dead give away. Also I have measured the specific gravity which comes to about 3.55 (or grams/cc). How do you do this? Why not buy my eBook "Playing Around with Minerals and Gemstones" and find out. Cheers, Allano