Only a very few know about Maxixe-type beryl (s), and often they are confused for aquamarine, iolite or even quartz. I have seen gem dealers getting puzzled when they have to deal with lots, and eventually they are sold as something else. You don't want to make god-like statement (s) when you don't have comparison stones or at times you go through 'momentary autism'--you just go blank/inert. Only a sophisticated lab with experienced staff will be able to recognize the tell-tale signs. Many labs do not have sample (s) of Maxixe-type beryl (s) for comparsion purposes so they get confused and misidentify them. At times it's like two blind walking the street (s).
(via The Journal of Gemmology, Vol.13, No.8, October 1973) K Nassau / D L Wood writes:
Abstract
Blue Maxixe beryl, kept in the dark since 1917, and current blue and green beryl showing similar characteristics have been examined b absorption spectroscopy, gamma ray spectroscopy, chemical analysis, and light, heat and irradiation treatments. All three show an anomalous dichroism (the ordinary ray is more blue than the extraordinary ray, while in aquamarine the reverse is true) and an unusual narrow band spectrum in the red and yellow regions. In all three cases the color is bleached by exposure to daylight or on heating and can be recovered by neutron or gamma ray irradiation. A color center not involving a transition metal such as Fe, Co, Cu, etc. is indicated. Examination of 23 faceted ‘sapphire’ blue beryl gemstones by gamma ray spectroscopy indicates that three had definitely been colored by neutron irradiation; the others may or may not have been treated by irradiation.
Introduction
About 1917 blue beryl was found in the Maxixe mine in Minas Gerais, Brazil, which had the following unusual properties: it showed a strong anomalous dichroism, a narrow band absorption spectrum for the ordinary ray which produces a pronounced ‘sapphire’ or ‘cobalt’ blue (distinctly different from the blue of aquamarine beryl); and the color faded on exposure to light. These and other properties were reported in 1933 and 1935. We consider any beryl to be ‘Maxixe-type’ beryl if it shows these three unusual properties: dichroism with blue in the ordinary ray; narrow-banded absorptions in the ordinary ray spectrum; and bleaching on exposure to light or heat. Some recent material of this type has become available, and our attention was drawn to the unusual absorption spectrum by Mr R Crowningshield.
Experimental
We have examined in detail the following: a piece of the original Maxixe find that has been kept from extended exposure to light since 1917, courtesy of Mr B W Anderson; 23 specimens of currently commercially available deep blue faceted stones (ranging from four to ten carats in weight) as well as blue rough, from an unspecified locality said to be in Brazil; and three dark green stones and some dark green rough, possibly from the same current locality. All exhibit the three properties just mentioned. Although there were some minor differences, all these specimens showed pronounced blue/colorless, blue/pale pink, or green/yellow dichroism with a similar characteristic w spectrum in the 5000 to 7500 Angstrom region.
Permission was obtained to expose to light four current deep blue stones, current deep blue and green rough, and part of the old Maxixe rough (either to daylight with intermittent sun or to a 100-watt frosted tungsten light bulb at a distance of six inches in an air-conditioned room) After one week all had faded significantly, ending with only about half of the original color or less. The bleaching was then completed by heating to a maximum of 235º (450ºF) for 30 minutes, resulting in a yellow or pale pink color. By comparison, aquamarine is customarily heated to a much higher temperature (400ºC - 750ºF) to improve the color, which remains stable to light.
Examination of all the specimens by gamma-ray spectroscopy using a lithium drifted germanium detector indicated in three of the faceted stones the presence of a small amount of Caesium-134, a radioactive species with a half life of 2 years. This is absent in nature, but produced by neutron irradiation of natural Caesium-133 in the specimens. These stones must therefore have been treated by neutron irradiation. The other specimens did not show this behavior and have probably not been irradiated with neutrons. On heating one of the partially bleached cut stones to 150ºC for 30 minutes there was no significant further change in color. However, after 30 minutes at 200ºC (about 400ºF) only a very pale pink color remained. Neutron irradiation (15 minutes at 10¹³ neutrons/cm²/sec) now returned the stone to a blue color even deeper than its original color. Another similar stone (blue/pale pink dichroism), when heated by Mr R Crowningshield, bleached completely to pale pink in less than 30 minutes at 95ºC (200ºF). This stone was exposed to gamma rays (2 x 107 rads from Cobalt 60) and also turned deep blue. This gamma ray irradiation does not leave any evidence of treatment, producing the usual characteristic w spectrum. As expected from the case of heat bleaching, this stone also bleached very rapidly in light (significantly in only 15 hours).
The green material, when bleached to a deep yellow by sunlight, could be returned to green by neutron irradiation, to a weak blue/green by X-rays, but was hardly changed by gamma rays from Cobalt-60. The recolored material (both blue and green) could be bleached again by light. The green could also be changed to yellow by a 30-minute heat treatment at 150ºC, while heating to 400ºC removed the yellow color as was previously noted in an ordinary yellow beryl; neutron irradiation returned this colorless material to green.
Analysis showed a high iron content in the green material (about 0.2%), but essentially none in the old Maxixe sample (0.000X%). This is consistent with the spectral evidence that the deep yellow component is due to Fe3+ in the octahedral Al site and indicates that Fe is not involved in the narrow banded w spectrum. Other transition metals such as Co, Cu, etc. are essentially absent. Since the blue material can be bleached by exposure to light or quite low temperatures and recovered by irradiation, a color center not involving a transition metal ion is indicated. The minor differences in the spectra may well be associated with differences in the total alkali content, the old Maxixe being high (about 2%), the green low (less than 0.1%).
Neutron irradiation was also tried on several of the beryl specimens used in our previous study. One of these, a colorless beryl showed a faint blue color after irradiation, and on examination showed a weak w spectrum of the Maxixe-type. Accordingly it appears that not any beryl can be irradiated to give a Maxixe-type color, but neither does it appear to be necessary to have material from a unique location. Investigation on this point is continuing.
Conclusions
There is some variation in spectrum, iron content, alkali content, color, and rate of bleaching by either light or heat. Nevertheless, in contrast to the many ordinary varieties of beryl known over the centuries, these specimens show sufficient similarity to merit a common designation, and we have used the term ‘Maxixe-type’ based on the first reported occurrence. At present there is not enough information to decide if this type of material originates from one or several localities. It appears that the color of some of this material may be as originally found, although some material has definitely been neutron irradiated either to form the color, to improve the color, or to return color which has been bleached by exposure to light or to heat. Some or all of the rest may have been colored by gamma rays.
Based on the observations here reported we believe that any blue or green beryl (particularly if the blue color is of ‘sapphire’ type) showing anomalous dichroism with the blue color in the ordinary ray and sharp absorption bands for the ordinary ray in the 5000 to 7500 Angstrom region should be designated as ‘Maxixe-type’. Such a beryl will face, either on exposure to light or on heating. Such a beryl may or may not have been irradiated with neutrons or with gamma rays. It is in fact not possible to determine whether a given stone has been treated or how fast it will fade.
In the words of Mr Crowningshield ‘potential buyers should be alerted to the possibility that any stone of this type, which they consider, may fade too rapidly to be a satisfactory jewelry stone.’
Appendix
A note on color centers
Most of the color in gems and minerals is caused by unpaired electrons in major ingredients such as the copper in malachite and turquoise, or in impurities such as the chromium in ruby and emerald or the iron in aquamarine and citrine. Alternatively there is color caused by physical structure, as in opal and labradorite (the optical diffraction grating effect).
But in some materials, where there is no such color causing ingredient or physical structure present, it is possible for ‘color centers’ to cause a variety of colors. Color centers have been studied intensively, but only few have been understood. Frequently this involves a vacancy (omitted atom) or some other type of defect (sometimes an impurity) which can hold (but does not of itself possess) an unpaired electron.
Examples of color centers occur in halite or sylvite (made purple to black by various treatments), fluorite (green, purple, etc.) and smoky quartz. A frequent characteristic of color centers is that exposure to light or to relatively low temperatures may permit the unpaired electrons to pair off, thus removing the color. Irradiation by X-rays, neutrons, or some other form of penetrating radiation may cause the color to return by unpairing the electrons again. An unusual, only partly understood color center is involved in the amethyst form of quartz which also contains iron as an impurity. Amethyst is turned yellow or green by heat, and can be recolored with X-ray irradiation. However not just any quartz colored green or yellow with iron will go to amethyst with irradiation—some specific defect must still be associated with the iron impurity. Synthetic quartz containing iron must be grown in one specific direction to produce this specific color center and enable amethyst to be produced on subsequent X-ray irradiation. The color of amethyst is unusually stable for a color center, although it will fade over a period of many years or in hours at 400 to 600ºC. The relative ease with which the color is produced by X-rays is consistent with this stability to light and to heat.
In the case of the deep blue beryl there does not seem to be any specific impurity present. It is likely therefore that a vacancy is involved which can hold an unpaired electron. The relative ease of fading implies that the electrons pair off readily, and the difficulty of returning the color is consistent with this instability.
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