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30/11/2007 A study of an unusual hackmanite from Myanmar <<< note: click on the images to see larger versions
By Thomas HAINSCHWANG, GEMLAB Research Gemologist
Hackmanite is a rather strange variety of the mineral sodalite. Sodalite is generally blue and translucent to transparent, but can occur also in colourless form. Hackmanite in contrast is generally colourless, translucent to transparent and rarely appears pale pink when kept in diffuse daylight. Stones are known from the Kola peninsula (Russia), Mount St. Hilaire (Canada), Afghanistan, and Myanmar. From these sources, only certain stones from Afghanistan and Myanmar seem to appear pink under regular daylight conditions. The vast majority of stones that we have seen in the past years, from no matter which origin, were colourless or white under regular daylight conditions.
Hackmanite is known to possess the unusual property to turn pink or to intensify the pink colour under UV irradiation, and turn back to the initial colour with a strongly varying decay time as soon as the UV source is turned off.
We have analyzed a spectacular hackmanite sample of 0.48 ct from Myanmar that we have purchased at the mineral show in Munich this year. The stone appeared distinctly pink under diffuse daylight (Fig. 1, top left) and turned dark purple by LWUV exposure and very dark purple by SWUV exposure (Fig. 1, bottom). The stone, when stored in the dark for 24 hours, appeared light violet, and would possibly decolourize when stored for prolonged periods of time.
The logics of the behaviour of the defect in hackmanite lead us to propose, that a very intense UV-free light source would accelerate this decolouration, thus we exposed the stone to an intense 532 nm (green) laser for 2 minutes. The result was as expected – the stone was near colourless with an extremely pale violetish blue appearance (Fig. 1, top right). After laser "decolorization", the sample returned back to its initial pink colour under regular diffuse daylight within a minute. The time for the dark colour obtained by UV excitation were in the order of a few seconds.
 - Fig. 1. The analyzed hackmanite after different light exposures
Fig. 1. The appearance of the hackmanite under various conditions:
top left - after exposure to regular diffuse daylight;
top right - after exposure to a green laser for 2 minutes;
bottom left - after exposure to longwave ultraviolet light for 1 minute;
bottom right - after exposure to shortwave ultraviolet light for 1 minute
Spectroscopic analysis has demonstrated that in the near colourless state three weak broad bands were present, at 537, 588 and 670 nm. Exposure to UV light strongly intensified the 537 nm band, but not the other two absorptions, which were practically undetectable in the spectra of the hackmanite in the dark purple state.
Fig. 2. The Vis-NIR spectra recorded from the hackmanite at the various colour stages shown in fig. 1;
the spectrum recorded after laser "decolorization" is somewhat inaccurate since the intense illumination of the
spectrometer has induced pink colour quite rapidly. Thus the absorption bands should be weaker than shown here.
In the colourless state the UV luminescence under LW was medium pinkish orange and under SW faint blue, withouth phosphorescence; in the dark purple state the luminescence changed to red orange under LW, weak blue under SW and the absence of phosphorescence changed to a persisting and distinct blue phosphorescence after the SW UV source was turned off.
The results from this study show, that some hackmanites contain colour centers activated by very low amounts of UV or possibly even by less energetic radiation, while others need high doses of UV in order to turn pink. The destruction of the colour center by a green laser shows, that radiation higher in energy (of lower wavelength) than 532 nm is necessary to activate the defect. A blue laser and a UV laser would be necessary to verify if a higher energy visible light source free of UV could create this colour, or if indeed UV is necessary for this unstable colour center.
The most recent top-inclusions and photographs by Gemlab <<< note: click on the images to see larger versions
"Superman" inclusion in a 0.53 ct vanadium bearing colour change garnet from Bekily, Madagascar.
The inclusion in the small perfectly colour changing garnet measures about 0.5 x 0.13 mm. This Super inclusion was identified as a doubly terminated quartz crystal. The photos on the left show the sample under daylight equivalent light and on the right under incandescent light. We found this inclusion in a sample of an old stock of small Bekily colour change garnets and think that this is one of the most amazing inclusions we have seen in such garnets so far; this "S" simply had to lead us to the association with the "Superman – S". The stone is now a new precious addition to the collection of the Gemlab laboratory.
Fig. 3. The "Superman" colour change garnet and a close-up of the "Superman" quartz inclusion under daylight
equivalent light (left) and incandecent light (right); the needle-like inclusions are rutile, an abundant mineral in garnet.
Quartz inclusion in a diamond
Quartz inclusions in diamond are rare, and until today this is the first time we had the chance to photograph an idiomorphous quartz crystal in a polished diamond. The diamond was very small, 1.1 mm in diameter, thus the dimensions of the quartz crystal are approx. 0.2 x 0.07 mm. The diamond was set in a client's jewel and therefore this small wonder of mother nature could unfortunately not be added to the lab collection.
Fig. 4. Left - The quartz inclusion in this tiny polished "single cut" diamond shows exceptionally well developped crystal faces;
right - a close-up of the amazing quartz inclusion; the dark inclusions on the sides of the crystal could not be identified.
©2007 GEMLAB Est - Liechtenstein.
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