Orange 'magnesio-' axinite likely from Mirerani, Tanzania
- Details
- Created: Sunday, 19 March 2017 10:58
The two cut stones (figures 1a, 1b) were faceted from crystals bought to be orange kyanite from Loliondo in Tanzania, then identified as orange sillimanite. Such orange color is unexpected for sillimanite! The material is relatively clean even if it contains a lot of inclusions. The observed inclusions (healed fracture) are unusual for kyanite as well as for sillimanite, orange kyanite usually hosts colorless, reddish-brown and black tiny crystals .
The classical gemology properties (table 1) definitely exclude kyanite and sillimanite materials. The data are consistent with axinite group and more precisely with magnesio-axinite that should be named axinite-(Mn) since 2008 as well as mangano- and ferro- aixinte should be respectively named axinite-(Mn) and axinite-(Fe). The table 2 gives published data for comparison to the measured properties of the gemstones analyzed here.
Another point is that the weak magnetic susceptibility confirms that Mg is predominant compared to Mn which usually causes strong magnetic susceptibility in minerals.
Even if there are few publications[1][2][3][4][5], there is almost no spectroscopy result published except the uv-vis spectrum of the axinite-(Mn) from Tanzania.
Figure 1a. 1.62 ct orange axinite-(Mg) likely from Mirerani, Tanzania.
Figure 1b. 1.52 ct orange axinite-(Mg) likely from Mirerani, Tanzania.| Shape | oval |
| Size | 9.0 x 6.8 x 3.6 and 9.2 x 6.6 x 3.7 |
| Color | orange with a moderate color shift from pale brownish-orange (daylight) to strong orange (incandescent light) |
| Lustre | vitreous |
| Weight | 1.52 and 1.62 |
| SG | 3.16 ± 0.02 |
| RI | 1.656 - 1.669 |
| DR | 0.013 B+ |
| Pleochroism | orange / pale yellow-green |
| Polariscope / Conoscope | Biaxial |
| SWUV | inert |
| LWUV | weak orange |
| Magnetic susceptibility | very weak with N52 magnet (loss weight is only 6%) |
Table 1. Observational and measured properties.
| The stones of this report | Manganaxinite[6] Jog Vol34 -3 2014, pp.191-192 |
Manganaxinite[4]
G&G SP 2008 p81 |
Magnesioaxinite[3]
G&G Wn 2007 p375 |
Magnesioaxinite[2]
G&G Su 2005 p170 |
Magnesioaxinite[1]
JoG Vol14, 1975, pp. 368–375 |
Ferroaxinite[3]
G&G Wn 2007 p375 |
|
| Locality | Tanzania | Tanzania | Tanzania | Tanzania | Tanzania | Tanzania | ? |
| Color | pale brownish-orange (D65)
strong orange (Inc.) |
greenish-blue (daylight) / lavender-purple (Inc.) | blue | pale violet to pale grayish violetish blue (D65) / Violet (Inc.) | brownish yellowish orange | blue | brown to reddish brown |
| Pleo | brownish-yellow-green / orange | violet-blue / pink / yellow | blue / brown / purple | light purple / yellow / yellowish green | moderate yellow / orange | strong purple / yellow / brownish orange | |
| RI | 1.656-1.669 | 1.672–1.687 | nα=1.665, nβ=1.672, nγ=1.679 | 1.660–1.672 | α=1.657, β=1.660, γ=1.668 | 1.656–1.668 | 1.668–1.680 |
| DR | 0.013 | 0.015 | 0.014 | 0.012 | 0.011 | 0.012 | 0.012 |
| Optical Char | B+ | ? | B ? | B+ | |||
| SG | 3.16 | 3.27 | 3.24 | 3.08-3.30 | 3.18 | 3.18 | 3.20 |
| LW | weak deep red | moderately strong orangish-red | moderate-to-strong orange-red | strong to very strong orangy pink | moderate orange | inert | |
| SW | inert | weak green | medium orangy pink | weak orange | inert | ||
| - | 6.9 wt.% MnO / 2.7 wt.% MgO Mn/Mg ~1.5 |
2% MnO / 6.27% Mg | 0.80% MnO / 6.73% MgO | 0.4% MnO | 7.80% FeO / 2.94% MgO / 0.62% MnO |
Table 2. Comparison of measured properties with published data about axinite.
Infrared reflectance spectroscopy:
IR reflectance spectra (figure 2) of the two gemstones were acquired from the gemstone's tables, the spectrum of the 1.52 ct gemstone is in red and the spectrum of the 1.62 ct is in blue. The spectra are identical, the band at 527 cm-1 indicates a high Mg content characterizing axinite-(Mg), axinite-(Mn) and axinite-(Fe) have the same band usually located between 518 and 522 cm-1. The later assumption is based on personal observations.
Figure 2. IR reflectance spectra of the two gemstones obtained from the tables (1.52 ct in red, 1.62 ct in blue). The spectra are identical, the band at 527 cm-1 indicates a high Mg content, axinite-(Mn) and axinite-(Fe) have the same band usually located between 518 and 522 cm-1.UV-VIS-NIR spectroscopy:
The UV-Vis-NIR spectrum (figure 3) was acquired with the light path being perpendicular to the table's plane. The spectrum is composed of strong absorption bands at 388, 483, 579, 743 and 835 nm. The given band position are calculated from a gaussian fitting. The 483 and 743 nm bands form a transmission window around 640 nm which causes the orange color of the stone.
Figure 3. UV-Vis-NIR spectrum of the 1.52 ct axinite-(Mg) showing absorption bands at 388, 483, 579, 743 and 835 nm. The given band position are calculated from a gaussian fitting. The 483 and 743 nm bands form a transmission window around 640 nm which causes the orange color of the stone.The UV-Vis-NIR spectrum of this axinite-(Mg) is different of that of the axinite-(Mn) published by JM. Arlabosse et al[4] and that of Pale-lilac axinite-(Mn) - 1.21 ct - Mirerani, Tanzania and Pale-lilac axinite-(Mn) - 3.36 ct - Mirerani, Tanzania.
Spectrum absorption bands cannot be ascribed yet, initial determination is under consideration at the time of writing.
Photoluminescence spectroscopy:
The photoluminescence spectrum (figure 4) was acquired with a 405 nm laser excitation source, the beam path being parallel to the length of the 1.52 ct gemstone.
Figure 4. Photoluminescence spectrum resulting of the 405 nm excitation acquired from the orange 1.52 ct axinite-(Mg). The spectrum shows a weak emission due to Mn2+ at 632 nm which is almost obfuscated by a strong emission at 657, 684, 689, 706, 710 and 736 nm characteristic of Cr3+.
The spectrum (figure 4) shows a weak emission due to Mn2+ at 632 nm which is almost obfuscated by a strong emission at 657, 684, 689, 706, 710 and 736 nm characteristic of Cr3+. Gaft et al[5] have published in their second edition book, the emission spectrum of an axinite likely axinite-(Mn) since referred in the Mn2+ bearing minerals paragraph where Mn2+ is mentioned to be capable of intrinsic luminescence (being part of the mineral and not as impurities). Rruff database has also some entries for axinite-(Mg): R070133 and for axinite-(Mn): R050299, R061121, all spectra being from broad scan with an excitation at 532 nm. PL spectra of axinite material are available in the Spec4Gem database at the following entries: Pale-lilac axinite-(Mn) - 1.21 ct - Mirerani, Tanzania and Pale-lilac axinite-(Mn) - 3.36 ct - Mirerani, Tanzania with a strong Mn2+ emission at 620-632 nm from which a sharp Cr3+ emission peak at 689/690 nm emerges.
Conclusion:
These orange gemstones are axinite-(Mg) formerly known as magnesio-axinite. This material is seldom available in the market and it is usually mixed within parcels of other materials, orange axinite-(Mg) in orange kyanite parcels, light blue axinite-(Mn) in blue-violet tanzanite parcels. The locality the stones come from is very likely Mirerani more usually spelled Mererani or Merelani, in the Lelatema Mts, Manyara region, Tanzania. There is no report of such material in Loliondo.
The material's properties are consistent with the published ones and from the luminescence spectroscopy observations, these axinite-(Mg) contain some Mn2+ and Cr3+ impurities. The color cause is not yet established and it should be worked on...
[1] Magnesioaxinite, a new mineral found as a blue gemstone from Tanzania, Jobbins et al., Journal of Gemmology, Vol. 14, 1975, pp. 368–375.
[2] Yellowish orange magnesioaxinite, E. P. Quinn and C. M. Breeding, Gems & Gemmology, 2005, Summer, p. 170.
[3] Two axinite species from Tanzania, HJ. Jang-Green, D. Beaton, B. M. Laurs William, S. and A. U. Falster, Gems & Gemmology, 2007, Winter, p. 375.
[4] A blue manganaxinite, J.-M. Arlabosse, B. Rondeau, E. Fritsch, Gems & Gemmology, 2008, Spring, p. 81.
[5] Modern Luminescence Spectroscopy of Minerals and Materials, 2nd Edition, M. Gaft, R. Reisfeld, G. Panczer, Springer Editor, ISBN: 9783319247632.
[6] Colour-change Axinite-(Mn) from Tanzania, C. and B. Williams , Journal of Gemmology, Vol. 34-3, 2014, pp. 191–192.