GEM NOTES

Spectroscopic Study of Serendibite from Sri Lanka

The mineral serendibite was discovered in 1902 near Sri Lanka). It was tested previously by Crystals Gallery Gangapitiya, Kandy District, Central Province, Sri Lanka, – Gem Lab, Ratnapura (report 1423 issued in October and its name is derived from serendib, an old Arabic 2012) and the Gemological Institute of America (report term for the country’s original name of Ceylon (Prior 7328158318 issued in March 2019). The rough piece & Coomáraswámy 1903). Gem-quality serendibite has weighs 0.052 g and has been polished on one side. It is appeared on the market only rarely (Reinitz & Johnson strongly pleochroic, ranging from vivid greenish blue to 1997; Schmetzer et al. 2002). The present study provides nearly colourless (Figure 7). The RIs are 1.697–1.702 and a spectroscopic investigation of a gemmy serendibite the SG (here reported as mass density) is ~3.43 g/cm3. specimen, and was motivated mainly by the lack of Chemical composition was determined by means of an reliable, appropriate-quality reference Raman spectra electron probe micro-analyser (EPMA) operated at 15 kV for this mineral in the published literature and in online and 10 nA, except for boron (Kα), which was measured databases. at 5 kV and 150 nA. More EPMA details are described The studied specimen originated from Kolonna elsewhere (Zeug et al. 2018) or can be obtained from (Ratnapura District, Sabaragamuwa Province, southern the authors upon request. Assuming 40 oxygen ions per

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Figure 7: These two images of the serendibite study specimen (about 4.7 mm long) display the sample’s strong pleochroism. Photos by Chutimun Chanmuang N.

formula unit, the chemical formula was calculated as (~465 nm) and 24,100 cm–1 (~415 nm) to Fe3+-related

Ca3.47Na0.59(Al6.24Mg5.53Fe0.19)(Si6.25Al3.16B2.59)O40. Slight absorption would only be possible with information on dominance of Al3+ over Mg2+ at the octahedral cation the valence state(s) of iron—obtainable by Mössbauer sites agrees with results of previous studies (Schmetzer and/or electronic paramagnetic resonance spectroscopy— et al. 2002; Grice et al. 2014 and references therein) and but is beyond the scope of this study. The PL electronic thus seems to be a general feature of serendibite. emission is dominated by a fairly narrow doublet band in Optical absorption and photoluminescence (PL) the red range, at 14,425 and 14,490 cm–1 (~693 and ~690 spectra (Figure 8; for analytical details see Zeug et al. nm, respectively), superimposed on a broad emission 2018) correspond to those reported by Schmetzer et al. feature in the range of 11,500–15,000 cm–1 (~870–667 (2002). The absorption in the visible range is dominated nm). The doublet is assigned to the split, spin-forbidden –1 2 4 3+ by an intense, broad band at about 14,300 cm (or ~700 E → A2 transition of Cr , the trace concentration of nm wavelength), which is presumably caused by Fe2+– which is, however, below the EPMA detection limit. The Fe3+ intervalence charge transfer. Reliable assignment of asymmetric broad hump consists of the spin-allowed –1 4 4 3+ this and additional low-intensity bands near 21,500 cm T2 → A2 transition of Cr and vibronic coupling (for

Optical Absorption Spectrum PL Spectrum

Wavelength (nm) Wavelength (nm)

2000 1000 800 600 500 400 900800 7005600 00

693 nm 690 nm

686 nm Absorption 14200 14400 14600 Emission Intensity

5000 10000 15000 20000 25000 12000 14000 16000 18000 20000

Wavenumber (cm–1) Wavenumber (cm–1)

Figure 8: Shown here are the optical absorption spectrum (left, recorded in the greenish blue direction; sample thickness about 1.9–2.3 mm) and PL spectrum (right, obtained with 473 nm excitation) of the serendibite specimen. Spectral ranges that are invisible to the human eye are shaded grey.

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Figure 9: The Raman Spectra unoriented Raman spectrum (473 nm excitation) of 889 serendibite is provided here in comparison 526 with reference spectra for gem materials 134 306 467 567 having similar physical 986 properties: sapphirine 422 629 (cf. Sato et al. 754 Serendibite 2009), dumortierite (Kolonna, Sri Lanka) (extracted from 207 673 Korsakov et al. 2019) and zoisite (cf. Weis et al. 2016).

Sapphirine Intensity (Embilipitiya, Sri Lanka)

Dumortierite (Kokchetav massif, Kazakhstan)

Zoisite (Sesia, Italy)

250 500750 1000 1250 Raman Shift (cm–1)

the analogous assignment of constituents of the Cr3+ Acknowledgements: We thank Andreas Wagner for emission of topaz, see Tarashchan et al. 2006). The cause sample preparation and Prof. Dr Gerald Giester (both of of the weak, broad emission near 17,850 cm–1 (~560 nm) the University of Vienna) for help with determining the remains unclear. general gemmological properties. The Raman spectrum (for analytical details, see Zeug Dr Chutimun Chanmuang N. et al. 2018) of the specimen is presented in Figure 9, and ([email protected]), the raw x,y data are available in The Journal’s online Prof. Dr Lutz Nasdala and Prof. Dr Manfred Wildner data depository. Serendibite is a relatively weak Raman University of Vienna, Austria scatterer and, therefore, Raman analysis is hampered considerably by background luminescence if present. Dr Radek Škoda In particular with 633 nm (He-Ne laser) excitation, the Masaryk University, Brno, Czech Republic Raman spectrum was heavily obscured by laser-induced PL. The least disturbing PL effects were observed when E. Gamini Zoysa FGA using blue (473 nm) laser excitation. The principal finger- Mincraft Co., Mount Lavinia, Sri Lanka print Raman pattern of serendibite is clearly distinct References from those of other gem minerals with similar physical Grice, J.D., Belley, P.M. & Fayek, M. 2014. Serendibite, properties, in particular sapphirine, dumortierite and a complex borosilicate mineral from Pontiac, zoisite/tanzanite (Reinitz & Johnson 1997; Schmetzer et Quebec: Description, chemical composition, and al. 2002; Heo & Kwak 2016). In conclusion, as long as crystallographic data. Canadian Mineralogist, 52(1), disturbance by laser-induced PL is avoided (by choosing 1–14, https://doi.org/10.3749/canmin.52.1.1. a short-wavelength excitation source), Raman spectros- Heo, M. & Kwak, K.-W. 2016. Two rare gems with similar copy provides a reliable and straightforward means of appearance: Serendibite and sapphirine. ICGL non-destructively identifying serendibite. Newsletter, No. 3, 4–5.

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Korsakov, A.V., Rezvukhina, O.V., Rezvukhin, D.I., Schmetzer, K., Bosshart, G., Bernhardt, H.-J., Gübelin, E.J. Greshnyakov, E.D. & Shur, V.Y. 2019. Dumortierite and & Smith, C.P. 2002. Serendibite from Sri Lanka. Gems tourmaline from the Barchi-Kol diamond-bearing & Gemology, 38(1), 73–79, https://doi.org/10.5741/ kyanite gneisses (Kokchetav massif): A Raman gems.38.1.73. spectroscopic study and petrological implications. Tarashchan, A.N., Taran, M.N., Rager, H. & Iwanuch, Journal of Raman Spectroscopy, 51(9), 1839–1848, W. 2006. Luminescence spectroscopic study of Cr3+ https://doi.org/10.1002/jrs.5699. in Brazilian topazes from Ouro Preto. Physics and Prior, G.T. & Coomáraswámy, A.K. 1903. Serendibite, a new Chemistry of Minerals, 32, 679–690, https://doi.org/ borosilicate from Ceylon. Mineralogical Magazine and 10.1007/s00269-005-0042-1. Journal of the Mineralogical Society, 13(61), 224–227, Weis, F.A., Lazor, P., Skogby, H., Stalder, R. & Eriksson, L. https://doi.org/10.1180/minmag.1903.13.61.04. 2016. Polarized IR and Raman spectra of zoisite: Reinitz, I. & Johnson, M.L. 1997. Gem Trade Lab Notes: Insights into OH-dipole orientation and the Serendibite, a rare gemstone. Gems & Gemology, 33(2), luminescence. European Journal of Mineralogy, 140–141. 28(3), 537–543, https://doi.org/10.1127/ejm/2016/ 0028-2528. Sato, K., Santosh, M. & Tsunogae, T. 2009. A petrologic and laser Raman spectroscopic study of sapphirine–spinel– Zeug, M., Nasdala, L., Wanthanachaisaeng, B., Balmer, quartz–Mg-staurolite inclusions in garnet from Kumiloothu, W.A., Corfu, F. & Wildner, M. 2018. Blue zircon southern India: Implications for extreme metamorphism from Ratanakiri, Cambodia. Journal of Gemmology, in a collisional orogen. Journal of Geodynamics, 47(2–3), 36(2), 112–132, https://doi.org/10.15506/ 107–118, https://doi.org/10.1016/j.jog.2008.07.003. JoG.2018.36.2.112.

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