A Raman microspectroscopic study of organic inclusions in “watermelon” tourmaline from the Paprok mine (Nuristan, Afghanistan)

An exceptional mineral specimen, particularly interesting for collectors and researchers, is “watermelon” tourmaline, mined in the Paprok region in Nuristan, Afghanistan. A pale pink core (zone III) is surrounded by a thin, colorless zone (zone II), and a green outer zone (zone I). Differences between zones are clearly seen in the SEM picture, i.e., diversity in the size of constituent forms, and also the presence of inclusions of alien mineral phases which range from a few dozen to a few μm. In the line mapping experiment the Raman spectra were collected from the inner core towards the outer rim. There are observed typical for tourmaline (elbaite) groups of bands: at ca. 3600, 1100, 700, 400 and 250 cm⁻¹. Raman measurements determine the differences in the local chemical composition of particular zones. Interestingly, there is a new band emerging in the outer zone at ca. 840 cm⁻¹, and it could be due to the SiO₄ stretching vibrational modes predisposed by the Fe ion presence in the mineral. The organic inclusions are present in the whole volume of the investigated gem. Organic inclusion content appears to be versatile. Raman spectra confirm the presence of hydrogen sulphide (2610 cm⁻¹), methane (2912 cm⁻¹), ethane (2950 cm⁻¹), and propane (2895 cm⁻¹). In the corresponding bending vibrations region apart from CH₂, CH₃ vibrations and C–O stretching (CO₂) at (1280, 1390 cm⁻¹), characteristic bands are also observed at ca. 1340, 1600 and 1660 cm⁻¹. The last ones possibly mark the evolution of graphitization process occurring from fluid saturated inclusions within condensed ring aromatics (1660 cm⁻¹ band), towards slightly more organized, although still amorphous forms of carbonaceous matter (1340 and 1600 cm⁻¹).