Vibrational characterization of the new gemstone Pezzottaite

Pezzottaite is a rare Cs-bearing mineral with ideal composition Cs(Be2Li)Al2Si6O18, discovered in November 2002. Pezzottaite is probably the only new mineral species with some relevance in gemology, thanks to its optical properties, rarity and beauty. It is considered as a member of the “beryl group”, along with beryl sensu-scricto (Be3Al2Si6O18;), bazzite (Be3Sc2Si6O18), stoppaniite (Be3Fe2Si6O18) and indialite (Mg2Al3(AlSi5O18)). The chemical composition and the spectroscopic features of pezzottaite from Ambatovita (central Madagascar) and a Cs-rich beryl from Monte Capanne (Isola d’Elba, Italy) were investigated by standard gemmological analysis, electron microprobe analysis in wavelength dispersive mode (EMPA-WDS), X-ray diffraction and micro-Raman spectroscopy. The density and the refractive index of pezzottaite were found to be higher than those of beryl due to the entrance of a large amount of alkali. However, an unambiguous distinction between pezzottaite and Cs-rich beryl cannot be done only on the basis of density and optical properties. Pezzottaite and Cs-rich beryl are usually distinguished on the basis of chemical analysis, considering a conventional upper-limit of caesium in Cs-rich beryl of Cs2O ~ 9 wt%, or by X-ray diffraction, as pezzottaite has different symmetry. In any case, the discrimination is not easy and requires advanced and expensive techniques. Chemical analysis of our samples showed an high amount of cesium (Cs2O 12.91 wt%) for pezzottaite, while the Cs-beryl has 1.27 wt%. The crystal structure of the samples has been investigated through X-ray diffraction. The pezzottaite has a trigonal symmetry (space group R-3c, with a~15.9 and c~27.8 Å), while beryl is hexagonal (space group P6/mcc, with a~9.2 and c~9.2 Å). The increase of cell parameters is due to the entrance of lithium, that replaces beryllium in the tethaedra. The replacement causes a positive charge deficit neutralized by cesium in the channels. The samples of pezzottaite and Cs-rich beryl were investigated by micro-Raman spectroscopy, a non-destructive and rapid tool of investigation. The un-polarized Raman spectrum of pezzottaite over the extended region 100-3650 cm-1 was collected for the first time, and compared with the spectrum of a Cs-beryl (Figure 1 and 2). In particular, Cs-beryl has showed only a intense peak at 3604 cm-1, ascribable to H2O stretching vibrations. On the other hand, two weak Raman bands at 3,591 and 3,545 cm-1, ascribable to the fundamental H2O or OH stretching vibrations respectively, were observed, despite the mineral should be nominally anhydrous. The Raman spectroscopy was useful to understand the type of water (type “I” or type “II”) and then to evaluate presence of alkali in the channels. In addition, the Raman spectrum of pezzottaite shows two intense and characteristic bands at 110-112 cm-1 and 1100 cm-1, which are not present in the beryl spectrum (Figure 1). Even if the true nature of the two bands is not completely understood, Raman spectroscopy appears to be a promising and inexpensive tool for a quicker identification of pezzottaite. Figure 1: Raman spectra of pezzottaite (above) and Cs-beryl (below) in the region 100-1,200 cm-1. Figure 2: Raman spectra of pezzottaite (above) and Cs-beryl (below) in the region 3,500-3,650 cm-1.