Raman and structural comparison between the new gemstone pezzottaite Cs(Be2Li)Al2Si6O18 and Cs-beryl†

The Raman spectrum over the extended region 100-3800 cm-1 of the new gemstone pezzottaite [ideal composition Cs(Be2Li) Al2Si6O18] from the type locality in Ambatovita (central Madagascar) is reported and compared with that of Cs-beryl from the Elba Island (San Piero in Campo). The extended Raman spectrum of pezzottaite has never been so far reported, and its vibrational features are described here in detail for the first time. The Raman spectrum of pezzottaite is characterized by two intense peaks at 111 and 1100cm-1, not present in the beryl spectrum, which allows a rapid identification of the stone. Moreover, in pezzottaite, two weak bands ascribable to the fundamental H2O stretching vibrations (i.e. 3591 and 3545 cm-1) are observed, despite that the mineral is nominally anhydrous, whereas beryl exhibits a single intense band at 3604 cm-1. In order to complete the spectroscopic study, electron microprobe analysis in wavelength dispersive mode, laser ablation inductively coupled plasma mass spectrometry, and single-crystal X-ray diffraction characterization were performed on the two samples. The major and trace elements confirmed the higher amount of alkali in pezzottaite. Furthermore, pezzottaite exhibits a trigonal symmetry (R3¯c) caused by an ordered Li/Be distribution at the tetrahedral sites, while Csberyl has a hexagonal symmetry (P6/mcc) with a disordered Li/Be distribution.