A single-crystal neutron and X-ray diffraction study of pezzottaite, Cs(Be2Li)Al2Si6O18

The chemical composition and the crystal structure of pezzottaite [ideal composition Cs(Be 2Li)Al 2Si 6O 18; space group: R3c, a = 15. 9615(6) Å, c = 27. 8568(9) Å] from the type locality in Ambatovita (central Madagascar) were investigated by electron microprobe analysis in wavelength dispersive mode, thermo-gravimetric analysis, Fourier-transform infrared spectroscopy, single-crystal X-ray (at 298 K) and neutron (at 2. 3 K) diffraction. The average chemical formula of the sample of pezzottaite resulted Cs1,Cs2(Cs 0. 565Rb 0. 027K 0. 017) Σ0. 600 Na1,Na2(Na 0. 101Ca 0. 024) Σ0. 125Be 2. 078Li 0. 922 Al1,Al2(Mg 0. 002Mn 0. 002Fe 0. 003Al 1. 978) Σ1. 985 Si1,Si2,Si3(Al 0. 056Si 5. 944) Σ6O 18·0. 27H 2O. The (unpolarized) IR spectrum over the region 3,800-600 cm -1 was collected and a comparison with the absorption bands found in beryl carried out. In particular, two-weak absorption bands ascribable to the fundamental H 2O stretching vibrations (i. e. 3,591 and 3,545 cm -1) were observed, despite the mineral being nominally anhydrous. The X-ray and neutron structure refinements showed: (a) a non-significant presence of aluminium, beryllium or lithium at the Si1, Si2 and Si3 sites, (b) the absence (at a significant level) of lithium at the octahedral Al1, Al2 and Al3 sites and (c) a partial lithium/beryllium disordering between tetrahedral Be and Li sites.