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(Be2Li)
Al2Si6O18
; space group: R3c, a = 15.9615(6) A?
, c =
27.8568(9) A?
] 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
(Cs0.565Rb0.027K0.017)R0.600
Na1,Na2
(Na0.101Ca0.024)R0.125Be2.078Li0.922
Al1,Al2
(Mg0.002
Mn0.002Fe0.003Al1.978
)R1.985
Si1,Si2,Si3
(Al0.056Si5.944
)R6O18

0.27H2O. 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
H2O 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.