We provide in this paper complete structural and micro-chemical characterization of two britholite samples with compositions (Mn 0.(from Los Islands, Guinea). The crystal-chemical formulae were calculated by combining electron-microprobe analyses for intermediate-Z elements (Na, Ca, Mn, Si, P), ion-microprobe analyses for low-Z (H, Li, Be, B, F) and high-Z elements (Ba, Y, REE and actinides), and high-quality (R obs 1.2-2.6) single-crystal structure refinements. Structure refinements indicate that the best approximation to the real symmetry is the P6 3 space group. In britholite, the lowering of symmetry with respect to the P6 3 /m space group of apatite means that the O3 and O3a atoms are no longer equivalent and allows the tetrahedron to rotate up to ~4° around the Si-O1 bond. Consequently, the O3a atom moves closer to the REE1a site, whereas the O3 atom moves farther from the REE1 site and closer to the REE2 site, which thus assumes a [7+1] coordination. The infrared spectrum of britholite from Latium shows a unique and very broad band in the OH-stretching region at 3437 cm -1 , which is consistent with the ordering of trivalent REE cations at the REE2 site. The calculated integral molar absorptivity ε i is 23600 L⋅mol -1 ⋅cm -2 . A remarkable constancy in the unit-cell volume along the whole apatite-britholite compositional range is observed for values of the aggregate ionic radius at the REE sites shorter than 1.12 and longer than 1.15 Å, whereas large variations are observed for intermediate values; this behavior suggests constraints due to the rigidity of the tetrahedral group, that are further enhanced at high symmetry.

This paper reports a single-crystal FTIR spectroscopic study of vishnevite, ideally [Na-6(SO4)] [Na-2(H2O)(2)](Si6Al6O24), a member of the cancrinite group of feldspathoids. The study was done on several crystals from various geological occurrences. Infrared spectra show that most samples, and in particular the specimens from the holotype locality at Vishnevye Mountains (Urals, Russia), contain molecular CO2 as the main carbon species in the structural pores, while the specimens from Loch Borolan (Scotland) were found to be CO3-rich. Polarized-light measurements show that the linear CO2 molecules are oriented perpendicular to the crystallographic c axis. Structure refinement of sample Pi4 from Latium (Italy) shows usual H2O center dot center dot center dot Na center dot center dot center dot H2O sequences within the undecahedral cages; however, difference Fourier maps suggest the presence of additional protons in the channels, possibly forming OH groups. The FTIR spectra show three absorptions in the 3800-3200 cm(-1) region. The first one, at 3590 cm(-1) is strongly polarized for E perpendicular to c while the second, at 3535 cm(-1), behaves almost isotropic. These two bands are assigned to the stretching Vibrations of an asymmetric water molecule in the structural cages. The third broad absorption at 3320 cm(-1), is predominantly polarized for E vertical bar vertical bar c and is assigned to additional OH groups in the channels. Detailed microspectroscopic mapping showed several samples from Latium (Italy) to be zoned with respect to the CO2/CO3 content, thus pointing to a possible use of the volatile content of these minerals for petrological modeling

We relate a single-crystal FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites. The structural refinements show that the oxygen site of the H(2)O molecule lies off the triad axis. The water molecule is almost symmetric and slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen bridges, with Ow center dot center dot center dot O donor-acceptor distances >2.7 angstrom. The FTIR spectra show two main absorptions. The first at 3,602 cm(-1) is polarized for E perpendicular to c and is assigned to the nu(3) mode. The second, at 3,531 cm(-1), is also polarized for E perpendicular to c and is assigned to nu(1) mode. A weak component at 4,108 cm(-1) could possibly indicate the presence of additional OH groups in the structure of cancrinite. Several overlapping bands in the 1,300-1,500 cm(-1) range are strongly polarized for E perpendicular to c, and are assigned to the vibrations of the CO(3) group

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We here present reults of a crystal-chemical study of a suite of pyrochlore-group minerals from the holocrystalline syenitic ejecta scattered within the Plio-Pleistocene pyroclastic deposits of Latium, in central Italy. Despite their high actinide content, the samples studied are non-metamict owing to their young age of formation, as suggested by the well-defi ned and sharp peaks in the X-ray powder-diffraction patterns. The measured a unit-cell dimension is correlated with the (Ti + Fe 3+) content at the B site of the structure. Following the IMA classifi cation scheme for pyrochlore-group minerals modifi ed according to the "50% rule" adopted in 1992, the samples are classifi ed as betafi te, pyrochlore and uranpyrochlore. In comparison with known data for pyrochlore-group minerals, the samples have extremely high Ti contents (up to 81.3 atom % Ti relative to the Ti + Nb + Ta sum) and signifi cant and variable contents of Zr (up to 8.44 wt.% ZrO 2) and W (up to 12.14 wt.% WO 3). Some crystals display strong compositional zoning, consistent with the substitution 3Nb 5+ + Na + 3Ti 4+ + U 4+. The accommodation of extremely high amounts of W 6+ can be related to the coupled substitution 2(Nb + Ta) 5+ Ti 4+ + W 6+ at the B site. Pyrochlore-group minerals from Latium have signifi cantly high REE contents (up to 7.75 wt.% oxide). Chondrite-normalized patterns follow the general trend typical of unaltered pyrochlore, i.e., with an extreme Ce-enrichment and a decrease toward the heavy REE.