Channel occupancy in an alkali-poor beryl from Serra Branca (Goias, Brazil); spectroscopic characterization

Abstract

Spectroscopic (micro FfIR, Raman, MAS NMR) and mass spectroscopic techniques have been used to examine, on both single-crystal and powder samples, the behavior of H20 and C02 molecules in the structural channels of an alkali-poor, volatile-rich beryl from Brazil. Polarized single-crystal FfIR and Raman spectra were obtained on oriented wafers of isolated crystals. Location and orientation of H20 and C02 molecules were determined from IR spectra. The proton-proton vectors of type-I and type-II H20 are parallel and perpendicular to the c axis, respectively; the molecular axis of the C02 molecule is perpendicular to the c axis. Relative proportions of both types of H20 were determined from their respective IR absorption-band intensities and were found to be nearly equivalent. There is, contrary to what is generally claimed in the literature, no relation between alkali and type-II H20 contents. Absorptivity coefficients for H20 and C02 were computed for every specific orientation. A lH MAS NMR spectrum resolved as a Pake doublet seems to confirm the absence of any anisotropic movement of the H20 molecules from one orientation to the other on the NMR time scale. Mass discrimination of volatiles released by pyrolysis under vacuum confirms the weak mobility of H20 and C02 molecules in the channel sites, although the plugging effect of alkalis in the channels can be neglected for such an alkali-poor beryl. The volatile vs. alkali content ratio in beryl could possibly be used as an environmental indicator.