MS14-P02 NMR crystallography of beryl minerals at 9.4 T and 19.6 TThe available description of the aluminosilicate beryl group of minerals suggests RX3Al2(Si6O18)·H2O as general formula, with beryl Be3Al2Si6O18 (P6/mcc) and pezzottaite Cs(Be2Li)Al2Si6O18 (R-3c or R3c) as end-members. It involves a wide diversity of intermediate chemical compositions because the structure can incorporate different alkali atoms in solid solution, located at channel R site and tetrahedral X sites, and H2O molecules in the structural channel. Reciprocal-space techniques have resolved only average structures in the intermediate members, related to one of the two the ideal crystal structure of the end-members, and have difficulties in the study of water molecules. Here, high-resolution multinuclear NMR spectroscopy, leading to 29Si, 27Al, 9Be, 133Cs, 7Li, 23Na and 1H spectra at 9.4 T as well as 27Al and 9Be spectra at 19.6 T, has been used to investigate the short- and medium-range structures in nine specimens along the solid solution of this group. The 1H NMR SP and CP experiments suggest that hydrogen atoms are actually as OH groups for compensation of local charge, instead of water molecules which were detected in very low concentration. The structural description by reciprocal-space techniques and NMR coincides only in the alkali-free perfectly long-range ordered P6/mcc beryl end-member, which in fact is extremely rare in Nature, having one tetrahedral site for Si atoms, one octahedral site for Al atoms, and one tetrahedral site for Be atoms. However a tectosilicate description seems more appropriated that a cyclosilicate classification from the values of the 29Si chemical shifts, as Be and Si atoms form a Q4 configuration. The most common mineral is alkali-rich beryl having a crystal structure with lower local symmetry than the ideal average hexagonal structure, with at least two sites for Si atoms, four sites for Be atoms, two R sites and two X sites for Li atoms, two R sites for Na atoms, one R site Cs atoms, and two types of OH groups. The NMR data of pezzottaite indicate six tetrahedral sites for Si atoms, two octahedral sites for Al atoms, one tetrahedral site for Be atoms, two R sites and two X sites for Li atoms, three R sites for Cs atoms, and two types of OH groups with one of them different from that of beryl. The lack of resolving power of diffraction techniques to resolve the structural detail of non stoichiometric beryl is similar to that found in order-disorder series of K-feldspars (Sánchez-Muñoz et al. 2013).
Sánchez-Muñoz, L.; Sanz, J.; Sobrados, I. & Gan Z.H. (2013) Am. Mineral., 98, 2115-2131Keywords: NMR crystallography, local structure, beryl