Hydrogen-atom positions in P4/nnc vesuvianite

Abstract

The crystal structure of a large crystal (11.0 x 7.8 x 4.1 nm) of gem-quality Fe-bearing vesuvianite [ɑ 15.533 (ll), c 11.785(16) Å, V 2843(5) Å^3; є 1.713(5), to ω 1.710(5), 2V 0°(5);D 3.43 g cm^(-3); 3.68 wt%FeO] from Kenya has been refined at 298 K by single-crystal neutron-diffraction methods to a weighted R value of 3.9% for 1448 unique Bragg reflections. No violations of P4/nnc space-group symmetry were observed on examination of the crystal by time-of-flight neutron methods. The protons are associated with oxygen atoms O(11)[H(1)0.4592(7),0.4814(9),0.3083(8)]and O(10)[H(2)0.25,0.25,0.280(10), as predicted by bond-valence calculations. Difference-Fourier maps show a double minimum between adjacent O(10) atoms along c, indicating a statistical distribution of H(2) between two sites. The partial occupancy of the H(1) site (85%) is consistent with the substitution of fluorine for oxygen at the O(11) site. Although hydrogrossular and vesuvianite have similar structures and occur in many of the same environments, the neutron-diffraction results rule out the possibility of a hydrogarnet-type substitution [(O_4H_4)^4 = (SiO_4)^(4-)] in this specimen. Its infrared spectrum (6500-7500 cm^(-1)) most closely resembles that of intermediate fluorine, boron-poor vesuvianite.