Hydrogen-atom positions in P4/nnc vesuvianite

The crystal structure of a large crystal (11.0 x 7.8 x 4.7 mm) of gem-quali ty Fe-bearing vesuvianite [a 15.533 (11), c 11.785(16) Angstrom, V 2843(5) Angstrom(3); epsilon 1.713(5), omega 1.710(5), 2V 0 degrees(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 B ragg reflections. No violations of P4/nnc space-group symmetry were observe d on examination of the crystal by time-of-flight neutron methods. The prot ons are associated with oxygen atom, O(11) [H(1) 0.4592(7), 0.4814(9), 0.30 83(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 adjace nt O(10) atoms along c, indicating a statistical distribution of H(2) betwe en two sites. The partial occupancy of the H(1) site (85%) is consistent wi th the substitution of fluorine for oxygen at the O(11) site. Although hydr ogrossular and vesuvianite have similar structures and occur in many of the same environments, theneutron-diffraction results rule out the possibility of a hydrogarnet-type substitution [(O4H4)(4-) = (SiO4)(4-)] in this speci men. Its infrared spectrum (6500-7500 cm(-1)) most closely resembles that o f intermediate-fluorine, boron-poor vesuvianite.