First accurate location of two proton sites in tourmaline: A single-crystal neutron diffraction study of oxy-dravite

A single-crystal neutron diffraction study of oxy-dravite from Osarara (Narok district, Kenya) was performed. Intensity data were collected in Laue geometry at 10 K and anisotropic-structure refinement was undertaken. For the first time, two independent H sites were refined unambiguously for a mineral belonging to the tourmaline supergroup and located at 0.26, 0.13, 0.38 (labelled as H3, site occupancy similar to 98%) and at 0, 0, 0.9 (labelled as H1, site occupancy similar to 25%). The H-bonding scheme can thus be defined as follows: (1) the O at the O3 site acts as a donor and the O at the O5 site as acceptor, the refined O3H3 bond distance is 0.972(2) angstrom (and 0.9946 angstrom corrected for riding motion), H3 center dot center dot center dot O5 = 2.263(2) angstrom, O3 center dot center dot center dot O5 = 3.179(1) angstrom and O3H3 center dot center dot center dot O5 = 156.6(1)degrees; (2) the oxygen at the O1 site acts as a donor and the O atoms at O4 and O5 as acceptors, the refined O1H1 bond distance is 0.958(8) angstrom (and 0.9833 angstrom corrected for riding motion), H1 center dot center dot center dot O4 = 2.858(6) angstrom, O1 center dot center dot center dot O4 = 3.378(1) angstrom and O1H1 center dot center dot center dot O4 = 115.12(1)degrees, whereas H1 center dot center dot center dot O5 = 2.886(6) angstrom, O1 center dot center dot center dot O5 = 3.444(1) angstrom and O1H1 center dot center dot center dot O5 = 118.23(1)degrees. A further test refinement was performed with the H1 site out of the three-fold axis (at 0.02, 0.01, 0.90); this leads to O1H1 = 0.995(8) angstrom (and 1.0112 angstrom corrected for riding motion), H1 center dot center dot center dot O4 = 2.747(6) angstrom and O1H1 center dot center dot center dot O4 = 121.7(4)degrees, whereas H1 center dot center dot center dot O5 = 2.654(9) angstrom and O1H1 center dot center dot center dot O5 = 136.5(6)degrees. Bond-valence analysis shows that the H-bonding strength involving O3 is stronger than that involving O1: similar to 0.11 and <0.05 valence units, respectively. The refined angle between the O3H3 vector and [0001] is 3.40(9)degrees. Such a small angle is in line with a pleochroic scheme for the OH-stretching absorption bands measured by infrared spectroscopy.