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 ∼98%) and at 0, 0, 0.9 (labelled as H1, site occupancy ∼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 O3–H3 bond distance is 0.972(2) Å (and 0.9946 Å corrected for “riding motion”), H3⋯O5 = 2.263(2) Å, O3⋯O5 = 3.179(1) Å and O3–H3⋯O5 = 156.6(1)°; (2) the oxygen at the O1 site acts as a ‘donor’ and the O atoms at O4 and O5 as ‘acceptors’, the refined O1–H1 bond distance is 0.958(8) Å (and 0.9833 Å corrected for “riding motion”), H1⋯O4 = 2.858(6) Å, O1⋯O4 = 3.378(1) Å and O1–H1⋯O4 = 115.12(1)°, whereas H1⋯O5 = 2.886(6) Å, O1⋯O5 = 3.444(1) Å and O1–H1⋯O5 = 118.23(1)°. 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 O1–H1 = 0.995(8) Å (and 1.0112 Å corrected for “riding motion”), H1⋯O4 = 2.747(6) Å and O1–H1⋯O4 = 121.7(4)°, whereas H1⋯O5 = 2.654(9) Å and O1–H1⋯O5 = 136.5(6)°. Bond-valence analysis shows that the H-bonding strength involving O3 is stronger than that involving O1: ∼0.11 and <0.05 valence units, respectively.The refined angle between the O3–H3 vector and [0001] is 3.40(9)°. Such a small angle is in line with a pleochroic scheme for the OH-stretching absorption bands measured by infrared spectroscopy.