Two proton positions in the very strong hydrogen bond of serandite, NaMn2[Si3O8(OH)]

The crystal structure and hydrogen positions of serandite, NaMn2[Si3O8(OH)] , have been refined from single-crystal X-ray and time-of-flight neutron di ffraction data at ambient conditions. The proton occupies an asymmetric, do uble-well position between O3 and O4, confirming one of the shortest asymme tric hydrogen bonds known in minerals with n(O3...O4) = 2.464(1) Angstrom ( X-ray) and 2.467(1) Angstrom (neutron). The proton position closest to O3 h as about 84% occupancy and an O-H distance of 1.078(3) Angstrom, and the po sition closest to O4 has an occupancy of 16% and an O-H distance of 1.07(1) Angstrom. The d(H...O) of these hydrogen bonds is 1.413(3) Angstrom and 1. 41(1) Angstrom, respectively. Hydrogen bond angles are 164 degrees for H1 a nd 168 degrees for H2. The Si-IV-OH bond length [1.628(1) Angstrom] is inte rmediate in length among the three other Si-O bonds in the dominantly (84%) hydrated Si1 tetrahedron. These new structure data for a very strong hydro gen bond may be useful for extending spectroscopy-structure correlation dia grams into the region of very low energy O-H stretching.