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 diffraction data at ambient conditions. The proton occupies an asymmetric, double-well position between O3 and O4, confirming one of the shortest asymmetric 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 has about 84% occupancy and an O-H distance of 1.078(3) Angstrom, and the position 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 and 168 degrees for H2. The Si-IV-OH bond length [1.628(1) Angstrom] is intermediate in length among the three other Si-O bonds in the dominantly (84%) hydrated Si1 tetrahedron. These new structure data for a very strong hydrogen bond may be useful for extending spectroscopy-structure correlation diagrams into the region of very low energy O-H stretching.