O-D center dot center dot center dot O bond geometry OD-chondrodite

The crystal structure of OD-chondrodite {Mg5Si2O8(OD)(2), P2(1)/b (a unique ), a = 4.74711(5), b = 10.34888(16), c = 7.90228(13) Angstrom, alpha = 108. 678(1)degrees] was refined to wR(p) = 0.0218, chi (2) = 3.545 at ambient co nditions using time-of-flight neutron powder data. The disordered H model p roposed for OH-chondrodite on the basis of single-crystal X-ray data is con firmed. The occupations of the D1 and D2 sites are, respectively, 0.52(1) a nd 0.48(1). The long O5-D1 [1.076(4) Angstrom] and O5-D2 [1.111(4) Angstrom ] bond lengths, which are two of the longest O-H(D) bonds observed in miner al structures, reflect the positional disorder of the O5 atom in the unshar ed OH-OH-edge. Both D1 [1.968(4), 2.489(4) Angstrom] and D2 [2.149(4), 2.25 1(4) Angstrom] atoms are involved in two hydrogen bonds. A re-examination o f the origin of positive OH frequency shifts in both F-bearing and OH-clnon drodite at high pressure is warranted in View of the crystallographic data.