O-D...O bond geometry in OD-chondrodite

• Published in: The American mineralogist Vol. 86; no. 1-2; pp. 176 - 180
• Main Authors: Lager, George A, Ulmer, Peter, Miletich, Ronald, Marshall, William G
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 01.02.2001; Walter de Gruyter GmbH
• Subjects: anisotropy; anvil cells; bonding; Chemistry; chondrodite; crystal structure; Crystals; fluorides; Geophysics; halides; High pressure; high temperature; humite; humite group; infrared spectra; lattice; mantle; Mineralogy; Minerals; nesosilicates; Neutrons; orthosilicates; P-T conditions; pressure; refinement; related to mantle; silicates; spectra; temperature; upper mantle; X-ray data
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-2001-0119

The crystal structure of OD-chondrodite [Mg5Si2O8(OD)2, P21/b (a unique), a = 4.74711(5), b = 10.34888(16), c = 7.90228(13) Å, α = 108.678(1)°] was refined to wRp = 0.0218, Χ2 = 3.545 at ambient conditions using time-of-flight neutron powder data. The disordered H model proposed for OH-chondrodite on the basis of single-crystal X-ray data is confirmed. The occupations of the D1 and D2 sites are, respectively, 0.52(1) and 0.48(1). The long O5-D1 [1.076(4) Å] and O5-D2 [1.111(4) Å] bond lengths, which are tow of the longest O-H(D) bonds observed in mineral structures, reflect the positional disorder of the O5 atom in the unshared OH-OH edge. Both D1 [1.968(4), 2.489(4) Å] and D2 [2.149(4), 2.251(4) Å] atoms are involved in two hydrogen bonds. A re-examination of the origin of positive OH frequency shifts in both F-bearing and OH-chondrodite at high pressure is warranted in view of the crystallographic data.