ALBERT

Description: The crystal structure, H bonding and chemical composition of hydroxylherderite from the Bennett pegmatite, Buckfield, Oxford County, Maine, USA were investigated by single-crystal X-ray diffraction and neutron Laue diffraction, electron microprobe analysis in wavelength-dispersive mode, inductively coupled plasma-atomic emission spectrometry and polarized Raman spectroscopy [ Ca (Na 0.01 Ca 1.01 ) 1.02 Be (Be 0.98 Li 0.01 ) 0.99 (Si 0.03 P 0.98 ) 1.01 O 4 (OH 0.67 F 0.33 ) 1 , Z = 4, a = 9.7856(5), b = 7.6607(5), c = 4.8025(3) Å, β = 90.02(3)°, V= 360.02(4) Å 3 , space group P 2 1 / a ]. The neutron-structure refinement converged with fully anisotropic displacement parameters to give a final agreement index R 1 = 0.0363 for 85 refined parameters and 1614 unique reflections with F o 〉4( F o ). The structure refinement was used to determine the H position and geometry of H bonding in the structure. One H site was found on the O 5 anion with an O–H interatomic distance, corrected for "riding motion", of 0.996(2) Å. The H bond of hydroxylherderite is bifurcated with O 2 and O 4 acceptors forming H bonds with O 5··· O 2 = 3.163(1) Å, H ··· O 2 = 2.544(2) Å and O 5– H ··· O 2 = 121.8(1)°; O 5··· O 4 = 3.081(1) Å, H ··· O 4 = 2.428(2) Å and O 5– H ··· O 4 = 124.4(1)°. The highly non-linear O–H···O hydrogen bonds in hydroxylherderite and in the isotypic datolite [ideally CaBSiO 4 (OH)] are constrained by the tetrahedral network topology. Two main O–H stretching modes were observed in the Raman spectra at 3565 and 3620 cm –1 , which are attributed to the bifurcated H bond. Two additional weak bands at 3575 and 3610 cm –1 are attributed to Si–P disorder in the tetrahedral sites. Results of this study will contribute to the correlation of H-bonding geometry and O–H stretching frequencies of highly non-linear H bonds.