Crystal structure and phase transition in noelbensonite: a multi-methodological study

• Published in: Physics and chemistry of minerals 2017-07, Vol.44 (7), p.485-496
• Main Authors: Cametti, G., Armbruster, T., Hermann, J., Churakov, S.
• Format: Article
• Language: English
• Subjects: Computer simulation; Crystal structure; Crystallography and Scattering Methods; Dependence; Earth and Environmental Science; Earth Sciences; Geochemistry; High temperature; Hydrogen bonds; Hydroxyl groups; Infrared spectra; Infrared spectroscopy; Low temperature; Mineral Resources; Mineralogy; Molecular dynamics; Original Paper; Phase transitions; Simulation; Single crystals; Spectrum analysis; Vibration analysis; Water chemistry; X-ray diffraction
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-017-0876-3

The crystal structure and the phase transition of noelbensonite ((Ba 0.72 Sr 0.31 Ca 0.01 ) Σ1.05 (Mn 1.82 Al 0.16 ) Σ1.98 [Si 2 O 7 ](OH) 2 ·H 2 O) were investigated by in situ single-crystal X-ray diffraction, ab initio simulations, and infrared spectroscopy. In contrast to previous assumptions, single-crystal X-ray diffraction data and molecular dynamic simulations at room temperature (RT) displayed the acentric space group P 2 1 cn, a  = 6.31303(2), b  = 9.0977(3), c  = 13.5820(4) Å, V  = 779.73(4) Å 3 . This corresponds to the low-temperature (−118 °C) structure of lawsonite (CaAl 2 [Si 2 O 7 ](OH) 2 ·H 2 O) and to the phase of hennomartinite (SrMn + 3 2 [Si 2 O 7 ](OH) 2 ·H 2 O) below 95 °C. At 225 °C, the structure changed to space group Cmcm , which corresponds to that of hennomartinite at >245 °C and of lawsonite above 0 °C. In this structure the oxygen site of the H 2 O molecule showed positional disorder. Molecular dynamic simulations indicated that the splitting of this site reflects the disordered arrangement of the hydroxyl groups and the H 2 O molecule in the high-temperature modification. Infrared spectra collected at RT showed similarities with those of lawsonite. The bands at 3566 and 3517 cm − 1 and the two broader bands between 3300 and 2930 cm − 1 agree with the stretching frequencies of the hydrogen bond system as calculated from X-ray diffraction data and theoretical computations. Normal mode analysis of molecular dynamic trajectories allowed to identify the origin of vibration bands and polarization dependence of the IR spectra.