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Title: A crystal chemical investigation of armenite, BaCa2Al6Si9O30·H2O: the behavior of extra framework Ca cations and H2O molecules in microporous silicates
Authors: Geiger, CA
Gatta, GD
Xue, X
McIntyre, GJ
Keywords: Monocrystals
X-ray diffraction
Temperature range 0065-0273 K
Issue Date: 22-Apr-2012
Publisher: European Geosciences Union
Citation: Geiger, C. A., Gatta, G. D., Xue, X., & McIntyre, G. J. (2012). A crystal chemical investigation of armenite, BaCa2Al6Si9O30·H2O: the behavior of extra framework Ca cations and H2O molecules in microporous silicates. Presented at the European Geosciences Union General Assembly 2012, Vienna, Austria, 22-27 April. In Geophysical Research Abstracts, 14, 1318. Retrieved from
Abstract: The crystal chemistry of armenite, ideally BaCa2Al6Si9O30·H2O, from Wasenalp, Valais, Switzerland was studied. Armenite typically forms in relatively low-temperature hydrothermal veins and fissures and has small pores containing Ca cations and H2O molecules as extra-framework species. Single-crystal neutron and X-ray diffraction measurements were made on armenite from Wasenalp for the first time. IR powder spectroscopic measurements were made from room temperature (RT) down to 10 K. 1H and 29Si NMR measurements were made at RT. Attention was given to investigating the behavior of the extra-framework species and hydrogen bonding. The neutron results also give the first static description of the protons, allowing bond distances and angles relating to the H2O molecules and H-bonds to de determined. The diffraction results indicate complete Al-Si order in the framework and four crystallographically independent Ca and H2O molecule sites. Both sites appear to have partial occupancies such that locally a Ca atom can have only a single H2O molecule bonded to it through an ion-dipole interaction. The Ca cation is further bonded to six O atoms of the framework forming a quasi cluster. The IR spectrum of armenite is characterized in the OH-stretching region at RT by two broad bands at roughly 3470 and 3419 cm−1 and by a single H2O bending mode at 1654 cm−1 and four intense OH bands at 10 K. The 1H MAS spectra contain a single main resonance near 5.3 ppm and a smaller one near 2.7 ppm. The extra-framework “Ca-oxygen-anion-H2O-molecule quasi-clusters” and the nature of H-bonding in the microporous zeolites scolecite, wairakite and epistilbite were also analyzed. The average OH stretching wavenumbers shown by the IR spectra of armenite and scolecite are, for example, not far removed from that observed in liquid H2O, but greater than that of ice. What remains poorly understood in microporous silicates is how the ion-dipole interaction in quasi clusters affects H-bonding strength between the H2O molecules and the aluminosilicate framework. © Author(s) 2012
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