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Titel |
A Crystal Chemical Investigation of Armenite, BaCa2Al6Si9O30â
H2O: The Behavior of Extra Framework Ca Cations and H2O Molecules in Microporous Silicates |
VerfasserIn |
C. A. Geiger, G. D. Gatta, X. Xue, G. J. McIntyre |
Konferenz |
EGU General Assembly 2012
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 14 (2012) |
Datensatznummer |
250059392
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Zusammenfassung |
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. |
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