A neutron and X-Ray diffraction and IR and 1H and 29Si NMR spectroscopic investigation of armenite : behavior of extra framework Ca cations and H2O molecules in microporous silicates

The crystal chemistry of armenite, ideally BaCa 2Al 6Si 9O 30 · 2 H 2O, 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 H 2O molecules as extra-framework species. Single-crystal neutron and X-ray diffraction measurements were made on armenite from the above locality 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 diffraction results show new features not observed before in published diffraction studies on armenite crystals from other localities. The neutron results also give the first static description of the protons, allowing bond distances and angles relating to the H 2O molecules and Hbonds to be determined. The diffraction results indicate Al/Si order in the framework. Four crystallographically independent Ca and H 2O molecule sites were refined, whereby both sites appear to have partial occupancies such that locally a Ca atom can have only a single H 2O 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 around it. The IR spectrum of armenite is characterized in the OH-stretching region at RT by two broad bands at roughly 3470 and 3410 cm -1 and by a single H 2O bending mode at 1654 cm -1. At 10 K four intense OH bands are located at 3479, 3454, 3401 and 3384 cm -1 and two H 2O bending modes at 1650 and 1606 cm -1. The 29Si MAS NMR spectra show four resonances at -81.9, -83.2, -94.9 and -101.8 ppm that are assigned to crystallographically different Si sites in an ordered structure, although their relative intensities deviate somewhat from those predicted for complete Al/Si order. The 1H MAS spectra contain a single main resonance near 5.3 ppm and a smaller one near 2.7 ppm, ...