A crystal-structure refinement of a hydrated NH4-exchanged barrerite was carried out on single-crystal X-ray diffraction data. The real symmetry Amma of natural barrerite became Fmmm for the NH4-exchanged variety, which is also its topological symmetry. This change of symmetry was due to the absence of an extraframework site, which, in natural barrerite, is responsible for the rotational displacement of the framework around a screw axis and the consequent lowering of symmetry. A slight increase of the unit cell volume (1%), analogous to that found in NH4-exchanged stilbites, was recognized. Many extraframework sites, all partially occupied, were located. A reasonable attribution of these sites to residual cations, H2O and NH4 molecules has been suggested. The residual cations occupied the cation site C1 of natural barrerite corresponding to the Ca site in stilbite and barrerite. A possible 7-fold coordination for this site was shown. The correspondence of many of extraframework sites with those found in natural barrerite indicates that, to some extent, memory of the initial location of the extraframework sites remained.
Crystal structure refinement of NH4-exchanged barrerite
MARTUCCI, Annalisa;ALBERTI, Alberto;SACERDOTI, Michele;
2000
Abstract
A crystal-structure refinement of a hydrated NH4-exchanged barrerite was carried out on single-crystal X-ray diffraction data. The real symmetry Amma of natural barrerite became Fmmm for the NH4-exchanged variety, which is also its topological symmetry. This change of symmetry was due to the absence of an extraframework site, which, in natural barrerite, is responsible for the rotational displacement of the framework around a screw axis and the consequent lowering of symmetry. A slight increase of the unit cell volume (1%), analogous to that found in NH4-exchanged stilbites, was recognized. Many extraframework sites, all partially occupied, were located. A reasonable attribution of these sites to residual cations, H2O and NH4 molecules has been suggested. The residual cations occupied the cation site C1 of natural barrerite corresponding to the Ca site in stilbite and barrerite. A possible 7-fold coordination for this site was shown. The correspondence of many of extraframework sites with those found in natural barrerite indicates that, to some extent, memory of the initial location of the extraframework sites remained.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
