Crystal-fluid interaction: the evolution of stilbite structure at high pressure

Natural stilbite, Ca4.00Na1.47(H2O)(30)[Al9.47Si26.53O72], space group F2/m, a=13.5978(3), b=18.2804(4), c=17.8076(4) angstrom, beta =90.685(2)degrees, V=4426.18(17) angstrom (3), Z=2, has been studied by single-crystal X-ray diffraction method at ambient conditions and under compression in penetrating (water-bearing) and non-penetrating (paraffin) media. In water-containing medium during the first compression stage (below 1 GPa) the pressure-induced hydration effect manifests in the additional occupation of partly vacant H2O positions; above 1 GPa the H2O position, which is vacant at ambient pressure and not linked to cations, becomes occupied. Above 2.6 GPa the composition of stilbite remains almost constant; apparently no further hydration is possible. The compressibility of stilbite in paraffin is expectedly higher compared to that in penetrating medium. The cations coordination changes mainly through minor shifts of water positions and some re-distribution of their occupancy. Above 3 GPa the structure abruptly contracts along the b axis; the structure symmetry is reduced to triclinic. The decompression experiments show full reversibility of structural changes on pressure release.

Automated summary

Natural stilbite was studied under different media, revealing pressure-induced hydration effect in water-containing medium and higher compressibility in paraffin. Structural changes were found to be reversible upon pressure release.