Influence of content of pressure-transmitting medium on structural evolution of heulandite: Single-crystal X-ray diffraction study.

The structural evolution of natural heulandite Ca 2.90 Na 2.07 Sr 0.25 K 0.16 Ba 0.06 (H 2 O) n [Al 8.65 Si 27.35 O 72 ] was studied upon compression in penetrating (water-containing) and non-penetrating (paraffin) media using single-crystal X-ray diffraction data in a diamond-anvil cell. Compression in a water-containing medium results in an additional hydration of heulandite from 23 molecules per unit cell at ambient conditions to ≈27 at 3.27 GPa. Water molecules enter both initially vacant and partly occupied positions. Additional hydration stabilizes the crystal structure decreasing its compressibility. [ABSTRACT FROM AUTHOR]