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_2.90Na_2.07Sr_0.25K_0.16Ba_0.06(H₂O)_n[Al_8.65Si_27.35O₇₂] 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.