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

• Heulandite demonstrated regular pressure-induced hydration.
• Additional hydration stabilizes the heulandite structure decreasing its compressibility.
• The chemical composition of heulandite influences on features of its high-pressure behaviour.

The structural evolution of natural heulandite Ca2.90Na2.07Sr0.25K0.16Ba0.06(H2O)n[Al8.65Si27.35O72] 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.

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