An X-ray single crystal study of alkaline cations influence on laumontite hydration ability: I. Humidity-induced hydration of Na,K-rich laumontite

The crystal structure of natural Na,K-rich variety of zeolite laumontite was refined from single-crystal X-ray data obtained at relative humidity (RH) of 30%, 45%, 70% and with crystal submerged in water. The strong evidences of humidity-induced hydration were obtained: the amount of H2O molecules per unit cell increased from 12.0 at RH of 30% to 13.4 at submerging in water, with corresponding increase of unit cell volume by 1.4%. It disproves the common opinion that Na,K-rich laumontite is characterized by incapability of hydration.The ordinary Ca-laumontite |Ca4(H2O)n|[Al8Si16O48] hydrates abruptly from 14 to 18 H2O molecules per unit cell at RH of 60–70% due to the sharp occupation of a vacant water site. In contrast, Na,K-rich laumontite demonstrated regular and less intensive hydration in studied RH range due to the slight occupancy increase of already populated water sites. The comparative analysis of H-bond nets in Ca-laumontite and Na,K-rich laumontite showed the similarity in H-bonding between Na,K-rich laumontite and high-hydrated form of Ca-laumontite.

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► Na,K-rich laumontite demonstrated regular humidity-induced hydration.
► The dependence of unit cell volume vs. H2O + cations sum is the same for Ca- and Na,K-rich laumontites.
► Na,K-rich laumontite does not hydrate completely at ambient temperature and pressure.