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Pressure-induced over-hydration of thomsonite: A synchrotron powder diffraction study

¹ Institute of Geology and Mineralogy SibD RAS, pr.ak.Koptyuga 3, 630090 Novosibirsk, Russia
² Institute of Inorganic Chemistry SibD RAS, ac.Lavrentieva av. 3, 630090 Novosibirsk, Russia
³ Institute of Solid State Chemistry SibD RAS, Kutateladze st. 18, 630218 Novosibirsk, Russia
⁴ Budker Institute of Nuclear Physics SibD RAS, ac.Lavrentieva av. 11, 630090 Novosibirsk, Russia

Correspondence: ^* E-mail: alih{at}uiggm.nsc.ru

The structural behavior of thomsonite compressed in aqueous medium up to 3 GPa was studied by means of in situ synchrotron powder diffraction with a diamond anvil cell. In the range between 0.0001 and 2 GPa, the compressibility of thomsonite is markedly lower than that reported previously, where a non-penetrating medium (with only 6% H₂O) was used. This indicates a pressure-induced hydration (PIH), which results in the transition to an over-hydrated phase observed at 2 GPa. The structure of over-hydrated thomsonite contains one additional, half-occupied H₂O position, coordinated by the calcium at the Ca2 site, with a scolecite-like coordination [CaO₄(H₂O)₃]. The appearance of new H₂O position causes a 4.5% volume expansion through the cooperative rotation of [T₂O₅] chains, leading to the enlargement of the cross-section of the main channels parallel to c axis. The observed deformation mechanism is similar to that found in high-hydrated and super-hydrated natrolite, although only a half of the channels are affected by PIH. The present data indicate that the over-hydration effect in fibrous zeolites strongly depends on the partial water pressure in compressing medium.

Key Words: Zeolite • thomsonite • high pressure • compressibility • phase transition • crystal structure • over-hydration

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