HIDDEN PHASE TRANSITIONS: EXPLORING T-P(H2O) SPACE WITH NAT-TOPOLOGY ZEOLITES, NATROLITE AND MESOLITE

The interaction of H₂O molecules with zeolite structures is a subject of great scientific and practical interest, as zeolites are useful in technological applications and are important in low-temperature metamorphic systems. In NAT-topology zeolites (e.g., natrolite and mesolite), H₂O molecules occupy much of the volume of the channel system and are ion-dipole attracted to the extraframework cations (Na or Ca). A gain or loss of H₂O molecules has a direct effect on the positions of the extraframework cations and aluminosilicate framework thermal stability. However, studies considering the effects of both temperature and P(H₂O) have been largely neglected to date. Most previous studies of dehydration-induced phase transitions simply increased temperature under partially saturated to dry conditions where the P(H₂O) conditions were seldom monitored or controlled, resulting in an incomplete picture of phase transition behavior. Our recent XRD studies illustrated that the evolution of the crystal structure of natrolite as a function of temperature follows two different phase-transition paths, depending on P(H₂O), with α1-metanatrolite occurring at elevated P(H₂O) and α2-metanatrolite occurring at low P(H₂O). This study showed that the absolute temperature of dehydration controls the nature of structural phase transitions in natrolite, e.g., based on contraction of the hydrous phase field under low P(H₂O) conditions. Similar P(H₂O)-dependent phenomena were also observed in mesolite, with a displacive phase transition to x-metamesolite occurring at low P(H₂O) and a phase transition to an amorphous phase occurring at elevated P(H₂O). The P(H₂O)-dependent phase transitions observed in natrolite and mesolite can be explained based on the existence of a separate phase field under low P(H₂O) conditions. When either zeolite is heated under high P(H₂O) conditions, this new field is bypassed due to the enhanced stability of the hydrous phase, going straight to the higher-temperature phase. Lower P(H₂O) conditions destabilize the hydrous phase relative to the new phase. Therefore, under low P(H₂O) conditions (P(H₂O)< ~ 5.2 mbar for natrolite, 20%RH at 22ºC, P(H₂O)< ~ 1.3 mbar for mesolite, 5%RH at 22ºC) we see these “hidden phase transitions” that cannot be observed at higher P(H₂O).