Structural transformations in natrolite and edingtonite

Abstract

Results of structural transformation studies in the natural fibrous zeolites natrolite and edingtonite are presented. The minerals were studied in situ in a wide range of temperatures, pressures and compositions by differential scanning microcalorimetry, thermogravimetry, dilatometry, X-ray diffractometry, nuclear magnetic resonance (NMR)and Raman-spectroscopy. The high-pressure experiments were done in diamond anvil cell and Be-bronze bomb for NMR using liquids with various dimensions of molecules as pressure-transmitting media. A number of structural transformations in natrolites and edingtonite have been found with constant or changing water content during transformation. Under high water pressures, some additional H₂O molecules entered the framework channels causing a framework deformation and an anisotropic “swelling” of the crystal. Under compression in nonpenetrating liquids no transformations were detected and above 70 kbar amorphization of the minerals was observed. The same displacive-tilt transformations were observed in zeolites at elevated temperatures as a result of the dehydration, e.g. natrolite ⇌ α-metanatrolite at 280° C. Fully reversible phase transitions at constant H₂O content were observed in natrolites and edingtonite at low temperatures (down to -120° C). These are connected with a variation in the mobility and position of exchange cations and water molecules within the framework channels and are followed by significant volume and thermal effects. In dehydrated zeolites, the transformations were found to be similar to the α ⇌ β transition in quartz (α-metanatrolite ⇌ β-metanatrolite). Heating of fibrous zeolites above 500 ÷ 700° C causes their amorphization and formation of porous quasiglass. The principal difference in structural behaviour of microporous crystals under compression in penetrating and nonpenetrating media has essential geochemical implications. Structural transformations of zeolites in P-T-X space demonstrate crystal chemical analogy of these parameters. Some deviations from this analogy depend on complex interactions between channel “filling”, H₂O and cations, and the [(Al,Si)-O_4/2] framework.