New insights on the hydration of the zeolite laumontite: a natural nano-sponge

Laumontite, |(Ca4-xNax)Kx(H2O)n|[Al8Si16O48], space group C2/m, is one of the most common natural zeolite occurring in a wide range of geological environments, including sedimentary deposits and deep-sea sediments. Remarkably, it is also present in oceanic basalts as well as in vugs of plutonic and volcanic rocks and in sedimentary rocks. Fully hydrated laumontite contains 18 H2O molecules per formula unit, although it can lose up to 4 H2O molecules per formula unit if exposed to air at relative humidity RH < 50%. Such a partially-dehydrated laumontite is formally referred as leonhardite (Yamazaki et al., 1991). To date, laumontite has been studied mainly via X-ray powder diffraction, in order to investigate the processes of hydration/dehydration controlling the RH or submerging samples in pure water or increasing temperature (e.g., Yamazaki et al., 1991; Fridriksson et al., 2004). Lee et al. (2004) investigated the high-pressure behavior of laumontite up to 7.5 GPa, by in situ synchrotron powder diffraction with a diamond anvil cell, using the 16:3:1 methanol-ethanolH2O pressure medium, and observed an instantaneous over-hydration effect at a relatively low pressure (< 0.5 GPa) with a potential additional phase transition at about 3 GPa (Lee et al., 2004). However, a number of open questions remain still open about: i) the possible phase transition observed by Lee et al. (2004) at about 3 GPa, ii) the elastic parameters of leonhardite, which both thermodynamic calculation and geological observations suggest being the stable form of laumontite under diagenetic and low- grade metamorphic conditions (e.g., Neuhoff & Bird, 2001; Coombs et al., 1959), and iii) the single-crystal hydration kinetics in H2O mixture.These parameters are critical considering the fact that laumontite is one of the most common zeolite in the oceanic basalts and, thereby, it can be an important H2Ocarrier in subduction zones. In this light, we performed in situ single-crystal synchrotron X-ray diffraction experiments using different pressure transmitting fluids, as well as a number of in situ single-crystal experiments at ambient pressure in different H2O rich-mixture. On the basis of these studies, we are able to describe: 1) the hydration mechanisms and kinetics of laumontite in large single-crystals, 2) the bonding configuration of the adsorbed H2O molecules and the structural deformation of the framework in response to the overhydration at ambient pressure; 3) the elastic parameters of leonhardite; 4) the different deformation behavior between leonhardite and the hydrated-laumontite.Coombs, D.S., Ellis, A.J., Fyfe, W.S., Taylor, A.M. (1959): The zeolite facies, with comments on the interpretation of hydrothermal syntheses. Geochim. Cosmochim. Acta, 17, 53-107. Fridriksson, T., Bish, D.L, Bird, D.K (2004): Hydrogen-bonded water in laumontite I: X-ray powder diffraction study of water site occupancy and structural changes in laumontite during room-temperature isothermal hydration/dehydration. Am. Mineral., 88, 277-287. Lee, Y., Hrilja, A.J., Vogt, T. (2004): Pressure-induced migration of zeolitic water in laumontite. Phys. Chem. Miner., 31, 421-428. Neuhoff, P.S. & Bird, D.K. (2001): Partial dehydration of laumontite: thermodynamic constraints and petrogenetic implications. Mineral. Mag., 65, 59-70. Yamazaki, A., Shiraki, T., Nishido, H., Otsuka, R. (1991): Phase change of laumontite under relative humidity-controlled conditions. Clay Sci., 8, 79-86.