A natural nanosponge: new insights on the crystal-fluid interactions in laumontite

Laumontite, |(Ca4-xNax)Kx(H2O)n| [Al8Si16O48], space group C2/m, is one of the most common natural zeolites occurring in a wide range of geological environments, including oceanic basalts, in vugs of plutonic and volcanic rocks and in sedimentary rocks. Fully hydrated laumontite contains 18 H2O molecules per formula unit (p.f.u.), although, if exposed to air at relative humidity (RH) < 50%, it can lose up to 4 H2O molecules p.f.u.. Partially dehydrated laumontite is formally referred to as “leonhardite” (e.g., Yamazaki et al., 1991). To date, a number of studies have been devoted, mainly by in-situ X-ray powder diffraction, to the processes of hydration/dehydration, controlling the RH or exposing samples to 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 a Ca-laumontite by in-situ synchrotron powder diffraction with a diamond anvil cell, using a 16:3:1 methanol-ethanol-H2O mixture as the pressure-transmitting medium, up to 7.5 GPa. In that experiment an instantaneous over-hydration, occurring at a relatively low pressure (< 0.3 GPa), was observed. Furthermore, the authors noticed a tripling of the unit-cell edge along [010] at ~ 3 GPa, which was interpreted as a phase transition (Lee et al., 2004).The authors themselves suggested that single-crystal data were needed to unravel the questions still open. Nowadays, a few open issues and missing information remain concerning: i) the possible phase transition observed by Lee et al. (2004) at about 3 GPa; ii) the elastic parameters of leonhardite, which both thermodynamic calculations and geological observations suggest is the stable form of laumontite under diagenetic and low-grade metamorphic conditions (e.g. Neuhoff and Bird 2001, Coombs et al., 1959); and iii) the single-crystal hydration kinetics in an H2O-ethanol mixture. Laumontite’s common occurrence in many geologic environments, for example in oceanic basalts, suggests that may be an important H2O carrier in the very first kilometers of subduction zones. In addition, the isothermal bulk modulus (KV0) of leonhardite, which is still unknown, is a critical parameter needed to model the thermodynamic stability of this mineral in geological environments of economic relevance (i.e., deposits related to oil reservoirs). In this light, we performed a series of experiments aimed to describe the crystal-fluid interaction in laumontite-leonhardite by in-situ high-pressure single-crystal synchrotron X-ray diffraction using different pressure-transmitting fluids, as well as a number of in-situ single-crystal experiments at ambient pressure in different H2O-ethanol mixtures