On the crystal-fluid interactions in laumontite

Laumontite, I(Ca4.xNa,)Kx(H20)nl [AlgSiI604SL space group C2/m, is one of the most common naturaI zeolite occurring in a wide range of geologicaI environments, including sedimentary deposits, deep-sea sediments and in sedimentary deposits related to oil reservoirs. Remarkbly, it is also present in oceanic basalts, as well as in vugs of plutonic and volcanic rocks. Fully hydrated laumontite contains 18 H 20 molecules per formula unit. If exposed to air relative humidity (RH) < 50%, laumontite can lose up to 4 H 20 molecules per formula unit. Such a partially-dehydrated laumontite is formally referred as leonhardite [I). Lee et al. [2J 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: l =methanol-ethanol-H20 mix as pressure-transmitting fluid, and observed an instantaneous over-hydration effect at a relatively low pressure « 0.3 GPa), with a potential additional phase transition at about 3 GPa [2). Others authors have investigated, mainly by in-si/II X-ray powder diffraction, the processes of hydration/dehydration, controlling the RH or submerging samples in pure water or increasing temperature [I, 3). However, no experiments have so far been devoted to the elastic behavior of leonhardite, which both thermodynamic calculation and geological observations suggest being the stable form of laumontite under diagenetic and low-grade metamorphic conditions [4). Furthermore, the bulk modulus K v of leonhardite is a critical parameter needed in order to model the thermodynamic stability of this mineral in geological environments of economic relevance (i.e., deposits related to oil reservoirs). Moreover, some questions are still open about e.g. the single-crystal hydration kinetics in H20 mixture and the possible phase transition observed by Lee ct al. [2] at about 3 GPa. In order to answer the open questions, we performed a series of in-st/u single-crystal synchrotron X-ray diffraction experiments using different pressure-transmitting fluids, as well as in-situ single-crystal X-ray experiments at ambient pressure in different H2O rich-mixture. On the basis of these studies, we are were to describe: I) the hydration mechanisms and kinetics of laumontite in 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 fully-hydrated laumontite.