Crystal-fluid interactions in laumontite

The elastic behavior and the structural evolution at high pressure of a natural Ca-laumontite and Ca-leonhardite, the partially dehydrated form of Ca-laumontite, have been investigated by in situ single-crystal synchrotron X-ray diffraction up to 2.7 GPa and 7.5 GPa, respectively, using a diamond anvil cell. Despite no phase transitions have been observed within the P-range investigated, an anomalous stiffening of the structure along the b crystallographic axis occurs at about 2.1 GPa in Ca-laumontite and 2.4 GPa in Ca-leonhardite. The isothermal bulk elastic parameters of Ca-laumontite, refined by a second order Birch–Murnaghan equation of state (BM-EoS) fit, are: V0 = 1393.9(6) Å3 and KV0 = 54.8(10) GPa; whereas the isothermal bulk elastic parameters of Ca-leonhardite, refined by a third order BM-EoS fit, are: V0 = 1348(1) Å3, KV0 = 36(1) GPa and KV’ = 2.4(3). The hydration process, at ambient P-T conditions, of Ca-leonhardite has also been studied by means of in-situ single crystal X-ray diffraction in several H2O-based mixtures. The results show that the hydration process is influenced by the fraction of H2O in the aqueous mixtures in which leonhardite is immersed, and an almost linear correlation between the occupancy of the crystallographic W1 site and the unit-cell volume has been found. The structure deformation mechanisms that govern the compression of Ca-laumontite and Ca-leonhardite at the atomic scale, as well as those related to the hydration process of Ca-leonhardite, are described.