LAUSR

Abstract 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: V 0  = 1393.9(6) A 3 and K V 0  = 54.8(10) GPa; whereas the isothermal bulk elastic parameters of Ca-leonhardite, refined by a third order BM-EoS fit, are: V 0  = 1348(1) A 3 , K V 0  = 36(1) GPa and K V ’  = 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 H 2 O-based mixtures. The results show that the hydration process is influenced by the fraction of H 2 O 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.