Phillipsite at high pressure: a single-crystal X-ray synchrotron diffraction study

Phillipsite is a low Si/Al natural zeolite, often found as autogenic mineral in both “close” and “open” hydrologic system or in vugs of basalt, as an alteration product of volcanic glass. Along with laumontite, it is one of the most common zeolites found in oceanic basalts. In order to investigate the high-pressure behavior of phillipsite and its structural evolution at the atomic scale, we performed an in situ single-crystal synchrotron X-ray diffraction experiment up to 10 GPa with a diamond anvil cell, using a nominally penetrating pressure-transmitting fluid (methanol:ethanol:H2O = 16:3:1 mix) (Gatta, 2010). The unit-cell parameters and the structure refinements within the P-range investigated show that: 1) phillipsite does not adsorb further H2O molecules from the penetrating-transmitting fluid within the P-range investigated; 2) the configuration of the extra-framework population changes with pressure (between 2 and 3 GPa), affecting the elastic behavior of the mineral. More in details, two distinct compressional regimes have been observed, in which the bulk moduli differs drastically (i.e., KV = 89(8) GPa between 0 and 2 GPa, KV = 18.8(7) GPa between 3 and 9 GPa); 3) phillipsite is crystalline at least up to 10 GPa, and this is surprising if we consider its microporous nature; 4) all the P-induced effects are completely reversible in decompression. The structural refinements allowed us to describe the mechanisms, at the atomic scale, that govern its elastic behavior, which are mainly governed by inter-tetrahedral tilting. The relatively low compressibility of phillipsite at room-P and its relatively wide P-stability shown in this experiment suggests that this zeolite is a potential H2O carrier during the first phase of the oceanic crust subduction or, toward the industrial front, its potential use in systems for the mechanical energy storage/dissipation (Eroshenko et al., 2001; Soulard et al., 2004). Acknowledgements: The author acknowledges the Italian Ministry of Education, MIUR-Project: “Futuro in Ricerca 2012 - ImPACT- RBFR12CLQD”.