Determination of P–V equation of state of a natural clinoptilolite using high-pressure powder synchrotron X-ray diffraction

Abstract

Characterization of the behavior of zeolites at high pressures is of interest both in fundamental science and for practical applications. For example, zeolites occur as a major mineral group in tuffaceous rocks (such as those at the Nevada Nuclear Security Site), and they play a key role in defining the high-pressure behavior of tuff in a nuclear explosion event. The crystal structure, Si/Al ratio, and type of pressure-transmitting media (PTM) used in high-pressure experiments influence the compressional behavior of a given zeolitic phase. The heulandite-type (HEU) zeolites, including heulandite and clinoptilolite, are isostructural but differ in their Si/Al ratios. Thus, HEU-type zeolites comprise an ideal system in unraveling the effects of Si/Al ratio and type of PTM on their pressure-induced structural behavior. In this study, we performed in situ high-pressure angle-dispersive powder synchrotron X-ray diffraction (XRD) experiments on a natural HEU zeolite, clinoptilolite, with a Si/Al ratio of 4.4, by compressing it in a diamond anvil cell (DAC) up to 14.65 GPa using a non-penetrating pressure-transmitting medium (KCl). Unit cell parameters as a function of pressure up to 9.04 GPa were obtained by Rietveld analysis. Unit cell volumes were fit to both a second and a third-order Birch–Murnaghan equation of state. The mean bulk modulus (K₀) determined from all the fittings is 32.7 ± 0.9 GPa. The zero-pressure compressibility of the a-, b-, and c-axes for clinoptilolite are 10.6 (± 0.8) × 10^–3 GPa^–1, 5.3 (± 0.7) × 10^–3 GPa^–1, and 17.1 (± 1.8) × 10^–3 GPa^–1, respectively. The pressure–volume equations of states of this type of zeolite are important for characterizing high-pressure behavior of the broader family of microporous materials and for developing reliable geophysical signatures for underground nuclear monitoring.