The elastic behavior and the phase stability of natural pollucite, (Cs, Na)16Al16Si32O96·nH2O, were investigated at hydrostatic pressure by in situ single-crystal X-ray diffraction with a diamond-anvil cell. Pollucite experiences a P-induced phase transition, not previously reported in the literature, at P = 0.66 ± 0.12 GPa from cubic (Iad) to triclinic symmetry (P1̅). The phase transition is completely reversible and without any appreciable hysteresis effect. No further phase transition has been observed up to 9 GPa. Fitting the pressure-volume data of the low-pressure cubic polymorph with a second-order Birch-Murnaghan Equation-of-State (BM-EoS), we obtain V0 = 2558.3(4) Å3, KT0 = 41(2) GPa, and KT′ = 4 (fixed). For the high-pressure triclinic polymorph, a third-order BM-EoS fit gives V0 = 2577.5(40) Å3, KT0 = 25.1(9) GPa, and KT′ = 6.5(4). The axial bulk moduli of the high-pressure triclinic polymorph were calculated with a third-order “linearized” BM-EoS. The EoS parameters are a0 = 13.699(12) Å, KT0(a) = 25.5(17) GPa, and KT′ (a) = 6.8(6) for the a axis; b0 = 13.728(12) Å, KT0(b) = 23.2(15)GPa, and KT′ (b) = 7.7(7) for the b axis; c0 = 13.710(7) Å, KT0(c) = 25.2(10) GPa, and KT′ (c) = 6.8(4) for the c axis [KT0(a):KT0(b):KT0(c) = 1.10:1:1.09]. Brillouin light-scattering was used to investigate the single-crystal elastic properties of pollucite at ambient conditions. The aggregate adiabatic bulk modulus (Ks) and shear modulus (G), calculated using the Voigt-Reuss-Hill averaging procedures, are Ks = 52.1(10) GPa and G = 31.5(6) GPa. The elastic response of pollucite and other isotypic materials (e.g., analcime, leucite, and wairakite) is compared. The high thermo-elastic stability of pollucite, reflected by the preservation of crystallinity at least up to 9 GPa (at room T) and 1470 K (at room P) in elastic regime, the large amount of Cs hosted in this material (Cs2O ~ 30 wt%), the immobility of Cs at high-temperature and high-pressure conditions, and the extremely low leaching rate of Cs, make of this open-framework silicate a functional material with potential use for fixation and deposition of Cs radioisotopes in high-level nuclear waste.

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