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Elastic behavior and phase stability of pollucite, a potential host for nuclear waste

¹ Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy
² CNR-Istituto per la Dinamica dei Processi Ambientali, 20133 Milano, Italy
³ Bayerisches Geoinstitut, Universität Bayreuth, Universitaetsstrasse 30, D-95447 Bayreuth, Germany
⁴ Institute for Mineralogy and Petrology, ETH Zürich, Clausiusstrasse 25, H-8092 Zürich, Switzerland

Correspondence: ^* E-mail: diego.gatta{at}unimi.it

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 (P). 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) ų, K_T0 = 41(2) GPa, and K_T' = 4 (fixed). For the high-pressure triclinic polymorph, a third-order BM-EoS fit gives V₀ = 2577.5(40) ų, K_T0 = 25.1(9) GPa, and K_T' = 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, K_T0(a) = 25.5(17) GPa, and K_T' (a) = 6.8(6) for the a axis; b₀ = 13.728(12) Å, K_T0(b) = 23.2(15)GPa, and K_T' (b) = 7.7(7) for the b axis; c₀ = 13.710(7) Å, K_T0(c) = 25.2(10) GPa, and K_T' (c) = 6.8(4) for the c axis [K_T0(a):K_T0(b):K_T0(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 (K_s) and shear modulus (G), calculated using the Voigt-Reuss-Hill averaging procedures, are K_s = 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.

Key Words: Pollucite • single-crystal X-ray diffraction • high-pressure • compressibility • phase transition • nuclear waste disposal material

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				G. D. Gatta, R. Rinaldi, G.J. McIntyre, G. Nenert, F. Bellatreccia, A. Guastoni, and G. D. Ventura On the crystal structure and crystal chemistry of pollucite, (Cs,Na)16Al16Si32O96{middle dot}nH2O: A natural microporous material of interest in nuclear technology American Mineralogist, November 1, 2009; 94(11-12): 1560 - 1568. [Abstract] [Full Text] [PDF]