Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Supplementary Data Info Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Gatta, G. D. Articles by Ventura, G. D.

On the crystal structure and crystal chemistry of pollucite, (Cs,Na)₁₆Al₁₆Si₃₂O₉₆·nH₂O: A natural microporous material of interest in nuclear technology

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, Milano, Italy
³ Dipartimento di Scienze della Terra, Università degli Studi di Perugia, Piazza Università 1, I-06100 Perugia, Italy
⁴ Institut Laue-Langevin, B.P. 156, F-38042 Grenoble Cedex 9, France
⁵ Dipartimento di Scienze Geologiche, Università degli Studi di Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy
⁶ Dipartimento di Geoscienze, Università degli Studi di Padova, Via Giotto 1, I-35137 Padova, Italy

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

The crystal structure and crystal chemistry of two natural pollucite samples, from Buckfield, Oxford County, Maine (sample M3), and from Kanzit Mawaie, Laghman, Nooristan, Afghanistan (sample N5), have been investigated by means of wavelength-dispersive X-ray microanalysis, thermogravimetric analysis, single-crystal X-ray and neutron diffraction, and single-crystal Fourier-transform infrared spectroscopy. The X-ray and neutron diffraction patterns of the two pollucite crystals show a metrically cubic unit cell [with a_M3 = 13.6914(6) Å and a_N5 = 13.6808(6) Å by neutron diffraction data; the deviation from isometry is <1.5(l_i), where l_i is the unrestrained unit-cell length] and the reflection conditions are consistent with the space group Iad. Anisotropic neutron structural refinements gave final agreement indices: R₁ = 0.0543 for 32 refined parameters and 372 unique reflections with F_o > 4(F_o) for M3 and R₁ = 0.0693 for 31 refined parameters and 331 unique reflections with F_o > 4(F_o) for N5. The structure refinements show a disordered Si/Al-distribution in the tetrahedral framework. The analysis of the difference-Fourier maps of the nuclear density confirms the presence of extra-framework water molecules with oxygen sharing the Cs site (at 1/8, 1/8, 1/8, Wyckoff-16b position). However, the minima, ascribable to the proton sites, are very weak in density. Two possible proton positions, leading to a reasonable H₂O configuration, are given, and the possible hydrogen bonding is described. Sodium is located at 1/4, 1/8, 0 (Wyckoff-24c position). The main IR absorption bands in the regions typical of H₂O are assigned, and the presence of hydroxyls in the studied samples is ruled out. Neutron diffraction and FTIR data agree with the presence of very weak hydrogen bonds in the structure. The detailed description of the crystal structure and crystal chemistry of pollucite (e.g., Si/Al-distribution, configuration of the extra-framework content, possible hydrogen bonding scheme) reported in this study is the key to understand the high thermo-elastic stability of pollucite, the immobility of Cs at non-ambient conditions, and the extremely low leaching rate of Cs, which make this open-framework silicate a promising material with potential use for fixation and deposition of Cs radioisotopes.

Key Words: Pollucite • zeolite • crystal chemistry • single-crystal X-ray diffraction • single-crystal neutron diffraction • single-crystal FTIR