The crystal chemistry of londonite, a rare Cs-bearing mineral [ideal chemical formula: (Cs,K)Al4Be4(B,Be)12O28; a = 7.3098(2) Å at 300 K, space group P4̅3m] from the granitic pegmatites of the Antsongombato Gem Mine, Betafo district, Madagascar, has been reinvestigated by means of wavelength dispersive X-ray spectroscopy, laser ablation-inductively coupled plasma-mass spectroscopy, inductively coupled plasma-atomic emission spectroscopy, X-ray powder diffraction, and single-crystal neutron diffraction at 300 and 20 K. Single-crystal anisotropic structural refinement at 300 K gave a final agreement index R1 = 0.0479 for 32 refined parameters and 416 unique reflections with Fo > 4σ (Fo). The analysis of the difference-Fourier maps of the nuclear density discounts the presence of hydroxyl groups, as wrongly reported in some of the previous studies. The structural refinements and the chemical analyses suggested that: the tetrahedral 4e site (at x ~ 0.258) is mainly occupied by Be, but a low amount of B (~3%) likely occurs; the tetrahedral 12h site (at x ~ 0.248) is mainly occupied by B, but a significant fraction of Be (~12%) is present; the octahedral 4e site (at x ~ 0.361) is fully occupied by Al; and the 1a site (at x = 0, with CN = 12) is mainly occupied by Cs, with Rb and K. The significantly high amount of B2O3 (~50 wt%) and Cs+Rb(CsO2+RbO2 ≥ 8 wt%) makes the synthetic counterpart of londonite of interest as a potential neutron absorber or a potential host for nuclear waste.

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