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The elastic and structural behaviour of the synthetic zeolite CsAlSi 5 O 12 (a = 16.753(4), b = 13.797(3) and c = 5.0235(17) A ˚, space group Ama2, Z = 2) were investigated up to 8.5 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase-transition occurs within the P-range investigated. Fitting the volume data with a third-order Birch-Murnaghan equation-of-state gives: V 0 = 1,155(4) A ˚3, K T0 = 20(1) GPa and K 0 = 6.5(7). The ''axial moduli'' were calculated with a third-order ''linearized'' BM-EoS, substituting the cube of the individual lattice parameter (a 3 , b 3 , c 3 ) for the volume. The refined axial-EoS parameters are: a 0 = 16.701(44) A ˚, K T0a = 14(2) GPa (b a = 0.024(3) GPa -1 ), K 0 a = 6.2(8) for the a-axis; b 0 = 13.778(20) A ˚, K T0b = 21(3) GPa (b b = 0.016(2) GPa -1 ), K 0 b = 10(2) for the b-axis; c 0 = 5.018(7) A ˚, K T0c = 33(3) GPa (b c = 0.010(1) GPa -1 ), K 0 c = 3.2(8) for the c-axis (K T0a :K T0b :K T0c = 1:1.50:2.36). The HP-crystal structure evolution was studied on the basis of several structural refinements at different pressures: 0.0001 GPa (with crystal in DAC without any pressure medium), 1.58(3), 1.75(4), 1.94(6), 3.25(4), 4.69(5), 7.36(6), 8.45(5) and 0.0001 GPa (after decompression). The main deformation mechanisms at high-pressure are basically driven by tetrahedral tilting, the tetrahedra behaving as rigid-units. A change in the compressional mechanisms was observed at P B 2 GPa. The P-induced structural rearrangement up to 8.5 GPa is completely reversible. The high thermo-elastic stability of CsAlSi 5 O 12, the immobility of Cs at HT/HP-conditions, the preservation of crystallinity at least up to 8.5 GPa and 1,000°C in elastic regime and the extremely low leaching rate of Cs from CsAlSi 5 O 12 allow to consider this openframework silicate as functional material potentially usable for fixation and deposition of Cs radioisotopes.

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The economic significance of pegmatites as a source of strategic rare metals for high-tech products and green energy motivated the present study on Ta–Nb oxides from Vishteritsa rare-element beryl–columbite LCT pegmatites of the Rila–West Rhodopes Batholith in the Western Rhodopes, Bulgaria. Here, we present the first U/Pb age data from columbite with application of the LA–ICP–MS U–Pb technique and a new X36 columbite standard reference material. The obtained Concordia age of 47.57 ± 0.32 Ma with a small spread of the individual 206Pb/238U ages between 45 and 51.3 Ma argues for Early Eocene magmatism and pegmatite formation. The host granite of the rare-element pegmatites is dated 51.94 ± 0.61 Ma with LA–ICP–MS U–Pb technique on zircon and suggests a fertile Early Eocene magmatic period in the Western Rhodopes. EPMA data for the composition of the columbite is used to refine the formula of the mineral (Mn0.554Fe0.427U0.006)0.987(Nb1.826Ta0.085Ti0.116)2.03O6 and define it as columbite-(Mn). Application of the in-situ LA–ICP–MS data technique establishes a series of typical trace elements (Ti, U, Zr, Hf, Y, W, and Zn) that are usually found in content above 500 ppm. The studied columbite is enriched in heavy rare earth elements (HREE sum: 306–697 ppm) and depleted in light REE and Eu. These geochemical characteristics are collectively interpreted as evidence for crystallization from highly fractionated fluid-rich magma. High UO2 content reaching 0.89 wt. % is characteristic for the Vishteritsa columbite. The decrease of U proximal to cracks and in outer crystal zones documents U-mobility during overprinting hydrothermal processes.