Xenotime-(Gd), a new Gd-dominant mineral of the xenotime group from the Zimná Voda REE–U–Au quartz vein, Prakovce, Western Carpathians, Slovakia

Abstract Xenotime-(Gd), ideally GdPO 4 , is a new mineral of the xenotime group. It was discovered at the Zimná Voda REE–U–Au occurrence near Prakovce, Western Carpathians, Slovakia. It forms rare crystal domains (≤20 μm, usually ≤10 μm in size) in Gd-rich xenotime-(Y) crystals (≤100 μm in size), in association with monazite-group minerals, uraninite, fluorapatite and uranyl arsenates–phosphates. The hydrothermal REE–U–Au mineralisation occurs in a quartz–muscovite vein, hosted in Palaeozoic phyllites near exocontact with Permian granites. The density is 5.26 g/cm 3 , based on calculated average empirical formula and unit-cell parameters. The average chemical composition ( n = 6) measured by electron microprobe is as follows (wt.%): P 2 O 5 30.1, As 2 O 5 0.5, SiO 2 0.2, UO 2 0.3, Y 2 O 3 15.7, (La, Ce, Pr, Nd) 2 O 3 0.5, Sm 2 O 3 5.7, Eu 2 O 3 1.4, Gd 2 O 3 29.2, Tb 2 O 3 3.9, Dy 2 O 3 10.4, Ho 2 O 3 0.4, (Er, Tm, Yb, Lu) 2 O 3 2.1, (Ca, Fe, Pb, Mn, Ba)O 0.1, total 100.5. The corresponding empirical formula calculated on the basis of 4 oxygen atoms is: (Gd 0.37 Y 0.32 Dy 0.13 Sm 0.08 Tb 0.05 Eu 0.02 Er 0.01 Tm 0.01 Nd 0.01 …) Σ1.01 (P 0.98 As 0.01 Si 0.01 )O 4 . The empirical formula of the Gd-richest composition is: (Gd 0.38 Y 0.31 Dy 0.13 Sm 0.08 Tb 0.05 Eu 0.02 Er 0.01 Nd 0.01 Ho 0.01 …) Σ1.01 (P 0.98 As 0.01 Si 0.01 )O 4 . The ideal formula is GdPO 4 . The xenotime-type structure has been confirmed by micro-Raman spectroscopy and a Fast Fourier-Transform pattern using HRTEM. Xenotime-(Gd) is tetragonal, space group I 4 1 / amd , a = 6.9589(5) Å, c = 6.0518(6) Å, V = 293.07(3) Å 3 and Z = 4. The new mineral is named as an analogue of xenotime-(Y) and xenotime-(Yb) with Gd dominant among the REE. The middle REE enrichment of xenotime-(Gd) is shared with the associated monazite-(Gd) and Gd-rich hingganite-(Y). This exotic REE signature and precipitation of Gd-bearing minerals is a product of selective complexing and enrichment in MREE in low-temperature hydrothermal fluids by alteration of uraninite, brannerite and fluorapatite on a micro-scale. The existence of xenotime-(Gd) and monazite-(Gd) is the first naturally documented dimorphism among REE phosphates. In addition, xenotime-(Gd) is only the third approved Gd-dominant mineral, after lepersonnite-(Gd) and monazite-(Gd).