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

Monazite-(Gd), ideally GdPO 4 , is a new mineral of the monazite group. It was discovered near Prakovce-Zimná Voda, ~23 km WNW of Košice, Western Carpathians, Slovakia. It forms anhedral domains (≤100 μm, mostly 10–50 μm in size), in close association with monazite-(Sm), Gd-bearing xenotime-(Y), Gd-bearing hingganite-(Y), fluorapatite and uraninite. All these minerals are hosted in a REE–U–Au quartz–muscovite vein, hosted in phyllites in an exocontact to granites. The density calculated using the average empirical formula and unit-cell parameters is 5.55 g/cm 3 . The average chemical composition measured by means of electron microprobe is as follows (wt.%): P 2 O 5 29.68, As 2 O 5 0.15, SiO 2 0.07, ThO 2 0.01, UO 2 0.04, Y 2 O 3 1.30, La 2 O 3 3.19, Ce 2 O 3 6.93, Pr 2 O 3 1.12, Nd 2 O 3 10.56, Sm 2 O 3 17.36, Eu 2 O 3 1.49, Gd 2 O 3 22.84, Tb 2 O 3 1.57, Dy 2 O 3 2.27, CaO 0.21, total 99.67. The corresponding empirical formula calculated on the basis of 4 oxygen atoms is: (Gd 0.30 Sm 0.24 Nd 0.15 Ce 0.10 La 0.05 Dy 0.03 Y 0.03 Tb 0.02 Eu 0.02 Pr 0.02 Ca 0.01 ) 0.98 P 1.01 O 4 . The ideal formula is GdPO 4 . The monazite-type structure has been confirmed by micro-Raman spectroscopy and selected-area electron diffraction. Monazite-(Gd) is monoclinic, space group P 2 1 / n , a = 6.703(1) Å, b = 6.914(1) Å, c = 6.383(1) Å, β = 103.8(1)°, V = 287.3(1) Å 3 and Z = 4. The middle REE enrichment of monazite-(Gd) is shared with the associated Gd-bearing xenotime-(Y) to ‘xenotime-(Gd)’ and Gd-bearing hingganite-(Y). This exotic REE signature and precipitation of Gd-bearing mineral assemblage is a product of selective complexing and enrichment in middle REE in low-temperature hydrothermal fluids by alteration of primary uraninite, brannerite and fluorapatite on a micro-scale. The new mineral is named as an analogue of monazite-(La), monazite-(Ce), monazite-(Nd) and monazite-(Sm) but with Gd dominant among the REE.