The title mineral has been discovered in a granitic pegmatite at the Stetind quarry, Tysfjord, Norway. Atelisite-(Y) forms pale brown to colourless, transparent crystals of dipyramidally terminated, short-prismatic habit with a diameter of up to 0.3 mm. Based on electron microprobe analysis and 16 anions per formula unit, the empirical formula is (Y3.11Yb0.46Er0.19Dy0.11Gd0.09Ho0.01Tb0.02)∑3.99Si2.40O16H10.43 The presence of hydroxyl-groups is inferred from the detection of OH-stretching modes near 3225 cm−1 in Raman spectra of the new mineral. The crystal structure of atelisite was solved and refined on single-crystal X-ray diffraction data in space group I4̄2d, with a = 6.947(4), c = 6.133(3) Å, V = 295.98(28) Å3. Site occupancy refinement at the 4a-site approaches 3/4 Si-occupancy, if the 4b-site is fully occupied by Y and Yb, with a refined Y/Yb ratio of 4. This suggests an idealized formula of Y4Si3O8(OH)8 for atelisite-(Y). The crystal structure of the new mineral is of particular interest with regard to the known occurrence of hydrous species in the nominally anhydrous minerals xenotime and zircon.

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