Xenotime-(Y) and Sn-rich thortveitite in miarolitic pegmatites from Baveno, Southern Alps, Italy

Xenotime-(Y), (Y, REE )PO4, and thortveitite, (Sc,Y)2Si2O7, in a miarolitic cavity in a niobium, yttrium, fluorine (NYF) granitic pegmatite at Baveno, Verbania, Southern Alps, Italy, were investigated by electron microprobe analysis and single-crystal X-ray diffraction. Fluorine has an important role as a complexing agent for Y and REE s in supercritical pegmatitic fluids; annite-siderophyllite and chamosite are the most likely sources of the Y and REE s. The thortveitite from Baveno contains up to 3.20 wt.% SnO2, an unusually high Sn content. Xenotime-(Y) is enriched in Gd in comparison to other Alpine xenotimes. Quantitative chemical data and measurements of the lattice parameters show that a higher scandium content results in a smaller unit-cell volume in thortveitite. The substitution of REE s for Y up to ~20 mol.% has little effect on the unit cell of xenotime-(Y). The textures of xenotime-(Y) and thortveitite provide information about the dissolution and crystallization processes in the miarolitic cavity.