Title: Żabińskiite, ideally Ca(Al<sub>0.5</sub>Ta<sub>0.5</sub>)(SiO<sub>4</sub>)O, a new mineral of the titanite group from the Piława Górna pegmatite, the Góry Sowie Block, southwestern Poland
Abstract: Abstract Żabińskiite, ideally Ca(Al 0.5 Ta 0.5 )(SiO 4 )O, was found in a Variscan granitic pegmatite at Piława Górna, Lower Silesia, SW Poland. The mineral occurs along with (Al,Ta,Nb)- and (Al,F)-bearing titanites, a pyrochlore-supergroupmineral and a K-mica in compositionally inhomogeneous aggregates, ∼120 μm × 70 μm in size, in a fractured crystal of zircon intergrown with polycrase-(Y) and euxenite-(Y). Żabińskiite is transparent, brittle, brownish, with a white streak, vitreous lustre and a Mohs hardness of ∼5. The calculated density for the refined crystal is equal to 3.897 g cm –3 , but depends strongly on composition. The mineral is non-pleochroic, biaxial (–), with mean refractive indices ≥1.89. The (Al,Ta,Nb)-richest żabińskiite crystal,(Ca 0.980 Na 0.015 ) ∑=0.995 (Al 0.340 Fe 3+ 0.029 Ti 0.298 V 0.001 Zr 0.001 Sn 0.005 Ta 0.251 Nb 0.081 ) ∑=1.005 [(Si 0.988 Al 0.012 )O 4.946 F 0.047 (OH) 0.007 ) ∑=5.000 ];60.7 mol.% Ca[Al 0.5 (Ta,Nb) 0.5 ](SiO 4 )O; is close in composition to previously described synthetic material. Żabińskiite is triclinic (space group symmetry A 1) and has unit-cell parameters a = 7.031(2) Å, b = 8.692(2) Å, c = 6.561(2) Å, α = 89.712(11)°, β = 113.830(13)°, γ = 90.352(12)° and V = 366.77 (11) Å 3 . It is isostructural with triclinic titanite and bond-topologically identical with titanite and other minerals of the titanite group.Żabińskiite crystallized along with (Al,Ta,Nb)-bearing titanites at increasing Ti and Nb, and decreasing Ta activities, almost coevally with polycrase-(Y) and euxenite-(Y) from Ca-contaminated fluxed melts or early hydrothermal fluids.