ideally Ca ( Al 0 . 5 Ta 0 . 5 ) ( SiO 4 ) O , a new mineral of the titanite group from the Pi ł awa Górna pegmatite , the Góry Sowie Block , southwestern Poland

Żabin ́skiite, ideally Ca(Al0.5Ta0.5)(SiO4)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-supergroup mineral 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). Żabin ́skiite is transparent, brittle, brownish, with a white streak, vitreous lustre and aMohs hardness of∼5. The calculated density for the refined crystal is equal to 3.897 g cm, but depends strongly on composition. The mineral is non-pleochroic, biaxial (–), with mean refractive indices≥1.89. The (Al,Ta,Nb)-richest żabin ́skiite crystal, (Ca0.980Na0.015)Σ=0.995(Al0.340Fe 3þ 0:029Ti0.298V0.001Zr0.001Sn0.005Ta0.251Nb0.081)Σ=1.005[(Si0.988Al0.012)O4.946 F0.047(OH)0.007)Σ=5.000]; 60.7 mol.% Ca[Al0.5(Ta,Nb)0.5](SiO4)O; is close in composition to previously described synthetic material. Żabin ́skiite 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. It is isostructural with triclinic titanite and bond-topologically identical with titanite and other minerals of the titanite group. Żabin ́skiite 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.

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