Pilawite-(Y), Ca₂(Y,Yb)₂[Al₄(SiO₄)₄O₂(OH)₂], a new mineral from the Piława Górna granitic pegmatite, southwestern Poland: mineralogical data, crystal structure and association

Abstract

Abstract Pilawite-(Y), ideally Ca 2 (Y,Yb) 2 Al 4 (SiO 4 ) 4 O 2 (OH) 2 , was discovered in a pegmatite near Piława Górna, Lower Silesia, Poland. The mineral occurs as white, translucent, brittle crystals up to 1.5 mm in size. It has a white streak, vitreous lustre and a hardness of 5 on Mohs scale. The calculated density is 4.007 g/cm 3 . Pilawite-(Y) is non-pleochroic, biaxial (+), with refractive indices α = 1.743(5), β = 1.754(5) and γ = 1.779(5), birefringence Δ = 0.03–0.04, 2V meas. = 65(2)° and 2V calc. = 68°. Pilawite-(Y) is monoclinic P 2 1 / c , with unit-cell parameters a = 8.558(3) Å, b = 7.260(3) Å, c = 11.182(6) Å, β = 90.61(4)o, V = 694.7(4) Å 3 . The crystal structure was refined to an R1 index of 2.76% and consists of chains of edge- and corner-sharing octahedra decorated by tetrahedra and having the stoichiometry [Al 2 (SiO 4 ) 4 O(OH)] that link by sharing corners to form an octahedron–tetrahedron framework with large interstices that contain Ca 2+ and (Y,Ln) 3+ . It is a graphical isomer of the Al–P framework in palermoite, Sr 2 Li 4 [Al 2 (PO 4 ) 2 (OH) 2 ] 2 . The pilawite-(Y)-bearing assemblage began crystallization at high Y + Ln activities and was modified progressively by a Ca-enriched fluid, resulting in the sequence: keiviite-(Y) → gadolinite-(Y) to hingganite-(Y) + hellandite-(Y) → pilawite-(Y) → allanite-(Y) → epidote/zoisite.