Sławomir Ilnicki

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 Pilawite-(Y), ideally Ca2(Y,Yb)2Al4(SiO4)4O2(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/cm3. 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, 2Vcalc. = 65(2)° and 2Vcalc. = 68°. Pilawite-(Y) is monoclinic P21/c, with unit-cell parameters a = 8.558(3) Å, b = 7.260(3) Å, c = 11.182(6) Å, β = 90.61(4)°, V = 694.7(4) Å3. 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 [Al2(SiO4)4O(OH)] that link by sharing corners to form an octahedron-tetrahedron framework with large interstices that contain Ca2+ and (Y,Ln)3+. It is a graphical isomer of the Al-P framework in palermoite, Sr2Li4[Al2(PO4)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.

Variscan prograde P-T evolution and contact metamorphism in metabasites from the Sowia Dolina, Karkonosze-Izera massif, SW Poland

Abstract Several bodies of moderately foliated and porphyroblastic metabasites crop out on the SE side of the metamorphic cover of the Karkonosze granite within metapelites of the Sowia Dolina area (West Sudetes, Saxothuringian zone). Depending on the microstructural setting of the Ca-amphiboles in the rocks, different mineral-chemical trends have been determined for Si, XMg, AlVI, A[Na+K] which serve as semi-quantitative indicators of temperature and pressure changes. Porphyroblasts and prisms oblique to the main foliation in schistose metabasites show zoning from Mg-hornblende and actinolite to tschermakite, and then to Mg-hornblende (or actinolite). Matrix amphiboles and those in pressure shadows around some porphyroblasts have tschermakitic cores and actinolitic rims. Rarely, Ca-amphibole is accompanied in schists by late- to post-tectonic cummingtonite. Thermobarometric calculations involving empirically calibrated amphibole equilibria enable a reconstruction of P-T paths for individual rocks and the unravelling of the metamorphic evolution of the metabasites. Peak metamorphic temperatures of 615-640°C and pressures of 7.3-8.2 kbar were preceded by a variably preserved earlier stage (T = 370-550°C, P = 2.8-6.2 kbar). The final metamorphic episode took place at 450-550°C and 2.5-4.8 kbar and is recorded particularly in rocks close to the Karkonosze pluton. The metabasites shed new light on the history of metamorphism in the Sowia Dolina area. The first two stages of MP-MT metamorphism, coeval with Variscan deformation events (continental collision, burial and subsequent exhumation), took place under epidote-amphibolite then amphibolite facies conditions. The last stage partly concurred with the final stages of Variscan deformation and overlapped the onset of thermal activity associated with the Karkonosze granite. This metamorphic event is documented by metabasites (occasionally cummingtonite-bearing) outcropping close to the granite. Finally, a prehnite-bearing assemblage reflects retrograde re-equilibration under greenschist/sub-greenschist facies conditions (T <300-350°C, P <2.5-3 kbar), which might also be partly due to hydrothermal activity around the pluton.

Bohseite, ideally Ca4Be4Si9O24(OH)4, from the Piława GóRNA quarry, the Góry Sowie Block, SW Poland

Abstract Bohseite is an orthorhombic calcium beryllium aluminosilicate with variable Al content and an endmember formula Ca4Be4Si9O24(OH)4, that was discovered in the Piława Górna quarry in the eastern part of the Góry Sowie Block, ∼50 km southwest of Wrocław, SW Poland. It occurs in a zoned anatectic pegmatite dyke in close association with microcline, Cs-rich beryl, phenakite, helvite, ‘lepidolite’, probably bertrandite and unidentified Be-containing mica as alteration products after a primary Be mineral, probably beryl. Bohseite forms fan-like or parallel aggregates (up to 0.7 cm) of white, platy crystals (up to 2 mmlong) with characteristic striations. It is white with a white streak, is translucent and has a vitreous lustre; it does not fluoresce under ultraviolet light. The cleavage is perfect on {001} and fair on {010}, and neither parting nor twinning was observed. Bohseite is brittle with a splintery fracture and Mohs hardness is 5-6. The calculated density is 2.719 g cm-3. The indices of refraction are α = 1.579, β = 1.580, γ = 1.597, all ± 0.002; 2Vobs = 24(3)°, 2Vcalc = 27°; the optic orientation is as follows: X ^ a= 16.1°, Y ^ b = 16.1°, Z ∥ c. Bohseite shows orthorhombic diffraction symmetry, space group Cmcm, a = 23.204(6), b = 4.9442(9), c = 19.418(6) Å, V = 2227.7(4) Å3, Z = 4. The crystal structure was refined to an R1 value of 2.17% based on single-crystal data, and the chemical composition was determined by electron-microprobe analysis. Bohseite is isostructural with bavenite. Bohseite was originally approved with an end-member composition of Ca4Be3AlSi9O25(OH)3, but subsequent discovery of compositions with Be > 3.0 apfu led to redefinition of its end-member composition, holotype sample and locality, as reported here. There is extensive solid solution in bavenite-bohseite according to the scheme O(2)OH- + T(4)Si4+ + T(3)Be2+ ↔ O(2)O2- + T(4)Al3+ + T(3)Si4+, and a general formula for the bavenite-bohseite minerals may be written as Ca4BexSi9Al4-xO28-x(OH)x, where x ranges from 2-4 apfu: Ca4Be2Si9Al2O26(OH)2 (bavenite) to Ca4Be4Si9O24(OH)4 (bohseite).

Żabińskiite, ideally Ca(Al0.5Ta0.5)(SiO4)O, a new mineral of the titanite group from the Piława Górna pegmatite, the Góry Sowie Block, southwestern Poland

Abstract Żabińskiite, 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). 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, (Ca0.980Na0.015)Σ=0.995(Al0.340Fe3+0:029Ti0.298V0.001Zr0.001Sn0.005Ta0.251Nb0.081)Σ=1.005[(Si0.988Al0.012)O4.946F0.047(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. Żabińskiite is triclinic (space group symmetry A1̄) 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) Å3. 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.