Adam Szuszkiewicz

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.

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.

New Occurrence of Clausthalite (PbSe) in the Sudetes (SW Poland)

New Occurrence of Clausthalite (PbSe) in the Sudetes (SW Poland) The presence of clausthalite in the area of old mining works near Dziećmorowice in the Sowie Mts (SW Poland) is reported here for the first time. The identification of the clausthalite is based on macro- and microscopic observations, reflectance measurements, chemical analyses and X-ray diffraction data. The clausthalite, together with uraninite, forms veinlets in a breccia comprising <50% calc-silicate rock fragments. Different polishing hardnesses suggest some variation in the mineral structure of individual clausthalite grains. Chemical spot analyses do not reveal elements other than Pb and Se though calculated unit-cell parameters may suggest minor substitution of S for Se.

Green to blue-green quartz from Rakowice Wielkie (Sudetes, south-western Poland) – a re-examination of prasiolite-related colour varieties of quartz

Abstract The green colour of prasiolite, defined as naturally occurring transparent macrocrystalline α-quartz with primary colouration, results from the optical absorption centred at ~13,660 cm−1 and attributed to the FeVI2+→FeVI3+ intervalence charge transfer (Platonov et al. 1992). However, optical absorption spectroscopy of blue-green to green quartz from Rakowice Wielkie, Sudetes, south-western Poland, shows that its primary colouration results from the combination of this band and absorptions at ~18,500 cm−1 and ~16,250 cm−1. The first is assigned to a hole centre Fe4+s(Fe3+s-e−) combined with an electron centre Fe2+I6(Fe3+I6+e−), while the second from AlO44− defects. The quartz is blue-green if the 18,500 cm−1 prevails and becomes pale green if the 16,250 cm−1 band predominates. These colours seem to represent intermediate colour varieties between amethyst and prasiolite. We also suggest that spectral features of coloured quartz varieties might be useful indicators of changes in the physical- and chemical characteristics of the mineral-forming fluids.