Dubińskaite, Ca4Sc2Al4[Be2Si8O30](OH)2, the first beryllosilicate mineral of the axinite group: description, crystal structure and origin

Abstract

Dubińskaite, ideally Ca4Sc2Al4[Be2Si8O30](OH)2, is the first beryllosilicate mineral isostructural with members of the axinite-group. It was discovered in a granitic pegmatite associated with rodingite-like calc-silicate rocks and metasomatized granitic bodies in a serpentinite quarry near Jordanów Śląski, Lower Silesia, SW Poland. The mineral occurs as subhedral to anhedral crystals, up to ∼180 μm in size, embedded in chlorite aggregates likely incorporated into the pegmatite from adjacent blackwall chlorite schists. Dubińskaite is pale brown, its color caused by minor amounts of Mn2+, Fe2+, and Ti4+, resembling Mn-bearing axinite-(Fe). It has a white streak, vitreous luster, Mohs hardness of ∼7, brittle tenacity, uneven fracture, and no visible cleavage. The calculated density is 3.246–3.251 g·cm-3 (holotype/cotype), and the calculated mean refractive index is close to 1.71. The holotype crystal J11 contains (wt%): 0.64(12) MgO, 17.68(65) Al2O3, 42.43(17) SiO2, 19.18(3) CaO, 10.15(33) Sc2O3, 0.61(26) TiO2, 0.90(7) MnO, 1.50(10) FeO), 0.39(11) SnO2, 0.28(3) Ta2O5, 4.44 BeOcalc., and 1.75 H2Ocalc.; total 99.95. Its empirical formula is X1,X2(Ca3.850Mn0.142Fe2+0.008)Σ4.000Y(Sc1.656Fe2+0.229Mg0.115)Σ2.000Z1,Z2(Al3.851Ti0.086Mg0.063)Σ4.000T5BeΣ2.000T1-4(Si7.949Al0.051)Σ8.000O29.818(OH)2.182Sn(Sn0.029Ta0.014)Σ0.043. The cotype J5 crystal has the formula: X1,X2(Ca3.868Mn0.128Fe2+0.004)Σ4.000Y(Sc1.699Fe2+0.218Mg0.083)Σ2.000Z1,Z2(Al3.845Mg0.116Ti0.039)Σ4.000T5BeΣ2.000T1-T4(Si7.866Al0.134)Σ8.000 O29.644(OH)2.356Sn(Sn0.026Ta0.011)Σ0.037. The simplified formula of dubińskaite is (Ca,Mn)4(Sc,Fe2+,Mg,)2(Sn,Ta)0.04(Al,Ti,Mg)4[Be2(Si,Al)8]O30-a(OH)2+a, where a < 0.3–0.4 apfu, and the ideal formula is Ca4Sc2Al4[Be2Si8O30](OH)2. Dubińskaite formed mainly via the coupled substitution T5Be2+ + YSc3+ → T5B3+ + Y(Fe,Mg,Mn)2+ likely accompanied by Y(Mg,Fe,Mn)2+ + H+ → YSc3+ + □ and T5Be2+ + H+ → T5B3+ + □. The crystal structure was refined to R1 = 0.0773 for 2077 reflections with Fo > 2σ(Fo) in the holotype crystal and to R1 = 0.0393 for 2756 reflections in the cotype crystal. Dubińskaite is triclinic, space group P-1, with unit-cell parameters (holotype): a = 7.2469(5) Å, b = 9.1246(5) Å, c = 9.0102(5) Å, α = 92.350(4)°, β = 98.558(5)°, γ = 77.132(5)°, V = 574.35(6) Å3; Z = 1. The cotype crystal has a = 7.2447(3) Å, b = 9.1177(3) Å, c = 9.0097(3) Å, α = 92.334(3)°, β = 98.492(3)°, γ = 77.099(3)°, V = 573.73(4) Å3.

We suggest that dubińskaite crystallized from metasomatic Ca-rich fluids circulating across the contact between the granitic pegmatite and the surrounding serpentinites, blackwall schists, and rodingite-type calc-silicate rocks under low- to moderate-temperature and pressure conditions. These fluids were poor in F and B but locally enriched in Be, Sc and Na. Beryllium and Na likely derived from alteration of pegmatitic beryl and plagioclase, whereas Sc was probably introduced into the pegmatite through decomposition of clinopyroxene in the mafic-ultramafic host rocks.