The dumortierite supergroup. II. Three new minerals from the Szklary pegmatite, SW Poland: Nioboholtite, (Nb_0.6□_0.4Al₆BSi₃O₁₈, titanoholtite, (Ti_0.75 □_0.25Al₆BSi₃O₁₈, and szklaryite, □Al₆BAs³⁺₃O₁₅

ABSTRACT

            Three new minerals in the dumortierite supergroup were discovered in the Szklary pegmatite, Lower Silesia, Poland. Nioboholtite, endmember (Nb_0.6□_0.4Al₆BSi₃O₁₈, and titanoholtite, end-member Ti_0.75 □_0.25Al₆BSi₃O₁₈, are new members of the holtite group, whereas szklaryite, end-member □Al₆BAs³⁺₃O₁₅, is the first representative of a potential new group. Nioboholtite occurs mostly as overgrowths not exceeding 10 μm in thickness on cores of holtite. Titanoholtite forms patches up to 10 μm across in the holtite cores and streaks up to 5 μm wide along boundaries between holtite cores and the nioboholtite rims. Szklaryite is found as a patch about 2 μm in size in As- and Sb- bearing dumortierite enclosed in quartz. Titanoholtite crystallized almost simultaneously with holtite and other Ta-dominant minerals such as tantalite-(Mn) and stibiotantalite and before nioboholtite, which crystallized simultaneously with stibiocolumbite during decreasing Ta activity in the pegmatite melt. Szklaryite crystallized after nioboholtite during the final stage of the Szklary pegmatite formation. Optical properties could only be obtained on nioboholtite, which is creamy-white to brownish yellow or grey-yellow in hand specimen, translucent, with a white streak, biaxial (−), n_α = 1.740-1.747, n_β ~ 1.76, n_γ ~ 1.76, and Δ < 0.020. Electron microprobe analyses of nioboholtite, titanoholtite and szklaryite give respectively in wt%: P₂O₅ 0.26, 0.01, 0.68; Nb₂O₅ 5.21, 0.67, 0.17; Ta₂O₅ 0.66, 1.18, 0.00; SiO₂ 18.68, 21.92, 12.78; TiO₂ 0.11, 4.00, 0.30; B₂O₃ (calc) 5.37, 5.45, 5.35; B₂O₃ (calc) 5.37, 5.45, 5.35; Al₂O₃ 49.74, 50.02, 50.74; As₂O₃ 5.92, 2.26, 16.02; Sb₂O₃ 10.81, 11.48, 10.31; FeO 0.51, 0.31, 0.19; Sum 97.27, 97.11, 96.54, corresponding on the basis of O = 18-As-Sb to  (o_0.44Nb_0.25Al_0.23Fe_0.05Ta_0.02Ti_0.01)_Σ1.00Al₆B(Si_2.02Sb_0.48As_0.39Al_0.09P_0.02)_Σ3.00(O_17.13o_0.87)_Σ18.00, (o_0.36Ti_0.32Al_0.25Nb_0.03Ta_0.03Fe_0.01)_Σ1.00Al₆B(Si_2.33Sb_0.50As_0.15Al_0.02)_Σ3.00(O_17.35o_0.65)_Σ18.00 and (o_0.52Al_0.43Ti_0.02Fe_0.02Nb_0.01)_Σ1.00 Al₆B(Si_1.38As_1.05Sb_0.46P_0.06Al_0.04)_Σ3.00(O_16.49o_1.51)_Σ18.00. Electron backscattered diffraction indicate that the three minerals are isostructural with dumortierite, that is, orthorhombic symmetry, space group Pnma (no. 62), and unit-cell parameters close to a = 4.7001 Å, b = 11.828 Å, c = 20.243 Å, with V = 1125.36 ų and Z = 4; micro-Raman spectroscopy provided further confirmation for titanoholtite. The calculated density is 3.72 g/cm³ for nioboholtite, 3.66 g/cm³ for titanoholtite and 3.71 g/cm³ for szklaryite. The strongest lines in X-ray powder diffraction patterns calculated from the cell parameters of dumortierite of Moore and Araki (1978) and the empirical formulae of nioboholtite, titanoholtite and szklaryite are, respectively [d, Å, I (hkl)]: 10.2125, 67, 46, 19 (011); 5.9140, 40, 47, 57 (020); 5.8610, 66, 78, 100 (013); 3.4582, 63, 63, 60 (122); 3.4439, 36, 36, 34 (104); 3.2305, 1000, 100, 95 (123); 3.0675, 53, 53, 50 (105); 2.9305, 65, 59, 51 (026); 2.8945, 64, 65, 59 (132). The three minerals have been approved by the IMA CNMNC (IMA 2012-068, 069, 070) and were named for their relationship to holtite and occurrence in the Szklary pegmatite, respectively.