The dumortierite supergroup. II. Three new minerals from the Szklary pegmatite, SW Poland: Nioboholtite,(Nb_(0.6)□_(0.4))Al_6BSi_30_(18), titanoholtite,(Ti_(0.75)□_(0.25))Al_6BSi_3O_(18), and szklaryite, □Al_6BAs_3~(3+)O_(15)

摘要

Three new minerals in the dumortierite supergroup were discovered in the Szklary pegmatite, Lower Silesia, Poland. Nioboholtite, endmember (Nb_(0.6)□_(0.4))Al_6B_3Si_3O_(18), and titanoholtite, endmember (Ti_(0.75)□_(0.25))Al_6B_3Si_3O_(18), are new members of the holtite group, whereas szklaryite, endmember □Al_6BAs_3~(3+)O_(15), 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 ~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 be obtained only from 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.%: P_2O_5 0.26, 0.01, 0.68; Nb_2O_5 5.21, 0.67, 0.17; Ta_2O_5 0.66, 1.18, 0.00; SiO_2 18.68, 21.92, 12.78; TiO_2 0.11, 4.00, 0.30; B_2O_3 4.91, 4.64, 5.44; A1_2O_3 49.74, 50.02, 50.74; As_2O_3 5.92, 2.26, 16.02; Sb_2O_3 10.81, 11.48, 10.31; FeO 0.51, 0.13, 0.19; H_2O (calc.) 0.05, -, -, Sum 96.86, 96.34, 97.07, corresponding on the basis of O = 18-As-Sb to {(Nb_(0.26)Ta_(0.02)□_(0.18))(Al_(0.27)Fe_(0.05)Ti_(0.01))□_(0.21)}_(Σ1.00)Al_6B_(0.92){Si_(2.03)P_(0.02)(Sb_(0.48)As_(0.39)Al_(0.07)}_(Σ3.00)(O_(17.09)OH_(0.04)□_(0.87))_(Σ18.00), {(Ti_(0.32)Nb_(0.03)Ta_(0.03)□_(0.10))(All_(0.35)Ti_(0.01)Fe_(0.01))□_(0.15)}_(Σ1.00)Al_6B_(0.86){Si_(2.36)(Sb_(0.51)As_(0.14))}_(Σ3.01)O_(17.35)□_(0.65)_(Σ18.00) and {□_(0.53)(Al_(0.41)Ti_(0.02)Fe_(0.02))(Nb_(0.01)□_(0.01))}_(Σ1.00)Al_6B_(0.01){(As_(1.07)Sb_(0.47)Al_(0.03)) Si_(1.37)P_(0.06)}z_(3.00)(O_(16.46)□_(1.54))_(Σ18.00)- Electron backscattered diffraction indicates that the three minerals are presumably isostructural with dumortierite, that is, orthorhombic symmetry, space group Pnma (no. 62), and unit-cell parameters close to a = 4.7001, b = 11.828, c = 20.243 A, with V= 1125.36 A~3 and Z = 4; micro-Raman spectroscopy provided further confirmation of the structural relationship for nioboholtite and 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 [d, A, 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, 100, 100, 95 (123); 3.0675, 53, 53, 50 (105); 2.9305, 65, 59, 51 (026); 2.8945, 64, 65, 59 (132), respectively. 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.