Shakhdaraite-(Y), ScYNb2O8, from the Leskhozovskaya granitic pegmatite, the valley of the Shakhdara River, southwestern Pamir, Gorno-Badakhshanskii Autonomous Region, Tajikistan: New mineral description and crystal structure

Abstract

ABSTRACT Shakhdaraite-(Y), ideally ScYNb2O8, is a new mineral from the Leskhozovskaya miarolitic granitic pegmatite at the Shakhdara River, southwestern Pamir (Tajikistan). Shakhdaraite-(Y) occurs mainly as grains from 10 to 150 μm in size in a near-miarolitic pegmatite complex in association with quartz, albite, pyrochlore-microlite, fersmite, and an unnamed Sc-Nb oxide; only one large, single, well-shaped crystal 200 μm long was found in a small cavity with quartz, albite, bertrandite, pyrochlore, and jarosite. Shakhdaraite-(Y) is black to dark-brown, streak is brown. Luster is vitreous semi-metallic. It is brittle with conchoidal fracture. Mohs hardness is 5. VHN100 = 436 kg/mm2. Dcalc. = 5.602 g/cm3. In reflected light, it is light gray and its reflective capacity is moderate to low. Anisotropy is distinct, without color effects. Pleochroism was not observed. Internal reflections are red-brown. Reflectance values were measured in air with SiC as reference material [λ(nm), Rmax, Rmin]: 470, 14.6, 13.9; 546, 14.0, 13.4; 589, 13.9, 13.3; 650, 13.8, 13.1. Electron probe microanalysis (WDS mode, 7 points) gives (wt.%): Nb2O5 50.70; Ta2O5 4.52; TiO2 0.08; WO3 0.79; SnO2 1.54; CaO 1.01; Sc2O3 11.35; MnO 1.38; FeO 0.01; Y2O3 12.00; Ce2O3 0.21; Pr2O3 0.04; Nd2O3 0.27; Sm2O3 0.32; Eu2O3 0.07; Gd2O3 0.86; Tb2O3 0.22; Dy2O3 2.07; Ho2O3 0.29; Er2O3 1.33; Tm2O3 0.35; Yb2O3 2.80; Lu2O3 0.32; PbO 0.24; ThO2 1.90; UO2 3.30, total 97.97. The empirical formula of shakhdaraite-(Y) based on O = 8 apfu (atoms per formula unit) is (Nb1.91Sc0.83Y0.53Ta0.10Mn0.10Ca0.09 Yb0.07U4+0.06Dy0.06Sn0.05Th0.04Er0.03Gd0.02W6+0.02Pb0.01Ce0.01Nd0.01Sm0.01Tb0.01Ho0.01Tm0.01Lu0.01Ti0.01)Σ4.00O8, Z = 2. The simplified formula is Sc(Y,Yb)Nb2O8, where Yb is the dominant lanthanoid. Shakhdaraite-(Y) is monoclinic, space group P2/c, a 9.930(2), b 5.6625(11), c 5.2108(10) Å, β 92.38(3)°, V 292.7(5) Å3, Z = 2. The crystal structure was solved by direct methods [R1 = 0.0269, 878 unique reflections (F > 4σF)]. There are three cation M sites: [6]M(1) = Nb2apfu, [6]M(2) = Sc apfu, and [8]M(3) = Y apfu, ideally M = ScYNb2apfu. The M(1) and M(2) octahedra each form a brookite chain along c. The Y-dominant [8]M(3A) polyhedra form a brookite-like kinked chain, and each M(3A) polyhedron of one brookite-like chain shares two edges with the two M(3A) polyhedra from the adjacent brookite-like chain, thus forming a [Y2O8]10– layer. In the structure of shakhdaraite-(Y), M(1A) and M(2) brookite chains and a layer of [8]-coordinated M(3A) polyhedra alternate along a. Shakhdaraite-(Y) is isostructural with samarskite-(Y), ideally YFe3+Nb2O8. Shakhdaraite-(Y) [Russian Cyrillic: шахдараит-(Y)] is named after its type locality: the valley of the Shakhdara River in the southwest of the Pamir Mountains.