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

Shalchdaraite-(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 mu m 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 mu m 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/mm(2). D-calc. = 5.602 g/cm(3). 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 [lambda(nm), R-max, R-min]: 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 Yb0.07U0.064+Dy0.06Sn0.05Th0.04Er0.03Gd0.02W0.026+Pb0.01Ce0.01Nd0.01Sm0.01Tb0.01Ho0.01Tm0.01Lu0.01Ti0.01)(Sigma 4.)(00)O8, Z = 2. The simplified formula is Se(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) angstrom, beta 92.38(3)degrees, V292.7(5) angstrom(3) , Z=2. The crystal structure was solved by direct methods [R-1 =0.0269, 878 unique reflections (F > 4 sigma F)]. There are three cation M sites: ([6]) M(1) = Nb-2 apfu, ([6]) M(2)=Sc apfu, and ([8]) M(3)= Y apfu, ideally M=ScYNb2 apfu. 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](1)(0-) 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: -(Y)] is named after its type locality: the valley of the Shakhdara River in the southwest of the Pamir Mountains.