Nacareniobsite-(Nd), Ca₂(CaNd)Na₃Nb(Si₂O₇)₂(OF)F₂, a New Rinkite-Group (Seidozerite-Supergroup) Mineral from the Darai-Pioz Alkaline Massif, Tajikistan: Description and Crystal Structure

Nacareniobsite-(Nd), Ca₂(CaNd)Na₃Nb(Si₂O₇)₂(OF)F₂, is a new rinkite-group (seidozerite-supergroup) TS-block mineral from the Darai-Pioz alkaline massif, Tien-Shan mountains, Tajikistan. It occurs as prismatic isolated grains up to 0.3 mm long and 0.03 mm thick in a quartz-pectolite aggregate in a silexite-like peralkaline pegmatite. Associated minerals are reedmergnerite, leucosphenite, baratovite, aegirine, orlovite, sokolovaite, mendeleevite-(Ce), odigitriaite, pekovite, zeravshanite, kirchhoffite, fluorite, and garmite. Crystals are colorless and transparent with a vitreous luster. Nacareniobsite-(Nd) has a white streak and uneven fracture and does not fluoresce under cathode or ultraviolet light. Cleavage is very good on {100}, no parting was observed, Mohs hardness is 5, and it is brittle, D_meas. = 3.58(2), D_calc. = 3.608 g/cm³. It is optically biaxial (+) with refractive indices α = 1.664(2), β = 1.670(2), γ = 1.692(2); 2V_calc. = 56°. Nacareniobsite-(Nd) is monoclinic, space group P2₁/c, a = 7.4195(15), b = 5.6567(11), c = 18.788(4) Å, β = 101.38(3)°, V = 773.0(3) ų. The six strongest reflections in the X-ray powder-diffraction data [d(Å), I, (h k l)] are: 3.059, 100, (0 0 6, −2 1 2, 2 1 0); 2.928, 36, (2 1 1, −2 1 3); 2.688, 27, (1 1 5, 0 1 6); 1.847, 23, (−4 0 2, 2 1 7, −2 1 9); 2.782, 21, (0 2 1); 1.674, 19, (−2 3 1). The empirical formula calculated on 18 (O + F) is Na_2.98Ca_2.79Sr_0.27Y_0.17(Nd_0.32Ce_0.26Sm_0.07Pr_0.05La_0.04Gd_0.04Tb_0.01Dy_0.01Tm_0.01Lu_0.01Ho_0.01Eu_0.01Er_0.01Yb_0.01)_Σ0.86(Nb_0.87Ti_0.10)_Σ0.97(Si_4.01O₁₄)O_1.00 F_3.00,Z = 2. The crystal structure was refined to R₁ = 3.85% on the basis of 2409 unique reflections (F_O > 4σ|F|) and is a framework of TS (Titanium-Silicate) blocks. The TS block consists of HOH sheets (H – heteropolyhedral, O – octahedral) parallel to (100). In the O sheet, the ^[6]M^O(1), ^[8]M^O(2), and ^[6]M^O(3) sites ideally give Nb, Na, and Na₂apfu, cations of the O sheet sum to Na₃Nb apfu. In the H sheet, the ^[7]M^H and ^[7]A^P sites are occupied by Ca_1.30(Ln_0.53Y_0.17)_Σ0.70 and Ca_1.16Sr_0.27Na_0.24Ln_0.33apfu, ideally (CaNd) and Ca₂apfu. The A^P + M^H sites ideally give Ca₂(CaNd) apfu. The M^H and A^P polyhedra and Si₂O₇ groups constitute the H sheet. Linkage of H and O sheets via common vertices of M^O(1–3), M^H and A^P polyhedra, and Si₂O₇ groups results in a TS block. In the structure, TS blocks link via common edges of M^H and A^P polyhedra and common vertices of M^H, A^P, and Si polyhedra of the H sheets belonging to two TS blocks. For nacareniobsite-(Nd), the ideal structural formula of the form A^P₂M^H₂M^O₄(Si₂O₇)₂(X^O_M)₂(X^O_A)₂ is Ca₂(CaNd)Na₃Nb(Si₂O₇)₂(OF)F₂. The mineral is structurally identical to nacareniobsite-(Y), Ca₂(CaY)Na₃Nb(Si₂O₇)₂(OF)F₂, and nacareniobsite-(Ce), Ca₂(CaCe)Na₃Nb(Si₂O₇)₂(OF)F₂.