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

Abstract 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, Dmeas. = 3.58(2), Dcalc. = 3.608 g/cm3. It is optically biaxial (+) with refractive indices α = 1.664(2), β = 1.670(2), γ = 1.692(2); 2Vcalc. = 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) Å3. 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 Na2.98Ca2.79Sr0.27Y0.17(Nd0.32Ce0.26Sm0.07Pr0.05La0.04Gd0.04Tb0.01Dy0.01Tm0.01Lu0.01Ho0.01Eu0.01Er0.01Yb0.01)Σ0.86(Nb0.87Ti0.10)Σ0.97(Si4.01O14)O1.00 F3.00,Z = 2. The crystal structure was refined to R1 = 3.85% on the basis of 2409 unique reflections (FO > 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]MO(1), [8]MO(2), and [6]MO(3) sites ideally give Nb, Na, and Na2apfu, cations of the O sheet sum to Na3Nb apfu. In the H sheet, the [7]MH and [7]AP sites are occupied by Ca1.30(Ln0.53Y0.17)Σ0.70 and Ca1.16Sr0.27Na0.24Ln0.33apfu, ideally (CaNd) and Ca2apfu. The AP + MH sites ideally give Ca2(CaNd) apfu. The MH and AP polyhedra and Si2O7 groups constitute the H sheet. Linkage of H and O sheets via common vertices of MO(1–3), MH and AP polyhedra, and Si2O7 groups results in a TS block. In the structure, TS blocks link via common edges of MH and AP polyhedra and common vertices of MH, AP, and Si polyhedra of the H sheets belonging to two TS blocks. For nacareniobsite-(Nd), the ideal structural formula of the form AP2MH2MO4(Si2O7)2(XOM)2(XOA)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.