Two Rinkite-Group (Seidozerite-Supergroup) Minerals: Nacareniobsite-(Y), a New Mineral from the Darai-Pioz Alkaline Massif, Tajikistan, and Crystal-Structure Refinement of Nacareniobsite-(Ce)

Abstract

Abstract Nacareniobsite-(Y), ideally Na3Ca3YNb(Si2O7)2OF3, is a new rinkite-group (seidozerite-supergroup) TS-block mineral from the Darai-Pioz alkaline massif, Tien-Shan mountains, Tajikistan. The mineral is of hydrothermal origin. It occurs as prismatic crystals up to 1 mm long and 0.1 mm thick embedded in an aggregate of coarse-grained reedmergnerite. Associated minerals are reedmergnerite, leucophanite, nordite-(Ce), microcline, zeravshanite, polylithionite, kentbrooksite, yusupovite, fluornatropyrochlore, and quartz. Crystals are pale yellow, transparent, with a vitreous to translucent luster. Nacareniobsite-(Y) has a white streak, uneven to conchoidal fracture, and does not fluoresce under cathode or ultraviolet light. Cleavage is {100} very good, no parting was observed, Mohs hardness is 5, and it is brittle, Dmeas. = 3.49(2) g/cm3, Dcalc. = 3.515 g/cm3. It is biaxial (+) with refractive indices (λ = 590 nm) α = 1.662(2), β = 1.668(2), γ = 1.690(2); 2Vcalc. = 56°. It is nonpleochroic. Nacareniobsite-(Y) is monoclinic, space group P21/c, a = 7.4069(15), b = 5.6540(11), c = 18.787(4) Å, β = 101.36(3)°, V = 771.3(3) Å3. The six strongest reflections in the X-ray powder diffraction data [d(Å), I, (h k l)] are: 3.068, 100, (0 0 6, 1 2, 2 1 0); 2.944, 45, (2 1 1, 1 3); 2.707, 32, (0 2 2, 0 1 6); 5.44, 31, (0 1 1); 1.853, 29, ( 0 2, 2 1 7, 1 9); 3.59, 26, (1 0 4, 0 1 4). The empirical formula calculated on the basis of 18 (O + F) is Na2.82Ca3.06Sr0.14[Y0.37(Nd0.16Ce0.15Dy0.08Gd0.06Sm0.05La0.04Tb0.02Ho0.02Lu0.02Pr0.01Eu0.01Tm0.01Er0.01Yb0.01)Σ0.65]Σ1.02(Nb0.63Ti0.38)Σ1.01(Si4.00O14)O1.00F3.00, Z = 2. The ideal formula is Na3Ca3YNb(Si2O7)2OF3. The crystal structure was refined on a twinned crystal to R1 = 3.50% on the basis of 1788 unique reflections (Fo > 4σFo). It is a framework of TS (Titanium-Silicate) blocks where the TS block consists of HOH sheets (H = heteropolyhedral, O = octahedral) parallel to (100). In the O sheet, the Nb-dominant [6]MO(1) site is ideally occupied by one Nb apfu. The [8]MO(2) and [6]MO(3) sites are ideally occupied by one Na and two Na apfu, respectively. The H sheet contains two unique sites: the [7]MH site, ideally (CaY), is occupied by Ca1.23(Y0.37Ln0.40)Σ0.77, with <MH–φ> = 2.424 Å, and the [7]AP site, ideally Ca2, is occupied by Ca1.61Sr0.14Ln0.25, with <AP–φ> = 2.469 Å. The AP+ MH sites ideally give Ca2(CaY) apfu. The MH and AP polyhedra and Si2O7 groups constitute the H sheet. Linkages of the H and O sheets via common vertices of the MH and AP polyhedra, and Si2O7 groups with MO(1–3) polyhedra, results in the TS block. The TS block in nacareniobsite-(Y) exhibits linkage 1 and has a stereochemistry typical for the rinkite group (Ti + Nb + Zr = 1 apfu) of the seidozerite supergroup. For nacareniobsite-(Y), the ideal structural formula of the form AP2MH2MO4(Si2O7)2(XOM)2(XOA)2 is Ca2(CaY)Na3Nb(Si2O7)2(OF)F2. The mineral is named nacareniobsite-(Y), as it is structurally identical to nacareniobsite-(Ce), ideally Na3Ca3CeNb(Si2O7)2OF3, with Y as the dominant REE. The crystal structure of nacareniobsite-(Ce), has been refined to R1 = 6.80% for 1421 unique (Fo > 4σFo) reflections: space group P21/c, a = 7.4684(15), b = 5.6891(11), c = 18.891(4) Å, β = 101.37(3)°, V = 786.9(3) Å3, Z = 2, Dcalc. = 3.539 g/cm3. The composition of the MH and AP sites in the H sheet has been reassigned as follows: the [7]MH site is occupied by Ca1.28(Ln0.68Y0.04)Σ0.72 where Ce is the dominant lanthanoid, ideally (CaCe) apfu, <MH–φ> = 2.458 Å, and the [7]AP site is occupied by (Ca1.44Na0.09Sr0.04)Σ1.57Ln0.43, ideally Ca2apfu, <AP–φ> = 2.48 Å. The AP+ MH sites ideally give Ca2(CaCe) apfu [cf. (Ca,REE)2(Ca,REE)2apfu, Sokolova & Hawthorne (2008)]. For nacareniobsite-(Ce), the revised ideal structural formula of the form AP2MH2MO4(Si2O7)2(XOM)2(XOA)2 is Ca2(CaCe)Na3Nb(Si2O7)2(OF)F2.