Rinkite-(Y), Na₂Ca₄YTi(Si₂O₇)₂OF₃, a seidozerite-supergroup TS-block mineral from the Darai-Pioz alkaline massif, Tien-Shan mountains, Tajikistan: Description and crystal structure

Abstract Rinkite-(Y), ideally Na 2 Ca 4 YTi(Si 2 O 7 ) 2 OF 3 , is a new rinkite-group (seidozerite-supergroup) TS-block mineral from the Darai-Pioz alkaline massif, Tian-Shan mountains, Tajikistan. The mineral is of hydrothermal origin. It occurs as aggregates (up to 1.5 cm long) of acicular crystals 0.1–1.0 mm thick, and as separate elongated columnar, flattened-prismatic crystals up to 1 cm long with rectangular or rhombic sections up to 0.5 mm across. Associated minerals are quartz, aegirine, microcline, neptunite, pectolite, calcite, eudialyte-group minerals, fluorite, titanite, turkestanite, kupletskite, galena, albite and pyrochlore-group minerals. Crystals are transparent and colourless to occasionally white, with a vitreous lustre. Rinkite-(Y) has a white streak, uneven, conchoidal fracture and does not fluoresce under a cathode or ultraviolet light. Cleavage is very good on {100}, no parting was observed, Mohs hardness is ~5, and it is brittle, D meas. = 3.44(2) g/cm 3 , D calc. = 3.475 g/cm 3 . It is biaxial (+) with refractive indices (λ = 590 nm) α = 1.662(2), β = 1.666(2), γ = 1.685(5); 2V meas. = 50(3) and 2V calc. = 49.7°. It is nonpleochroic. Rinkite-(Y) is monoclinic, space group P 2 1 / c , a = 7.3934(5), b = 5.6347(4), c = 18.713(1) Å, β = 101.415(2)° and V = 764.2(2) Å 3 . The six strongest reflections in the X-ray powder diffraction data [ d (Å), I , ( hkl )] are: 3.057, 100, (006, $\bar{2}$ 12, 210); 2.688, 28, (016); 9.18, 24, (002); 2.929, 17, ( $\bar{2}$ 13, 211); 3.559, 15, (104, 014) and 2.783, 14, (021). The empirical formula calculated on 18 (O + F) is Na 2.11 (Ca 3.74 Sr 0.03 Mn 0.03 ) Σ3.80 (Y 0.50 Nd 0.16 Ce 0.16 Gd 0.07 Dy 0.06 Sm 0.05 Pr 0.03 La 0.03 ${\rm U}_{0.01}^{{\rm 4 + }} {\rm )}_{\Sigma 1.07}{\rm (T}{\rm i}_{0.85}{\rm N}{\rm b}_{0.17}{\rm W}^{6+}_{0.01}{\rm T}{\rm a}_{0.01}{\rm )}_{\Sigma 1.04}\left( {{\rm S}{\rm i}_{4.03}{\rm O}_{14}} \right){\rm O}_{1.40}{\rm F}_{2.60}$ with Z = 2. The ideal formula is Na 2 Ca 4 YTi(Si 2 O 7 ) 2 OF 3 . The crystal structure was refined on a twinned crystal to R 1 = 4.59% on the basis of 1489 unique reflections ( F > 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 Ti-dominant [6] M O 1 site ideally gives 1 Ti apfu. The [8] M O 2 and [6] M O 3 sites are ideally occupied by Na and (NaCa) apfu. In the H sheet, the [7] M H site is occupied by Ca 1.13 Y 0.50 REE 0.37 , ( REE = rare-earth element), ideally (CaY), <M H –φ> = 2.415 Å and the [7] A P site is occupied by Ca 1.81 REE 0.19 , ideally Ca 2 , <A P –φ> = 2.458 Å. The M H + A P sites ideally give (Ca 3 Y) apfu. The M H and A P polyhedra and Si 2 O 7 groups constitute the H sheet. Linkage of H and O sheets via common vertices of M H and A P polyhedra and Si 2 O 7 groups with M O 1–3 polyhedra results in a TS block. The TS block in rinkite-(Y) exhibits linkage 1 and stereochemistry typical for the rinkite group (Ti = 1 apfu) of the seidozerite supergroup. For rinkite-(Y), the ideal structural formula of the form A P 2 M H 2 M O 4 (Si 2 O 7 ) 2 $ \left( {{\rm X}_{\rm M}^{\rm O} } \right)_2\left( {{\rm X}_{\rm A}^{\rm O} } \right)_2{\rm is }\;\left( {{\rm C}{\rm a}_3{\rm Y}} \right){\rm Na}\left( {{\rm NaCa}} \right){\rm Ti}\left( {{\rm S}{\rm i}_2{\rm O}_7} \right)_2\left( {{\rm OF}} \right){\rm F}_2 $ with Z = 2. The mineral is named rinkite-(Y) as it is structurally identical to rinkite-(Ce) and Y is the dominant rare-earth element.

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