IMA 2017-043 = rinkite-(Y)

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Marco E. Ciriotti
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IMA 2017-043 = rinkite-(Y)

Messaggio da Marco E. Ciriotti » lun 02 lug, 2018 10:29

Prossima pubblicazione.

Referenza:
▪ Pautov, L.A., Agakhanov, A.A., Karpenko, V.Yu., Uvarova, Yu.A., Sokolova, E., Hawthorne, F.C. (2018): 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. Mineralogical Magazine, 82, (in press).

Abstract:
microcline, neptunite, pectolite, calcite, eudialite-group minerals, fluorite, titanite, turkestanite, kupletskite, galena, albite and pyrochlore-group minerals. Crystals are transparent and colourless to occasionally white, with a vitreous luster. Rinkite-(Y) has a white streak, uneven, 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. = 3.44(2) g/cm3, Dcalc. = 3.475 g/cm3. It is biaxial (+) with refractive indices (λ = 590 nm) α = 1.662(2), β = 1.666(2), γ = 1.685(5); 2Vmeas.= 50(3) and 2Vcalc. = 49.7°. It is nonpleochroic. Rinkite-(Y) is monoclinic, space group P21/c, a = 7.3934(5), b = 5.6347(4), c = 18.713(1) Å, β = 101.415(2)°, V = 764.2(2) Å3. The six strongest reflections in the X-ray powder diffraction data [d(Å), I, (h k l)] are: 3.057, 100, (0 0 6, -2 1 2, 2 1 0); 2.688, 28, (0 1 6); 9.18, 25, (0 0 2); 2.929, 17, (-2 1 3, 2 1 1); 3.559, 15, (1 0 4, 0 1 4); 2.783, 14, (0 2 1). The empirical formula calculated on 18 (O + F) is Na2.11(Ca3.74Sr0.03Mn0.03)Σ3.80(Y0.50Nd0.16Ce0.16Gd0.07Dy0.06Sm0.05Pr0.03La0.03U4+0.01)Σ1.07(Ti0.85Nb0.17W6+0.01Ta0.01)Σ1.04(Si4.03O14)O1.40F2.60, Z = 2. The ideal formula is Na2Ca4YTi(Si2O7)2OF3. The crystal structure was refined on a twinned crystal to R1 = 4.59% on the basis of 1489 unique reflections (F > 4σF) 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]MO(1) site ideally gives 1 Ti apfu. The [8]MO(2) and [6]MO(3) sites are ideally occupied by Na and (NaCa) apfu. In the H sheet, the [7]MH site is occupied by Ca1.13Y0.50REE0.37, ideally (CaY), <MH–> = 2.415 Å and the [7]AP site is occupied by Ca1.81REE0.19, ideally Ca2, <AP–> = 2.458 Å. The MH + AP sites ideally give (Ca3Y) apfu. The MH and AP polyhedra and Si2O7 groups constitute the H sheet. Linkage of H and O sheets via common vertices of MH and AP polyhedra and Si2O7 groups with MO(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 AP2MH2MO4(Si2O7)2(XOM)2(XOA)2 is (Ca3Y)Na(NaCa)Ti(Si2O7)2(OF)F2, Z = 2. The mineral is named rinkite-(Y) as it is structurally identical to rinkite-(Ce) and Y is the dominant rare-earth element.
Marco E. Ciriotti

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