Bortolanite, Ca2(Ca1.5Zr0.5)Na(NaCa)Ti(Si2O7)2(FO)F2, a New Rinkite-Group (Seidozerite Supergroup) TS-Block Mineral from the Bortolan Quarry, Poços de Caldas Massif, Minas Gerais, Brazil

Abstract

Bortolanite (IMA 2021–040a), ideally Ca2(Ca1.5Zr0.5)Na(NaCa)Ti(Si2O7)2(FO)F2, is a rinkite-group (seidozerite supergroup) TS-block mineral from Poços de Caldas massif, Minas Gerais, Brazil. Associated minerals are götzenite, nepheline, alkali feldspar, aegirine, natrolite, analcime, and manganoan pectolite. Bortolanite shows complex compositional zoning with götzenite and is visually indistinguishable from götzenite. Bortolanite is pale-yellow to brown and has a vitreous luster. Cleavage is perfect parallel to {100}. Mohs hardness is 5. Bortolanite fluoresces weak yellow under ultraviolet light (100–280 nm). Dcalc. = 3.195 g/cm3. Bortolanite is biaxial (+) with refractive indices (λ = 589.3 nm) α = 1.673(2), β = 1.677(2), γ = 1.690(2); 2Vmeas. = 56(2)°, 2Vcalc. = 58.4°. Chemical analysis by electron microprobe gave Nb2O5 1.07, HfO2 0.20, ZrO2 6.70, TiO2 9.94, SiO2 32.49, Gd2O3 0.12, Nd2O3 0.37, Ce2O3 1.25, La2O3 0.65, Y2O3 0.31, FeO 0.59, MnO 1.46, CaO 31.15, Na2O 8.36, F 6.95, O=F –2.93, sum 98.68 wt.%. The empirical formula based on 18 (O + F) apfu is (Ca1.88La0.03Ce0.06Nd0.02Gd0.01)Σ2[Ca1.56(Zr0.41Hf0.01Y0.02)Σ0.44]Σ2(Na0.85Ca0.15)Σ1(Na1.18Ca0.60Mn0.16Fe2+0.06)Σ2(Ti0.94Nb0.06)Σ1(Si4.07O14)(O1.24F0.76)Σ2F2, Z = 1. The simplified formula is Ca2(Ca,Zr)2Na(Na,Ca)2Ti(Si2O7)2(O,F)2F2. Bortolanite is triclinic, space group P, a 9.615(3), b 5.725(2), c 7.316(2) Å, α 89.91(1), β 101.14(1), γ 100.91(1)°, V 387.7(3) Å3. The crystal structure was refined to R1 = 3.19% on the basis of 2194 unique reflections [F > 4σ(F)] measured using a Bruker APEX II ULTRA three-circle diffractometer with a rotating-anode generator (MoKα), multilayer optics, and an APEX II 4K CCD detector. The crystal structure of bortolanite is a framework of TS (Titanium-Silicate) blocks [structure type B3(RG)], where the TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The TS block exhibits linkage and stereochemistry typical for the rinkite group where Ti (+ Nb + Zr) = 1 apfu. The O sheet is composed of Ti-dominant MO(1) octahedra, [8]Na-dominant MO(2) polyhedra and (Na,Ca) MO(3) octahedra. In the H sheet in bortolanite, Si2O7 groups link to (Ca1.5Zr0.5) MH and Ca-dominant AP octahedra. Along a, TS blocks link directly through 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. The name bortolanite is after the locality: the Bortolan quarry in the Poços de Caldas massif, Brazil. Bortolanite is isostructural with three rinkite-group minerals: fogoite-(Y), hainite-(Y), and götzenite.