Fogoite-(Y), Na3Ca2Y2Ti(Si2O7)2OF3, a Group-I TS-block mineral from the Lagoa do Fogo, the Fogo volcano, the São Miguel Island, the Azores: Description and crystal structure

Fogoite-(Y), Na3Ca2Y2Ti(Si2O7)2OF3, is a new mineral from the Lagoa do Fogo, the São Miguel Island, the Azores. It occurs in cavities as highly elongated (on [001]) prisms, up to 2000 μm long and 50 x 50 μm in cross-section, associated to sanidine, astrophyllite, fluornatropyrochlore, ferrokentbrooksite, quartz and ferro-katophorite. Crystals are generally transparent and colourless, with vitreous luster, occasionally creamy white. Fogoite-(Y) has a white streak, splintery fracture and very good {100} cleavage. Mohs hardness is ~5. Dcalc. = 3.523 g/cm3. It is biaxial (+) with refractive indices (λ = 590 nm) α = 1.686(2), β = 1.690(2), γ = 1.702(5); 2Vmeas.= 57(1)° and 2Vcalc. = 60°. It is nonpleochroic. Fogoite-(Y) is triclinic, space group P1 , a = 9.575(6), b = 5.685(4), c = 7.279(5) Å, α = 89.985(6), β = 100.933(4), γ = 101.300(5)°, V = 381.2(7) Å3. The six strongest reflections in the X-ray powder diffraction data [d (Å), I, (h k l)] are: 2.954, 100, (⎯1⎯1 2,⎯3 1 0); 3.069, 42, (3 0 0, 0⎯1 2); 2.486, 24, (3 1 0, 2⎯1 2); 3.960, 23, (⎯1⎯1 1,⎯2 1 0); 2.626, 21, (⎯2 2 0); 1.820, 20, (⎯1 0 4). Electron microprobe analysis gave the following empirical formula calculated on 18 (O + F)(Na2.74Mn0.15)Σ2.89Ca2[Y1.21(La0.01Ce0.03.03Sm0.02Gd0.08Dy0.08Er0.05Yb0.04Lu0.01)Σ0.35 Mn0.16Zr0.11Na0.09Fe2+0.07Ca0.01]Σ2(Ti0.76Nb 0.23Ta0.01)Σ1(Si4.03O14)O1.12F2.88, Z = 1. The crystal structure was refined on a twinned crystal to R1 = 2.81% on the basis of 2157 unique reflections (Fo > 4σFo) and is a framework of TS (Titanium Silicate) blocks, which consists of HOH sheets (H-heteropolyhedral, O-octahedral) parallel to (100). In the O sheet, the the [6]MO(1) site is occupied mainly by Ti, = 1.980 Å and the [6]MO(2) and [6]MO(3) sites are occupied by Na and Na plus minor Mn, = 2.490 Å and = 2.378 Å. In the H sheet, the two [4]Si sites are occupied by Si, with = 1.623 Å; the [6]MH site is occupied by Y and REE (Y > REE), with minor Mn, Zr, Na, Fe2+ and Ca, = 2.271 Å and the [6]AP site is occupied by Ca, = 2.416 Å. The MH and AP octahedra and Si2O7 groups constitute the H sheet. The ideal compositions of the O and two H sheets are Na3Ti(OF)F2 and Y2Ca2(Si2O7)2 a.p.f.u. Fogoite-(Y) is isostructural with götzenite. The mineral is named after the type locality, the Fogo volcano in the Azores.