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

Abstract

AbstractFogoite-(Y), Na3Ca2Y2Ti(Si2O7)2OF3, is a new mineral from the Lagoa do Fogo, São Miguel Island, the Azores. It occurs in cavities as highly elongated (on [001]) prisms, up to 2000 μm long and 50 μm× 50 μm in cross-section, associated with sanidine, astrophyllite, fluornatropyrochlore, ferrokentbrooksite, quartz and ferro-katophorite. Crystals are generally transparent and colourless, with vitreous lustre, 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 groupP1,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 powder X-ray diffraction data [d(Å), I, (hkl)] are: 2.954, 100, (1̄1̄2, 3̄10); 3.069, 42, (300, 01̄2); 2.486, 24, (310, 21̄2); 3.960, 23, (1̄1̄1, 2̄10); 2.626, 21, (2̄20); 1.820, 20, (1̄04). Electron microprobe analysis gave the following empirical formula calculated on 18 (O + F) (Na2.74Mn0.15)∑2.89Ca2[Y1.21(La0.01Ce0.03Nd0.03Sm0.02Gd0.08Dy0.08Er0.05Yb0.04Lu0.01)∑0.35Mn0.16Zr0.11Na0.09Fe0.072+Ca0.01]∑2(Ti0.76Nb0.23Ta0.01)∑1(Si4.03O14)O1.12F2.88,Z= 1. The crystal structure was refined on a twinnedcrystal toR1= 2.81% on the basis of 2157 unique reflections (Fo> 4σFo) and is a framework of TS (Titanium Silicate) blocks, which consist 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, <MO(1)–ϕ> = 1.980 Å, and the[6]MO(2) and[6]MO(3) sites are occupied by Na and Na plus minor Mn, <MO(2)–ϕ>= 2.490 Å and <MO(3)–ϕ> = 2.378 Å. In the H sheet, the two[4]Si sites are occupied by Si, with <Si–O> = 1.623 Å; the[6]MHsite is occupied by Y and rare-earth elements (Y >REE), with minor Mn, Zr, Na, Fe2+and Ca, <MH–ϕ> = 2.271 Å and the[6]APsite is occupied by Ca, <AP–ϕ> = 2.416 Å. The MHand APoctahedra and Si2O7groups constitute the H sheet. The ideal compositions of the O and two H sheets are Na3Ti(OF)F2and Y2Ca2(Si2O7)2apfu. Fogoite-(Y) is isostructural with götzenite and hainite. The mineral is named after the type locality, the Fogo volcano in the Azores.