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 Article Swipe

Fogoite-(Y), Na 3 Ca 2 Y 2 Ti(Si 2 O 7 ) 2 OF 3 , 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. D calc. = 3.523 g/cm 3 . It is biaxial (+) with refractive indices (λ = 590 nm) α = 1.686(2), β = 1.690(2), γ = 1.702(5); 2V meas. = 57(1)° and 2V calc. = 60°. It is nonpleochroic. Fogoite-(Y) is triclinic, space group P 1, 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) Å 3 . 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) (Na 2.74 Mn 0.15 ) ∑2.89 Ca 2 [Y 1.21 (La 0.01 Ce 0.03 Nd 0.03 Sm 0.02 Gd 0.08 Dy 0.08 Er 0.05 Yb 0.04 Lu 0.01 ) ∑0.35 Mn 0.16 Zr 0.11 Na 0.09 Fe 0.07 2+ Ca 0.01 ] ∑2 (Ti 0.76 Nb 0.23 Ta 0.01 ) ∑1 (Si 4.03 O 14 )O 1.12 F 2.88 , Z = 1. The crystal structure was refined on a twinnedcrystal to R 1 = 2.81% on the basis of 2157 unique reflections ( F o > 4σ F o ) 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] M O (1) site is occupied mainly by Ti, <M O (1)–ϕ> = 1.980 Å, and the [6] M O (2) and [6] M O (3) sites are occupied by Na and Na plus minor Mn, <M O (2)–ϕ>= 2.490 Å and <M O (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] M H site is occupied by Y and rare-earth elements (Y > REE ), with minor Mn, Zr, Na, Fe 2+ and Ca, <M H –ϕ> = 2.271 Å and the [6] A P site is occupied by Ca, < A P –ϕ> = 2.416 Å. The M H and A P octahedra and Si 2 O 7 groups constitute the H sheet. The ideal compositions of the O and two H sheets are Na 3 Ti(OF)F 2 and Y 2 Ca 2 (Si 2 O 7 ) 2 apfu. Fogoite-(Y) is isostructural with götzenite and hainite. The mineral is named after the type locality, the Fogo volcano in the Azores.