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
Articolo
Data di Pubblicazione:
2017
Citazione:
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 / F. Cámara, E. Sokolova, Y.A. Abdu, F.C. Hawthorne, T. Charrier, V. Dorcet, J. Carpentier. - In: MINERALOGICAL MAGAZINE. - ISSN 0026-461X. - 81:2(2017 Apr), pp. 369-381. [10.1180/minmag.2016.080.103]
Abstract:
Fogoite-(Y), Na3Ca2Y2Ti(Si2O7)2OF3, is a new mineral from the Lagoa do Fogo, Saõ 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 crosssection, associated with sanidine, astrophyllite, fluornatropyrochlore, ferrokentbrooksite, quartz and ferrokatophorite. 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 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 powder X-ray diffraction data [d (Å), I, (hkl)] are: 2.954, 100, (112, 310); 3.069, 42, (300, 012); 2.486, 24, (310, 212); 3.960, 23, (111, 210); 2.626, 21, (220); 1.820, 20, (104). 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)∑0.35Mn0.16 Zr0.11Na0.09Fe2+ 0.07Ca0.01]∑2(Ti0.76Nb0.23Ta0.01)∑1(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 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, = 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 rare-earth elements (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 apfu. Fogoite-(Y) is isostructural with götzenite and hainite. The mineral is named after the type locality, the Fogo volcano in the Azores.
Tipologia IRIS:
01 - Articolo su periodico
Keywords:
chemical formula; crystal-Structure refinement; electron microprobe analysis; Fogo volcano; Fogoite-(Y); Group I; GöTzenite; Hainite; the Azores; TS-Block minerals
Elenco autori:
F. Cámara, E. Sokolova, Y.A. Abdu, F.C. Hawthorne, T. Charrier, V. Dorcet, J. Carpentier
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