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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

Published online by Cambridge University Press:  02 January 2018

F. Cámara ,
E. Sokolova ,
Y. A. Abdu ,
F. C. Hawthorne ,
T. Charrier ,
V. Dorcet  and
J. -F. Carpentier
F. Cámara*
Affiliation:
Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, I-10125, Torino, Italy CrisDi, Interdepartmental Centre for the Research and Development of Crystallography, via Pietro Giuria 5, I-10125, Torino, Italy
E. Sokolova
Affiliation:
Department of Geological Sciences, University of Manitoba, 125 Dysart Road, Winnipeg, MB, R3T 2N2 Canada
Y. A. Abdu
Affiliation:
Department of Applied Physics, University of Sharjah, P.O. Box 27272, Sharjah, United Arab Emirates
F. C. Hawthorne
Affiliation:
Department of Geological Sciences, University of Manitoba, 125 Dysart Road, Winnipeg, MB, R3T 2N2 Canada
T. Charrier
Affiliation:
Le Mange Cailloux, 1 rue de la Belle Allée, 85290 Mortagne-sur-Sèvre, France
V. Dorcet
Affiliation:
Institut des Sciences Chimiques de Rennes – UMR 6226 CNRS-Universite de Rennes 1, Campus de Beaulieu, F-35042 Rennes Cedex France
J. -F. Carpentier
Affiliation:
Institut des Sciences Chimiques de Rennes – UMR 6226 CNRS-Universite de Rennes 1, Campus de Beaulieu, F-35042 Rennes Cedex France
*
*E-mail: [email protected]
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Abstract

Fogoite-(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 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, (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 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, <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]MH site 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]AP site is occupied by Ca, <AP–ϕ> = 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.

Keywords

fogoite-(Y)crystal-structure refinementelectron microprobe analysischemical formulaTS-block mineralsgötzenitehainiteGroup IFogo volcanothe Azores
Type
Research Article
Information
Mineralogical Magazine , Volume 81 , Issue 2 , April 2017 , pp. 369 - 381
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2017

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