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

F. Camara (1, 2) , E. Sokolova (3) , Y. A. Abdu (4) , F. C. Hawthorne (3) , T. Charrier (5) , V. Dorcet (6) , J. -F. Carpentier (6)

1 UNITO - Università degli studi di Torino = University of Turin
2 Univ Turin, CrisDi Ctr Crystallog, Dept Chem, Turin, Italy
3 University of Manitoba [Winnipeg]
4 American University of Sharjah
5 Le Mange Cailloux
6 ISCR - Institut des Sciences Chimiques de Rennes

Fogoite-(Y), Na3Ca2Y2 Ti(Si2O7)(2) OF3, is a newmineral from the Lagoa do Fogo, SaoMiguel Island, theAzores. It occurs in cavities as highly elongated (on [001]) prisms, up to 2000 mu m long and 50 mu m x 50 mu m 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 similar to 5. D-calc. = 3.523 g/cm(3). It is biaxial (+) with refractive indices (lambda = 590 nm) alpha = 1.686(2), beta = 1.690(2), gamma = 1.702(5); 2V(meas).= 57(1)degrees and 2V(calc). = 60 degrees. It is nonpleochroic. Fogoite-(Y) is triclinic, space group P1 , a = 9.575(6), b = 5.685(4), c = 7.279(5)angstrom, alpha = 89.985(6), beta = 100.933(4), gamma = 101.300(5)degrees, V = 381.2 (7)angstrom(3). The six strongest reflections in the powder X-ray diffraction data [d (angstrom), I, (h k l)] are: 2.954, 100, ((1) over bar (1) over bar 2, (3) over bar 1 0); 3.069, 42, (3 0 0, 0 (1) over bar 2); 2.486, 24, (3 1 0, 2 (1) over bar 2); 3.960, 23, ((1) over bar (1) over bar 1, (2) over bar 1 0); 2.626, 21, ((2) over bar 2 0); 1.820, 20, ((1) over bar 0 4). Electron microprobe analysis gave the following empirical formula calculated on 18 (O + F) (Na2.74Mn0.15)(Sigma 2.89)Ca-2[Y-1.21(La0.01Ce0.03 Nd0.03Sm0.02Gd0.08Dy0.08Er0.05Yb0.04Lu0.01)(Sigma 0.35)Mn-0.16 Zr0.11Na0.09 Fe0.072+Ca0.01](Sigma 2) (Ti 0.76Nb0.23Ta0.01)(Sigma 1) (Si4.03O14)O1.12F2.88, Z = 1. The crystal structure was refined on a twinned crystal to R-1 = 2.81% on the basis of 2157 unique reflections (F-o > 4 sigma 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 M-[6](O)(1) site is occupied mainly by Ti, < M-O(1)-phi> = 1.980 angstrom, and the ([6]) M-O(2) and M-[6](O)(3) sites are occupied by Na and Na plus minorMn, < M-O(2)-phi> = 2.490 angstrom and < M-O(3)-phi> = 2.378 angstrom. In the H sheet, the two Si-[4] sites are occupied by Si, with = 1.623 angstrom; the (MH)-M-[6] site is occupied by Yand rare-earth elements (Y > REE), with minor Mn, Zr, Na, Fe2+ and Ca, < M-H- phi> = 2.271 angstrom and the ([6])AP site is occupied by Ca, < A(P)-phi> = 2.416 angstrom. 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)F-2 and Y2Ca2(Si2O7)(2) apfu. Fogoite-(Y) is isostructural with gotzenite and hainite. The mineral is named after the type locality, the Fogo volcano in the Azores.

Mots clés: fogoite-(y); crystal-structure refinement; electron microprobe analysis; chemical formula; ts-block minerals; gotzenite; hainite; group i; fogo volcano; the azores

Domaines: Chimie organique

F. Camara, E. Sokolova, Y. A. Abdu, F. C. Hawthorne, T. Charrier, et al.. Fogoite-(Y), Na3Ca2Y2Ti(Si2O7)(2)OF3, a Group I TS-block mineral from the Lagoa do Fogo, the Fogo volcano, Sao Miguel Island, the Azores: Description and crystal structure. Mineralogical Magazine, 2017, 81 (2), pp.369--381. ⟨10.1180/minmag.2016.080.103⟩. ⟨hal-01542757⟩