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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 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, O(1)–ϕ> = 1.980 Å, and the[6]MO(2) and[6]MO(3) sites are occupied by Na and Na plus minor Mn, O(2)–ϕ>= 2.490 Å and O(3)–ϕ> = 2.378 Å. In the H sheet, the two[4]Si sites are occupied by Si, with = 1.623 Å; the[6]MHsite is occupied by Y and rare-earth elements (Y >REE), with minor Mn, Zr, Na, Fe2+and Ca, H–ϕ> = 2.271 Å and the[6]APsite is occupied by Ca, = 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.

The Angolan alkaline&ndash, carbonatite complex of Monte Verde has a semi-circular shape and is comprised of a central intrusion of foidolite rocks surrounded by concentrically arranged minor bodies of other alkaline rocks and carbonatite magmatic breccias. This rock association is hosted by fenitized Eburnean granites. Concentric swarms of alkaline dykes of late formation, mostly of nepheline trachyte composition, crosscut the previous units. Most high-field strength elements (HFSE) and rare earth elements (REE) are concentrated in pyrochlore crystals in the carbonatite and alkaline breccias. Magmatic fluornatropyrochlore is replaced and overgrown by five secondary generations of pyrochlore formed during subsolidus stages and have higher Th, REE, Si, U, Sr, Ba, Zr, and Ti contents. The second, third, and fourth pyrochlore generations are associated with late fluids also producing quartz and REE rich minerals, whereas fifth and sixth pyrochlore generations are linked to the fenitization process. On the other hand, minerals of the rinkite, rosenbuschite, w&ouml, hlerite, eudialyte groups, as well as loparite-(Ce), occur in accessory amounts in nepheline trachyte, recording low to moderate agpaicity. In addition, minor REE-bearing carbonates, silicates, and phosphates crystallize as late minor secondary minerals into carbonatite breccia and alkaline dykes. In conclusion, the scarcity of HFSE and REE minerals at the Monte Verde carbonatite-alkaline-agpaitic complex suggests low metallogenetic interest and economic potential for the outcrops analysed in this study. However, the potential for buried resources should not be neglected.

The crystal structure of mosandrite, ideally Na2Ca4REETi(Si2O7)2OF3,a =7.4184(8),b =5.6789(6), c = 18.873(2) Å, β = 101.410(2)°,V =779.35(5) Å3, space groupP21/c, Z =2, Dcalc= 3.363 g.cm-3, from the type locality, Låven (Skådön), Langesundsfjorden, Larvik, Vestfold, Norway, has been refined toRi =6.33% on the basis of 1113 unique reflections F°≥4σF. Electron microprobe analysis gave the empirical formula Na1.99(Ca3.93Sr0.02)Σ3.95(Ce0.41La0.16Nd0.13Pr0.04Sm0.02Dy0.01Y0.13)Σ0.90(Ti0.864+Nb0.08Zr0.05)Σ0.99(Si207)2(F1.20O0.80)Σ2.00F2, Z = 2, calculated on the basis of 18 (O + F) a.p.f.u.. The crystal structure of mosandrite is a framework of TS (titanium silicate) blocks. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The TS block in mosandrite exhibits linkage and stereochemistry typical for Group I (Ti = 1 a.p.f.u.) Ti-disilicate minerals: two H sheets connect to the O sheet such that two (Si2O7) groups link to thetransedges of a Na polyhedron of the O sheet. The O sheet cations give Na(NaCa)Ti (4 a.p.f.u.). The TS blocks linkviacommon vertices of (Si2O7) groups and common vertices and edges of Ca-dominant MHand Appolyhedra. Two adjacent TS blocks are related by the glide plane cy. Composition and topology of the TS block in mosandrite and rinkite are identical. The crystal structure of mosandrite from the type locality is topologically and chemically identical to that of rinkite from the type locality of Kangerdluarssuk, Greenland.

Rinkite, ideallyNa2Ca4REETi(Si2O7)2OF3, is a common mineral in alkaline and peralkaline rocks. The crystal structures of five rinkite crystals from three alkaline massifs: Ilimaussaq, Greenland; Khibiny, Kola Peninsula, Russia and Mont Saint-Hilaire, Canada, have been refined as two components related by the TWIN matrix (–1 0 0, 0 –1 0, 1 0 1)(Mo-αaradiation). The crystals,a =7.4132-7.4414,b =5.6595-5.6816,c =18.8181-18.9431 Å, β = 101.353-101.424(2)°,V=776.1-786.7 Å3, space groupP21lc, Z =2,Dcalc= 3.376—3.502 g cm–3, were analysed using an electron microprobe subsequent to collection of the X-ray intensity data. Transmission electron microscopy confirmed the presence of pseudomerohedral twinning in rinkite crystals. The crystal structure of rinkite is a framework of TS (titanium silicate) blocks. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The TS block in rinkite exhibits linkage and stereochemistry typical for Group I (Ti = 1 a.p.f.u.) of Ti disilicate minerals: two H sheets connect to the O sheet such that two (Si2O7) groups link to thetransedges of a Na polyhedron of the O sheet. The crystal chemistry of rinkite and nacareniobsite-(Ce) is discussed.