minerals Article Non-Metamict Aeschynite-(Y), Polycrase-(Y), and Samarskite-(Y) in NYF Pegmatites from Arvogno, Vigezzo Valley (Central Alps, Italy) Alessandro Guastoni 1,*, Luciano Secco 1, Radek Škoda 2 , Fabrizio Nestola 1 , Mariangela Schiazza 3 , Milan Novák 2 and Giorgio Pennacchioni 1 1 Department of Geosciences, Padova University, Via Gradenigo 6, 35131-I Padova, Italy; [email protected] (L.S.); [email protected] (F.N.); [email protected] (G.P.) 2 Department of Geological Sciences, Faculty of Science, Masaryk University, Kotlarska 2 CZ 611 37, Brno, Czech Republic; [email protected] (R.Š.); [email protected] (M.N.) 3 DiSPuTer, Chieti-Pescara University, Via dei Vestini 31, 66100-I Chieti, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-0498272006 Received: 19 April 2019; Accepted: 16 May 2019; Published: 21 May 2019 Abstract: At Arvogno, Vigezzo valley in the Central Alps, Italy, pegmatite dikes are unique in the scenario of a tertiary alpine pegmatite field because they show marked geochemical and mineralogical niobium–yttrium–fluorine features. These pegmatites contain AB2O6 aeschynite group minerals and ABX2O8 euxenite group minerals as typical accessory minerals including aeschynite-(Y), polycrase-(Y), and samarskite-(Y). They are associated with additional typical minerals such as fluorite, Y-dominant silicates, and xenotime-(Y). The Y–Nb–Ti–Ta AB2O6 and ABX2O8 oxides at the Arvogno pegmatites did not exhibit any textural and compositional features of oxidation or weathering. They are characterized by low self-radiation-induced structural damage, leading to the acquisition of unit-cell data for aeschynite-(Y), polycrase-(Y), and samarskite-(Y) by single-crystal X-ray diffraction. Aeschynite-(Y) and polycrase-(Y) crystals allowed for both to provide space groups whereas samarskite-(Y) was the first crystal from pegmatites for which cell-data were obtained at room temperature but did not allow for the accurate determination of the space group. According to the chemical compositions defined by Ti-dominant content at the B-site, the cell parameters, respectively, corresponded to polycrase-(Y), aeschynite-(Y), and the monoclinic cell of samarskite-(Y). Emplacement of Alpine pegmatites can be related to the progressive regional metamorphic rejuvenation from east to west in the Central Alps, considering the progressive cooling of the thermal Lepontine Barrovian metamorphic dome. Previous studies considered magmatic pulses that led to emplace the pegmatite field in the Central Alps. As an example, the pegmatites that intruded the Bergell massif were aged at 28–25 millions of years or younger, around 20–22 m.y. Keywords: aeschynite-(Y); Arvogno; Central Alps; euxenite-(Y); NYF pegmatites; polycrase-(Y); samarskite-(Y); Vigezzo valley 1. Introduction Complex REE–Nb–Ta–Ti oxides (aeschynite-, euxenite-, and samarskite-group minerals) are uncommon but locally important accessory phases occurring typically in the NYF (niobium, yttrium, fluorine) family and REL–REE (rare-element and rare earths) class (as per the listed pegmatites of allanite-, euxenite-, and gadolinite-type by Cernˇ ý and Ercit [1]). Aeschynite- and euxenite-group minerals have a general formula AB2O6 where the eight-fold coordinated A-site, a square antiprism, is occupied by Y, REE, Ca, U, Th, Fe, while the six-fold coordinated B-site has a typical octahedra to form double chains in a zigzag pattern along the b-axis [2] occupied by Ti, Nb, Ta. and W. Aeschynite-group Minerals 2019, 9, 313; doi:10.3390/min9050313 www.mdpi.com/journal/minerals Minerals 2019, 9, 313 2 of 23 Minerals 2019, 9, x FOR PEER REVIEW 2 of 21 mineralsform double (AGMs) chains and in euxenite-group a zigzag pattern minerals along (EGMs)the b-axis are [2] orthorhombic, occupied by Ti, but Nb, the Ta. BO 6andstacking W. octahedraAeschynite-group is different, minerals leading (AGMs) to the Pbnm and spaceeuxenite-g grouproup in AGMsminerals and (EGMs) to the arePbcn orthorhombic,space group but in EGMs.the Historically,BO6 stacking samarskite-group octahedra is different, minerals leading have to the Pbnm general space formula group ABOin AGMs4 and and an toα -PbOthe Pbcn2 primitive space structuregroup [in3, 4EGMs.]; Nb isHistorically, dominant atsamarskite-group the six-fold coordinated minerals have B-site, the and general the A-site formula is mostlyABO4 and occupied an byα REE,-PbO Ca,2 primitive U, Fe2+ structure, and Fe 3[3,4];+, although Nb is dominant some studies at the six-fold placed Fecoordinated3+ in the B-siteB-site, andand the Ti can A-site occupy is bothmostly sites occupied [5]. The by A-site REE, inCa, samarskite U, Fe2+, and isFe occupied3+, although by some REE, studies usually placed with Fe a3+ dominancein the B-site and of Y, Ti but samarskite-(Yb)can occupy both has sites also [5]. been The reported A-site in [ 6samarskite]. is occupied by REE, usually with a dominance of Y,Kjelmann but samarskite-(Yb) [7] proposed has a also new been nomenclature reported [6]. for samarskite-group minerals with a general formula Kjelmann [7] proposed a new nomenclature for samarskite-group minerals with a general ABC2O8. Recently, a new structural investigation of non-metamict samarskite-(Y) from Laacher See, formula ABC2O8. Recently, a new structural investigation of non-metamict samarskite-(Y) from Eifel volcanic region, Germany, revealed a unique cation-ordered niobate structure related to layered, Laacher See, Eifel volcanic region, Germany, revealed a unique cation-ordered niobate structure double tungstenates ABW2O8 by Britvin et al. [8]. These authors refined the crystal structure of related to layered, double tungstenates ABW2O8 by Britvin et al. [8]. These3 authors+ refined the crystal samarskite-(Y) as monoclinic P2/c and redefined the ideal formula to YFe Nb2O8. The nomenclature structure of samarskite-(Y) as monoclinic P2/c and redefined the ideal formula to YFe3+Nb2O8. The wasnomenclature approved by thewas Commissionapproved by on Newthe Minerals,Commission Nomenclature on New Minerals, and Classification, Nomenclature International and MineralogicalClassification, Association International (IMA-CNMNC) Mineralogical and Association is leading to (IMA-CNMNC) the end-member and formula is leading of samarskite-(Y), to the basedend-member on an ordered formula formula, of samarskite-(Y), ABX2O8. based on an ordered formula, ABX2O8. 4+ 4+ Yttrium-rareYttrium-rare earths-oxide earths-oxide minerals minerals typically typically contain contain elevated elevated concentrations concentrations of of U Uand4+ and Th Th4+and undergoand undergo self-radiation-induced self-radiation-induced structural structural damage damage leading leading to amorphization to amorphization of formerly of formerly crystalline lattice.crystalline This crystalline-to-amorphous lattice. This crystalline-to-amorphous transition (metamictization) transition (metamictization) is followed by substantialis followed changes by of physicalsubstantial properties changes [ 9of,10 ].physical Such metamict properties phases [9,10]. are Such very pronemetamict to hydrothermal phases are very alteration, prone whichto commonlyhydrothermal leads alteration, to substantial which changes commonly in the leads chemical to substantial composition changes of ainff ectedthe chemical domains composition [11–15]. ofThe affected Arvogno domains pegmatite [11–15]. dikes (Figure1) are unique within the tertiary alpine pegmatite field [ 16,17] showingThe marked Arvogno geochemical pegmatite and dikes mineralogical (Figure 1) are NYF unique features. within These the pegmatites tertiary alpine contain pegmatite aeschynite-(Y), field polycrase-(Y),[16,17] showing and samarskite-(Y)marked geochemical associated and withmineralogical further typical NYF features. NYF minerals These suchpegmatites as Y-rich contain fluorite, Y-richaeschynite-(Y), spessartine, polycrase-(Y), allanite-group and minerals, samarskite-(Y) gadolinite-(Y), associated and with xenotime-(Y). further typical This NYF mineral minerals assemblage such as Y-rich fluorite, Y-rich spessartine, allanite-group minerals, gadolinite-(Y), and xenotime-(Y). This and the chemical composition of minerals suggest that the examined pegmatites are typical examples mineral assemblage and the chemical composition of minerals suggest that the examined pegmatites of the NYF family enriched in Nb, Y, and other REE and F, fitting very well the definition of the NYF are typical examples of the NYF family enriched in Nb, Y, and other REE and F, fitting very well the family [18,19]. definition of the NYF family [18,19]. FigureFigure 1. Simplified1. Simplified structural structural map map of the of Centralthe Central Alps Alps with with the field the offield the of Alpine the Alpine pegmatites pegmatites showing theshowing locations the of locations the Vigezzo, of the Vigezzo, Codera, Codera, and the and Bodengo the Bodengo areas. areas. The The thick thick black black lines lines represent represent the Periadriaticthe Periadriatic Fault (FaultPF), the(PF Giudicarie), the Giudicarie Fault Fault (GF), ( andGF), theand Engadinethe Engadine Fault Fault (EF ).(EF The). The light-grey light-grey areas representareas represent quaternary quaternary deposits deposits along major along valleys. major valley The tertiarys. The tertiary batholith batholith of Bergell of (Br)Bergell is in (Br) dark-grey, is in dark-grey, and the smaller, younger Novate stockwork intrusion (No) is in black. A detailed and the smaller, younger Novate stockwork