Mesto E.*1, Laurita S.2, Lacalamita M.1, Schingaro E.1, Rizzo G.2, Sinisi R.2 & Mongelli G.2
1 Dipartimento di Scienze della Terra e Geoambientali. Università degli Studi di Bari “Aldo Moro”. Bari, Italy.
2 Dipartimento di Scienze, Università della Basilicata, Potenza, Italy Corresponding email: [email protected]
Keywords: Fe-carpholite, crystal-chemistry, thermal behavior.
The carpholite group encompasses hydrated inosilicates with general formula A0-1M12M22M32[(OH, F)4(Si2O6)]2 where A =[], K, Ba and Na, M1 = Mn, Mg, Fe, Al, Li and Na, M2 = Al, V3+, Fe3+ and Ti, and M3 = Al, Mg, V3+, Fe3+ and Ti. In the present study the Fe-carpholite from the Pollino Massif, southern Apennines (Liguride Complex, Frido Unit) has been investigated by combining Secondary Electron Microscopy (SEM), Single Crystal X-Ray Diffraction (SCXRD), µRaman spectroscopy and in situ High Temperature X-Ray Powder Diffraction (HTXRPD). Fe-Mg carpholite is known as an index mineral in LT-HP conditions for blueschist facies rocks, but, in metasedimentary rocks may be the main, or locally the only, evidence of LT-HP metamorphism (Pourteau et al., 2013).
The results of SEM analyses on thin sections show that the studied carpholite is embedded in quartz and quartz-calcite veins cross-cutting fine-grained greyish metapelites cropping out close to Viggianello (Potenza, Italy). The mineral usually exhibits a thin, 200-300 µm long, hair-like, habit. The SCXRD confirms that the studied Fe-carpholite has Ccce symmetry, as previously found by Fuchs et al. (2001) and references therein, with a~13.77 Å, b~ 20.16 Å, c~5.11 Å and V~1419 Å3. The structure refinements, that converged at R values between 2.3 and 3.1%, provided ~38% Mg and ~62% Fe at M1, that corresponds to XFe = 0.6; the M2 and M3 sites are fully occupied by Al.
Raman spectra, collected on unoriented crystals, evidence, in the high frequency region, two peaks at 3630 and 3593 cm-1 which account for the presence of two independent OH groups as also derived from the structure refinement. Similar results were also reported for the Fe-carpholites in Ferraris et al. (1992) In situ HT XRD patterns were collected from 30 to 630°C. No modifications are observed in the diffraction patterns collected from 30 to 380°C whereas a splitting of the reflections occurs starting from 405°C. The phase is stable up to 630°C and transforms to a spinel phase at 680°C. All these findings, and especially the in situ HT XRPD results, add further constraints on our knowledge of the thermal behaviour of this mineral in relation to the thermometamorphic history of metasediments.
Ferraris G., Ivaldi G. & Goffé B. 1992. Structural study of a magnesian ferrocarpholite: Are carpholites monoclinic? N.
Jb. Mineral., Mh., 337-347.
Fuchs Y., Mellini M. & Memmi I. 2001. Crystal-chemistry of magnesiocarpholite: controversial X-ray diffraction, Mössbauer, FTIR and Raman results. Eur. J. Mineral., 13, 533-543.
Pourteau A., Sudo M., Candan O., Lanari P., Vidal O. & Oberhänsli R. 2013. Neotethys closure history of Anatolia:
insights from 40Ar-39Ar geochronology and P-T estimation in high-pressure metasedimentary rocks. J. Metamorph.
Geol., 31, 585-606.
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Disorder and modulation in first aragonite precipitates from Obstanser Eishöhle (Austria)
Mugnaioli E.*1, Gemmi M.1 & Németh P.21 Center for Nanotechnology Innovation@NEST, Istituto Italiano di Tecnologia, Italy.
2 Institute of Materials and Environmental Chemistry, Hungarian Academy of Sciences, Hungary.
Corresponding email: [email protected]
Keywords: aragonite, electron crystallography, crystal nucleation.
Aragonite is thermodynamically metastable at near-surface conditions, and still it is relatively widespread in marine and terrestrial sediments. In this contribution we propose the detailed chemical and crystallographic analysis of fresh aragonitic precipitates. Wet samples, collected from a dolomitic cold cave in the East Alpine range, were directly taken from underground dripping water and from the surface of aragonite speleothems.
Calcium carbonate nano- and microcrystals are always found in association with magnesite and hydromagnesite and incorporate variable amounts of magnesium and possibly hydroxyl groups.
The typical size of the analyzed precipitates ranges from some tens of nanometers to few microns. Advanced electron crystallographic tools were therefore necessary for a proper structural characterization. Indeed, in the last ten years electron diffraction (ED) turned into a robust protocol for phase identification and ab-initio structure determination. Such evolution was mostly propelled by the development of semi-automatic routines for 3D data collection (Mugnaioli & Gemmi, 2018). The concept at the basis of 3D ED is the same as for single-crystal X-ray diffraction, but electrons allow sampling single crystals 10 to 1000 times smaller, despite the presence of surrounding crystals of other mineralogical phases.
3D ED revealed that first calcium carbonate precipitates have a structure strictly related to conventional aragonite. Still, diffuse scattering and satellite reflections appear along aragonite {110} and point to a reduction of symmetry into the monoclinic system (Németh et al., 2018). Following the order-disorder description of aragonite proposed by Makovicky (2012), such disorder can be associated with the same mechanism responsible for the twinning in mature aragonite. The frequent (or systematic) inversion of the stacking vector can be imposed by the incorporation of magnesium in the structure, whose atomic radius and coordination significantly differ from those of calcium. In turn, the necessity to include magnesium and hydroxyl groups in the lattice may be the very factor that favors the crystallization of aragonite in respect to calcite, which should otherwise be the stable mineral phase at near-surface conditions. Such ‘monoclinic-aragonite’ seeds might therefore represent the key step for the formation of large amount of metastable aragonite sediments.
Makovicky E. (2012) - Twinning of aragonite —The OD approach. Mineral. Petrol., 106, 19-24.
Mugnaioli E. & Gemmi M. (2018) - Single-crystal analysis of nanodomains by electron diffraction tomography:
mineralogy at the order-disorder borderline. Z. Kristallogr., 233, 163-178.
Németh P., Mugnaioli E., Gemmi M., Czuppon G., Demény A. & Spötl C. (2018) - A nanocrystalline monoclinic CaCO3 precursor of metastable aragonite. Sci. Adv., 4, eaau6178.
Peculiar mineralogy of the Água de Pau syenites (São Miguel, Azores islands, Portugal)
Nazzareni S.1, Nestola F.2, Bindi L.3, Scricciolo E.*1, Pacheco J.4, Zanon V.4, Zanatta M.5 & Giuli G.61 Dipartimento di Fisica e Geologia, Università di Perugia, Perugia, Italy.
2 Dipartimento di Geoscienze, Università di Padova, Padova, Italy.
3 Dipartimento di Scienze della Terra, Università di Firenze, Firenze, Italy.
4 IVAR, Universidade dos Açores, Ponta Delgada, Portugal.
5 Dipartimento di Informatica, Università di Verona, Verona, Italy.
6 Scuola di Scienze e Tecnologie – sez. Geologia, Università di Camerino, Camerino, Italy.
Corresponding email: [email protected] Keywords: Syenite xenoliths, Agua de Pau, mineralogy.
Syenite xenoliths frequently occur in pyroclastic sequences erupted during the Holocene volcanic activity of the Azores archipelago (Portugal). In particular syenite xenoliths occurring at Água de Pau volcano (São Miguel island) are characterised by a peculiar mineralogy of minor components: apatite, titanite, aenigmatite, eudyalite, zircon, chevkinite, pyrochlore, and dalyite. Furthermore, several REE and REE-bearing minerals have been also reported and recently new minerals were discovered in the Agua de Pau syenites (chiappinoite, Kampf & Housley, 2015; fogoite Cámara et al., 2017).
In the course of a research project dealing with the mineralogical and petrographic characterization of minerals from Azores rocks (Nazzareni et al., 2019), we carried out a preliminary combined chemical (SEM-EDS) and structural (single-crystal X-ray diffraction) study. During this study, we identified: zircon, pyrochlore, Fe-katophorite, Nb-bearing titanite, zircon, monazite, Zr- and Nb-bearing astrophyillite, chevkinite-(Ce), steacyite, chiappinoite, euxenite, dalyite, wulfenite, moissanite and native Zn.
Here we focused on pyrochlore and monazite, which have a large chemical variation, as well as chevkinite, steacyite and chiappinoite. A still unidentified phase with V,Ti,Fe composition associated to moissanite is under investigation. A detailed crystal-chemical study of the minor phases present in the Agua de Pau syenites may help to understand the processes related to the REE enrichment of the syenitic magma.
Cámara F., Sokolova E., Abdu Y.A., Hawthorne F.C., Charrier T., Dorcet V. & Carpentier J.-F. (2017) - 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. Min. Mag., 81(2), 369-381.
Kampf A.R. & Housley R.M. (2015) - Chiappinoite-(Y), Y2Mn(Si3O7)4, a new layer silicate found in peralkaline syenitic ejecta from the Água de Pau volcano, Azores. Eur. J. Mineral., 27(1), 91–97.
Nazzareni S., Nestola F., Zanon V., Bindi L., Scricciolo E., Petrelli M., Zanatta M., Mariotto G. & Giuli G. (2019) - Discovery of moissanite in a peralkaline syenite from the Azores Islands. Lithos, 324-325, 68-73.
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