A first report on anion vacancies in a defect MgAl2O4 natural spinel

The chemical and structural features of a natural spinel sensu stricto (s.s.) sample were studied by a multi-analytical approach, including electron microprobe analysis (EMP), Fourier transform infrared spectroscopy (FTIR), and single crystal X-ray diffraction structural refinement (SREF). The sample, coming from impure dolomitic marbles of Pegu (Myanmar), has an anomalous chemistry with an Mg-content exceeding that of the ideal formula. In addition, a chemical zoning along a line-scan of EMP analyses was observed, with Mg and Al amounts showing opposite trends. The comparatively high and low concentrations, respectively, of divalent and trivalent cations lead to a deficit of positive charges. Thus, the requirement of neutrality of global charges for crystal structures appears to be violated, in this case. The possible reasons accounting for the anomalous chemistry are discussed. Based on the combined EMP, FTIR and SREF results, it is concluded that anion vacancies can adequately compensate for the observed deficit of positive charges. Thus, the analysed sample is a defect spinel. This is the first report of anion vacancies in a natural spinel s.s. With reference to the ideal formula MgAl2O4, the formation of anion vacancies, coupled to an excess of Mg and a deficiency of Al, may be described by the substitution mechanism 2Mg(2+)+(V)square -> 2Al(3+)+O2-, where (V)square represents an oxygen vacancy

XRDT Study of Structural Defects of 6H-SiC Crystals

X-Ray Diffraction Topography (XRDT) and Optical Microscopy (OM) are adopted to study extended structural defects in 6H-SiC bulky crystals. Topographs are taken by means of White Beam Synchrotron Radiation Source (WB-SRS-XRDT) and by means of monochromatic radiation (MoKα1) with conventional source (Lang method). All studied samples are characterised by the presence of linear defects, dislocations and microchannels, uniformly distributed in the crystal. Such defects draw a net of independent systems of parallel lines, with different orientation and different contrast widths. Micro-channels are parallel to the çaxis, whereas dislocations are perpendicular or nearly parallel to the c axis. The last are unit screw dislocations. It has been concluded that the growth mechanism is driven by screw dislocations and that channels results from the coalescence of parallel dislocations

Mineralogical Characterization of Zoned Sphalerite from the Zn-Pb Ore Deposit of Longobucco (Sila Massif, Calabria, Southern Italy)

Polymetallic ore deposits of the Sila Massif were subjected to raw metals exploitation starting from the Bronze and Iron ages. In spite of this, the scientific literature on their mineralogical characterization is not comprehensive and mostly out-of-date, as recently reported (Fregola et al., 2023). A mineral chemistry study with special focus on some phases could certainly contribute to decipher the origin of these mineral deposits. In this contribution, we present a mineralogical characterization of zoned sphalerite samples collected from the mineralized bodies outcropping along a fault zone cross-cutting monzodiorites at the “Torrente La Manna” site in the Longobucco area (Sila Massif, Calabria). In a separate contribution, we also present results on the whole mineralogical association of this mineralized site, including paragenetic considerations (Ciccolella et al., 2023; Session P.74). Sphalerite from Longobucco has been analysed by optical microscopy, micro-Raman spectroscopy, scanning electron microscopy (SEM), electron probe micro analyses (EPMA), and single crystal X-ray diffraction. Chemical micro-analyses revealed that sphalerite contains minor to trace amounts of Fe (0.803±0.021 to 9.550±0.064 wt%), Cd (up to 0.685±0.020 wt%), Hg (0.176±0.033 to 0.461±0.039 wt%), Cu (up to 0.454±0.011 wt%), and Co (up to 0.023±0.005 wt%). Sphalerite from Longobucco systematically shows colour and chemical zoning, with darker coloured growth bands due to enriched Fe-content. Sector zoning of Cd is also observed. The optical and paragenetic study allowed us to associate the different colour zones to different growth episodes. In particular, sphalerite of the first generation (sphalerite-I) is lighter coloured and Fe-poorer, with respect to the darker and Fe-richer one of the second generation (sphalerite-II). Micro-Raman spectroscopy and single crystal X-ray diffraction were performed on the different colour zones. In particular, Raman spectra show that number, position and relative intensity of the bands in the spectral range 250-400 cm-1 are related to the colour zoning and Fe-content of the sphalerite growth bands. These results are in good agreement with the previous literature (Buzatu et al., 2013). Preliminary single crystal X-ray diffraction data detected a slight difference between the lattice parameters of the lighter sphalerite-I (5.4140 ± 0.0009 Å) and of the darker sphalerite-II (5.4194 ± 0.0007 Å). Based on the relationship between Fe-content and cell parameters found in synthetic FexZn(1-x)S solid solutions (Osadchii & Gorbaty, 2010), our results would correspond to a difference from 0.06 to 0.15 mole fraction of FeS. Zoned sphalerite indicates a variation in the composition of the mineralizing fluid(s) during the geological history of the Longobucco ore deposit. Buzatu A., Buzgar N., Damian G., Vasilache V. & Apopei A.I. (2013) – The determination of the Fe content in natural sphalerites by means of Raman spectroscopy. Vibrational Spectroscopy 68, 220-224. https://doi.org/10.1016/j.vibspec.2013.08.007 Ciccolella A., Fregola R.A., Ruggieri G., Tursi F., Festa V., Ventruti G. & Schingaro E. (2023) - Polymetallic mineral deposits of the Sila Massif (Calabria, southern Italy): The case study of Longobucco. Congresso SGI-SIMP, “The Geoscience paradigm: Resources, Risks and future perspectives”. Potenza, 19-21 sept. 2023, Abstract Book, p. . Fregola, R.A., Ciccolella, A., Festa, V., Ruggieri, G., Schingaro, E., Tursi, F., & Ventruti, G. (2023) - Review of Polymetallic Mineralization in the Sila and Serre Massifs (Calabria, Southern Italy). Minerals, 13(3), 439. https://doi.org/10.3390/min13030439 Osadchii E.G. & Gorbaty Y.E. (2010) - Raman spectra and unit cell parameters of sphalerite solid solutions (FexZn1-xS). Geochim. Cosmochim Acta, 74, 568-573. https://doi.org/10.1016/j.gca.2009.10.02

Mineralizations of the Sila Massif (Calabria, southern Italy): The case study of the Zn-Pb ore deposit of Longobucco

Although the mineral deposits of the Longobucco area in the Sila Massif (Calabria, southern Italy) are historically known as metal exploitation sites since ancient times, scientific interest in their study has only recently been renewed (Fregola et al., 2023a). In particular, a detailed mineralogical characterization using modern methodologies and framed in an updated geological context, would help to decipher their genesis. We have collected samples of the mineralized bodies outcropping along a fault zone crosscutting monzodiorites at the “Torrente La Manna” site in the Longobucco area, where Vighi (1953) first observed a sphalerite-galena- pyrite-quartz-calcite mineral association. We have characterized such minerals using optical microscopy, scanning electron microscopy (SEM), electron probe microanalyses (EPMA), and micro-Raman spectroscopy. Sphalerite is the dominant polymetallic mineral, showing color and chemical zoning, and can be used as fingerprint to differentiate the evolution stages of the mineralized deposit; noteworthy its mineralogical characterization is presented in a separate contribution to this congress (Fregola et al., 2023b; session P.40). Our reconstructed paragenetic sequence comprises almost five growth stages. Early minerals include a light- colored Fe-poor sphalerite-I, followed by a darker Fe-richer sphalerite-II formed in association to euhedral quartz-I crystals. The latter show color and chemical zoning of Al, Na, K, Ca. Moreover, we have observed a Mn-rich (1.19±0.03 to 0.49±0.02 wt%) and Mg-poor (up to 0.08±0.01 wt%) calcite, partly hosting REE- fluorcarbonates. The latter minerals are reported here for the first time in the Longobucco area, and consist of synchysite, with 20 to 60 μm grain size, and total REE (Ce, La, Y, Nd, Sm, Pr, Gd, Dy) concentrations ranging between 42.64 and 2.40 wt%. Late stages minerals comprise sphalerite-III, quartz-II, pyrite, and Ag- free galena with low Sb-content (up to 0.09±0.03 wt%). Fregola, R.A., Ciccolella, A., Festa, V., Ruggieri, G., Schingaro, E., Tursi, F., & Ventruti, G. (2023a) - Review of Polymetallic Mineralization in the Sila and Serre Massifs (Calabria, Southern Italy). Minerals, 13(3), 439. https://doi.org/10.3390/min13030439 Fregola R.A., Ciccolella A., Ruggieri G., Ventruti G., Mesto E. & Schingaro E. (2023b) – Mineralogical characterization of zoned sphalerite from polymetallic mineralization of the Sila Massif (Calabria, Southern Italy): The Longobucco case study. Congresso SGI-SIMP, “The Geoscience paradigm: Resources, Risks and future perspectives”. Potenza, 19-21 sept. 2023, Abstract Book, p. . Vighi L. (1953) Sulla geologia e sulle mineralizzazioni metallifere della regione di Longobucco in Calabria. Mem. e Note Ist. Geol. Appl. Napoli, 5, 3–61