TRACE ELEMENT AND REE BEHAVIOR OF GEM-QUALITY GREEN ANDRADITE FROM VARIOUS LOCALITIES (Comunicazione a convegno)

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  • TRACE ELEMENT AND REE BEHAVIOR OF GEM-QUALITY GREEN ANDRADITE FROM VARIOUS LOCALITIES (Comunicazione a convegno) (literal)
Anno
  • 2011-01-01T00:00:00+01:00 (literal)
Alternative label
  • ADAMO Ilaria1, BOCCHIO Rosangela1, DIELLA Valeria2 (2011)
    TRACE ELEMENT AND REE BEHAVIOR OF GEM-QUALITY GREEN ANDRADITE FROM VARIOUS LOCALITIES
    in Geoitalia 2011, VIII forum italiano di Scienze della Terra, Torino, 19-23 settembre
    (literal)
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  • ADAMO Ilaria1, BOCCHIO Rosangela1, DIELLA Valeria2 (literal)
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  • 317 (literal)
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  • 317 (literal)
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  • Geoitalia 2011, VIII forum italiano di Scienze della Terra, Torino (literal)
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  • 4 (literal)
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  • 4 (literal)
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  • 1 (literal)
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  • 1 - Dipartimento di Scienze della Terra, Universit√† degli Studi di Milano 2 - CNR, Istituto per la Dinamica dei Processi Ambientali, Milano (literal)
Titolo
  • TRACE ELEMENT AND REE BEHAVIOR OF GEM-QUALITY GREEN ANDRADITE FROM VARIOUS LOCALITIES (literal)
Abstract
  • Andradite, of ideal composition Ca3Fe2Si3O12 and space group Ia3d, is a rock-forming garnet typically found in metamorphic rocks (i.e. serpentinites and skarns). The structure consists of a framework of alternating SiO4 tetrahedra and FeO6 octahedra, with cavities containing Ca cations in eightfold coordination (Novak and Gibbs, 1971). Because relatively few studies are devoted to the trace-element composition, including REE profiles, of Ca-Fe3+ garnets (Gaspar et al., 2008), we have undertaken a study of andradite, in particular focusing on gem-quality green-colored crystals (variety \"demantoid\"). A suite of 28 andradite samples, occurring both in serpentinites and skarns, coming from the most notable localities worldwide (Italy, Russia, Iran, Pakistan, Madagascar, Namibia, and Romania) has been investigated by means of EMPA and LA-ICP-MS. More details about geology and garnet formation are reported by Bocchio et al. (2010) and Pezzotta et al. (2011). Our work aims to provide a chemical characterization of these stones and so contribute to a quantitative evaluation of the crystal chemistry of andradite. The analyzed crystals cover a restricted range of composition, from almost pure andradite (Adr 98 mol%) to members of andradite-uvarovite (Adr 81-96-Uv3-18) or andradite-grossular (Adr86-94-Grs6-12) solid-solution series. They are homogeneous, with the only exception of a sample from the skarn deposit of Namibia, showing a composition varying from pure andradite up to almost pure grossular (Grs89Adr11), which frequently occurs SESSIONE GEO6 Epitome 2011 - Geoitalia 2011 317 in garnets from skarn (Smith et al., 2004; Gaspar et al., 2008). The chondrite-normalized diagrams of selected trace elements (Sc, Ti, Cr, Co, Ni, Zn, Sr, Y, Zr) indicated that all the examined garnets, both associated with serpentinites and skarn deposits, display a comparable pattern of distribution. However, the garnets occurring in the skarn rocks (i.e. samples from Madagascar, Namibia, and Romania) contain minor amounts of the chromophore elements, in particular of chromium, which is almost absent. The chromium contents detected in specimens from serpentinites are instead very variable, ranging from a few ppm up to 1 wt%. With regard to the profile of REE, we observed that (i) all the samples showing a composition close to pure andradite, both associated with serpentinites and skarns, exhibit LREE-enriched and HREE-depleted patterns with a strong positive Eu anomaly, whereas (ii) the uvarovite-enriched samples show flatter patterns with a small positive Eu anomaly, and (iii) grossular rich samples are LREE-depleted with no Eu anomaly or negative one. These results indicate that the incorporation of REE in andradite is mainly controlled by the crystal chemistry. In particular, the substitution of Fe3+ (and Cr3+) for Al expands the framework and consequently favors the incorporation of the larger LREE. However, also the bulk composition of the host rocks and the physicochemical conditions occurring during the growth may contribute to the REE incorporation (Whitney and Olmsted, 1998; Smith et al., 2004; Gaspar et al., 2008). References Bocchio, R., Adamo, I., Diella, V. (2010) Can. Mineral., 48, 1205-1216. Gaspar, M., Knaack, C., Meinert, L.D., Moretti, R. (2008) Geochim. Cosmochim. Acta, 72, 185-205. Novak, G.A. & Gibbs, G.V. (1971) Am. Mineral., 56, 791-825. Pezzotta, F., Adamo, I., Diella, V. (2011) Gems Gemol., 47, 2-14. Smith, M.P., Henderson, P., Jeffries, T.E.R., Long, J., Williams, C.T. (2004) J. Petrol., 45, 457-484. Whitney, P.R. & Olmsted, J.F. (1998) Geochim. Cosmochim. Acta, 62, 2965-2977. (literal)
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