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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)
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- 2011-01-01T00:00:00+01:00 (literal)
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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
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- ADAMO Ilaria1, BOCCHIO Rosangela1, DIELLA Valeria2 (literal)
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- Geoitalia 2011, VIII forum italiano di Scienze della Terra, Torino (literal)
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- 1 - Dipartimento di Scienze della Terra, Università 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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