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Barium-rich phengite in eclogites from the Voltri Group (northwestern Italy)
No full textA white mica with high barium
content has been found for the first time in eclogites from a single body cropping out near the village of Vara Inferiore (Voltri Group,Italy).Petrographic observations reveal that the modal abundance of mica
is low and that its crystallisation took place after the climax of the eclogite-facies metamorphism.This study reports data on bulk rock chemistry,obtained
by ICP-MS,as well as the results of EMP and
LA-ICP-MS analyses on major and selected trace elements carried out in situ on handpicked separate grains of mica.The most significant features of the
bulk rock chemistry are the high content of Na2O and the enrichment in LIL elements (Ba and Cs over K and Rb)compared with other eclogites of the area. The white mica is a “phengite ” that contains 3.17-3.30 Si a.p.f.u,and 0.19 -0.31 Mg a.p.f.u.All the
analysed grains show marked heterogeneities in both major and trace element distribution.In particular,
the content of BaO varies from 1.65 to 4.55 wt% and from 2.46 to 4.59 wt% in two examined samples, thus leading to a range of ̃4-12 % Ba atoms in the cation interlayer (I )of the mica.The high barium content
in white mica is coupled with a high abundance in other LILE (Cs,Rb)and light elements such as Li and B.On the contrary,mica has low concentration
of REE, Sr, P and HFSE (e.g. Nb, Zr, Ti, Y). Phengite
chemistry indicates that its trace element signature is controlled by a fluid activity that also prevented the achievement of complete homogenisation
Nephrite jade from Val Malenco, Italy: Review and update
No full textAlpe Mastabia, in the Val Malenco district of northern Italy, has been a source of nephrite jade since the early 2000s. Twenty-one samples from this locality were investigated by classical gemological methods; X-ray powder diffraction, combined with quantitative phase analysis; scanning electron microscopy in combination with energy-dispersive spectrometry; electron microprobe analysis; mass spectrometry; and mid-infrared spectroscopy. From a mineralogical standpoint, this jade consists mainly of tremolite amphibole, with variable amounts of other constituents, especially calcite (up to approximately 30 wt.%), but also pyroxene, apatite, and sulfide minerals. Its pale green color is related to the low iron content of the tremolite amphibole, whereas the other minerals are responsible for different colors (calcite for white, molybdenite and galena for gray). On the basis of minor and trace-element composition, we can classify this jade as dolomite-related nephrite (para-nephrite). Although new material could be recovered from this area, future production will probably be limited by access difficulties
Biotiti e cloriti nelle diverse facies del granito di Baveno-Mottarone (Verbania)
No full textIl plutone di Baveno – Mottarone è costituito da diverse varietà di granito, caratterizzate dalla diversa colorazione dei feldspati. La porzione maggiore e altimetricamente più bassa del plutone è costituita da granito bianco, mentre la parte alta è costituita dal tipico granito a feldspato rosa, da granito rosso (sia K-feldspato che oligoclasio colorati), entrambi miarolitici, e da una facies “di transizione” in cui la colorazione del K-feldspato varia irregolarmente da rosa tenue a bianco. I graniti sono tutti metalluminosi o leggermente peralluminosi; quelli a feldspati colorati si distinguono per un tenore più elevato in alcali e minore in Al2O3 (Boriani et al., 1992). Questa differenza tra granito bianco e graniti colorati è evidenziata anche dai diversi caratteri tipologici degli zirconi (Caironi, 1985). Nella facies bianca la biotite è pleocroica sul bruno e altera in una clorite debolmente pleocroica sul verde pallido. Nelle facies colorate la biotite è fortemente pleocroica sul bruno-verde scuro e anche la clorite è nettamente pleocroica sul verde scuro; inoltre, vicino alle lamelle cloritizzate sono presenti fluorite e micro-miaroli a miche chiare.
Cristalli di biotiti e cloriti, opportunamente selezionati da campioni rappresentativi delle diverse facies, sono stati analizzati mediante microsonda elettronica (EMP) per gli elementi maggiori, e mediante microsonda ionica Cameca IMS 4f (SIMS) per gli elementi in traccia Li, Be, B e F, in accordo a Ottolini et al. (2002).
Le biotiti del granito bianco, rispetto alle altre biotiti, risultano chiaramente più ricche di Mg (MgO = 3.26 – 4.63 wt % contro 1.61 – 2.53 wt %) e più povere di Fe (FeOtot=25.87 - 29.29 wt % contro 26.05 – 31.32 wt %); inoltre sono leggermente più ricche di Al (valore medio di Al2O3 = 18.17 wt % contro 16.23 wt %) e in generale un po’ più povere di Ti (valore medio di TiO2 = 2.08 contro 2.50 wt %). Nei diagrammi classificativi proposti da Abdel Rahman (1994) i due gruppi di biotiti sono sempre ben separati; i campioni provenienti dal granito bianco cadono nei campi caratteristici delle biotiti dei graniti peralluminosi, mentre le biotiti dei graniti colorati sono spostate verso, e in parte entro, i campi delle biotiti dei graniti alcalini. Inoltre le biotiti dei graniti a feldspati colorati si caratterizzano per la presenza di un tenore più elevato di Li2O (valore medio 1.20 vs 0.34 wt %) e di F (valore medio 0.92 vs 0.36 wt %). Gli altri elementi in traccia analizzati (Be e B), sempre molto bassi, non sembrano essere discriminanti per le biotiti dei graniti delle varie facies in quanto variano irregolarmente. Per quanto riguarda le cloriti, si può osservare che i loro caratteri composizionali riflettono quelli delle coesistenti biotiti soprattutto per quanto riguarda i contenuti di MgO e FeOtot, che risultano anche in questo caso discriminanti tra le cloriti della facies bianca e quelle delle facies colorate. Inoltre, nonostante il limitato numero di analisi di cloriti effettuate mediante SIMS, è possibile evidenziare la presenza di Li2O e F in quantità maggiore nei campioni appartenenti alle facies a feldspati colorati.
La diversa composizione in termini di elementi maggiori (soprattutto Fe, Mg e Al) sembra essere un carattere acquisito durante la cristallizzazione magmatica e legato ad un diverso chimismo del magma. Il maggior contenuto in F e Li delle biotiti dei graniti a feldspati colorati è da attribuire a fluidi arricchiti in F e Li, presenti probabilmente già a partire dalla fase tardomagmatica, come indica la presenza di fluorite nella roccia e la cristallizzazione nelle geodi, insieme a fluorite e miche litinifere, degli stessi minerali costituenti il granito
New chemical data on the clinopyroxene-garnet pair in the Alpe Arami eclogite, Central Alps, Switzerland
No full textIn the area of Gorduno, near Bellinzona, Central Alps, Switzerland, well-preserved eclogites crop out only at the margin of the
garnet peridotite massif of Alpe Arami (AA). These bodies of eclogite were not derived from cognate mafic layers of mantle
origin, and their thermal evolution is different from that inferred for the peridotite. Some questions are still open about the P–T
evolution of AA eclogites, and the data given in literature cover a broad range of variation. We have investigated the bulk
composition and the clinopyroxene–garnet trace-element distribution in a sample of eclogite from a lens occurring at the rim of
the peridotite body. We report new data on trace elements in the bulk rock, obtained by ICP–MS, as well as the results of SIMS
analyses on REE, Ti, V, Cr, Y, Sr, Zr, Sc carried out on the two major rock-forming minerals, i.e., clinopyroxene and garnet. We
also provide AAS data for Co, Ni and Cu in purified clinopyroxene and garnet concentrates, and O isotope ratios both on the bulk
rock and on clinopyroxene and garnet. We found that: i) the trace-element abundance in garnet and clinopyroxene may be
strongly influenced by the major-element composition; ii) their distribution within both mineral phases is inhomogeneous. In
particular, their highly variable contents highlight the complex evolution of the AA eclogite and emphasize the existence, at least
for some elements and at a very local scale, of chemical disequilibrium, also confirmed, in an independent way, by a significant
difference in 18 O between clinopyroxene and garnet. Therefore, the low temperature of equilibration estimated in eclogite
sample 70–AM–10 is not an artifact of the computational strategy adopted, but is the record of the lack of complete equilibration
between clinopyroxene and garnet with changing P–T conditions during the various metamorphic events.
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The profile of trace elements, including the REE, in gem-quality green andradite from classic localities
No full textCrystals of gem-quality andradite (variety “demantoid”) from Val Malenco, Italy, have been analyzed for major, rare-earth and other trace elements (Sc, Ti, V, Cr, Co, Ni, Zn, Sr, Y, Zr), together with samples from Aosta Valley, Italy, and from the classic deposits of the world (Russia, Iran, Pakistan, Namibia, Madagascar). They are all of gem quality and vary from bright green to yellow-green in color. The samples are homogeneous within the limit of the analytical error and cover a restricted range of composition from almost pure andradite (Adr ≥ 98 mol.%) to members of andradite–uvarovite (Adr81–96Uv3–18) or andradite–grossular (Adr92–94Grs6–8) solid solution, with the sole exception of the sample from Namibia, showing a composition varying from pure andradite to almost pure grossular (Adr11Grs89). All the samples have a low abundance of most trace elements, except for Cr, which ranges from a few ppm to more than 5 wt% Cr2O3. The substitution of Fe3+ and Cr3+ for Al at the [Y] site significantly controls the geometry of the structural sites and the incorporation of the REE. In particular, (i) the samples showing a composition close to pure andradite 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 a negative one. However, such a compositional variation may also arise from differences in the bulk composition of the host rocks and from changes in the physicochemical conditions during growth
In situ high temperature single crystal X-ray diffraction study of a natural omphacite
No full textIn situ high temperature single crystal X-ray diffraction (XRD) experiments have been performed on a chemically quasi-ideal omphacite sample [(Ca0.49Na0.51)(Σ1)(Mg0.46Al0.48Fe0.063+)(Σ1)(Si1.97 Al0.03)(Σ2)O6], up to 1000°C. The lattice parameters were studied as a function of temperature, and their thermal expansion coefficients determined. The b and c cell edges show discontinuities as a function of temperature which are interpreted here in terms of intracrystalline cation diffusion processes. Structure refinements have been carried out using data collected at room temperature, at 800°C and at ambient conditions after cooling. The structural behaviour as a function of temperature of chemically quasi-ideal omphacites is compared with those of jadeite and diopside
Trace element distribution in eclogites and their clinopyroxene/garnet pair:a case study from Soazza (Switzerlan)
No full textRare earth element (REE) abundances have been determined in six samples of metabasites from the Pennidic Adula nappe in the area of Soazza (Mesolcina Valley, Switzerland). The samples cover a broad range of mineralogical composition from eclogite to epidote amphibolite through symplectitic garnet amphibolite. The REE patterns do not vary significantly and indicate a common origin of these rocks as "normal" mid-ocean ridge basalts (N-MORB).
Two main eclogitic phases, clinopyroxene (cpx) and garnet (grt), have also been analysed for REE and other trace elements (Sc, V, Cr, Co, Ni, Cu, Zn, Sr, Y, Zr). The incorporation of REE and most trace elements in structural sites of cpx and grt is in accordance with that expected mainly from an evaluation of site geometry, ionic radius and charge balance and is therefore dependent on the major-element composition of the host phases. The changes in cpx/grt partition coefficients (Di) for HREE are particularly associated with the temperature condition of the equilibration of the pair
Geochemistry of eclogitised Fe-Ti gabbros from different lithological setting (Aosta valley ophiolites, italian western Alps). Protolith composition and eclogitic paragenis
No full textGems corals : classification and spectroscopic features
No full textSeveral gem corals, belonging to the classes Hydrozoa and Anthozoa of the phylum Cnidaria, have been analyzed. These have been previously classified from a zoological point of view. In this paper surface structures, physical data, FTIR- and Raman-spectra of corals with gem potential have been investigated. Spectrophotometric characterisation proved to be a useful tool for determining the main features of the mineral phases (calcite or aragonite) and of the organic content in each sample. Chromophore complexes belonging to the caroteno-proteins of the carotenoid family also could be detected
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