Demantoid from Val Malenco, Italy: Review and update

Adamo, Ilaria; Bocchio, Rosangela; Diella, Valeria; Pavese, Alessandro; Vignola, Pietro; Prosperi, Loredana; Palanza, V.

doi:10.5741/gems.45.4.280

Cited by 19 publications

(26 citation statements)

References 7 publications

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“…Val Malenco is an extremely interesting geological and mineralogical district (Adamo et al, 2009) situated in the Rhetic Alps near the Italian-Swiss border between the Southern Alps and the so-called "root zone" of the Alpine nappes. The main geological unit is an ultramafic body (the "Malenco unit") that is one of the largest ophiolitic masses of the Alps (figure 2).…”

Section: Geologic Settingmentioning

confidence: 99%

Nephrite Jade From Val Malenco, Italy: Review and Update

Adamo¹,

Bocchio²

2013

G&G

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Alpe 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.

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Section: Geologic Settingmentioning

confidence: 99%

Nephrite Jade From Val Malenco, Italy: Review and Update

Adamo¹,

Bocchio²

2013

G&G

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“…2b). Iron-rich garnets show the maximum spread of Cr content, varying from ~2 to 4189 ppm, in correlation with the color variations observed in these samples (see for details Adamo et al 2009). Chromium, together with titanium and vanadium, are extremely low in the samples from Madagascar and Namibia (NA 1 ), which is ascribed to the low concentration of these elements in the metasedimentary rocks that host the garnet.…”

Section: Data On the Trace Elements Including The Rare-earth Elementsmentioning

confidence: 86%

“…This variation is due to the internal diffusion of chromium from some inclusions of Cr-bearing magnetite found in the analyzed sample and already documented in andradite from Val Malenco by Bedogné & Pagano (1972). The presence of chromium in the garnet adjacent to these inclusions and its depletion with increasing distance from the Cr-bearing magnetite grains has been described in detail by Adamo et al (2009).…”

Section: Major-element Compositionmentioning

confidence: 95%

The Profile of Trace Elements, Including the Ree, in Gem-Quality Green Andradite From Classic Localities

Bocchio¹,

Adamo²,

Diella³

2010

The Canadian Mineralogist

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Crystals 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 (Adr 81-96 Uv 3-18 ) or andradite-grossular (Adr 92-94 Grs 6-8 ) solid solution, with the sole exception of the sample from Namibia, showing a composition varying from pure andradite to almost pure grossular (Adr 11 Grs 89 ). 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% Cr 2 O 3 . The substitution of Fe 3+ and Cr 3+ 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.soMMaIRe Nous avons analysé des cristaux gemmes d'andradite (variété "démantoïde") provenant de Val Malenco, en Italie, pour les éléments majeurs, les terres rares et autres éléments trace (Sc, Ti, V, Cr, Co, Ni, Zn, Sr, Y, Zr), ainsi que des échantillons de la vallée d'Aosta, en Italie, et des gisements classiques à travers le monde (en Russie, en Iran, au Pakistan, en Namibie, et au Madagascar). Ces cristaux sont tous de qualité gemme et vont du vert brillant au jaune-vert. Les cristaux sont homogènes compte tenu des erreurs analytiques, et leur champ de composition va de l'andradite presque pure (Adr ≥ 98%, base molaire) à des compositions de la série andradite-uvarovite (Adr 81-96 Uv 3-18 ) ou andradite-grossulaire (Adr 92-94 Grs 6-8 ), avec une seule exception, un échantillon de Namibie dont la composition varie de l'andradite presque pure à grossulaire (Adr 11 Grs 89 ). Tous les échantillons possèdent la plupart des éléments cités en faibles teneurs, sauf dans le cas du Cr, qui va de quelques ppm à plus de 5% de Cr 2 O 3 (poids). La substitution de Fe 3+ et de Cr 3+ au Al au site [Y] exerce un contrôle déterminant sur la géometrie des sites structuraux et sur le taux d'incorporation des terres rares. En particulier, (i) les échantillons ayant une composition voisine de l'andradite pure montrent un enrichissement en terres rares légères et un appauvrissement en terres rares lourdes, avec une forte ano...

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“…S1g. Sample VM-11, from the well-known demantoid locality at Val Malenco, Italy, also appears to contain very minor amounts of a fine hair-like phase (see also Adamo et al, 2009). Sample Bal-1, occurring as euhedral crystals with elongated {1 1 0} faces, from Balochistan, Pakistan, contains, as well, very small amounts of chrysotile and, in addition, tiny Cr-bearing magnetite crystals, as described by Adamo et al (2015).…”

Section: Sample Description and Optical Propertiesmentioning

confidence: 99%

“…The magnetic behavior of andradite from Val Malenco, Italy, was studied by low temperature 57 Fe-Mössbauer measurements (Murad, 1984), which show that the electron spins of the Fe 3þ cations order from the paramagnetic to antiferromagnetic state with a Néel transition temperature of 11.5 ± 0.1 K. The nature of this transition has also been investigated via ab initio calculations (Meyer et al, 2010). Finally, the UV/VIS spectrum of andradite has been measured multiple times (e.g., Manning, 1969;Burns, 1993;Adamo et al, 2009;Taran & Langer, 2000) focusing on the Fe 3þ -O chargetransfer as well as on the spin-forbidden bands.…”

Section: Introductionmentioning

confidence: 99%

Heat capacity and entropy behavior of andradite: a multi-sample and −methodological investigation

Geiger¹,

Dachs²,

Vielreicher³

et al. 2018

ejm

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Andradite, ideal end-member formula Ca 3 Fe 3þ 2 Si 3 O 12 , is one of the common rock-forming garnets found in the Earth's crust. There are several outstanding questions regarding andradite's thermodynamic and physical property behavior. Three issues are: i) Could there be differences in the thermodynamic properties, namely heat capacity, C p , between synthetic and natural andradite crystals, as observed in the Ca-garnet grossular, Ca 3 Al 2 Si 3 O 12 ? ii) What is the thermal nature of the low-temperature magnetic-phasetransition behavior of andradite? and iii) How quantitative are older published calorimetric (i.e., adiabatic and DSC) heat-capacity results? In this work, four natural nearly end-member single crystals and two synthetic polycrystalline andradite samples were carefully characterized by optical microscope examination, X-ray powder diffraction, microprobe analysis, and IR and UV/VIS single-crystal spectroscopy. The IR spectra of the different samples commonly show a main intense OH À stretching band located at 3563 cm À1 , but other OH À bands can sometimes be observed as well. Structural OH À concentrations, calculated from the IR spectra, vary from about 0.006 to 0.240 wt% H 2 O. The UV/VIS spectra indicate that there can be slight, but not fully understood, differences in the electronic state between synthetic and natural andradite crystals. The C p behavior was determined by relaxation calorimetry between 2 and 300 K and by differential scanning calorimetry (DSC) methods between 150/300 and 700/950 K, employing the same andradite samples that were used for the other characterization measurements. The low-temperature C p results show a magnetic phase transition with a Néel temperature of 11.3 ± 0.2 K, which could be slightly affected by the precise electronic state of Fe 2þ/3þ in the crystals. The published adiabatic calorimetry results on andradite do not provide a full and correct thermal description of this magnetic transition. The calorimetric C p measurements give a best estimate for the standard third-law entropy at 298.15 K for andradite of S o ≈ 324 ± 2 J/mol · K vs. the value of 316.4 ± 2.0 J/mol · K, as given in an early adiabatic investigation. Both natural and synthetic crystals give similar S o values within experimental uncertainty of about 1.0%, but one natural andradite, richer in OH, may have a very slightly higher value around S o ≈ 326 J/mol · K. Low-temperature DSC measurements made below 298 K agree excellently with those from relaxation calorimetry. The DSC measurements above 298 K show a similarity in C p behavior among natural and synthetic andradites. A C p polynomial for use above room temperature to approximately 1000 K was calculated from the data on synthetic andradite giving: C p (J/mol · K) = 599.09 (±14) À 2709.5 (±480) · T À0.5 À 1.3866 (±0.26) · 10 7 · T À2 þ 1.6052 (±0.42) · 10 9 · T À3 .

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Demantoid from Val Malenco, Italy: Review and update

Cited by 19 publications

References 7 publications

Nephrite Jade From Val Malenco, Italy: Review and Update

Nephrite Jade From Val Malenco, Italy: Review and Update

The Profile of Trace Elements, Including the Ree, in Gem-Quality Green Andradite From Classic Localities

Heat capacity and entropy behavior of andradite: a multi-sample and −methodological investigation

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