The Vesuvianite Gems of the Val d’Ala (Piedmont, Italy)

Abstract

In Val d’Ala (Western Alps in Piedmont, Italy), the most interesting rocks for mineralogical research are represented by rodingite (rich in mineralized veins and fractures) associated with serpentinite in the eclogitized oceanic crust of Piedmont Zone, south of Gran Paradiso Massif. Among the vein-filling minerals, vesuvianite is well appreciated for its potential as gem-quality materials, even though it has never been characterized in detail. This study provides a gemological characterization of eleven vesuvianite crystals from different localities of the Val d’Ala. The refractive index (1.717–1.708) and density (1.705–1.709) values of our vesuvianite are in the range of those in the literature. Scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) established that the samples are pretty compositionally homogeneous in terms of major elements, while trace and rare earth elements (REE) contents are more variable. All REE patterns are characterized by pronounced positive Eu anomalies. The variations in color (from olive green to dark green with chocolate (reddish-brown color shades and polychrome bands) are due to the relevant presence of Fe and, to a lesser extent, Ti and Cr. The X-ray powder diffraction (XRPD) analyses and SEM/EDS quantitative study indicate that the other phases associated with vesuvianite are represented by diopside, garnet, clinochlore.

In situ Sr–Nd isotope analysis of vesuvianite by LA-MC-ICP-MS: methodology and application

“…To assess the reliability of our methods, an in-house vesuvianite Fig. 5 Plots of (a) Rb-Sr and (b) Sm-Nd for the vesuvianite samples, including data from Halama et al, 6 Fukuyama et al, 7 Guo et al, 8 Caucia et al, 9 Xing et al, 13 Zhang et al, 14 Ma et al, 21 Su et al, 74 Xu et al, 75 and He et al The M6635 vesuvianite has a homogeneous Sr isotopic composition and was used to externally calibrate the other data during this study. Aer the corrections, the 87 Sr/ 86 Sr ratios calculated from the LA data are consistent with those obtained from the solution analyses (Fig.…”

“…10 The high isomorphic capacity and stability of vesuvianite favours the incorporation of variable amounts of U (several tens or hundreds of mg g −1 ), making it a potential mineral for U-Pb dating. [11][12][13][14] As a Ca-rich mineral, vesuvianite has moderate or high Sr (several tens to thousands of mg g −1 ) and low Rb (<1 mg g −1 ) contents, 6,7,9 which mean the measured 87 Sr/ 86 Sr ratio represents the initial Sr isotopic composition of the uid or melt from which the vesuvianite crystallised. 15,16 Furthermore, vesuvianite can be enriched in light rare earth elements (LREEs).…”

“…2,6 However, these complexities make it possible to constrain the formation environment of vesuvianite and the geochemistry of the uid or melt from which it crystallised. [6][7][8][9] Vesuvianite can also act as a geothermometer owing to the sensitivity of its symmetry to the crystallisation temperature. 10 The high isomorphic capacity and stability of vesuvianite favours the incorporation of variable amounts of U (several tens or hundreds of mg g −1 ), making it a potential mineral for U-Pb dating.…”

“…2,6 However, these complexities make it possible to constrain the formation environment of vesuvianite and the geochemistry of the fluid or melt from which it crystallised. 6–9 Vesuvianite can also act as a geothermometer owing to the sensitivity of its symmetry to the crystallisation temperature. 10…”

“…Fig.5Plots of (a) Rb-Sr and (b) Sm-Nd for the vesuvianite samples, including data from Halama et al,6 Fukuyama et al,7 Guo et al,8 Caucia et al,9 Xing et al,13 Zhang et al,14 Ma et al,21 Su et al,74 Xu et al,75 and He et al76 …”