Andradite variety demantoid from Val Malenco, Sondrio, Italy

Demantoid is the yellowish green to green variety of andradite, a species belonging to the garnet group, with ideal chemical composition Ca3Fe2(SiO4)3. Because its rarity and incomparable brilliance, resembling that of diamond, demantoid is the most highly prized of all garnets. Among the most notable localities for andradite variety demantoid, there is Val Malenco, in the province of Sondrio, Lombardy, northern Italy, where the mineral is found in well-shaped crystals contained into asbestos-filled lithoclases crosscutting the outcrops of the serpentinites formation (Amthauer et al., 1974; Bedogné et al., 1993). This study aims to provide an overview of andradite variety demantoid from Val Malenco, Italy, investigating a suite of gem-quality samples from this locality, including both faceted gemstones and roughs, by means of EMP and LA-ICP-MS chemical analyses, X-ray powder diffraction, and UV-Vis-NIR and mid-IR spectroscopic measurements. For comparison, we studied also demantoids from other significant localities, including Ural Mountains (Russia), Erongo Mountains (Namibia), Kerman Province (Iran), Pakistan, and Val d’ Aosta (Italy). The investigated samples from Val Malenco have a wide range of colors, varying from yellowish or brownish green to green. The faceted gems are all singly refractive with moderate to strong anomalous double refraction, with refractive indices >1.79 and density from 3.79 to 3.88 g/cm3. The space group Ia3d with unit cell parameters a=b=c= 12.0623(1)-12.0629(1) Å, and V= 1755.05(4)-1755.33(4) Å3 are typical of andradite garnet (Deer et al., 1997), as well as the mid-IR pattern (Moore et al., 1971). The spectrum also shows absorption features related to hydroxide, incorporated as minor component, in the form of structurally bonded OH- groups (Amthauer and Rossman, 1998). In a sample we also found dark nuclei of chromium-bearing magnetite which spread their Cr contents into the garnet. Chemical analyses show that all samples from different localities are almost pure andradites (Adr≥ 90%), containing only minor amounts of Mg, Al, Cr, Ti and Mn. The Cr content is variable between different specimens and it is positive correlated with the intensity of green coloration. Demantoids are characterized by variable ∑REE and by LREE-enriched and HREE-depleted patterns, with a positive Eu anomaly in most samples, typical of Fe-rich garnets (Gaspar et al., 2008). The exceptions are demantoids from Val d’ Aosta and Russia, containing relatively higher Al amounts and whose patterns show a general tendency to be enriched in HREE, with no Eu anomaly. The UV-Vis-NIR spectra of various demantoids, containing different Cr amounts, indicate that the chromophore ions Fe3+, intrinsic to the structure, and Cr3+, as trace element, are the primary causes of color.