Green andradite stones: gemmological and mineralogical characterisation

Abstract

A multi-methodological investigation of natural gem-quality andradite crystals occurring in serpentinites (Val Malenco, Italy; Nizhniy Tagil, Russia; Kerman, Iran, and Kaghan, Pakistan), or in skarn rocks (Erongo, Namibia, and Antetezambato, Madagascar) has been performed by means of gemmological standard testing, electron-microprobe chemical analyses in wavelength-dispersive mode (EMPA-WDS), ultraviolet, visible and near-infrared (UV-Vis-NIR) and mid-infrared (MID-IR) spectroscopy, and single-crystal X-ray diffraction, in order to describe the gemmological properties and the crystal chemistry of these marketable green stones. The EMPA-WDS data show that these green garnets are almost pure andradite [Adr (Ca3Fe2Si3O12)>96.1 mol%]. The Cr-content ranges, as Cr2O3, from <0.01 up to ~1.0 wt%. This chromophore element, if present, influences the garnet’s colour, which is mainly due to Fe3+. The Cr-content does not affect the unit-cell constant, at least within the range found here. The refined electron content at the Fe-site suggests that chromium occupies the octahedral site along with iron, as found in previous investigations. The MID-IR spectra show the presence of hydroxyl groups. The refined site occupancy factors of the Si-site, modelled with the scattering curve of silicon alone, range between 99.2(4) and 99.9(2), which barely suggests potential hydrogarnet substitution [ i.e. , (SiO4)4− ↔(O4H4)4−].