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 EMPAWDS data show that these green garnets are almost pure andradite [Adr (Ca3Fe2Si3o 12)>96.1 mol%]. The Cr-content ranges, as Cr2O 3, 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 Crcontent 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-].
Green andradite stones: gemmological and mineralogical characterisation / I. Adamo, G.D. Gatta, N. Rotiroti, V. Diella, A. Pavese. - In: EUROPEAN JOURNAL OF MINERALOGY. - ISSN 0935-1221. - 23:1(2011 Feb), pp. 91-100. [10.1127/0935-1221/2011/0023-2079]
Green andradite stones: gemmological and mineralogical characterisation
I. AdamoPrimo
;G.D. Gatta
;N. Rotiroti;A. PaveseUltimo
2011
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 EMPAWDS data show that these green garnets are almost pure andradite [Adr (Ca3Fe2Si3o 12)>96.1 mol%]. The Cr-content ranges, as Cr2O 3, 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 Crcontent 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-].File | Dimensione | Formato | |
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