Natural and synthetic gem-quality spinel single crystals were investigated by electron-microprobe analysis and Raman spectroscopy to study the dependence of Raman spectra on chemical composition.The composition of natural samples is dominanted by spinel s.s. (MgAl2O4) or gahnite (ZnAl2O4) end-member component. The remaining composition consists in hercynite (FeAl2O4) or Magnesiochromite (MgCr2O4) component, with a minor content of Mn2+, Co2+, V3+, Fe3+, which strongly control the colours assumed by these samples. The composition of synthetic samples belong to the MgAl2O4-CoAl2O4 (blue), MgAl2O4-FeAl2O4 (green) and MgAl2O4-MgCr2O4 series (red).Raman spectra of the natural and synthetic spinels display only few of the five Raman-active modes predicted for the general oxide spinel group of minerals. Natural spinels with spinel s.s. composition show four peaks at ~310, 405, 660, and 765 cm-1 attributed to the T2g (1), Eg, T2g (3) and the A1g modes, respectively. Natural spinels with gahnite composition show only three of the five Raman-active modes at ~ 415, 510, 655 cm-1 attributed to the Eg, T2g (2) and T2g (3) modes, respectively. Consequently, discrimination among spinels with different principal component is easily possible looking at the Raman shift. On the contrary, discrimination among spinels with the same principal component and different minor elements, which cause the difference in colors, is not straightforward and requests more studies. What is possible and quite successful is discrimination between natural, treated and synthetic spinels with similar composition. In fact, synthetic spinels, grown in the 1200-900°C thermal range, show a small cation disorder which cause a broadening of the bands and the occurrence of a new peak at ~ 727 cm-1, not predicted by the group theory. In the photoluminesce (PL) spectra, almost all natural and synthetic spinels show the characteristic Cr3+ bands between 5700 and 8000 cm-1, known by gemologists as 'organ pipes'. However, in the synthetic samples the disorder causes a broadening of more intense bands, a merging of the less intense ones and a shift of ~ 60 cm-1 towards IR of all of them.
Raman investigation of natural and synthetic gem-quality spinels / D'Ippolito, Veronica; Andreozzi, Giovanni Battista; P. P., Lottici. - (2013). (Intervento presentato al convegno Gemme: dalla identificazione alla tracciabilità. Problematiche gemmologiche, commerciali e legislative tenutosi a Napoli nel 29/09/2013-01/10/2013).
Raman investigation of natural and synthetic gem-quality spinels
D'IPPOLITO, VERONICA;ANDREOZZI, Giovanni Battista;
2013
Abstract
Natural and synthetic gem-quality spinel single crystals were investigated by electron-microprobe analysis and Raman spectroscopy to study the dependence of Raman spectra on chemical composition.The composition of natural samples is dominanted by spinel s.s. (MgAl2O4) or gahnite (ZnAl2O4) end-member component. The remaining composition consists in hercynite (FeAl2O4) or Magnesiochromite (MgCr2O4) component, with a minor content of Mn2+, Co2+, V3+, Fe3+, which strongly control the colours assumed by these samples. The composition of synthetic samples belong to the MgAl2O4-CoAl2O4 (blue), MgAl2O4-FeAl2O4 (green) and MgAl2O4-MgCr2O4 series (red).Raman spectra of the natural and synthetic spinels display only few of the five Raman-active modes predicted for the general oxide spinel group of minerals. Natural spinels with spinel s.s. composition show four peaks at ~310, 405, 660, and 765 cm-1 attributed to the T2g (1), Eg, T2g (3) and the A1g modes, respectively. Natural spinels with gahnite composition show only three of the five Raman-active modes at ~ 415, 510, 655 cm-1 attributed to the Eg, T2g (2) and T2g (3) modes, respectively. Consequently, discrimination among spinels with different principal component is easily possible looking at the Raman shift. On the contrary, discrimination among spinels with the same principal component and different minor elements, which cause the difference in colors, is not straightforward and requests more studies. What is possible and quite successful is discrimination between natural, treated and synthetic spinels with similar composition. In fact, synthetic spinels, grown in the 1200-900°C thermal range, show a small cation disorder which cause a broadening of the bands and the occurrence of a new peak at ~ 727 cm-1, not predicted by the group theory. In the photoluminesce (PL) spectra, almost all natural and synthetic spinels show the characteristic Cr3+ bands between 5700 and 8000 cm-1, known by gemologists as 'organ pipes'. However, in the synthetic samples the disorder causes a broadening of more intense bands, a merging of the less intense ones and a shift of ~ 60 cm-1 towards IR of all of them.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
