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Physico-chemical characterization of iranian turquoises : a tentative to trace middle-eastern turquoise-bearing artifacts

Abstract : This study consists in the comparison of turquoise CuAl6 (PO4) 4 (OH) 8.4H2O from various historic mines in Iran (Persia) to those of other regional localities in order to potentially find discriminating characteristics making it possible to identify the origin of turquoise-bearing jewelry or artifacts. Turquoise mine samples were collected from three main sources: Neyshapour, Damghan and Kerman and of different colors. In addition, turquoise jewels from the Musée des Arts Décoratifs de Paris (MAD) were as well analyzed. Both Raman and FTIR vibrational spectroscopy allows to identify the structure of turquoise. Although FTIR does not show any particular differences between the different colors of turquoise, Raman analyzes of the blue and green turquoise samples show that the linewidth of the blue sample seems much more resolved than that of green. This indicates that the network and the modes of vibration OH and H2O are sensitive to the substitutions inducing a relative crystalline disorder. Absorption spectroscopy reveals that even the turquoise with the best blue color has a slight absorption of Fe3 +. The influence of divalent copper ions and trivalent iron on the color of turquoise is explained. According to the optical absorption results, Fe2+ has no particular effect on the color of the turquoise; the substitution Fe3+ / Al changes the color of the turquoise to a green color. In addition, it has been shown that elementary micro laser induced degradation spectroscopy (LIBS) maps allow rapid identification of the turquoise phases (correlated Cu, Al and P) as well as those of pyrite (FeS2) and silicate. Slight interference between P, Al and Fe reveals the slight substitution of Al by Fe. Non-destructive chemical analyzes carried out by XRF, made it possible to quantify the major elements (Al, P, Cu) and the trace elements (Fe, Zn, As, Ca, Ti). Correlation and PCA analyzes were conducted using measured data as well as bibliographic data. MAD turquoise jewelry overlays the turquoise domains of Neyshapour (Fe / Cu vs Zn / Cu; Zn / Cu vs As / Cu; Zn / Cu vs Ca / Cu). The Kerman turquoises do not match, as do the Chinese turquoises. It is therefore possible to suggest that the turquoise stones in MAD jewelry prior to the 19th century were turquoise stones of Persian origin
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Niloofar Mousavipak. Physico-chemical characterization of iranian turquoises : a tentative to trace middle-eastern turquoise-bearing artifacts. Material chemistry. Université de Lyon, 2020. English. ⟨NNT : 2020LYSE1021⟩. ⟨tel-02487545⟩

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