The nature of pigments in naturally colored pearls is still under discussion. For this study, Raman scattering measurements were obtained for 30 untreated freshwater cultured pearls from the mollusk Hyriopsis cumingi covering their typical range of colors. The originality of this work is that seven different excitation wavelengths (1064 nm, 676.44 nm, 647.14 nm, 514.53 nm, 487.98 nm, 457.94 nm, 363.80 nm) are used for the same samples at the highest possible resolution. All colored pearls show the two major Raman features of polyenic compounds assigned to double carbon-carbon (C C) -at about 1500 cm −1 -and single carbon-carbon (C-C) -at about 1130 cm −1 -bond stretching mode, regardless of their specific hue. These peaks are not detected in the corresponding white pearls, and therefore seem directly related to the major cause of body color. Additionally, the exact position of C C stretching vibration shows that these compounds are not members of the carotenoid family. Moreover, some changes are observed in intensities, shape and positions of the two main characteristic polyenic peaks from one sample to the next. Similar changes are observed also using several excitation wavelengths for the same point of the same pearl. The exact position of C-C stretching vibration of polyenic molecules depends strongly on the number of double bonds (N) contained in their polyenic chain. Hence, using a constrained decomposition of this band for different excitation wavelengths, up to nine different pigments may be detected in the same pearl. Their general chemical formula is R-(-CH CH-) N -R with N = 6-14. All our colored samples contained at least four pigments (N = 8-11). Different colors are explained by different mixtures, not by a simple change of pigment. The chemical nature of the chain ends is still unknown, because it cannot be detected with Raman scattering. However, it is possible that these polyenes are complexed with carbonate molecules of the nacre. Similar coloration mechanisms are found in products from other living organisms (e.g. parrots feathers). Moreover, it seems that a similar series of pigments is found in other pearls also, as well as in some marine animals living in similar environments (e.g. corals).

The exact nature of pigments present in cultured freshwater pearls is still not well known. We examined 21 untreated cultured freshwater pearls from Hyriopsis of typical colors by diffuse reflectance UV-Vis-NIR and Raman scattering measurements, at high resolution. The objective was to establish the relation between color and the nature of the pigment mixture in pearls, using strictly non-destructive methods. All natural color samples show the two major Raman resonance features of unmethylated (unsubstituted) polyenes, not carotenoids. Their general chemical formulae are R-(-CH¼CH) N-R 0 with N ¼ 6 to 14 and they give absorptions from violet to yellow-green. Each color is due to a mixture of pigments, not a single pigment. Different colors are explained by different mixtures. Each pigment identified by Raman spectroscopy can be related to a specific absorption with apparent maximum in the range 405-568 nm, thus absorbing from violet to yellow-green. This is the first study of the precise relation between Raman and diffuse reflectance spectra of cultured freshwater pearls.

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Late palaeozoic magmatism in the basement rocks Southwest of Mt. Olympos, Central Pelagonian zone, Greece: Remnants of a permo-carboniferous magmatic arc. ABSTRACTWe dated basement rocks from several localities southwest of Mt. Olympos, as well as from a locality near the top of the mountain using the single zircon Pb/Pb evaporation technique. For the samples southwest of the mountain, the ages obtained range from ca. 280 to 290 Ma, with only a few zircon grains being around 300 Ma. By contrast, the sample from near the top of the mountain appears to be slightly younger, with ca. 270 Ma. These ages imply that the granitoids crystallized during Late Carboniferous -Early Permian times, and are therefore younger than the basement gneisses of other regions of the Pelagonian zone, which yielded zircon ages of around 300 Ma (e.g. Yarwood & Aftalion 1976, Mountrakis 1983, De Bono 1998, Engel & Reischmann 2001. However, the ages obtained in the present study are identical, within error, to the muscovite Ar-Ar cooling ages from Mt. Ossa (Lips 1998). Our geochronological data show that the magmatic evolution for this part of the basement of the Pelagonian Zone lasted at least 30 Ma. 985http://epublishing.ekt.gr | e-Publisher: EKT | Downloaded at 11/07/2020 07:36:31 | -988http://epublishing.ekt.gr | e-Publisher: EKT | Downloaded at 11/07/2020 07:36:31 | -990http://epublishing.ekt.gr | e-Publisher: EKT | Downloaded at 11/07/2020 07:36:31 | -992http://epublishing.ekt.gr | e-Publisher: EKT | Downloaded at 11/07/2020 07:36:31 |