Natthapong Monarumit

Oxidation states of Fe and Ti in blue sapphire

X-ray absorption near-edge spectroscopy (XANES) can be used to study the oxidation state of a dilute system such as transition metal defects in solid-state samples. In blue sapphire, Fe and Ti are defects that cause the blue color. Inter-valence charge transfer (IVCT) between Fe2+ and Ti4+ has been proposed to describe the optical colors origin. However, the existence of divalent iron cations has not been thoroughly investigated. Fluorescent XANES is therefore employed to study K-edge absorptions of Fe and Ti cations in various blue sapphire samples including natural, synthetic, diffused and heat-treated sapphires. All the samples showed an Fe absorption edge at 7124 eV, corresponding to the Fe3+ state; and Ti at 4984 eV, corresponding to Ti4+. From these results, we propose Fe3+-Ti4+ mixed acceptor states located at 1.75 eV and 2.14 eV above the valence band of corundum, that correspond to 710 nm and 580 nm bands of UV–vis absorption spectra, to describe the cause of the color of blue sapphire.

Oxidation State of Ti Atoms and Ti-O Bond Length on Natural Sapphire Gem-Materials Probed by X-Ray Absorption Spectroscopy

Sapphire, an inorganic gem-material in a variety of corundum, mainly consists of alpha-alumina (α-Al2O3) structure. The geological origins of sapphire are related to either basaltic or metamorphic rocks. The causes of the color on sapphire are some trace elements such as Cr, Fe, and Ti. It could be mentioned that Ti atoms have cooperated with Fe atoms for creating the blue color. In this study, X-ray absorption spectroscopy (XAS) technique focused on the x-ray absorption near edge structure (XANES) and the extended x-ray absorption fine structure (EXAFS) is employed to identify the oxidation state of Ti atoms and Ti-O bond length on sapphire samples. The Ti K-edge XANES and EXAFS spectra of natural sapphires were carried out using the 13-channel array germanium detector in fluorescence mode. The XANES spectra showed that the oxidation state of Ti was Ti4+ regardless of Fe content. Moreover, the Ti-O bond length on a-Al2O3 was equal to the Ti-O bond length on rutile (TiO2) analyzed from the EXAFS spectra. From these results, it could be concluded that the oxidation state of Ti atoms on natural sapphires was Ti4+ which substitutes Al3+ on the sapphire structure.

FWHM Calculation of Zircon Gem-Materials before and after Thermal Enhancement

Zircon samples from Ubon Ratchathani, Thailand; Rattanakiri, Cambodia and Dak Nong, Vietnam change their color from light brown and reddish-brown to blue color after thermal enhancement at 1000 ๐C in reducing condition for 60 min. The high temperature is one of the factors for the zircon structure to recrystallize. The objective of this study is to describe the crystal structure of zircon samples before and after thermal enhancement. Zircon is a metamict mineral whose structure is destroyed by some trace elements. There are radioactive elements such as U and Th in the zircon structure. In this study, Raman spectroscopy was used to analyze the molecular vibration in zircon structure before and after thermal enhancement. As a result, the Raman spectra of zircon samples after thermal enhancement show the Raman shift at peak position of V3(SiO4) stretching around 1008cm-1to higher wavenumber concerning to the full width at half maximum (FWHM) values calculated by PyMCA software. The results could be summarized that the metamict zircon will be recrystallized to the crystalline zircon after thermal enhancement. The advantage of this study is about the identification of zircon before and after thermal enhancement.

Ancient Glass Bead from U-Thong Ancient City Site, Central Thailand

The goal of this research is to build a database for the ancient glass beads from U-Thong excavation site (Dvaravati Period, 6th to 13th centuries). We can compare the chemical properties among the ancient glass beads from different sites in order to learn about the ancient trading route of those glass beads. The 30 samples and 12 colors (translucent to opaque and various colors) were collected from U-Thong archaeological site, Suphanburi Province, Central Thailand. The ancient glass bead samples were studied about the physical and chemical characteristics as well as the surface features. The glass beads were analyzed by the scanning electron microscope coupled with energy dispersive x-ray spectrometer (SEM-EDS), Fourier transform infrared spectrometer (FTIR) and Atomic Force Microscope (AFM). The chemical composition of the dark blue bead for example contained 2.25 wt% MgO, 90.98 wt% SiO2, 1.26 wt% K2O, 4.17 wt% CaO and 0.59 wt% CuO. The FTIR spectrum of ancient glass beads is rarely to interpret their molecular vibration. The AFM allowed us to see the glass corrosion in nano scale on the surface. This research can help Thai archaeologists to identify the characteristics of ancient glass bead in the Central Thailand.

Internal Features of Glass Filled Ruby Samples Probed by EPMA

Glass filled ruby is one of the treated rubies that consist of the additive substance to improve the clarity of ruby. There are many variations of glass filled rubies based on their chemical additives to fulfill the fractures during heat treatment process. However, the quality and the price of ruby are related to the different types of glass materials. Thus, the aim of this study is important to classify the type of glass materials inside the ruby samples by a non-destructive property such as their internal features. In this research, the glass filled ruby samples were collected including lead-glass filled rubies and high alumina-glass filled rubies. The surface and internal features in macro scale of the samples were observed by gem microscope. Moreover, their internal features were magnified to detect the chemical content of fractures and groundmass using electron probe micro analyzer (EPMA). As the results, there are high PbO and SiO2 in the fractures of lead-glass filled rubies. Besides, there are high Al2O3 mixing with SiO2 in the fractures of high alumina-glass filled rubies. It could be summarized that the varieties of glass filled ruby samples have been classified by the glass materials in their fractures.

The Phase Transition of Corundum-Eskolaite Mineral Series Probed by X-Ray Absorption Spectroscopy

The red color of ruby is caused by Cr3+ impurities substituting Al3+ in Al2O3 structure. The complete substitution of Al3+ by Cr3+ produces Cr2O3 that has the green color. In this study, we focused on the phase transition of solid solution between corundum (Al2O3) and eskolaite (Cr2O3) mineral series. A series of Al2O3-Cr2O3 samples were prepared by mixing fine powder of Al2O3 and Cr2O3 in various ratios. They were ground together and then recrystallized. The obtained crystals were ground again in powder form for X-ray absorption near edge structure (XANES) measurements to avoid polarization dependence. The measurements were carried out at the Cr K-edge absorption energy using a 13-channel array Ge detector in the fluorescence mode. The XANES spectra showed line-shape transitions for 0.125 to 100% variation of Cr2O3. Significant variations in transitions were found when the Cr2O3 content was between 20% and 80%. With Cr2O3 concentration increasing, the sample colors apparently turned from pink to dark green. CIELAB color index measurements were performed. The variations of line-shape spectra of the Al2O3-Cr2O3 samples were found to be consistent with the CIELAB color index results. This Cr K-edge XANES study could be useful for the research in mineral sciences.

Combination of FTIR and SEM for Identifying Freshwater-Cultured Pearls from Different Quality

The freshwater-cultured pearl (Chamberlainia hainesiana species) is an organic gemstone mainly composed of calcium carbonate mineral including calcite, aragonite and vaterite phases. Generally, the quality of freshwater-cultured pearl is based on its luster. The high luster pearl is full of the aragonite phase without vaterite phase. On the other hand, the low luster pearl consists of aragonite and vaterite phases. These data could be proved by the Fourier Transform Infrared (FTIR) spectroscopy combined with the scanning electron microscopy (SEM). As the results, the high luster pearl similarly shows the FTIR spectrum of aragonite phase, and also, it shows the hexagonal shape of aragonite for the SEM image. On the other hand, the FTIR spectrum of low luster pearl has been pointed to the mixture component among aragonite and vaterite phases, and based on the SEM image; the irregular form is also interpreted to the mixture of aragonite and vaterite phases. This research concludes that the quality of freshwater-cultured pearls can be identified by the combination data of FTIR spectra and SEM images. These techniques are suitable for applied gemology.