Characterization of tourmalines enhancement and study on correlations between structural and chemical behavior by neutron diffraction

다국어 초록 (Multilingual Abstract)

Tourmaline is an interesting gem mineral due to can be found in some of the most vibrant colors, as well as every color of the rainbow. Most color centers of tourmaline are related to transition metal ions. Oxidation/reduction of these ions has been known to be related with color enhancement of tourmaline. However, the knowledge concerning the microscopic structure of the color centers remain confusing, especially in mechanism of color changing under enhancement process. Therefore, the information on crystals and chemicals behavior of tourmaline enhancement by irradiation and heat treatment have been examined by using neutron scattering techniques such as HRPD including with XRD, EDXRF, XPS, EPR UV-vis and FTIR spectroscopy in this study. The mechanism of color change on three types of tourmaline gemstones: two pink from Afghanistan and one green from Nigeria were described.
Firstly, γ-irradiations were performed on three tourmaline samples. The pink tourmaline of the higher Mn ions (T2, 0.24 wt%) showed significant improvement in pink color quality (rubellite) after irradiation of 800 kGy, while the pink tourmaline of the low MnO content (T1, 0.08 wt%) showed a color adulteration. Pink color enhancement in T2 showed the increase of the two absorption bands peaking at 396 and 522 nm after irradiation which respond to darker pink. These absorption bands ascribed to d-d transitions of divalent manganese. T1 with color enhancement due to oxidation of Mn2+ show slightly larger in the <Y-O> distance. The green one containing a much higher amount of both Mn (T3) and Fe ions, 2.59 wt% and 5.7 wt% respectively, changed to a yellow color after irradiation 800 kGy. The refined structural parameters of this sample revealed the distortions in Z site. The <Z-O> distance decreased from 2.033 to 2.0192 Å. In addition, the unit-cell parameter decreased after irradiation. The color change in green tourmaline (T3) is ascribed to the decrease of the absorption band intensity in the red color region (600~750 nm). XPS measurement results also supported that the relative ratios of Fe2+/Fe3+ [Fe3+ (Fe2p3/2 711.2 and Fe2p1/2 724.3 eV), Fe2+ (Fe2p3/2 710.2 and Fe2p1/2 722.8 eV)] and Mn2+/Mn3+ [Mn2+ (Mn2p3/2 641.4 and Mn2p1/2 652.3 eV), Mn3+ (Mn2p3/2 641.9 and Mn2p1/2 653.3 eV)] peak intensities decreased after irradiation.
Next step, the recovering color mechanism of heat treatment tourmaline by electron irradiation were observed in three tourmaline samples. The pink tourmaline of the higher Mn ions (T2, 0.24 wt%) can recovery pink color by irradiated with electron 800 kGy due to behavior of Mn2+ and Mn3+ which respond in absorption band at 396 and 522 nm after became colorless by heating at 600 °C in air for 3 h. Both absorption bands disappear after heat treatment and appear again after electron irradiation. The EPR spectra exhibiting resonance signals at g » 2.0 and 3.5 indicated the valence state change of Mn2+ after treatment. The signal at g » 2.0 significantly increased indicated that reduction of Mn3+ occur during heating. When subsequently irradiated with electron 800 kGy decreasing in intensity indicate occurring the oxidation of Mn2+ by electron capture. Mid-infrared spectra indicate affect of heat treatment and electron irradiation to YM and ZM metallic ions. For the pink tourmaline of the low MnO content (T1, 0.08 wt%) showed yellow color after heating at 600 °C in air for 3 h and subsequently irradiated with electron 800 kGy. The UV-Vis spectra show a tail extending into the UV spectral range, which originated from an edge of a small broad absorption band at 522 nm for this color change. These behavior correlated with the O- hole trap center which located in the OH site. After heating at 600 °C in air for 3 h, the Mid-infrared bands at 4600, 4530, 4440 and 4340 cm-1 show increasing in intensity due to receiving excited energy. After subsequently irradiated with electron 800 kGy, the OH- molecule in the OH site were decomposed by electron into O- (hole trap) and Ho (electron trap) resulting in decreasing of intensity. In case of green tourmaline, the green color appearing due to the absorption band intensity in the red color region (600~750 nm), which can be ascribed to the valence state change Fe2+ ® Fe3+. The effect of various treatment response to this absorption band intensity but color changed invisible. The EPR spectra respond to broad resonance signals due to high composition of Mn and Fe metal ions. Mid-infrared spectra in the ranges of 3800-3400 cm-1 and 4800-4000 cm-1 are related to hydroxyl stretching modes.
Finally, heat treatment and irradiation (electron/gamma ray) can alternate in the process of color enhancement for tourmaline. The transformation of color may occur differently depending on the various conditions, such as the temperature and time of heat treatment, the dose and type of irradiation, the procedure of irradiation and heat treatment, and the quantity of impurities present in the mineral.

목차 (Table of Contents)

• ABSTRACT
• LIST OF TABLES
• LIST OF FIGURES
• CHAPTER
• 1. Introduction
• 2. Tourmaline enhancements
• 3. Methodology for characterization of Tourmaline enhancements
• 4. Crystal Structure and Color Enhancement Studies in γ-Irradiated Tourmaline by Neutron Diffraction
• 5. UV-Visible, EPR, and Mid-IR Spectroscopic Characterization of Elbaite Tourmaline Heat Treatment and Electron Beam Irradiation
• 6. Summary
• REFERENCES
• ACKNOWLEDGEMENTS
• CURRICULUM VITAE