Ultraviolet-Visible, near Infrared and Mid Infrared Reflectance Spectroscopy of Turquoise

Abstract

Near infrared (NIR) spectroscopy of turquoise minerals from Arizona and Senegal with a formula of Cu(Al6–x,Fex)(PO4)4(OH)8.4H2O has been studied and a comparison of their NIR, ultraviolet-visible (UV-vis) and mid-IR spectra made. UV-vis and NIR reflectance spectroscopy is the most effective approach for the detection and determination of transition metal ions that cause colour to minerals and semi-precious gems such as turquoise. The UV-vis spectra of the two turquoise minerals (blue and green) reported in this research are distinctly different. The spectra confirm the difference in the origin of colour of the minerals. The paramagnetism of the blue mineral arises mainly from divalent copper. Turquoise from Senegal, West Africa has a replacement of some Al(III) by Fe(III) in the structure. NIR spectroscopy identifies the bands attributed to the hydroxyl units. The NIR spectroscopic technique is most sensitive in detecting Fe2+, even though it is present in very low concentrations in turquoise from Senegal. Mid-IR spectra of the two mineral samples are very similar in the 1200 to 900 cm−1 region but strong differences are observed in the 900 to 100 cm−1 region. The effect of substitution of Fe for Al in turquoise from Senegal shifts the bands to lower wavenumbers. Factor group analysis (FGA) implies four OH stretching vibrations for both the water and hydroxyl units. Two bands ascribed to water in these minerals at ∼ 3280 and 3070 cm−1 and three hydroxyl stretching vibrations are observed. The combination of these bands and their fundamental overtones give rise to many of the features in the NIR spectra.