Abstract

Near infrared (NIR) spectroscopy of turquoise minerals from Arizona and Senegal with a formula of Cu(Al_6–x,Fe_x)(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.

© 2006 NIR Publications

Near-infrared, visible, and ultraviolet lidar echo emulator

Mónica Rodríguez-Cortina, Pawel Adamiec, Juan Barbero, Manuel Caño-García, Xabier Quintana, and Morten Andreas Geday
Opt. Express 30(2) 2173-2185 (2022)

In situ chemical analysis of geology samples by a rapid simultaneous ultraviolet/visible/near-infrared (UVN) + longwave-infrared laser induced breakdown spectroscopy detection system at standoff distance

Clayton S. C. Yang, Feng Jin, Sudhir Trivedi, Ei Brown, Uwe Hömmerich, Laszlo Nemes, and Alan C. Samuels
Opt. Express 27(14) 19596-19614 (2019)

Mid-infrared near-field spectroscopy

Sergiu Amarie, Thomas Ganz, and Fritz Keilmann
Opt. Express 17(24) 21794-21801 (2009)

Multiple Monochromators. IV. A Triple Monochromator and Its Application to Near Infrared, Visible, and Ultraviolet Spectroscopy

A. Walsh and J. B. Willis
J. Opt. Soc. Am. 43(11) 989-992 (1953)

Toward a UV–visible–near-infrared hyperspectral imaging platform for fast multiplex reflection spectroscopy

Jianping Li and Robert K. Y. Chan
Opt. Lett. 35(20) 3330-3332 (2010)