Affiliation:
1. School of Gemmology, China University of Geosciences (Beijing), Beijing 100083, People's Republic of China
Abstract
Thirty-five gem-quality turquoise samples with various colours were investigated using energy-dispersive X-ray fluorescence spectroscopy, ultraviolet–visible spectroscopy, Fourier-transform infrared spectroscopy and scanning electron microscopy. Sample chemical and spectral analyses indicate that Fe
3+
contributes to green hue of turquoise, whose absorption band exhibits a bathochromic shift from 426 to 428 nm with increasing V content in the solid-solution series turquoise-chalcosiderite. V
3+
enhances absorption in the blue and orange regions, and Cr
3+
increases absorption in the green region, both of which are responsible for the vivid greenish yellow in faustite. Substitutions of Al by medium-sized trivalent cations (primarily Fe
3+
and V
3+
) enhance polarity of the phosphate group (PO
4
)
3−
, resulting in strong absorption in the infrared spectra for analogues of turquoise. The reflectivity ratio (
R
OH
) of the double absorption peaks at 781 and 833 cm
−1
allows faustite to be distinguished from turquoise and chalcosiderite, with a value greater than 1, while V-rich faustite only has a single absorption peak at 798 cm
−1
. An increasing amount of absorbed water contributes to blue chroma in turquoise and has a negative effect on lightness based on the CIE 1976
L
*
a
*
b
* colour system. Loose turquoise with a low specific gravity tends to display greater colour differences with a significant decrease in lightness.
Cited by
15 articles.
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