The impact of trace metal cations and absorbed water on colour transition of turquoise.

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 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 enhances absorption in the blue and orange regions, and Cr 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 and V) enhance polarity of the phosphate group (PO), resulting in strong absorption in the infrared spectra for analogues of turquoise. The reflectivity ratio ( ) of the double absorption peaks at 781 and 833 cm 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. An increasing amount of absorbed water contributes to blue chroma in turquoise and has a negative effect on lightness based on the CIE 1976 *** colour system. Loose turquoise with a low specific gravity tends to display greater colour differences with a significant decrease in lightness.