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

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.