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(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.