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 Fe3+ 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. V3+ enhances absorption in the blue and orange regions, and Cr3+ 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 Fe3+ and V3+) enhance polarity of the phosphate group (PO4)3−, resulting in strong absorption in the infrared spectra for analogues of turquoise. The reflectivity ratio (ROH) 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.
Highlights
Turquoise is one of the most well-known and ancient types of jade, prevalent in Chinese literature, religion, politics and arts [1]
A quantitative analysis of the chemical composition of 35 turquoise samples based on FP standard curve shows turquoise oxide content to be the following: w(P2O5) ∈ (32.827, 45.547), w(Al2O3) ∈ (22.100, 32.437), w(CuO) ∈ (1.403, 22.491), w(Fe2O3) ∈ (0.621, 15.192), w(ZnO) ∈ (0.089, 8.296), where w(Fe2O3) represents the total iron content detected in turquoise
The turquoise group structurally consists of distorted XO6 octahedra, MO6 octahedra and PO4 tetrahedra, where the X site is occupied by medium-sized divalent cations (e.g. Cu2+ and Zn2+) and the P site is occupied by phosphorus
Summary
Turquoise is one of the most well-known and ancient types of jade, prevalent in Chinese literature, religion, politics and arts (e.g. jewellery) [1]. A complex chemical composition produces a wide colour range in turquoise [8]. Zn2+ or Ca2+ can be present at the A-position and V3+ or Cr3+ can occur at the B-position in some rare turquoises [9]. Previous research suggests that the chemical composition, structure compactness and adsorbed water content of turquoise are the main internal factors affecting its colour—explained by crystal field theory and charge transferring [10]. For example, comprises Cu octahedra in crystalline turquoise, and a colour transition from blue to yellow is determined by Fe content [11]. There are other trace elements found in turquoise, which may affect the colour of turquoise. UV–Vis lends itself to the study of colours in various gemstones, such as emerald [13], variscite [14], demantoid [15], uvarovite [16] and turquoise [6,17]
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