Mineralogical and Spectral Characteristics of Turquoise and Its Common Imitations

This article mainly used modern testing techniques such as conventional gemmological tests, infrared spectrometer, Raman spectrometer, ultraviolet visible spectrometer, and energy spectrum analysis to systematically study the mineralogical and spectra characteristics of the turquoise and its imitations purchased from mining areas, manufacturers, and trusted channels. We have come to the following conclusion. The density of natural turquoise is between 2.45-2.75 g/cm³, and the refractive index is between 1.61-1.63. The vibration frequencies of structural water, crystalline water, and phosphate groups determine the infrared absorption spectra and Raman spectra of natural turquoise. The infrared spectrum tests show the IR absorption peaks around 3 508, 3 463, 3 287, 3 086, 1 652, 1 112, 1 060, 841, 784, 648, 569 cm^-1 and 483 cm^-1; Raman spectrum tests show the peaks around 3 496, 3 471, 3 448, 3 265, 3 069, 1 618, 1 159, 1 102, 1 039, 937, 812, 643, 589, 547, 470 cm^-1 and 414 cm^-1; The absorption peaks appear near 267, 430, 670 nm and 799 nm in the UV visible spectrum.

There are many varieties of turquoise's imitation products, commonly seen in the market mainly including trihydrate alumina, barite, magnesite, calcite and mineral mixtures. The most effective and rapid method for distinguishing them from natural turquoise is infrared absorption spectrum of turquoise in the range of 3 600-3 000 cm^-1 and 1 200-900 cm^-1. The presence of an absorption peak at 430 nm in the UV visible spectrum and the presence of Cu and Fe of colouring elements in turquoise can be used as auxiliary evidence.