Mineralogical Characteristics and Colouration Mechanism of Blue Opals from Peru

In recent years, the study of blue opal has been limited to the mineral composition and coloration mechanism. However, the chemical composition, infrared spectrum, and Raman spectrum of the blue opal have not been studied in details. Combined with previous research, Fourier Transform Infrared Spectroscopy (FTIS), X-ray Diffraction (XRD), Electron Microprobe Analyzer (EMPA), and UV Vis Absorption Spectroscopy (Uv-Vis) and Raman Spectroscopy were used to investigate vibration spectra, functional group characterization, mineral composition, and coloring mechanism of blue opal samples. Results show that the main mineral composition of blue opal is amorphous opal and the vibrational spectra show some peak frequency shifts relative to natural opal. EMPA shows that the main elements of blue opals are Si and Cu, whereas the Uv-Vis spectrum shows a broad band with higher absorption intensity near 742 nm. Combined EMPA and Uv-Vis spectrum results further indicate that the coloration element for the blue color of the opal is Cu, with the typical planar square structure of[Cu²⁺(H₂O)₄]²⁺. Moreover, the blue color and the Cu content are positively correlated, and with the increasing Cu content the blue color is more intense.