Mineral Characteristics and Color-Causing Mechanisms of Beryl from Xinjiang, Northwest China: Insights from Multi-Spectroscopic Analyses and Chemical Compositions

Beryllium (Be), a critical strategic metal element, is predominantly extracted from beryl, which serves as a key mineral combining significant strategic importance with essential industrial applications. Significant debate remains, however, regarding the mineralogical characteristics and color-causing mechanisms of beryl. In this study, we integrate Electron Probe Microanalysis (EPMA), Fourier transform infrared spectrometer (FTIR), laser Raman spectrometer (LRS), X-ray diffractometer (XRD), and ultraviolet–visible spectrophotometer (UV-VIS) to elucidate the mineralogy and spectral characteristics of pegmatitic beryl from Xinjiang, Northwest China. The results indicate that the beryl mainly presents a yellowish-green color, associated with minerals such as feldspar, quartz, and garnet. The EPMA results confirm the chemical composition of the typical beryl and indicate that the Al content is lower than the theoretical value, reflecting the substitution of Al3+. The FTIR shows characteristic vibrations of Si-O tetrahedral groups within the range of 1400~400 cm−1, along with distinct bending and stretching vibration peaks of H2O molecules observed in the range of 1700~1500 cm−1 and 3500~3800 cm−1, respectively. Combined with spectral analysis, it can be determined that both Type I water and Type II H2O are present in the samples. Raman spectroscopy reveals that the two distinct peaks of beryl are located at approximately 685 cm−1 (attributed to the stretching vibration of Be-O) and 1067 cm−1 (corresponding to the bending vibration of Si-O), respectively. The XRD analysis shows that the ratio of unit cell parameters c/a of the samples ranges from 0.9950 to 1.0068, and the isomorphous substitution in its structure is mainly manifested as the replacement of octahedral coordination sites by Al3+. The UV-VIS shows that Fe3+ exhibits a broad absorption band in the range of 200~300 nm, while no obvious absorption peaks are observed in the range of 300~800 nm. The above characteristics indicate that Fe3+ has a significant impact on the color of beryl. For green beryl samples, a portion of Fe3+ occupies the structural channel sites and interacts with H2O molecules within the channels, which contributes to the yellowish hue of beryl. Our study highlights crucial data for mineralogical identification, genetic tracing, as well as efficient utilization of beryl resources.