The Effect of Heat Treatment on Yellow-Green Beryl Color and Its Enhancement Mechanism

• Published in: Crystals (Basel) Vol. 15; no. 8; p. 746
• Main Authors: Hao, Binru, Zhao, Shuxin, Guo, Qingfeng
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 21.08.2025
• Subjects: Absorption spectra; Beryl; Beryllium aluminum silicates; Color; Crystal structure; Dehydration; Electron probe microanalysis; Fourier transforms; Heat treating; Heat treatment; Molecular weight; Oxidation; Phase transitions; Raman spectroscopy; Reducing atmospheres; Spectrum analysis; Temperature; Trace elements; Unit cell; Valence; Japan; Germany
• ISSN: 2073-4352; 2073-4352
• DOI: 10.3390/cryst15080746

Beryl is classified as a cyclosilicate mineral, and its color is primarily determined by the type and oxidation state of trace elements. In this study, natural yellow-green beryl was used as the research subject, and heat treatment experiments were performed at various temperatures under both oxidizing and reducing atmospheres. A combination of analytical techniques, including electron probe microanalysis (EPMA), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and ultraviolet-visible spectroscopy (UV-Vis), were employed to systematically investigate the composition, structure, and chromogenic mechanisms of beryl before and after heat treatment. The experimental results indicate that heat treatment under both atmospheres can lead to the transformation of yellow-green beryl into blue, with 500–600 °C under a reducing atmosphere identified as the optimal treatment condition. With increasing temperature, beryl gradually dehydrates, resulting in a faded blue color and reduced transparency. Even after treatment at 700 °C, no significant changes in unit cell parameters were observed, and both type I and type II water were retained, indicating that the color change is not attributed to crystal structure transformation or phase transitions. The study reveals that the essential mechanism of color modification through heat treatment lies in the valence change between Fe2+ and Fe3+ occupying channel and octahedral sites. The observed color variation is attributed to changes in absorption band intensity resulting from charge transfers of O2− → Fe3+ and Fe2+ → Fe3+. This study provides theoretical insights and technical references for the color enhancement of beryl through heat treatment.