Comparison of the Microscopic Characteristics of Beryl before and after Heat Treatment

The thermal treatment process of beryl is quite simple, and this method is widely applied for colour modification of beryl. The colour of heat-treated beryl is stable, and heat-treated beryl, such as aquamarine, is comparatively common in the market. The internal microscopic features of beryl may change somewhat after heat treatment. A muffle furnace was used for heat treatment experiments in the range of 400~1 000 ℃ under air conditions. Microscopic photographs were used to record the microscopic characteristics of the samples before and after heat treatment; laser Raman spectrometer was also applied to determine the potential composition changes of gas-liquid inclusions. The natural beryl was characterized by gas-liquid inclusions with CO₂+N₂+CH₄ as the gas composition and H₂O with a small amount of dissolved CO₂ as the liquid phase composition. The results showed that after heat treatment, some of the gas-liquid inclusions burst, causing the fluid to escape and leaving only the cavity. In addition, the burst inclusions lead to some changes in their original shape, mainly manifested as part of the flattened gas-liquid two-phase inclusions changes into flattened fractures, elongated tubular gas-liquid inclusion cavity walls appear local fracture and shrinkage. Fluid inclusions burst is related to the heat treatment temperature and inclusion size. When the heat treatment temperature is 400 ℃, burst was only observed in gas-liquid inclusions of relative larger size in beryl. In contrast, when the heat treatment temperature is equal or higher than 650 ℃, almost all the gas-liquid inclusions burst. Raman analysis did not detect the Raman peaks of the initially captured gas and liquid compositions in the bursted fluid inclusions, further confirms the formation of inclusion cavity after heat treatment. Microscopic observation of gas-liquid inclusions in beryl is an effective means to identify whether it has been heat-treated or not. The variation of gas-liquid inclusions of smaller sizes may indicate the heat treatment temperature. The systematic study of the internal microscopic features of beryl before and after heat treatment may contribute to the effective identification of heat-treated beryl.