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Summary
Beryl group belongs to the hexagonal system and contains a series of beryllium-aluminosilicate minerals. Beryl gems normally show various colors, due to the occurrence of different trace elements. Greenish-yellow beryl and aquamarine are usually colored by iron ions in different valence states. Fe3+ and Fe2+ may occupy tetrahedral position (Fe-4(2+)), octahedral position (Fe-6(3+)) and crystalline channel ([Fe-2 (OH)(4)](2+)). Light blue aquamarine can be obtained by heating treatment of greenish-yellow beryl: (1) in 300 similar to 550 degrees C step heating-treatment experiments, the color of samples changes successively from light green, pale yellowish-green, through light yellow, pale blue to light blue; (2) the color turns into pale blue in 550 similar to 600 degrees C; (3) desirable color modification occurs in 500 similar to 550 degrees C. Mid FT-IR, UV-Vis and EPR spectroscopic analyses were conducted on beryl samples before and after heating treatment. The mid-FT-IR spectra of heated and unheated samples show similar molecular vibration spectroscopy characteristics. The absorption peaks in the region of 1250 similar to 600 cm(-1) which correlate to the typical vibration of [Si6O18] group shift to the direction of low wave number, along with the reduction of absorption intensity, indicating changes in Fe3+-O-2 charge transfer and the bending vibration of the structural water during heating: (1) the weakening of Fe3+-O-2 charge transfer was caused by the reduction of Fe3+ in the structural channel; (2) the weakening of bending vibration of structural water was caused by the water loss in heating treatment. The UV-Vis spectra show that same characteristics in both reductive and oxidative environment. However, there are some changes indicating the weakening of Fe3+ forbidden transition which leads to the disappearance of yellow color compared to unheated samples: ( 1) the absorption peak which attributed to O-2 -> Fe3+ charge transfer shifts towards the ultraviolet-light region and forms 322 nm absorption; (2) (6)A(1g) -> T-4(2g) transfer shifts towards violet-light region and form 370 nm absorption; (3) (6)A(1g) -> E-4(g) + (4)A(1g) transfer shifts towards red-light region and form 427 nm absorption. Also, a wide absorption band near 823 nm was enhanced and shifted towards the visible-light region indicating a content increase of Fe2+ in structural channels which led the appearance of blue color. The ESR spectrum of natural greenish yellow beryl shows that Fe3+, Mn2+ and OH radical exist both in Al-O octahedron and structural channels. Following conclusions can be drawn from combined UV-Vis spectra: (1) Fe3+ in structural channels has been reduced to Fe2+ in heating treatment; (2) the forbidden transition of Fe3+ in Al-O octahedron is weakened during heating treatment; (3) the blue color acquired from heating treatment is caused by a combination of the above two changes in beryl crystals.
