FTIR study of aquamarines after gamma irradiation, heat treatment and electrodiffusion

Abstract

Beryl, Be 3 Al 2 (SiO 3 ) 6 , is a natural gemstone with many colors. Some of these colors can be induced or modified by exposure to ionizing radiation, by heating, and maybe by electrodiffusion. Small contents of chromophore chemical elements are related to the colors of beryl, like iron, chromium, vanadium, manganese, and others. There is great interest in relation to methods of improving or inducing colors in beryl. There is evidence that infrared spectroscopy (FTIR) can contribute to foresee beryl behavior submitted to procedures for color change. It was observed that electrodiffusion with or without contaminant ions did not alter the FTIR spectrum of aquamarines, unlike heat treatment. Green samples have a higher content of type I water molecules, whereas blue samples have a higher content of type II water molecules. Significant changes in FTIR spectra of aquamarines were observed only in green samples after exposure to gamma rays or to heat. The vanishing of the band at 3633 cm -1 may be related to the position of Na + ion in the crystal lattice of beryl.

Gemological characteristics and inclusions of green rutilated quartz from Huanggangliang, Inner Mongolia

“…The peak at 800 cm −1 belongs to the symmetric stretching vibration peak of Si–O. 24,25 The infrared beam directed toward the crystal along the c -axis direction produced an infrared spectrum with only one peak at 800 cm −1 , while the spectrum obtained along the other directions showed an additional peak at 779 cm −1 , which reflected the directionality of the symmetric stretching vibration of the Si–O. 26,27…”

Incorporation and substitution of ions and H2O in the structure of beryl

“…A yellow beryl can be heated and turned into a blue aquamarine. This practice is commonly applied in the jewellery and gemstone trade and influences the market value the stone has (Blak et al, 1982;Alkmim et al, 2017). It is rooted in the reduction of Fe 3+ to Fe 2+ ions (Blak et al, 1982;Alkmim et al, 2017).…”

“…This practice is commonly applied in the jewellery and gemstone trade and influences the market value the stone has (Blak et al, 1982;Alkmim et al, 2017). It is rooted in the reduction of Fe 3+ to Fe 2+ ions (Blak et al, 1982;Alkmim et al, 2017). Though this procedure yields a more desirable blue colour (Chankhantha et al, 2016), it can be reversed by irradiation with high energy, such as γ -rays (Alkmim et al, 2017).…”

“…It is rooted in the reduction of Fe 3+ to Fe 2+ ions (Blak et al, 1982;Alkmim et al, 2017). Though this procedure yields a more desirable blue colour (Chankhantha et al, 2016), it can be reversed by irradiation with high energy, such as γ -rays (Alkmim et al, 2017). This does not only hold true for the colour, but also for the spectral changes induced by heat treatment, which have been observed to be restored by irradiation (Goldman et al, 1978).…”

Study of the mechanism of color change of prehnite after heat treatment

“…Fe 2+ and Fe 3+ can exist in prehnite at the same time, and the relative content for Fe 2+ and Fe 3+ will also affect the color of the prehnite. In general, there are several mechanisms for heat treatment to change the color of gemstones, such as changing the valence state of coloring ions, 14 destroying the original color center of gemstones, 15 causing dehydration of some watery gemstones, 16 diffusing the coloring ions in the gemstone, 17 changing the lattice structure. 18 It remains to be explored whether the color of prehnite can be improved by changing the valence state of the chromic element Fe through heat treatment.…”