Spectral, electron microscopic and chemical investigations of gamma-induced purple color zonings in amethyst crystals from the Dursunbey-Balıkesir region of Turkey

Abstract

Amethyst crystals on matrix specimens from the Dursunbey-Balıkesir region in Turkey have five representative purple color zonings: dark purple, light purple, lilac, orchid, and violet. The purple color zonings have been analyzed with optical absorption spectra in the visible wavelength region, chemical full trace element analyses (inductively coupled plasma-atomic emission spectroscopy and inductively coupled plasma-mass spectroscopy), and scanning electron microscopic images with high magnification. It can be proposed that the production of the purple color in amethyst crystals is due to three dominant absorption bands centered at 375, 530, and 675 nm, respectively. In addition, the purple color zonings are also due to four minor absorption bands centered at 435, 480, 620, and 760 nm. X-ray diffraction graphics of the investigated amethyst crystals indicate that these crystals are composed of a nearly pure alpha-quartz phase and do not include any moganite silica phase and/or other mineral implications. Trace element analyses of the amethyst crystals show five representative purple color zonings, suggesting that the absorption bands can be mainly attributed to extrinsic defects (chemical impurities). However, another important factor that influences all structural defects in amethyst is likely to be the gamma irradiation that exists during amethyst crystallization and its inclusion in host materials. This gamma irradiation originates from the large underlying intrusive granitoid body in the region of amethyst formation. Irradiation modifies the valence values of the impurity elements in the amethyst crystals. It is observed that the violet-colored amethyst crystals have the most stable and the least reversible coloration when exposed to strong light sources. This situation can be related to the higher impurity content of Fe (2.50 ppm), Co (3.1 ppm), Ni (38 ppm), Cu (17.9 ppm), Zn (10 ppm), Zr (3.9 ppm), and Mo (21.8 ppm).

The Mössbauer spectra of prasiolite and amethyst crystals from Poland

“…In the amethyst crystals, the contents were average or lower. Levels of residual elements are similar to those reported in Hatipoğlu et al (2011).…”

“…This color is stable up to 600 °C. Color change in different quartz crystals caused by irradiation and heating has been demonstrated mainly with optical, FTIR and Raman spectra (Lameiras et al 2009;Guttler et al 2009;Hatipoğlu et al 2011;Alkmim et al 2013;Nunes et al 2013). Nassau and Prescott (1977) found the green and violet colors reflects structurally independent causes and the crystals to be different from "greened amethyst" as described in Lehmann and Bambauer (1973).…”

Unstable Colouration of Padparadscha-like Sapphires

“…type 2 of Nassau and Valente, 1987), thus resulting in a yellow colour that fades slowly upon exposure to daylight (Schiffmann, 1981;Nassau and Valente, 1987;Hughes et al, 2017). Additional cases of colour centres that are unstable in daylight are not uncommon in mineralogy, as seen, for example, in some amethyst (Hatipoğlu et al, 2011), and also quite dramatically in Maxixe-type beryl (Nassau et al, 1976) and in hackmanite-a rare sulphur-bearing variety of sodalite that becomes stunningly purple after brief exposure to UV radiation before fading (rather quickly) to greyish white in daylight (Medved, 1954;Kondo and Beaton, 2009).…”

Study on the effect of heat treatment on amethyst color and the cause of coloration

“…It is widely distributed in the crust and the origins are all over the world, such as Brazil, Uruguay, Canada, Sri Lanka 1 , Rwanda 2 , Morocco 3 and Arizona 4 , etc. When the quartz contains impurities such as iron, aluminum and titanium, it will make quartz appear rich in color 5 . The colors of amethyst 6 , 7 , citrine 8 and prasiolite 7 , 9 , 10 are all related to iron.…”