Origin of violet colour in agates from Chihuahua, Northern Mexico
摘要
Agate samples from the Rancho Coyamito (Chihuahua, Mexico) were investigated to reveal the origin of their conspicuous violet coloured chalcedony bands. The comprehensive geochemical and spectroscopic investigations comprised trace element analyses by inductively coupled plasma-mass spectroscopy (ICP-MS), optical absorption spectroscopy, electron paramagnetic resonance (EPR) spectroscopy as well as cathodoluminescence (CL) microscopy and spectroscopy. The results show that the violet colour is probably caused by a colour centre similar to the amethyst Fe4+ centre in macrocrystalline quartz, while in yellowish and reddish chalcedony bands of the same agates iron is mainly incorporated as micro-inclusions of goethite and hematite, respectively. Absorption measurements on the violet agate bands confirmed the typical absorption bands for amethyst at ~ 350 nm, 545 nm, and 950 nm. EPR spectra of violet agates measured at room temperature revealed a suite of previously reported Fe3+ signals, and those collected at a liquid-helium temperature of 30 K and microwave powers up to ~ 66.5 mW suggest the amethyst Fe4+ centre. The Fe4+ centre, which is most likely responsible for the violet colour, was probably transformed from trivalent iron via high-energy radiation. Elevated concentrations of uranium (up to 32.5 ppm) were measured in violet chalcedony bands, and uranyl ions (UO22+) were detected by CL microscopy and spectroscopy. Annealing treatments at 430 °C and 550 °C for 24 h resulted in complete bleaching of the former violet colour. EPR spectra of these annealed agate samples revealed the same suite of Fe3+ centres as in the initial spectra, but at enhanced intensities.