Amethyst: Optical Properties and Paramagnetic Resonance.
Color centers in amethyst are produced by the action of ionizing radiation on precursor centers that arise from substitution of Fe(+3) for Si(+4) in the alpha-quartz structure. These Fe(+3) centers provide the dominant features of the electron paramagnetic resonance spectrum of amethyst. The three equivalent Si(+4) sites are unequally occupied by Fe(+3), a circumstance that explains the optical biaxiality of amethyst.
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