Paramagnetic Resonance and Optical Properties of Amethyst

Color centers in amethyst are produced by action of ionizing radiation on precursor centers which arise from substitution of Fe3+ for Si4+ in the α-quartz structure, with charge neutralization usually achieved by an alkali-metal ion on the twofold axis of the center. These Fe3+ centers provide the dominant features of the electron paramagnetic resonance spectrum of amethyst, which was measured in the X-band at room and liquid-nitrogen temperatures. The spectrum was fitted to an orthorhombic spin Hamiltonian with g=2.0032, D=−2.0463, E=5.5277, F=−0.0282 Gc/sec. The optical biaxiality of amethyst, with biaxial angle 2V=6 to 12°, is due to unequal occupation of the three equivalent Si4+ sites in the structure by Fe3+ during crystal growth. This phenomenon may help to specify geological conditions during hydrothermal growth of natural amethyst.