Paramagnetic Resonance and Optical Properties of Amethyst

Abstract

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.