Optical and Paramagnetic Properties of Iron Centers in Quartz

Abstract

Several different paramagnetic centers have been identified and studied in specimens of natural and synthetic α quartz. The dominant feature of the EPR spectrum of natural amethyst and citrine is a center S₁, which was previously identified as substitutional Fe³⁺ with a charge‐compensating alkali‐metal ion on a neighboring interstitial site. Synthetic brown and green quartz also contain a small proportion of S₁ centers, but the dominant feature of their EPR spectra is a center I which is identified as an interstitial Fe³⁺ lying in one of two possible interstitial sites in the structure, with a charge neutralizing ion in a neighboring substitutional site. This spectrum is fitted to a spin Hamiltonian with D=2.333 Gc/sec, E=0.63 Gc/sec, F=0.60 Gc/sec, the z axis of the center coinciding with the c axis of the crystal. Interconversion of S₁ and I centers by heat treatment indicates that the latter are more stable. In a crystal of synthetic amethyst, a new EPR center, denoted S₂, has been characterized, which is converted to S₁ by annealing at 350° and obtained from S₁ by irradiation with x rays. Citrine and heat‐treated amethyst display a broad isotropic EPR absorption at g=2, believed to be caused by nuclei of precipitated Fe₂O₃. The brown color is partly due to these inclusions and partly to a shift of the charge‐transfer band [Fe³⁺+O^2—→Fe²⁺+O⁻] into the visible as S₁ centers are converted to less constricted I centers.