Optical and Paramagnetic Properties of Iron Centers in Quartz

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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++O2—→Fe2++O−] into the visible as S1 centers are converted to less constricted I centers.