The structures of three centers of trivalent iron in α-quartz

Trivalent iron is known to occur in amethysts in three different environments: substitutional with an as yet unidentified alkali ion in an adjacent interstitial site on the twofold axis (so-called S₁ center), substitutional with a proton on one of the four oxygen neighbors (S₂ center) and without detectable charge compensation (so-called I center because it was proposed to be in interstitial sites, but its nature is still disputed). The alkali ion in the S₁ center was now identified as Li and the Li and Fe superhyperfine (shf) tensors were determined by electron paramagnetic resonance at low temperatures in samples of unusually high quality. The fine structure and ¹H shf structure tensors for the S₂ center with the proton on an oxygen with longer bond distance were evaluated and the geometry of the oxygen coordination was estimated by application of the superposition model. For the I center the ⁵⁷Fe and ²⁹Si shf splittings were determined and the evidence in favor of assignment to an interstitial site is reviewed.