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

Abstract

Abstract. 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 $1 center), substitutional with a proton on one of the four oxygen neighbors ($2 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 Sz 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 ~H shf structure tensors for the $2 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 zgsi shf splittings were determined and the evidence in favor of assignment to an interstitial site is reviewed.

Dynamic Effects in a New Substitutional Center of Trivalent Iron in Quartz

“…3 sets an upper limit of 0.5% for the 3s spin density on Na, in the same range as the value obtained for 'Li in the Lif compensated center [4]. However, due to the facts that this hyperfine splitting was measured for a fine-structure signal of unknown assignment far from the high-field limit and that the dipolar contribution is not known this value can only be considered as an approximate order of magnitude in the case of Na.…”

“…This alkali ion in this so-called S, center was recently identified as Li [4]. Since results of trace analyses for natural amethysts indicate that Fe, Li, and Na are always present in almost equal numbers [5, 61 it is surprising that no sodium-compensated center of Fe3+ has so far been observed.…”

EPR of iron centres in silicon dioxide

“…Thus Stegger and Lehmann [74,75] discovered that in [FeO4/Li] ~ the EPR signals for ~Li (7.42% abundance, I= 1) and 7Li (92.58%, I--3/2) at certain crystal orientations in field B are actually separated (Fig. 5c).…”

“…For a detailed discussion of the problem, one can consult various publications I7, 14,15,61,62,66,75].…”

The Mössbauer spectra of prasiolite and amethyst crystals from Poland

“…Cox (1976) used measurements of amethyst done with K and Q-band spectrometers to prove the presence of not only the Fe 3+ ion but also of Fe 4+ in the crystals studied. Moreover, Stegger and Lehmann (1989) Mössbauer spectroscopy is the most suitable method for researching the valence and coordination of Fe ions and is used most often for Fe-bearing minerals. Examples of the many good publications on the application of Mössbauer spectroscopy to the study of minerals are those of Greenwood and Gibb (1971), Zhe et al (2001), Darby Dyar et al (2006), Berry (2005), Grodzicki and Lebernegg (2011), Golubeva et al (2009) and Fridrichova et al (2015).…”