Beryl’s various coloring make it a sought-after and precious gemstone. Knowledge on the physical origin behind the various colorings is essential to differentiate artificial colorings from natural ones. The blue color of the variety aquamarine has been proposed to be due to charge transfer between adjacent Fe3+-Fe2+ pairs on the Al site and its adjacent 6g interstitial position, respectively. The present work presents the first experimental confirmation of such an arrangement.

Single-crystal electron paramagnetic resonance (EPR) spectra of a light blue beryl (Springbok, South Africa), before, and after annealing at 800 °C, have been measured at temperatures from 25 to 295 K and reveal two Fe3+-Fe3+ pairs related to an axial Fe3+ center at the effective g = ~2.00 as well as a subordinate rhombic Fe3+ center at g = ~4.28. Powder EPR spectra show that the intensity of the axial Fe3+ center increases after annealing from 400 to 800 °C but becomes constant after 800 °C, whereas the rhombic Fe3+ center is not affected by annealing. One Fe3+-Fe3+ pair is the same as the one investigated previously by Edgar and Hutton (1982) and arises from Fe3+ ions at the two nearest Al sites along the c axis. The best-fit spin Hamiltonian parameters show that the second Fe3+-Fe3+ pair is also oriented parallel to the c axis and has a separation of 2.4 Å, corresponding to Fe3+ ions at an Al site and its nearest 6g interstitial position. The increased intensities of both Fe3+-Fe3+ pairs after annealing suggest their formation from Fe3+-Fe2+ and Fe2+-Fe2+ precursors. Modeling of synchrotron Fe K-edge X-ray absorption spectra measured on the sample annealed at 1000 °C also shows occupancies of Fe3+ at both the Al site and the 6g interstitial position. The assignment of the rhombic Fe3+ center to the Be site remains tentative.

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