Spectroscopic study of synthetic hydrothermal Fe3+-bearing beryl

A synthetic hydrothermal beryl Fe-4-51, investigated previously by Taran and Rossman (Am Miner 86:973–980, 2001 ), was additionally studied by microprobe, Mössbauer, optical absorption, Raman and IR spectroscopy. For comparison, polarized spectra of natural blue aquamarine and Cr 3+ , Fe 3+ -bearing alexandrite, both from Brazil, are also presented. Fe-4-51 is a nearly pure Fe 3+ -bearing beryl, with a homogeneous composition as shown by electron microprobe. Averaging over 22 points gives a formula of Be 3.07 (Al 1.94 , Fe 0.07 3 + ) Σ=2.01 Si 5.95 O 18 , with Fe 3+ replacing Al 3+ in the octahedral site of the structure. The Mössbauer spectrum is dominated by a broad disordered pattern with beryl-suitable parameters; for Fe 2+ , IS = 1.21 mm/s, QS = 2.71 mm/s, area ≈ 5% and for Fe 3+ , IS = 0.34 mm/s, QS = 0.71 mm/s, and area ≈ 67%—are distinguished overlying a broad disordered continuum. The optical absorption spectrum is typical of octahedral Fe 3+ . From it, the crystal field strength Dq is derived as ~ 1520 cm −1 and the values of Racah parameters of interelectronic repulsion B and C are found to be 665 and 3415 cm −1 , respectively. This rather low B value, compared with that of a free Fe 3+ ion, 814 cm −1 , suggests a comparatively high degree of covalency in the octahedral Fe 3+ –O bond. Infrared spectra show the presence of channel H 2 O of both I and II structural type in comparable quantities, about 0.5 and 1 mass%, respectively. Raman data show the expected five bands in the energy range from 300 to 1200 cm −1 .