Characterization of beryl (aquamarine variety) by Mössbauer spectroscopy

The Mössbauer spectra of several blue beryls have been obtained in the temperature range of 4.2-500 K. A common feature observed in all room-temperature spectra is the presence of an asymmetric Fe²⁺ doublet (ΔE_Q ∼ 2.7 mm s^-1, δ ∼ 1.1 mm s^-1), with a very broad low-velocity peak. This asymmetry seems to be related to a relaxation process involving ferrous ions and water molecules in the structural channels, as suggested by Price et al. (1976). Surprisingly, the spectrum at 500 K also shows a broad, but symmetrical, doublet, with a clear splitting of the lines indicating the presence of at least two Fe²⁺ components. The room-temperature spectrum obtained after the 500 K run shows the same features as prior to the heating. At 4.2 K the spectrum of a deep blue beryl was well fitted with four symmetrical doublets, one of which could be related to Fe²⁺ in the structural channels. Ferrous ion was also found to occupy the octahedral and tetrahedral sites, whereas ferric ion is most probably located in the octahedral site. A meaningful fit of the room-temperature spectrum, as well as an explanation for the temperature dependence of the Mössbauer spectra, are discussed. Finally, it is believed that the color in beryl will be dictated by the relative proportions of Fe³⁺ in the octahedral sites and of Fe²⁺ in the channels.