Study of Fe ions in aquamarine and the effect of dichroism as seen using UV-Vis, NIR and x-ray

The oxidation states and coordination environment of Fe in 4 aquamarine samples with different color shades were studied. The C axis along with plane A and plane C of the samples were identified and the measurements were made with plane A oriented facing incoming beam and plane C oriented facing incoming beam. NIR and Raman spectroscopy were employed to confirm the structure of the samples. XRD was used to confirm the polished faces. UV-Vis and X-ray absorption near edge structure (XANES) were used to identify the presence of both Fe2+ and Fe3+ in the samples. The results indicated that Fe resided predominantly in 6-fold coordinated sites and the deep blue shade depended on the amount of Fe2+ in 6-fold coordinated sites. The effect of dichroism was noticeable in NIR, Raman spectroscopy, UV-Vis and XANES. Clearer Raman bands were observed when samples were oriented with plane A facing laser source. NIR and UV-Vis analysis could benefit from the complementarity of spectra measured from both orientations. Discrepancies between XANES spectra measured from the two orientations could lead to uncertainties in obtained values of oxidation states.