The color of blue sapphire has long been explained using the intervalence charge‐transfer (IVCT) [Fe2+‐Ti4+ and/or Fe2+‐Fe3+] theory, in which the blue can be reduced by thermal (heat) treatment in oxidizing condition to alter Fe2+ (FeO) to Fe3+ (Fe2O3). However, the color mechanism is not well understood. Recently, the band gap theory proposes that iron (Fe) in sapphire always presents as Fe3+, and the blue is caused by Fe3+‐Ti4+ pair, hence thermal treatment does not alter Fe oxidation. The UV–Vis–NIR, Proton‐Induced X‐ray Emission (PIXE), and X‐ray Absorption Near‐Edge Structure (XANES) techniques are carried out before and after treatment to prove this issue. The absorption spectra are slightly lower after treatment, corresponding with the reduction of blue color in all samples. XANES data reveal mixture of Fe2+ and Fe3+ (mainly Fe3+) in all samples, and show insignificantly change after heated which rather supports the band gap theory. However, calculation based on linear combination fitting (LCF) technique shows a slight change in the oxidation from Fe2+ to Fe3+ which rather supports the IVCT theory. It is noted that the highest change is found in the lowest ratio of Fe:Ti sample, while the lowest change is conversely.
Kunzite, the pink variety of spodumene is famous and desirable among gemstone lovers. Due to its tenebrescent properties, kunzite always remains a hot research candidate among physicists and mineralogists. The present work is continuing the effort towards value addition to kunzite by enhancing its color using different treatments. Before color enhancement, it is essential to identify the chromophores and their oxidation states. In this paper, the authors investigated the main impurities in natural kunzite from the Nuristan area in Afghanistan and their valence states. Some impurities in the LiAlSi2O6 spodumene structure were identified and quantified by using sensitive techniques, including Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), UV−VIS and X-ray absorption near-edge structure (XANES). LA-ICP-MS indicated many trace elements as impurities in kunzite, among which Fe and Mn are the main elements responsible for coloration. The oxidation states of these two transition elements were determined by the XANES technique. The study reveals that Mn is present in both Mn2+ and Mn3+ oxidation states, while Fe is present only in Fe3+ oxidation state.
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