An X-ray Absorption Near-Edge Structure (XANES) Study on the Oxidation State of Chromophores in Natural Kunzite Samples from Nuristan, Afghanistan

Abstract

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.

Study on the Color-Influencing Factors of Blue Iolite

“…Mn 2+ and Fe 3+ have the same electronic structure, both are chromophores, and will only produce obvious color at higher concentrations [39]. Mn 2+ is less abundant in iolite, compared to Fe 2+ , only a small amount of Mn 2+ can replace Mg 2+ on the octahedral M-site.…”