Spectroscopic Study Of Kunzite From Nuristan,Afghanistan,and It's Color Enhancement

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.Kunzite,one of the most attractive members of the colorful family of spodumene,has its color ranging from a light pink hue to lilac pink.Some of them have very light colors while others are so dark that the real hue can only be identified when the stones are held against the strong light.Fine jewelry-grade kunzite is relatively rare because most of the material has a very light color;therefore,color enhancement is normally carried out on natural samples.The present work is continuing the effort towards value addition to kunzite by enhancing its color using different treatments.Before doing any treatment for the color enhancement,it is essential to identify the color-causing chromophores and their oxidation states present in the kunzite.In the first part of the current research,the main color causing impurities in natural kunzite from the Nuristan area in Afghanistan and their valence states were determined.The chemical analysis of major and trace elements in spodumene(kunzite)structure was identified and quantified by using sensitive techniques,including Laser Ablation Inductively Coupled Plasma Mass Spectrometry(LA-ICP-MS),UV-Vis-NIR for checking of the absorption peak of color-cause elements,and X-ray absorption near-edge structure(XANES)for checking of their oxidation states.LA-ICP-MS and UV-Vis-NIR 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 a mixed-valence state i.e.,both Mn²⁺and Mn³⁺oxidation states,while Fe is present only in the Fe³⁺oxidation state.After knowing the color causing chromophores and their oxidation states,two different techniques were used for the color enhancement,which are:i.Ion beam implantation(O-ions and N₂-ions)ii.Electron beam irradiation treatment followed by heat treatmentIn our study for the first time,we used the ion implantation technique for the color enhancement of kunzite from Nuristan,Afghanistan.The ion implantation was performed by using two different types of ions i.e.,O-ion with dose 3x1018 ions/cm² and accelerating voltage80 ke V for 3 hours,while N₂-ion with dose rate 2x1017 ions/cm² with an accelerating voltage70 ke V energy for 2 hours.With the O-ion implantation there observed an obvious color enhancement in the color of the kunzite,and the color was enhanced from pink to more darker pink.While with N2-ions treatment followed by heat treatment at 100oC for 1 hour,the kunzite color changed from pink to light pink color with the appearance of smokey color tint in it,which results in the decrease of its value.X-ray absorption near-edge spectroscopy(XANES),UV-visible,and Raman spectroscopy techniques were used to investigate the cause of color change that occurred in kunzite.The XANES K-edge results show that after O-ion beam implantation treatment the Mn still exists in the mixed-valence state i.e.,Mn²⁺and Mn³⁺,but there arises a noticeable increase in the concentration of Mn³⁺concentration.Similarly,after the O-ion beam implantation,Fe was also detected in a mixed-valence state i.e.,Fe²⁺and Fe³⁺.XANES K-edge of Mn and Fe was also checked after the N₂-ion implantation,but we didn't observe any change in the oxidation states of Mn and Fe and found it in the same state as that before the treatment.UV-Vis-NIR spectra show that a considerable increase occurred in the absorption peak at 530 nm after the O-ion implantation,which is due to the increase in the concentration of the Mn³⁺ions,resulting from the inter-valence charge transfer(IVCT)between Mn and Fe,RAMAN analysis shows that a significant shift change occurred due to O-ion implantation while no change is observed due to the N₂-ion beam implantation treatment.In any point of view,the bombardment by using an O-ion beam is a promising technique for the color improvement of the kunzite from Nuristan,Afghanistan.In the next phase,electron beam irradiation treatment followed by heat treatment was carried out.The samples were characterized before and after the color enhancement.Conclusive remarks were drawn upon the modification of their color appearance,optical absorption,and oxidation states of the chromophores.After preparation,the samples were divided into three groups:the first group was irradiated with 15 MGy,the second group with20 MGy,while the third with 25 MGy.After the irradiation,the pink color of kunzite was changed to an emerald-green color.After doing the heat treatment at 250oC for 2 hours,the green color changed to pink color again,but this time the pink color produced is darker as compared to the previous natural one.The Mn and Fe K-edge XANES spectrum was checked after the e-beam irradiation treatment.XANES Mn K-edge spectrum shows that there occurred an increase in the concentration of Mn³⁺ions after the treatment,while the Fe K-edge spectrum shows the formation of Fe²⁺ions after the treatment.Before treatment,the samples contain Fe³⁺but after the treatment,it contains a mixed oxidation state of Fe i.e.,Fe²⁺and Fe³⁺.The idea of the green color of the irradiated kunzite is due to Mn⁴⁺proposed by some researchers does not full in our case.Because when we plotted the obtained XANES data for Mn K-edge against Mn O₂(Mn⁴⁺)standard we did not find any traces of Mn⁴⁺after the irradiation treatment.The Raman analysis shows that the green color is caused due to the defects produced due to the high energetic radiation.UV-vis-NIR spectra of the untreated samples displayed a very broad absorption band at530 nm related to Mn³⁺.After the electron beam irradiation treatment,the 530nm band disappeared and a new band at 625 nm and 490 nm appeared.But,after annealing the irradiated samples and checking the UV-Vis-NIR spectra of these annealed samples shows that the 625nm was disappeared while the two bands at 520 nm and 470 nm appeared,but the peak intensity of 520nm band is more as compared to the 530 nm of the untreated sample.It was noticed that both electron beam irradiation and heat treatment at current conditions did not affect any inclusions in the specimens.The pink color of kunzite is enhanced,both after the O-ion implantation treatment and electron beam irradiation treatment followed by heat treatment.Ion beam treatment of gemstone applies ion implantation to the stone samples to achieve effects in gemological modification comparable to or even better those of other treatments.Although each method has its own advantages and disadvantages,ion implantation seems more advantageous.If we compare the ion beam treatment with other treatments,like electron beam,gamma,and x-ray irradiation treatment already reported in the literature by the researchers then this method is faster and more economic in case of time and money.As in the case of ion beam treatment,there are no worries that the stone will become radioactive due to long exposure in front of the irradiation.That's also the one reason that ion implantation of gemstones has been recently paid more attention to and practiced in laboratories.