Investigation of Blue Coloration in Sapphires Using X‐Ray Absorption Near‐Edge Structure Spectroscopy (XANES)

Abstract

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.

Changes in Blue Color of Sapphire Compared with Oxidation State Changes

“…32.71788). These values are typical for sapphire from magmatic origin [7,31,34]. The data are presented in Table 3.…”

“…In the fields of gemology and mineralogy, XANES spectroscopy has been employed to ascertain the oxidation states of Fe and Ti in heat-treated blue sapphire [14][15][16][17][18]34], the oxidation state of Fe in quartz [35,36], the Fe valence in beryl [37,38], and the Fe valence in heat treated spinel [39]. XANES spectroscopy is conducted using synchrotron radiation generated from a synchrotron light source radiation facility, as illustrated in Figure 3.…”

The blue color mechanism on sapphires from different gem deposits before and after heating under oxidizing atmosphere

“…From the energy position and pre-edge centroid position, it can be confirmed that the presence or absence of blue color on sapphire has not been involved with the IVCT process [10,13,14] because there is no evidence of the transition among Fe 2+ and Fe 3+ oxidation state on the sapphire structure during the heating process.…”

“…Naturally, there are many color shades of blue sapphire from milky to dark blue based on color intensity relating to the content of trace elements and internal features as well as micro-and nano-inclusion [8]. The blue intensity of blue sapphire can be changed after heat treatment [9][10][11][12][13][14][15][16] relating to the important parameters including oxidation-reduction atmospheric condition, heating temperature, soaking time and chemicals in gemstone [9].…”

“…The iron and titanium dissolved from Fe-bearing and Ti-bearing mineral inclusions were the most important trace elements for the color-causing mechanism on blue sapphire after heating under oxidizing condition and the intensity of blue color after heating corresponded to the stronger and weaker intensity of the Fe 2+ -Ti 4+ IVCT absorption band [13]. The slight changing from Fe 2+ to Fe 3+ on blue sapphire after heating under oxidizing atmosphere at 1400 °C were proposed by linear combination fitting calculation [14]. However, the oxidation state of Fe and Ti on blue sapphire was reported as Fe 3+ and Ti 4+ before and after heating under oxidizing atmosphere no matter what temperature was used [15,16].…”