Study of impurity in blue spinel from the Luc Yen mining area, Yen Bai province, Vietnam

Vietnam Journal of Earth Sciences, 40(1), 47-55, Doi:10.15625/0866-7187/40/1/10915 47 (VAST) Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences Study of impurity in blue spinel from the Luc Yen mining area, Yen Bai province, Vietnam Le Thi Thu Huong*1,2, Tobias Haeger3, The-Long Phan4 1Faculty of Geology, VNU University of Science, 334, Nguyen Trai, Hanoi 2Institut of Earth science, Karl Franzens University, Universitaetsplatz 2, 8010-Graz, Austria 3Institute of Geosciences, Johannes Gutenberg University, 55099- Mainz, Germany 4Department of Physics, Hankuk University of Foreign Studies, Yongin 17035, South Korea Received 6 March 2017; Received in revised form 24 October 2017; Accepted 28 November 2017 ABSTRACT In this paper, we present the main impurities in natural blue spinel from the Luc Yen mining area in Yen Bai province, Vietnam. Together with identifying impurities and their valence, we have taken into account the site of some impurities in the MgAl2O4 spinel structure by using sensitive techniques, including electron microprobe analy- sis (EMPA), UV-Vis-NIR, electron spin resonance (ESR), and X-ray absorption (XAS). EMPA results indicated that blue spinel contains different impurities, including V, Mn, Ni, Zn, Ti, Cr, Fe, Co and Ca. UV-Vis-NIR, ESR and XAS studies revealed that the oxidation state of Mn, Co, Fe and Ni is 2+ while that of Cr is 3+. Particularly, Co2+, Fe2+, and Cr3+ ions are identified. They contribute to the color variation of the blue spinel samples. Our study also indicated Co2+ and Cr3+ located in the octahedral position of Al3+ of the MgAl2O4 structure. Keywords: Blue spinel; impurity; coloration. ©2018 Vietnam Academy of Science and Technology 1. Introduction1 The study about lattice defects and impuri- ties in natural spinel has been carried out by some research groups to identify the origin of the color. For blue spinel, the coloring mech- anism is very complex and has been a matter of intense debate because the color displays different hues and saturations, such as light to dark, greyish to violetish blue, greenish and cobalt blue. The same visibly appeared color *Corresponding author, Email: letth80@gmail.com of stones could be resulted from different col- oring mechanisms. Prior to the study of Shigley and Stockton (1984), it was believed that iron is the main factor causing the blue color in natural spinel, whereas the detection of cobalt in the samples was a proof to the conclusion of synthetic origin. These authors, however, elucidated that both cobalt and iron attributed to the blue color in natural spinel. More recently, D’Ippolito et al. (2015) studied on the influ- ence of Co and Fe on different blues of natu- Le Thi Thu Huong, et al./Vietnam Journal of Earth Sciences 40 (2018) 48 ral spinel, and reported that the color of natu- ral spinel is enhanced by the presence of Co, even with a very low concentration. When iron and cobalt amounts are comparable, elec- tronic transitions in cobalt have influence on the spinel color much stronger than those re- lated to iron. Only when the Co content is ex- tremely low (below 10 ppm), the total iron content has its main role in the color. Particularly, according to Chauvire et al. (2015), Co2+ is the main chromophore of blue spinel from the Luc Yen area, Yen Bai prov- ince, Vietnam. The presence of Fe2+ makes the stone appeared greyer, and the hue of blue depends on iron/cobalt ratio. In our study, we based on UV-Vis spectroscopy to assess the valence of typical impurities, including Cr, Fe and Co. We have taken into account the va- lence and the site of coloring impurities in blue spinel. We detected Co2+, Fe2+, Mn2+, Ni2+ and Cr3+. Performing Fourier transform for Co and Cr K-edge XAS spectra indicate Co2+ and Cr3+ ions occupying the octahedral site of the spinel structure. The color variation of Luc Yen blue spinel is discussed in details. 2. Location and geologic background The Luc Yen district (Yen Bai province) is located 270 km north-west of the capital city Hanoi in northern Vietnam. The Yen Bai province is composed of two different geolog- ical units, namely the Lo Gam zone to the northeast and the Day Nui Con Voi to the southwest (Figure 1). Figure 1. Local access and geological map of Luc Yen district (After Chauvire et al., 2015) All of the gem deposits are located in the Lo Gam zone. The Lo Gam formation consists of a sedimentary series metamorphosed into marble, gneiss, calc-silicates, micaschist, and Vietnam Journal of Earth Sciences, 40(1), 47-55 49 amphibolite which are sometimes intruded by granitic and pegmatitic dykes (Leloup et al., 2001; Giuliani et al., 2003; Garnier et al., 2008). The marbles are mainly calcitic and in- terlayered with Al-, V-, and Cr-rich amphibo- lites. Blue spinel is found in a layer of marble which is more than 500 meters thick. It occurs in discontinuous series of lenses, tens of mil- limeters thick and meter-sized in length, roughly following the regional foliation. Alt- hough ruby and spinel of other colors are also found in marbles but they are not associated with blue spinel. 3. Materials and Methods 3.1. Material Two cobalt-blue spinel samples collected from the Luc Yen mining area (Yen Bai, Vietnam) were used for this study (Figure 2). They display homogeneous cobalt-blue color, and could be classified into two typical sys- tems of lighter cobalt blue (denoted hereafter as LY-1) and saturate cobalt blue (LY-2). The LY-2 samples were extracted from marble host rock from Bai Son mine by the authors weighing 4.2 ct, while LY-1 samples were collected from alluvial in An Phu weighing 1.1 ct and 7.3 ct. The samples were then cut into smaller pieces and polished for EMPA and UV-VIS-NIR studies. ESR and XAS measurements were performed on powder samples. 3.2. Research methods Electron microprobe analysis (EMPA): Microscopic images were performed on an electron microscope (JEOL JXA-8900RL) equipped with energy-dispersive X-ray spec- troscopy, using acceleration voltage of 20 kV and filament current of 20 nA. Each sample was analysed 3 different positions to obtain the average composition. For most elements, the detection limit for wavelength-dispersive (WD) spectrometers is in the range of 30~300 parts per million (ppm). Figure 2. Original samples of (a) LY-1 and (b) LY-2 used in our study. UV-Vis-NIR measurement: UV-Vis-NIR absorption spectra in the wavelength range of 200-1600 nm were recorded by using a Perkin Elmer Lambda 900 spectrophotometer. A sweeping speed of 300 nm/min and a slit width 2.5 mm of the spectrometer were held during measurement. The data were analyzed by using the Perkin Elmer Spectrum V.5.0.1 program. Electron spin resonance (ESR): ESR spec- tra were recorded by using a JEOL JES- TE300 spectrometer. For this measurement, an amount 20 mg of the samples in powder was loaded into a quartz-tube holder, and put in a microwave cavity of the ESR spectrome- ter. Microwave frequency was fixed at 9.45 GHz (within the X band), and the magnetic field (H) could be swept from 0 to 10 kOe. X-ray absorption spectroscopy: We used the extended X-ray absorption (XAS) tech- Le Thi Thu Huong, et al./Vietnam Journal of Earth Sciences 40 (2018) 50 nique to investigate geometric and electronic structures of transition-metal impurities pre- sent in the samples. A light source was oper- ated with energy of 2.5 GeV and maximum current of 160 mA. The spectra in the trans- mission configuration of some impurity 3d- elements for the K edge, consisting of Ti (with the binding energy E0 = 4966 eV), V (E0 = 5465 eV), Cr (E0 = 5989 eV), Mn (E0 = 6539 eV), Co (E0 = 7709 eV), Fe (E0 = 7112 eV) and Ni (E0 = 8333 eV) were checked. For ref- erence, some foils and oxides of these ele- ments were recorded under the same condi- tions. The analyses of XAS data afterwards were based on an IFEFFIT package. 4. Results 4.1. EMPA analysis Chemical compositions of LY-1 and LY-2 obtained from EMPA analyses revealed that they are sensu stricto end members. Main chemical elements are Al and Mg with the amounts of respective oxides up to 70.48 wt% and 28.07 wt%, respectively. The dominant impurities detected in these samples are Fe, Cr and Ni. Other impurities include V, Mn, Zn, Ti, Co, and Ca. Particularly, LY-2 contains averagely 0.09 wt% Co, and 0.5 wt% Fe while LY-1 contains 0.05 wt% Co and 0.71 wt% Fe, as shown in Table 1. 4.2. UV-Vis-NIR UV-Vis-NIR spectra of LY-1 and LY-2 shown in Figure 3 reveal absorption peaks lo- cated in the ranges 500-650 nm, and 400-500 nm (the violet-blue region). As indicated by Chauvire et al. (2015), the absorption peaks at 371, 386, 455 and 590 nm are due to the elec- tronic transitions of Fe2+ ions. Meanwhile, three intense absorption peaks at 550, 580 and 625 nm are associated with electronic transi- tions of Co2+ ions. Another weak peak at ~427 nm reported by Chauvire et al. (2015) is also seen in our UV-Vis-NIR spectra, which has not been assigned to any impurity. To learn more about these impurities, and others unde- tectable by UV-Vis-NIR absorption spectros- copy, we additionally used other sensitive tools, as shown below. Table 1. Average chemical compositions of 2 blue spi- nel samples from Luc Yen, obtained by EMPA. Light cobalt blue (LY-1) Saturate cobalt blue (LY-2) Oxides (wt%) V2O3 0,07 0,08 MnO 0,10 0,09 NiO 0,08 0,23 ZnO nd 0,02 Al2O3 70,22 70,48 TiO2 0,01 0,01 Cr2O3 0,05 0,14 FeO 0,71 0,50 CoO 0,05 0,09 MgO 28,07 27,65 CaO nd 0,01 Total 99,35 99,29 Cation (4 Oxygen) V 0,001 0,002 Mn 0,002 0,002 Ni 0,002 0,004 Zn 0,000 0,000 Al 1,982 1,989 Ti 0,000 0,000 Cr 0,001 0,003 Fe 0,014 0,010 Co 0,001 0,002 Mg 1,002 0,987 Figure 3. UV-Vis-NIR spectra of LY-1 and LY-2 show- ing the presence of Co2+ and Fe2+ ions. Vietnam Journal of Earth Sciences, 40(1), 47-55 51 4.3. ESR analyses ESR is a resonant technique based on the microwave absorption of unpaired electron spins in an applied magnetic field (H). The resonance occurs at the resonant field (Hr) as h = g/BHr, where h,, g, and B are the Planck constant, microwave frequency, Lande factor, and Bohr magneton, respectively. This is proven to be a sensitive tool to study elec- tronic structures, lattice defects, impurities, and magnetic phases of a material (Ikeya, 1993). For color centers (lattice defects and/or impurities) present in gemstones, depending on color-center types and oxidation states, their resonant spectra could be observed at low or room temperatures. Using this tech- nique, we recorded room-temperature ESR spectra of the samples LY-1 and LY-2 (Figure 4). It appears that their spectral features are quite the same, with two resonant regions. The first region of 1650~1950 Oe shows a single line (the insets of Figure 4) with Hr  1970 Oe, which is assigned to a Cr3+ forbid- den transition (M = 2) (Padlyak et al., 2003). The other one of 3000~4000 Oe shows the sextet due to Mn2+ ions isolated in crystal fields of the MgAl2O4 structure (Ikeya, 1993). The spectral splitting (the distance between any two neighboring lines) is about 75 Oe. Originally, these lines are generated from the electron-nuclear interaction (i.e., the hyperfine structure) of Mn2+ that can be understood as follows: Mn2+ with the high-spin electron con- figuration 3d5 has the spin numbers S = 5/2 (electron spin) and I = 5/2 (nuclear spin). Six energy levels (2M + 1 = 6) of the electron spin with the magnetic quantum number M = 5/2, 3/2, and 1/2 generated by an external mag- netic field are further split by the magnetic fields due to six nuclear spin states (2I + 1 = 6) with the nuclear spin magnetic quantum number m = 5/2, 3/2, and 1/2. This leads to the resonance at six different magnetic fields, corresponding to the allowed transi- tions (M = 1 and m = 0) of (-5/2, -3/2), (- 3/2, -1/2), (-1/2, +1/2), (+1/2, +3/2), and (+3/2, +5/2) (Ikeya, 1993). The observation of these transitions depends on the crystal orien- tation versus the applied field direction. For the present samples in powder, we only ob- serve the central transition of (-1/2, +1/2), which is angular-independent of the crystal orientation. Figure 4. ESR spectra of the samples of (a) LY-1 and (b) LY-2, which show the presence of Cr3+ and Mn2+ ions. The insets show an enlarged view of the Cr3+- related spectral region If more attention is given to the resonant- signal intensity, one can see that the intensity of the Mn2+ hyperfine lines in LY-2 is smaller than that of LY-1, proving less Mn2+ ions pre- sent in LY-2. However, the Cr3+ amount in LY-2 is higher than that in LY-1. These re- sults are in good agreement with those record- ed from the EMPA study, as shown in Table 1. Apart from Mn2+ and Cr3+ ions, the ESR in- vestigation did not detect other impurities as Le Thi Thu Huong, et al./Vietnam Journal of Earth Sciences 40 (2018) 52 found in the EMPA and UV-Vis studies be- cause their resonant signal is invisible at room temperature or their concentration in the sam- ples are out of the detection limit of the spec- trometer. Notably, the ESR study found the presence of Mn2+ and Cr3+ ions, but could not identify their site in the spinel structure. We thus used another spectroscopic technique, as shown below. 4.4. XAS analyses The study of how X-ray beams absorbed by an atom at energy levels near and above core-level binding energies is known as X-ray absorption fine structure (XAFS). It is related to chemical and physical states of the absorb- ing atom, and thus sensitive to valence state, bond lengths, and the coordination number of neighbor atoms (Teo, 1986). The study of matter based on XAFS provides important in- formation on the atomic scale. Basically, each XAFS spectrum of an absorbing atom is di- vided into three characteristic regions: the pre- edge, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS). The pre-edge region is re- lated to transitions from core electrons to bound states, such as 1s to nd, (n+1)s or (n+1)p orbitals for the K edge. The XANES region gives information related to valence states of the absorbing atom. Different oxida- tion states result in a chemical shift in the ab- sorption edge. For the EXAFS region, it is the sum of all outgoing and incoming waves, and dependent on the immediate environment sur- rounding the absorbing atom. EXAFS analysis thus gives information on the bond distances, geometric structure, and coordination number of neighbor atoms. We performed XAFS experiments for the K edge of elements Ti, V, Cr, Mn, Co, Fe, Zn and Ni present in a typical sample LY-2. Among these elements, only XAFS spectra of Cr, Co, Fe and Ni were recorded clearly, as shown in Figure 5. The spectra of other ele- ments (Ti, V, Zn and Mn) were very weak, and undistinguishable from background nois- es, probably due to their low concentration, and/or their small emission energy absorbed by ambient air. Analyzing the XAFS spectra, we obtained the following results: (i) For ele- ment Cr, the absorption edges of LY-2, Cr2O3 (containing Cr3+), and Cr3+-MgAl2O4 (red spi- nel), in the energy range 5990-6000 eV, over- lap each other, see Figure 5(a). This proves that the oxidation state of Cr is 3+. (ii) For el- ement Fe, the absorption spectra of LY-2, Fe foil (Fe0) and Fe-doped LaMnO3 (with a coex- istence of Fe3+ and Fe2+) do not show any overlap at the K-edge in the range of 7105- 7130 eV, Figure 5(b), reflecting that the Fe oxidation state in LY-2 could not be 0, 3+ and 4+. We found the absorption edge of Fe in LY-2 located at around 7117.5 eV, which is very close to that of Fe2+ in FeO (Nguyen et al., 2011). This means that Fe impurities in our LY-2 sample are Fe2+ rather than other states. (iii) For element Co, we also recorded the XAFS spectra of Co foil (Co0), CoO (Co2+), and Co3O4 (Co2+, Co3+) together with LY-2. The results shown in Figure 5(c) reveal the absorption edge of LY-2, in the range 7710-7717 eV, almost overlapping with that of CoO, proving the existence of Co2+ ions in LY-2. Finally, for element Ni, the spectra in Figure 5(d) show the overlap of the absorption edges of LY-2 and NiO (Ni2+), demonstrating the presence of Ni2+ ions in LY-2; its concen- tration is quite small, leading to noisy signals at high energies far from the edge absorption. In short, detailed XAFS studies have provided a evidence that there coexist of Cr3+, Fe2+, Co2+, and Ni2+ ions in the sample LY-2. Vietnam Journal of Earth Sciences, 40(1), 47-55 53 Figure 5. K-edge XAFS spectra of (a) Cr, (b) Fe, (c) Co, and (d) Ni dopants present in LY-2, compared with those of standard samples (foils or oxides) Together with identifying impurities and their valence, we have taken into account their site in the MgAl2O4 spinel structure. This could be carried out in terms of performing the Fourier transform (FT) in the real space R for the EXAFS spectra (Teo, 1986). Here, we are only successful for the FT for the EXAFS data of the Cr and Co; other elements have a poor profile of the EXAFS region. The FT spectra of the Cr and Co K-edges are plotted in Figure 6. It appears from Figure 6(a) that the average bond length between Cr and O, RCr-O, in the spinel LY-2 is about 1.46 Å. Meanwhile, the bond distance between Cr and Al, RCr-Al is about 2.29 Å. These distances are very close to those between Cr with O, and Cr with Al of a red spinel (Cr3+: MgAl2O4, the reference sample). It should be noticed that RCr–O and RCr-Al values are shifted by ~0.5 Å on the R axis from their true values because of the phase shift of backscattered photoelec- trons. If considering the FT spectra of Co (Figure 6b), one also can see that the bond distances RCo-O and RCo-Al (dotted lines) are almost close to those of RCr–O and RCr-Al in the samples LY-2. Juhin et al. (2007) have stud- ied MgAl2O4:Cr3+ samples (where Cr3+ substi- tuted for Al3+) by using the XAS technique Le Thi Thu Huong, et al./Vietnam Journal of Earth Sciences 40 (2018) 54 and theoretical calculation, and showed that the distances RCr–O and RCr-Al are 1.98~1.99 Å and 2.88~2.91 Å, respectively. These distanc- es are in good agreement with our work re- sults, as shown above, proving that both Cr3+ and Co2+ ions have the same position, and are located at the Al3+ site of the MgAl2O4 spi- nel. Figure 6. The FT spectra of LY-2 for (a) Cr and (b) Co K-edges comparing with MgAl2O4:Cr3+ 5. Discussions The presence of Cr3+, Fe2+, Co2+, Mn2+ and Ni2+ in MgAl2O4 were determined. Particular-ly, we identified the location of Cr3+ and Co2+ in the Al3+ site. In MgAl2O4 spinel structure, it has been believed that 2+ ions (Fe2+, Co2+, Mn2+, and/or Ni2+) occupy the Mg tetragonal (Chauviré et al., 2015; Gaft et al., 2005). However, it is now indicated that Co2+ could be also located in octahedral sites of spinel. In reference to the color of the samples LY-1 and LY-2, based on the features of UV-Vis spectra shown in Figure 3, we agree with the opinion that Co2+ ions are the main coloring agent for the cobalt-blue color (Chauviré et al., 2015). The saturate level of cobalt-blue color de- pends on Co2+ concentration. A lower Co2+ concentration in LY-1 compared with LY-2 makes the color of LY-1 brighter than that of LY-2. Apart from Co2+ ions, we believe that ions Fe2+ and Cr3+ also influence strongly the color of cobalt-blue spinel. For example, Chauvire et al. (2015) pointed out that a high- er iron/cobalt ratio makes spinel grayer. With increasing Cr content, spinel changes the col- or from red to green (Juhin et al., 2007). For Mn2+ (and Ni2+) ions, they seem to contribute insignificantly to the color change of the sam- ples because LY-1 has a higher Mn2+ content, but its cobalt color is lighter than LY-2. 6. Conclusions Impurities present in light and saturated cobalt-blue spinel samples collected from Luc Yen mining area (Vietnam) have been studied in details. By using different techniques, in- cluding EMPA, UV-VIS-NIR, ESR and XAS, the existence of impurities V, Mn, Ni, Zn, Ti, Cr, Fe, Co, and Ca were identified. Among those impurities, the oxidation state of Mn, Co, Fe and Ni is 2+, while that of Cr is 3+. Based on the FT, we found that Cr3+ and Co2+ ions located at the Al3+ site of the MgAl2O4 spinel structure. We believe that the variation of cobalt-blue color is due to Co2+, Fe2+ and Cr3+, depending on their concentration. Other impurities (Mn2+ and Ni2+) contributed insig- nificantly to the color variation of blue spinel. Acknowledgements This study is supported by a NAFOSTED project grants (105.99-2013.13). References D'Ippolito V., Andreozzi G.B., Hålenius U., Skogby H., Hametner K., Günther D., 2015. Color mechanisms in spinel: cobalt and iron interplay for the blue color. Physics and Chemistry of Minerals, 42, 431-439. Chauviré B., Rondeau B., Fritsch E., Ressigeac P. and Devidal J.L., 2015. Blue spinel from the Luc Yen district of Vietnam, Gems & Gemology, 51, 2-17. Garnier V., Giuliani G., Ohnenstetter D., Fallick A.E., Dubessy J., Banks D., Vinh H.Q., L’homme T., Ma- luski H., Pêcher A., Bakhsh K.A., Long P.V., Trinh Vietnam Journal of Earth Sciences, 40(1), 47-55 55 P.T., Schwarz D., 2008. Marble-hosted ruby depos- its from Central and South-East Asia: Toward a new genetic model. Ore Geology Reviews, 34, 169-191. Gaft M., Reisfeld R., Panczer G., 2005. Modern lumi- nescence spectroscopy of minerals and materials, Springer-Verlag Berlin Heidelberg, 96-97. Giuliani G., Dubessy J., Banks D., Vinh H.Q., Lhomme T., Pironon J., Garnier V., Trinh P.T., Long P.V., Ohnenstetter D., Schwarz D., 2003. CO2-H2S-COS- S8-AlO(OH)-bearing fluid inclusions in ruby from marble-hosted deposits in Luc Yen area, North Vietnam. Chemical Geology, 194, 167-185. Ikeya1 M., 1993. New Applications of Electron Spin Resonance, World Scientific Publishing, 34-57. Juhin A., G. Calas, D. Cabaret, L. Galoisy and J. L. Hazemann, 2007. Structural relaxation around sub- stitutional Cr3+ in MgAl2O4. Physical Review B, 76, 54-105. Nguyen H.M., Dang N.V., Chuang P.Y, Thanh T.D., Hu C.W, Chen T.Y., Lam V.D., Lee C.H., Hong L.V., 2011. Tetragonal and hexagonal polymorphs of BaTi1-xFexO3- δ multiferroics using x-ray and Ra- man analyses. Applied Physics Letters, 99, 202-501. Leloup P.H., Arnaud N., Lacassin R., Kienast J.R., Har- rison T.M., Trinh P.T., Replumaz A., Taponnier P., 2001. New constraints on the structure, thermochro- nology, and timing of the Ailo Shan-Red River shear zone, SE Asia. Journal of Geophysical Research, 106, 4, 6683-6732. Padlyak B.V., Grinberg M., Lukasiewicz T., Kisielewski J., Swirkowicz M., 2003. EPR spectroscopy of the Cr3+ centers in LLGG:Cr single crystals. Journal of Alloys Compounds, 361, 6-12. Shigley J. E. and Stockton C. M., 1984. Cobalt-blue gem spinels. Gems & Gemology, 20, 34-41. Teo B.K., 1986. EXAFS: Basic principles and data anal- ysis, Springer-Verlag.