The Color Change of Natural Green Sapphires by Heat Treatment

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A. Mungchamnankita, T. Kittiauchawalb×, J. Kaewkhaoc, P. Limsuwand

aDepartment of Physics, Faculty of Science, Rangsit University, Pathumtani, 12000, Thailand

bFaculty of Science and Technology, Thepsatri Rajabhat University, Lopburi, 15000, Thailand

cCenter of Excellence in Glass Technology and Materials Science, Faculty of Science and Technology,Nakhon Pathom Rajabhat University, Nakhon Pathom, 73000, Thailand

dThailand Center of Excellence in Physics, CHE, Ministry of Education, Bangkok, 10400, Thailand

Elsevier use only: Received 30 September 2011; Revised 10 November 2011; Accepted 25 November 2011.

Abstract

This research aimed to investigate the effects of heat treatment on color and clarity of green sapphires. The Green sapphires were heated at sequentially increasing temperature of 1300, 1400, 1500 and 1600 °C in oxygen atmosphere. At each treatment temperatures, the samples were treated for 40 hours with their color and clarity were measured before and after heat treatment at each temperature. It was found that, when green sapphires were heated at high temperatures, the color of the untreated sapphire crystal changed to yellow at higher temperatures. Whereas, there were no significant changes in the clarity of all samples found in the temperature range of 1300-1600 °C.

© 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of I-SEEC2011

Open access under CC BY-NC-ND license.

Keywords: Green sapphires, heat treatment

1. Introduction

The structure of Green sapphire is related to corundum (Al2O3) and involves the Al3+ ions being distributed in an ordered fashion in 2/3 of the distorted (trigonal) octahedral sites within a frame work of hexagonal close-packed O2- ions. The color of ruby arises mainly from Al3+ ions being replaced by 3d transition ions during a crystallization process. This is possible because the size of 3d transition ions is very cose to that of Al3+ (0.57 Å) [1].

* Corresponding author. Tel.: +663-642-4175; fax: +663-642-4175.

E-mail address: k_treedej@hotmail.com.

1877-7058 © 2012 Published by Elsevier Ltd. Open access under CC BY-NC-ND license.

doi:10.1016/j.proeng.2012.02.037

Fig. 1. A polyhedral model of the structure of corundum [1]

The presence of varying amounts of Cr3+ ions gives color to ruby from pink (0.01 mole %) to deep red (2 mole %) [1]. Iron usually exists in both Fe2+ as well as Fe3+ depending on the number of O2- vacancies and other point defects [2]. Fe3+ and Ni3+ in corundum yield yellow color whereas Fe2+ gives sapphire green color instead [3]. The charge transfer mechanism between 3d6 (Fe2+) and 3d0 (Ti4+) bands effectively gives sapphire its blue color. A combination of these Fe2+, Ti4+ and Cr3+ ions is clearly responsible for bluish red in ruby [4]. More detailed coloring mechanisms and impurity ions present in sapphires are listed in Table 1.

Sapphire ColorVerneuil SyntheticsNatural Coloring IonsColoring Mechanisms
ColourlessPureNoneNone
Ruby Cr3+Cr3+Electronic transitions & Fluorescence
Pink Cr3+Cr3+Electronic transitions &
   
   
   
   
   
  

Table 1. 3d Transition ions coloring gem corundum (Al2O3) [5]

Fluorescence(R-line)

Blue Fe2+,Ti4+ Fe2+, Ti4+ Charge transfer

Purple and

3+ 2+ 4+

3+ 2+

Electronic transitions ,

4+

Violet Cr

,Fe

,Ti

Cr , Fe

, Ti

Charge transfer & Fluorescence(R-line)

Yellow Ni3+ Fe3++ color centers Electronic transitions

Orange Ni3+, Fe3+ Fe3+, Cr3+ +color centers Electronic transitions &

Fluorescence(R-line)

Green Cobalt, Vanadium + Nickel Fe2+ Electronic transitions

The goal of this work was to investigate the effect of heat treatment on color of natural green sapphire measurements by spectrophotometer.

2. Materials and Methods

The natural green sapphires were cleaned thoroughly with acid and solvent to remove all stains and other impurities on their surfaces. Four crystal samples were chosen to determine trace elements by LA- ICP-MS. Then, the transmission spectrum of samples were measured by a spectrophotometer(Perkin Elmer Instruments model Lambda 800) before and after 40 hours of each temperature of sequential heat

treatment in electric furnace at 1200, 1300, 1400, 1500 and 1600 oC under oxygen atmosphere. Then, the transmission spectra were converted to CIE color index. The CIE color index used Lab color space in three dimensions is shown in Fig. 2(a) and in two dimensions is shown in Fig. 2(b).

  • (b)
(a)
(b)

Fig. 2. CIEL*a*b* color index, (a) in three dimensions and (b) in two dimensions

3. Results and discussion

The results on the investigation of the trace elements in natural green sapphires before heat treatment are shown in Table 2.

Table 2. the trace elements concentration (ppm) of natural Vietnamese blue sapphires

SampleBMgCaTiVCrFeGa
144.9810.0892.56113.479.314.306310.44191.69
240.6519.0215.02568.3314.224.396789.78212.94
333.8520.5322.24176.459.1636.931627.41190.91
437.8211.2246.5188.8115.412.886198.53165.97
Average39.3215.2144.08236.7612.0212.125231.54190.37

From Table 2, it was found that the natural Vietnamese blue sapphires contain 98 %wt of Al2O3, B (39.32 ppm), Mg (15.21ppm), Ca (44.08 ppm), Ti (236.76 ppm), V (12.02 ppm), Cr (12.12 ppm), Fe

(5231.54 ppm) and Ga (190.37 ppm) as trace elements.

The absorbance spectra of samples before and after the heat treatment are depicted in Fig. 3 and Fig. 4, respectively. In Fig. 3, the evolution shows the appearance of many absorption bands, the absorbance bands near 450, 388 and 377 nm are attributed to Fe3+/ Fe3+ pairs, Fe3+ and Fe3+/ Fe3+ pairs, respectively. A small poorly resolved peak at about 860 nm was assigned to Fe2+ and a broader peak around 560 nm is associated with Fe2+-Fe3+ charge transfer. On the contrary, the absorption band corresponded to iron in ferrous state (Fe2+) is not present in the absorbance spectra of after treated samples at 1600 oC, as shown in Fig. 4.

Fig. 3. Absorption spectra from 300 to 900 nm of untreated blue sapphire samples (2 pieces)

Fig. 4. Absorption spectra from 300 to 900 nm of Be-treated samples which became yellow sapphires (2 pieces)

The results of color measurements of three samples for the study of color change and clarity before  and after heat treatment are shown in Table 3 and Figure 5.

Table 3. CIEL*a*b* color index of three samples before and after heat treatment at the temperatures range 1300 – 1600 qC

Temperature CIE L*a*b* color index

(oC)L*a*b*
RT -4.559.44
120058.45-4.7411.87
130052.21-4.0212.34
140052.17-4.9619.95
150053.13-4.3513.53

   1600 50.28 -4.87 17.19

From Table 3 and Fig. 5, the green sapphires were heated at higher temperatures, the b* index of samples increases, or green color of the untreated sapphire crystal (RT) changed to yellow after heat treatment at high temperatures in oxygen atmosphere.

Fig. 5. CIEL*a*b* color index of green sapphires, before and after heat treatment in oxygen atmosphere

4. Conclusion

From results, it can be concluded and confirmed that the yellow coloration by heating under O2 is of the same oxidation process which iron acts as the most important agency. By heating under oxidizing conditions, the oxidation processes of Fe2+ by gaseous oxygen to be considered are

4Fe2+ + O2 o 4Fe3+ + 2O2-

or, as the oxide:

or in corundum

4FeO + O2 o 2Fe2O3

2Al2-2xFe2xO3-x + xO2 o 2Al2-2xFe2xO3

The above three equations are equivalent and result in an electrically neutral state containing only Fe3+; in corundum, the result now corresponds to Fe2O3 being present in Al2O3, having a pale-to-medium yellow color, depending on the concentration. Heat treatment can reduce or eliminates the green color.

In conclusion, the cause of yellow coloration in heated green sapphires is actual reason is the charge transfer between the ferrous and ferric states of iron. The entails heating of the stone in an oxidizing atmosphere produces the following reaction:

Fe2+

heat in an oxidizing Fe3+ atmosphere

with the conversion of the ferrous state to the ferric state, resulting in a yellow color replaces the green. Fe3+ may be present as isolated ions scattered around the substance. Generally, as the concentration of

these iron ions increases, the intensity of the yellow color increases proportionally [6]. Furthermore, the produced color is stable under the fading test.

Acknowledgements

The authors would like to thank the National Research Council of Thailand (NRCT) for financial support.

References

  • Hughes, R. W. Corundum. Courier International Ltd. England. 1990.
  • Kittel, C. Introduction to Solid State Physics, 6th Ed. John Wiley & Sons Inc. New York.1991.
  • Nassau, K. The Physics and Chemistry of Color, 2nd Ed. John Wiley & Sons, Inc. New York. 2001.
  • Winotai, P., T. Wichan, I.M. Tang and J. Yaokulbodee. Heat Treatments of Tanzania Ruby as Monitored by ESR Spectroscopy. IJMPB 2000;14(16):1693 –1700.
  • Winotai, P., S. Saiseng, and T. Sudyoadsuk. Optimization of Heat Treatments of African Green Sapphires. Modern Physics Letters B 2001;15(20):873-882.

Emmett, J.L., Scarratt, K., McClure, S.F., Moses, T., Douthit, T.R., Hughes, R., Novak, S., Shigley, J.E., Wang, W., Bordelon, O. and Kane, R.E., Beryllium Diffusion of Ruby and Sapphir

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