588 Chiang Mai J. Sci. 2018; 45(1)

Chiang Mai J. Sci. 2018; 45(1) : 588-600 http://epg.science.cmu.ac.th/ejournal/ Contributed Paper

Heat Treating Sapphire of Pailin, Cambodia Kanyarat Kwansirikul*, Panjawan Thanasuthipitak, Siwakon Chimnakphant and Opor Saidum Department of Geological Sciences, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand. * Author for correspondence; e-mail: [email protected]

Received: 26 May 2016 Accepted: 19 October 2016

ABSTRACT A total of 54 rough blue sapphire samples from Pailin gem field of Cambodia were studied. The samples can be classified into three groups on the basis of their colors and distinct appearance. They consist of trapiche-like blue sapphire, and blue to dark blue groups. The main aim of this study is to study heat treatment process of blue sapphire samples from Pailin gem field of Cambodia. Gemological properties and chemical compositions of these samples are also investigated. For heat treating experiments, the samples were divided into two batches according to the purpose of the heat treatments. Forty-five samples were heated in oxidizing condition to lighten dark blue color of the samples while nine samples were heated in reducing condition to intensify or develop the blue color. The results show that heating at 1200°C can lighten dark blue color and heating over 1400°C can develop or intensify blue color. The UV-Vis-NIR absorption spectra of all samples exhibit Fe3+ absorption peaks, Fe2+/Ti4+ IVCT and Fe2+/Fe3+ IVCT absorption bands. The more intense blue color intensified after heating, the stronger the intensity of Fe2+/Ti4+ IVCT absorption band. The Fe2+/Fe3+ IVCT absorption band probably contributes to the darker tone in the samples. Chemical compositions reveal iron (Fe) and titanium (Ti) are the most significant minor and trace elements because both of them are necessary for the color-causing mechanism in blue sapphire.

Keywords: heat treatment, blue sapphire, corundum

1. INTRODUCTION Blue sapphire is a variety of corundum distributed in many areas of the world such and one of the most economical colored as Thailand, Cambodia, Myanmar, Vietnam, gemstones used in jewelry due to its beauty, Laos, Sri Lanka, Australia, China, and USA good hardness, and durability. Corundum [3-6]. Pailin is a province in western Cambodia can be grouped into two varieties that are and Pailin gemstone mining area is located ruby and sapphire. Ruby is referred to near the Chanthaburi-Trat gemfield in corundum with -apparent color while the eastern part of Thai border. The Pailin sapphire is referred to corundum of all gemfield was discovered probably by other colors [1-2]. Corundum deposits are Burmese merchants as late as 1874 [7]. Chiang Mai J. Sci. 2018; 45(1) 589

Corundums of this area come from small rounding of the crystal faces due to partial basaltic bodies similar to those of Thailand corrosion from the lava. Healed fractures in [7]. They occur as both eluvial and alluvial the sapphires are numerous and make the gravel derived from Tertiary to Pleistocene stones lack of clarity or transparency [4, 7]. basaltic intrusion in older sedimentary or Heat treatment is commonly used to remove metamorphic rocks. The famous Pailin mines undesirable colors and improve clarity and produce only ruby and blue sapphire. also value. Heat treatment of ruby and The internal features of the ruby and blue sapphire is a thermochemical process and sapphire from Pailin gemfield resemble has been known, performed and documented those found in the ruby and blue sapphire for many centuries [13]. Vast majority of ruby from the Chanthaburi-Trat gemfield [4, 7]. and sapphire in the jewelry market today Corundum is a single crystal of aluminum have been heat treated to enhance quality oxide (Al2O3) with mineral structure and value [13-15]. Although heat treatment of hematite group. Pure corundum is processes of corundum were reported colorless. Its transparency ranges from through many publications, the exact heating approximately160 nm in the far ultraviolet conditions for each deposits cannot usually to 5500 nm in the infrared region of the be specified for two main reasons. First, most spectrum. All colors in corundum are the heat treatment processes have never been results of color-producing transition metals fully revealed. The success of heat treatment in form of dispersed metal ions and charge has still remained the treaters’ experience. transfer mechanism, other point defects in Second, the stones from each deposit have the crystals, and physical phenomena e.g. wide variations of materials and also chemical the presence of colored inclusions [8-10]. compositions [16]. The main aim of this Many things can happen to the light entering study is to determine the appropriate heat the crystal. It can be reflected, refracted, treatment configuration for low-quality blue diffracted, scattered, absorbed, or simply sapphires from Pailin, Cambodia. Physical transmitted. Although absorption is by far and optical properties and chemical the most important factor in determining compositions are also studied. color, several combinations of these different processes are possible [9-10]. Corundum 2. MATERIALS AND METHODS structure is based upon hexagonal close 2.1 Sample Classification and Preparation packing of six oxygen atoms, with cation Blue sapphire samples used in this study (Al3+) in octahedral coordination sites between were bought by one of the authors directly them. When the Al3+ in corundum is replaced from miners during the visit to Pailin gem by a trace of transition metal element such as mining area in Cambodia. A total of 54 rough Cr3+, Fe3+, Fe2+ or Ti4+, the resulting complexes samples ranging from 0.28 to 4.54 ct. was are often colored [11-12]. chosen for this heating experiment. Color of the Pailin blue sapphires ranges The majority of the samples are rounded or from medium to dark blue. Most of them broken fragments of hexagonal crystals. are non-gem or low quality sapphires. They show various , ranging Color zoning is extremely sharp and follows from light blue to very dark blue. The samples the hexagonal contours of the crystal. are semitransparent to nearly opaque and Euhedral crystals are relatively rare. Most blue not of good gem qualities. All samples sapphires exhibit a slight to large degree of were cleaned by soaking in hydrochloric acid 590 Chiang Mai J. Sci. 2018; 45(1)

and then hydrofluoric acid to remove stains trapiche-like blue sapphire (23 samples), and other impurities that might be attached light blue (10 samples) and blue to dark blue to the samples. Then, they were polished on (21 samples) groups. The trapiche-like blue one side perpendicular to the c-axis of the sapphire group shows distinct brown core, crystal for easily observation the change brown fixed star-like appearance (arm), and after heat treatment and photographed to blue sector between arms. The light blue keep as reference before heat treatment group has light blue color with unevenly process. The studied blue sapphires were distributed blue or white patches or zones. divided into three groups according to The blue to dark blue group shows blue to their colors and distinct appearance on the dark blue color with uneven distribution of polished surface (Figure 1). They consist of white or milky color patches or zones.

Figure 1. The studied blue sapphire samples from Pailin, Cambodia, in the trapiche-like blue sapphire (left), the light blue sapphire (middle), and the blue to dark blue sapphire groups (right).

2.2 Analytical Techniques spectrophotometer with beam condenser The standard gemological techniques and polarizing filter, for polarized used in this study include specific gravity spectroscopy in the Ultraviolet-Visible determination by hydrostatic balance, through Near Infrared range. Light sources refractive index and birefringence used are deuterium (D2) and tungsten (W) determination by Duplex II refractometer lamps which produce continuous radiation with sodium equivalent light source, of UV and Vis-NIR regions, respectively. ultraviolet luminescence observation by Photomultiplier (for UV-Vis region) and ultraviolet lamp (using both long-wave cooled-type PbS cell (for NIR region) (365.4 nm) and short-wave (253.7 nm) detectors were used to obtain the spectra radiation), and gemological microscope with with wavelength accuracies of +/-0.2 nm in addition fiber optic illumination to examine UV-Vis region and +/-1.0 nm in NIR region the internal features and surface of the samples. incorporated with automatic wavelength These standard gemological techniques calibration function. The absorption spectra were carried out at the Gemological section, were recorded over a range of 250 to 1200 Department of Geological Sciences, Faculty nm at a scan speed of 300 nm/minute and of Science, Chiang Mai University, Chiang a slit width of two millimeters. The intensity Mai, Thailand. of light passing through a crystal at a given The absorption spectra were obtained wavelength was measured in absorbance from Hitachi U-4001 UV-Vis-NIR unit. The data were compiled by Hitachi Chiang Mai J. Sci. 2018; 45(1) 591

spectrophotometer UV solution program. were 20 kV accelerating voltage, 60 nA Scanning Electron Microscope with current, 5 micron spot size, and counting energy dispersive X-ray fluorescent times of 10 and 60 seconds for major and spectrometry (SEM-EDS) was used to trace elements, respectively. The standards analyze and identify mineral inclusions used were a combination of natural minerals present in the samples. Two samples from and synthetics. Detection limits of Al2O3, the trapiche-like blue sapphire and the blue FeO (total), TiO2, Ga2O3, SiO2, MgO, V2O5, to dark blue sapphire groups were selected and Cr2O3 are 0.015, 0.012, 0.010, 0.013, for examination of mineral inclusions exposed 0.009, 0.008, 0.016, and 0.013 wt%, to the sample surface using SEM-EDS. respectively. They were polished on one side perpendicular to the c-axis of the crystals and coated 2.3 Heating Experiments with carbon to be electrically conductive. The studied blue sapphires were divided This analytical technique were carried out into three groups according to their colors from a JEOL Model, JSM 5900LV scanning and distinct appearance on the polished electron microscope with energy dispersive surface as mentioned in the section 2.1. X-ray fluorescent spectrometer, at the They consist of trapiche-like blue sapphire Department of Mineralogy, Natural History (23 samples), light blue (10 samples) and Museum, London, England. Oxford’s INCA blue to dark blue (21 samples) groups. software was used for spectrum acquisition The studied sapphire samples were heated in through backscattered electron image. electric furnace at Department of Geological The machine conditions for analysis include Sciences, Faculty of Science, Chiang Mai an accelerating voltage of 20 kV and a probe University, Chiang Mai, Thailand. The heating current of 2 nA. experiments were conducted under two The chemical composition of 35 atmospheric conditions: oxidizing and sapphire samples was analyzed using reducing conditions, depending on the CAMECA SX50 and CAMECA SX100 objective of the experiment whether to electron microprobe with wavelength lighten or intensify the blue color of the dispersive X-ray fluorescent spectrometry studied samples. Forty-five samples from at the Department of Mineralogy, Natural all samples in the trapiche-like blue sapphire History Museum, London, England. The and blue to dark blue groups, and one sample analysed samples selected from the from the light blue group were heated in three groups were polished on one side oxidizing condition to lighten the blue color perpendicular to the c-axis of the crystals while nine samples from the light blue group and coated with carbon. They were analyzed were heated in reducing condition to intensify for Al2O3, FeO (total), TiO2, Ga2O3, SiO2, MgO, the blue color. The heating atmosphere of

V2O5, and Cr2O3. One to three analysis the oxidizing condition was carried out points were measured on each sample, with the oxygen content in the chamber and a total of 121 analysis points were approximately of 35% oxygen gas level. measured. The measured points on each The reducing condition was created sample were chosen on the basis of through continuous purging of Ar gas into different apparent colors. The analyses were the heating chamber to reduce oxygen gas operated with the Sunray program for level retained in the heating chamber, by data management. Operating parameters which the oxygen content in the chamber 592 Chiang Mai J. Sci. 2018; 45(1)

was lowered to 0.5-1 %. For both conditions, atomic% of elements composing the mineral the samples were placed in alumina crucibles. inclusions was made and the results were The maximum temperatures are set at used to identify the inclusions by comparing 1200, 1300, 1400, 1500, 1600, 1700 and with chemical formula of minerals using 1800 °C for oxidizing condition, and at 1200, an index of mineral species and varieties 1400 and 1600 °C for reducing condition [17-18]. The SEM-EDS analysis reveals the ° with the heat-up rate of 3-7 C/minute and presence of mangan-ilmenite [(Fe, Mn)TiO3], five hours soaking time. When the set ilmenorutile [(Ti, Nb, Ta, Fe)O2], zircon temperature and soaking time were reached, (ZrSiO4), monazite [(La, Ce, Th, Nd, U)PO4] the furnace was switched off. with minor amounts of thorium,

uranpyrochlore [(U)(Nb, Fe, Mn)2O6(OH)] 3. ESULTS AND DISCUSSION R and ferrocolumbite [(Fe, Mn)(Ta, Nb)2O6]. 3.1 Gemological Properties Specific gravity (S.G.) of all blue samples ranges from 3.96 to 4.03. Refractive indices

(R.I.) vary from 1.760 to 1.762 for ne and from 1.768 to 1.772 for no with a birefringence (B.F.) of 0.007 to 0.010. There are no significant variations in each group mentioned in previous section. The samples are uniaxial negative and appear inert when exposed to ultraviolet radiation both long wave and short wave. Some samples show moderate to strong dichroism (blue to violetish blue). The determined values are typical for Figure 2. Healed fractures which are sapphire from all other locations world-wide. commonly seen in the studied blue sapphire Microscopic examination reveals healed samples. fractures, which are common in the studied samples (Figure 2), mineral inclusions, negative crystals, and color zoning. The color zoning that is commonly observed is alternating blue and light blue or white/brown zones. Black and dark brown mineral inclusions occur as small particles commonly scattered in core and arms of the trapiche- like blue sapphires (Figure 3). Some healed fractures present in the samples are filled with iron stain. Two samples from the trapiche-like blue sapphire and the blue to dark blue sapphire groups were selected for examination of mineral inclusions exposed Figure 3. Small particles of black and dark to the sample surface using SEM-EDS. brown mineral scattered in core and arms of Semi-quantitative analysis in weight% and the trapiche-like blue sapphires. Chiang Mai J. Sci. 2018; 45(1) 593

3.2 Spectroscopic Features absorption (Figure 4). They commonly show Absorption spectra were determined in a peak at 450 nm assigned to Fe3+ pair on the all the blue sapphire samples. The samples nearest-neighbor lattice sites, a broad band which had been heat treated were observed between 500 and 650 nm with a maximum again in the same area. Comparison was approximately at 580 nm due to Fe2+/Ti4+ made for the spectra before and after heat IVCT, and an absorption band between 700 treatment of the same samples. Generally, and 980 nm with a maximum approximately the absorption spectra of the samples were at 875 nm due to Fe2+/ Fe3+ IVCT [2, 15, recorded depending on whether the light 21-23]. The position at around 880 nm of propagating in the crystal has its polarization the absorption spectra in this study is caused parallel or perpendicular to the c-axis of the by the change of detector between crystal. Thus, each sample was run twice, ultraviolet-visible and near infrared regions. one for the extraordinary (e) spectrum The absorption spectra of the samples in (parallel to the c-axis) and the other for the light blue sapphire group were recorded ordinary (o) spectrum (perpendicular to the from the light blue area. They show some c-axis). The absorption spectra of the heated peaks at 388 due to Fe3+ and 450 nm, a broad samples are presented in section 3.4. band between 500 and 650 nm with a The recorded spectra have features typical of maximum approximately at 580 nm, and an those seen in blue sapphire, with variations in absorption band between 700 and 980 nm the relative intensities of the absorption bands with a maximum approximately at 875 nm associated with color-causing mechanisms. (Figure 5). Absorption spectra of the The intensities of the spectra are also affected samples in blue to dark blue sapphire by the saturation of color, transparency and group were recorded from the blue or dark thickness. Additionally, most samples with blue area including white zone. They show uneven color distribution or the different some peaks at 388 and 450 nm, a broad apparent colors were measured in many band between 500 and 650 nm with a areas. Blue coloration in sapphire is essentially maximum approximately at 580 nm, and an caused by intervalence charge-transfer absorption band between 700 and 980 nm processes (IVCT) of the ion pairs Fe2+/Ti4+ with a maximum approximately at 875 nm and Fe2+/Fe3+ [9, 10, 17, 18]. The Fe3+ and (Figure 6). pairs of Fe3+ spectra can be seen in most samples [9-10, 19-20]. Absorption spectra of the samples in the trapiche-like blue sapphire group were recorded from three areas: brown core, brown arm, and blue sector between arms. However, a few samples were recorded from only two areas because of the small size of the samples (approximately 3 to 4 mm wide) and the limitation of the slid width of two Figure 4. UV-Vis-NIR absorption spectra in millimeters. The overall patterns of the the trapiche-like blue sapphire group before absorption spectra are similar for the three and after heating under oxidizing condition areas with variations in the intensity of the and at different temperatures. 594 Chiang Mai J. Sci. 2018; 45(1)

3.3 Chemical Compositions The data shown in this section summarized the overall results of trace elements in wt% oxide concentration for the three sample groups (Table 1). The information provided consist of the number of samples and analysis points for each group of samples, the maximum, minimum, arithmetic mean, and the number of analysis Figure 5. UV-Vis-NIR absorption spectra in points with value above detection limit the light blue sapphire group before and after for each trace element. heating under reducing condition and at The electron probe microanalysis reveals different temperatures. the presence of various major, minor and

trace elements. Al2O3 is the sole major element. Iron (Fe) and titanium (Ti) are the most significant minor and trace elements because both of them are necessary for the color- causing mechanism in blue sapphire. Gallium (Ga) and silicon (Si) are also detected as trace elements present in the studied sapphires. Magnesium (Mg) and vanadium (V) are present in negligible amounts while Figure 6. UV-Vis-NIR absorption spectra in chromium (Cr) is only present in one analysis the blue to dark blue sapphire group before point from a sample of the blue to dark blue and after heating under oxidizing condition group. and at different temperatures.

Table 1. Chemical compositions of the elements in wt% oxide with minimum and maximum values along with the arithmetic mean values (M) in parenthesis obtained from the analyses of the sapphire samples by the EPMA-WDS.

Group Number Wt% oxide of of point Al2O3 FeO(total) TiO2 Ga2O3 SiO2 MgO V2O5 Cr2O3 samples analyses (N) Trapiche- 47 95.590- 0.274-1.773 0.010-1.992 0.015-0.039 0.009-0.300 0.010-0.018 0.018 bdl like blue 100.420 (M=0.446) (M=0.106) (M=0.024) (M=0.028) (M=0.010) n=1 n=0 sapphire (M=98.606) n=47 n=39 n=44 n=43 n=2 n=47 Light blue 28 97.470- 0.082-0.560 0.010-0.083 0.016-0.040 0.010-0.045 0.010-0.010 0.020 bdl sapphire 99.631 (M=0.334) (M=0.030) (M=0.024) (M=0.022) (M=0.010) n=1 n=0 (M=98.727) n=28 n=22 n=23 n=21 n=7 n=28 Blue to 46 97.243- 0.214-1.515 0.010-0.275 0.013-0.046 0.009-0.043 0.010 0.016-0.018 0.013 dark blue 100.230 (M=0.445) (M=0.057) (M=0.028) (M=0.021) n=1 (M=0.017) n=1 sapphire (M=98.995) n=46 n=36 n=42 n=43 n=3 n=46 Chiang Mai J. Sci. 2018; 45(1) 595

The maximum mean value of FeO (total) 3.4 Effect of Heat Treatment on the Blue content is 0.446 wt% found in the samples Sapphire from Pailin, Cambodia from the trapiche-like blue sapphire group A total of 54 sapphire samples were while the minimum mean value is 0.334 wt% divided into two batches. They were heated found in the samples from the light blue in oxidizing condition to lighten the blue group. The maximum mean value of TiO2 color and in reducing condition to intensify content is 0.106 wt% found in the sample the blue color as mentioned in section from the trapiche-like blue sapphire group 2.3 (heating experiments). The maximum whereas the minimum mean value is 0.030 temperatures are set at 1200, 1300, 1400, wt% found in the samples from the light blue 1500, 1600, 1700 and 1800 °C for oxidizing group. Overall, the dark brown color of the condition (Table 2), and at 1200, 1400 samples in the trapiche-like blue sapphire and 1600 °C for reducing condition group show higher iron oxide and titanium (Table 3). In oxidizing condition after oxide contents than the samples in the other heating at 1200 and 1300 °C, the blue color two group. The mean values of iron oxide intensity of the samples was reduced but and titanium oxide contents in the samples more white patches or zones in some from blue to dark blue group are higher samples appeared. The brown core and than those in the samples from light blue brown arm of the trapiche-like blue sapphire group. The results of chemical analysis in samples were unchanged. After heating this study indicate that high iron contents at 1400 and 1500 °C, the blue color intensity and titanium contents are probably related of the samples including the brown core with brown color and dark tone of blue color and brown arm of the trapiche-like blue because Fe2+/Ti4+ IVCT and Fe2+/Fe3+ IVCT sapphire samples started to develop. are the causes of blue coloration and Fe3+ However, the core and arm sectors could is responsible for yellow to brown coloration. still be seen. More white patches or zones in The very dark blue sapphires, especially from some samples also turned to blue shades. magmatic affiliation e.g. China, Cambodia, After heating at 1600 °C, more blue color etc., normally contain large amounts of iron was developed in the samples. After heating (Fe2+) or titanium (Ti4+) [24-25] whereas pale at 1700 and 1800 °C, blue color intensity blue sapphires have iron (Fe3+) in excess was slightly developed. In reducing condition, [26-27]. The colour and clarity of the sapphires the samples heated at 1200, 1400 and depend on the amount and oxidation state 1600 °C showed similar results with the other of titanium and iron contained. There is a high heating experiment at the same temperatures content of iron in dark sapphire, especially under oxidizing condition. Figures 7 to 9 Fe2+, which leads to the sapphire looking dark show progressive change in the samples of blue. By decreasing the iron content or each group after heat treatments at different oxidizing Fe2+ to Fe3+, the dark sapphires maximum temperatures. would turn to lighter blue and the clarity would be improved [24]. 596 Chiang Mai J. Sci. 2018; 45(1) res. 1800 intensity intensity intensity developed. developed. developed. was slightly was slightly was slightly Blue colour Blue colour Blue colour 1700 slightly slightly slightly developed. developed. developed. Blue colour Blue colour Blue colour intensity was intensity was intensity was 1600 zones zones. colour in all sectors. patches or patches or More blue more blue More blue more blue More Blue colour with colour with 1500 zones. zones. C) ° with blue with blue patches or patches or Blue colour Blue colour Blue colour in all sectors. 1400 zones. zones. Temperature ( Temperature with blue with blue patches or patches or Blue colour Blue colour Blue colour in all sectors. 1300 after heat treatment under oxidizing condition at difference temperatu after heat treatment under oxidizing zones. zones. arm were Light blue Light blue patches or Light blue patches or color with color with and brown unchanged. blue sector. more white more white color in the Brown core 1200 zones. zones. arm were Light blue Light blue patches or Light blue patches or color with color with and brown unchanged. blue sector. more white more white color in the Brown core zones. Before* or zones. dark blue patches or Blue color distributed distributed and blue to brown arm, blue colour Blue to dark Brown core, colour sector blue or white white patches between arms. with unevenly with unevenly 1 of 23 21 samples Number Blue to samples sapphire sapphire dark blue Group of Light blue Trapiche-like blue sapphire Before*: Before heat treatment Progressive in colours of changes the samples before and Table 2. 2. Table Chiang Mai J. Sci. 2018; 45(1) 597

Table 3. Progressive changes in colours of the samples before and after heat treatment under reducing condition at difference temperatures.

Group of Number Temperature (°C) sample of samples Before* 1200 1400 1600 Light blue Light blue color with Very light blue colour Blue colour with blue More blue colour sapphire 9 unevenly distributed with more white patches or zones in the in all samples with blue or white patches patches or zones samples showing white blue patches or or zones. in some samples. patches or zones. zones.

Before*: Before heat treatment

Figure 7. Progressive changes in trapiche-like blue sapphire samples before and after heat treatment under oxidizing condition and at different maximum temperatures.

Figure 8. Progressive changes in light blue sapphire samples before and after heat treatment under reducing condition and at difference maximum temperatures.

Figure 9. Progressive changes in blue to dark blue sapphire samples before and after heat treatment under oxidizing condition and at difference maximum temperatures. 598 Chiang Mai J. Sci. 2018; 45(1)

The development of blue color after low transparency of some samples makes the heating at higher temperatures is due to change spectra rather noisy, especially the peak at 388 of oxidation state of iron and dissolved nm. A band between 700 and 980 nm appears titanium. The change of oxidation state from after heating. After heating experiments, Fe2+ to Fe3+ still occurs but titanium in the the inclusions in the samples were examined presence of titanium-containing inclusions by the gemological microscope again to verify in the samples can be activated. Titanium may whether they had changed. Most samples be present in the form of micro-inclusions show darker colors after heating making before heat treatment. When the samples the inclusions much more difficult to be were heated at high temperatures, these observed. Color zoning in some samples micro-inclusions dissolve and diffuse to areas are clearly seen. Some healed fractures grow where Fe2+ is available, and Fe2+/Ti4+ IVCT larger. After the heat treatment, there is no is activated. Thus, blue color in the samples significant variation of the specific gravity and can be intensified after heating at higher refractive indices of all samples. The samples temperatures although they are heated under are still inert under short-wave and long-wave oxidizing condition. Moreover, the higher ultraviolet radiation. intensity of Fe2+/Fe3+ absorption band after The quality of rough blue sapphires heating probably contributes to the darker generally depends on color and clarity. blue color and makes the samples look dull The color of most samples in this heating [2, 15, 21]. experiments can be improved. Titanium- Absorption spectra of the heated containing inclusions present in some samples were observed again after each samples can dissolve back into the sapphire temperature of heat treatment and compared samples after heating at high temperature with the spectra before heat treatment. and also improve their clarity. However, The samples show changes in their absorption heating to high temperature alone as in this patterns or intensities of peaks or bands study will not improve the clarity of the corresponding with the changes in color or stone that has multiple fractures. It will not transparency. In the trapiche-like blue close and seal a significant portion of the sapphire group, a broad band between 500 fractures. Fracture filling with some additives and 650 nm and a peak at 450 nm are e.g. sodium tetraborate, Pb-glass, etc. is often persistent throughout the heat treatment part of the heat-treatment process for process. A band between 700 and 980 nm is commercial purpose [15]. Some treaters claim more developed after heating (Figure 4). that the additive mixture acts as an insulator The peak or band intensity varies depended that allows the corundum crystals to be heated on the developed color and change of slowly and evenly [13, 16]. transparency. The transparency of some samples seems lower after heat treatment. 4. CONCLUSIONS In the light blue group and blue to dark blue From the heat treating experiments of group, the broad band between 500 to the blue sapphires from Pailin, Cambodia, 650 nm gradually decreases in intensity it can be concluded that the gemological when the blue color faded after heating and properties of all samples are in the same this band increases in intensity again when ranges as those from other deposits the blue color developed after heating at higher world-wide. There is no significant variation temperatures (Figure 5 and 6). Additionally, of these properties among the three sample Chiang Mai J. Sci. 2018; 45(1) 599

groups, both before and after heat treatment. [2] Emmett J.L., Scarratt K., McClure S.F., Inclusions found in the samples are mainly Moses T., Douthit T.R., Hughes R., healed fractures and some mineral inclusions Novak S., Shigley J.E., Wang W., which are iron-and/or titanium-containing Bordelon O. and Kane R.E., Gems Gemol., minerals and often occur as micro-inclusions 2003; 39: 84-135. DOI 10.5741/GEMS. 39.2.84. scattering in the samples. These mineral inclusions can affect the blue color of the [3] Coenraads R.R., Aust. Gemol., 1992; 18: samples after heating. The absorption spectra 70-68. of all samples exhibit Fe3+ absorption peaks [4] Hughes R.W., Ruby and Sapphire, RWH and Fe2+/Ti4+ IVCT and Fe2+/Fe3+ IVCT Publishing, Colorado, 1997. absorption bands. The more intense blue [5] Sutherland F.L. and Schwarz D., color intensified after heating, the stronger Gems Gemol., 1997; 33: 302. the intensity of Fe2+/Ti4+ IVCT absorption band. Fe2+/Fe3+ IVCT absorption band [6] Sutherland F.L. and Schwarz D., probably contributes to the darker tone in the Aust. Gemol., 2001; 21: 30-33. samples. Chemical compositions reveal that [7] Jobbins E.A. and Berrange J.P., iron and titanium are the most significant J. Gemmol., 1981; 17: 555-567. minor and trace elements because both of [8] Kr ger F.A., Solid State Ion., 1984; 12: them are necessary for the color-causing 189-199. mechanism in blue sapphire. The heating [9] Fritsch E. and Rossman G.R., experiments in this study indicate that heating Gems Gemol., 1987; 23: 126-139. samples under both oxidizing and reducing conditions yield similar results. Heating at [10] Fritsch E. and Rossman G.R., 1200 °C under the both conditions can Gems Gemol., 1988; 24: 3-15. lighten dark blue color and heating over [11] Hurlbut C.S. and Kammerling R.C., 1400 °C under the both conditions can Gemology, 2nd Edn., John Wiley & Sons, develop or intensify blue color. However, New York, 1991. the trapiche appearance is still persisted after [12] Nassau K., The Physics and Chemistry heat treatment although the brown color in of Color, 2nd Edn., John Wiley & Sons, the core and arm sectors can change to blue New York, 2001. color. [13] Themelis T., The Heat Treatment of Ruby and Sapphire, 1st Edn., Gemlab Inc., ACKNOWLEDGEMENTS New York, 1992. The authors would like to thank [14] Crowningshield R. and Nassau K., Chiang Mai University for providing the J. Gemmol., 1981; 17: 528-541. financial support of this research project. Thanks are also extended to the Geological [15] Emmett J.L. and Douthit T.R, Sciences, Faculty of Science, Chiang Mai Gems Gemol., 1993; 29: 250-272. University for providing the facilities. DOI 10.5741/GEMS.29.4.250. [16] Nassau K., Gemstone Enhancement, REFERENCES History, Science and State of the Art, 2nd Edn., Butterworth-Heineman, Oxford, [1] Lehmann G. and Harder H., Am. Miner., 1994. 1970; 55: 98-105. 600 Chiang Mai J. Sci. 2018; 45(1)

[17] Hey M.H., An Index of Mineral Species and [22] Ferguson J. and Fielding P.E., Varieties Arranged Chemically, 2nd Edn., Aust. J. Chem., 1972; 10: 1371-1385. British Museum (Natural History), DOI 10.1071/CH9721371. London, 1975. [23] Khotchanin K., Heat Treatment of [18] Gaines R.V., Skinner H.C.W., Foord E.E., Non-Gem Blue Sapphire, PhD Thesis, Mason B. and Rosenzweig A., Dana’s Chiang Mai University, Thailand, 2010. New Mineralogy, 8th Edn., John Wiley & [24] Wang C., Yang Y. and Li G., J. Gemmol., Sons Inc., 1997. 1992; 23: 195-197. [19] Fritch E. and Mercer M., Gems Gemol., [25] Guo J., Wang F. and Yakoumelos G., 1993; 29: 151. Gems Gemol., 1992; 28: 255-260. [20] Smith C.P., Kammerling R.C., Keller A.S., [26] Gunaratne H.S., J. Gemmol., 1981; 17: Peretti A., Scarratt K.V., Khoa N.D. 292-300. and Repetto S., Gems Gemol., 1995; 31: 168-186. DOI 10.5741/GEMS.31.3.168. [27] Ediriweera R.N. and Perera S.I., J. Gemmol., 1989; 21: 403-404. [21] Ferguson J. and Fielding P.E., Chem. Phys. Lett., 1971; 10: 262-265. DOI 10.1016/ 0009-2614(71)80282-8.