Heat Treating Blue Sapphire of Pailin, Cambodia
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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 Blue 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, light blue 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 red-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 shades of blue, 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.