Low-Temperature Heat Treatment of Pink Sapphires from Ilakaka, Madagascar

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Low-Temperature Heat Treatment of Pink Sapphires from Ilakaka, Madagascar

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Here's an interesting one from GIA on low heat treatment of pink sapphires that I hope you find as interesting as I did. I left the last two categories, the results and conclusions, for you to read via the link. :)

Low-Temperature Heat Treatment of Pink Sapphires from Ilakaka, Madagascar

Sudarat Saeseaw, Charuwan Khowpong, and Wim Vertriest

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Figure 1. These pink sapphires from Ilakaka, Madagascar, underwent low-temperature heat treatment. From top to bottom, they weigh 1.16 ct, 1.17 ct, and 1.06 ct. Photo by Robert Weldon/GIA; courtesy of Jeff Hapeman, Earth’s Treasury.

"ABSTRACT

Low-temperature heat treatment is often applied to gem corundum to improve its appearance. Pink sapphires from Madagascar, one of the most important sapphire-producing countries, are no exception. Therefore, characterizing Madagascan pink sapphire before and after heat treatment helps laboratories develop criteria for treatment detection. This study showed that after heat treatment at 800ºC, a subtle blue tint was removed, making the color a purer pink. Only slight alterations of monazite inclusions and iron stains in fissures were observed following these heating conditions. Other common inclusions such as needles, particles, etch tubes, and mica and zircon crystals remained unaffected. Interestingly, the monazite crystal inclusions changed color from orange-brown to near-colorless after heat treatment. Raman spectroscopy was used to detect the effect of heat treatment on zircon and monazite crystals. While the Raman spectra of zircon remained unchanged, except in the region of 1000 cm–1 after heating at 1000°C, the Raman peaks of monazite crystals became sharper with increasing temperature. We measured full width half maximum at ~976 cm–1 (ν1, related to the PO4 group of monazite) after all annealing steps. The results showed that the width decreased around 5 cm–1 after heating at 1000°C. This study also showed that Fourier-transform infrared (FTIR) spectroscopy, specifically the peak at 3232 cm–1, is a useful technique to detect low-temperature heat treatment in pink sapphires from Madagascar.


INTRODUCTION

Heat treatment has been used to improve the color and/or clarity of corundum for more than a thousand years. Various parameters such as temperature, heating and cooling time, and oxidizing or reducing atmosphere will affect the final color (Emmett and Douthit, 1993; Emmett et al., 2003; Hughes et al., 2017). The border between high- and low-temperature heat treatment has been defined by Emmett (in Hughes et al., 2017) as the temperature needed to dissolve second-phase microcrystals, which is somewhere between 1200° and 1350°C. Heating at high temperatures will damage most inclusions in ruby and sapphire and is often detectable by trained gemologists. In contrast, heating corundum at low temperatures, sometimes below 700°C, will only subtly affect their internal inclusions, making the treatment much more challenging to detect without advanced instrumentation (figure 1).

Previous studies on the effect of low-temperature heat treatment on inclusions have focused on rubies from Mozambique (Pardieu et al., 2015; Sripoonjan et al., 2016; Saeseaw et al., 2018) and blue sapphires from Madagascar (Krzemnicki, 2010; Hughes and Perkins, 2019). In these studies, rubies showed slight inclusion alterations when heated to 900°C and developed clear FTIR signatures after heat treatment. This article focuses on the effects of low-temperature heat treatment on inclusions, such as zircon and monazite, and the FTIR spectra of pink sapphire from Madagascar. This is the first study to report the use of Raman spectroscopy to analyze monazite inclusions in corundum during heat treatment experiments.

MATERIALS AND METHODS

Samples and Instruments. Fourteen samples reportedly from Ilakaka, Madagascar, were selected and polished with at least two windows for data analysis. Sample sizes ranged from 0.41 to 2.15 ct. The first set of samples (PS01–PS11) consisted of 11 pink sapphires (table 1). Samples PS01–PS08 and PS11 were purchased directly from the miner, but not at the mine site near the Taheza River in Ilakaka. These samples are classified as C-type samples in GIA’s colored stone reference collection (Vertriest et al., 2019). Sample PS09 was acquired from a trader in the Ilakaka market and was reportedly from the Ambalavihy mining area. This sample is classified as an E-type sample. PS10 was purchased from miners at the Sakameloka washing site, which makes it a B-type sample.

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A second set of three pink sapphires, containing zircon and monazite inclusions (PS21–PS23), was used for a follow-up heating experiment (table 2). Sample PS21 was purchased from the miners at the Ambarazy mine; therefore, it is C-type. GIA field gemologists witnessed the mining of samples PS22 and PS23 in the Esoki and Besatra areas, respectively. These samples are classified as B-type (Vertriest et al., 2019).

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To compare the true color of the samples before and after treatment, we used a Canon EOS 5D camera with a Canon Macro MP-E 65 mm lens to produce consistent results. Photographs were taken under identical lighting conditions, with the reference samples placed in a Logan Electric Tru-View 810 color-corrected lightbox (5000 K lamp). A neutral-density filter was used to calibrate the camera/lightbox combination to produce a neutral gray background.

Photomicrographs of internal inclusions were captured at different magnifications with a Nikon SMZ18 system and under different types of illumination, together with a fiber-optic light source.

Non-polarized FTIR spectra were collected using a Thermo Fisher Nicolet 6700 FTIR spectrometer equipped with an XT-KBr beam splitter and a mercury-cadmium-telluride (MCT) detector operating with a 4× beam condenser accessory. The resolution was set at 4 cm–1 with 1.928 cm–1 data spacing. Infrared spectra were collected in the same area of each sample before and after heat treatment.

Raman spectra were obtained using a Renishaw inVia Raman microscope fitted with a Stellar-REN Modu Ar-ion laser producing highly polarized light at 514 nm. For internal inclusion analysis, the system was operated in confocal mode with 20× and 50× objectives. Calibration using neon emission lines and the silicon signal at 520 cm–1 provided wavenumber accuracy within ±0.5 cm–1. Spectra were collected in the 1500–200 cm–1 region.

Heating Experiment. The pink sapphires were separated into two experimental groups. First, 11 samples (PS01–PS11) were subjected to heat treatment in air at 800°C for 160 minutes to study the effect on color, inclusions, and infrared spectroscopy. The second set consisted of three samples (PS21–PS23) that were heated at 200°, 400°, 600°, 800°, and 1000°C for 120 minutes at each step to study the changes in monazite and zircon inclusions using Raman spectroscopy.

Samples were placed on an alumina ceramic felt (99.8% Al2O3) in a Thermo Scientific FB1400 Thermolyne benchtop 1100°C muffle furnace and heated to the desired temperature at a rate of 5°C/min. At the end of each heating cycle, the furnace was switched off. When the temperature of the furnace dropped below 600°C, the samples were removed and allowed to cool to room temperature. Samples were analyzed at room temperature after each heating cycle.

RESULTS AND DISCUSSIONS
CONCLUSIONS ... "

https://www.gia.edu/gems-gemology/winte ... -treatment
PinkDiamond
ISG Registered Gemologist


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