Orangey-red to orangey-pink gem spinels from a new deposit at Lang Chap (Tan Huong-Truc Lau), Vietnam

Effects of heat treatment on red gemstone spinel: single-crystal X-ray, Raman, and photoluminescence study

“…Raman spectra at ambient conditions or at high temperature on natural or synthetic MgAl 2 O 4 spinel (Cynn et al 1992;Slotznick and Shim 2008;Malsy et al 2012) and also Cr 3+ photoluminescence spectra (Wood et al 1968;Mikenda and Preisinger 1981a, b;Mikenda, 1981;Derkosch and Mikenda 1983;Strek et al 1988;Phan et al 2004;Malsy et al 2012) of natural and synthetic spinel have been repeatedly measured and interpreted. In contrast, the number of combined crystal structure and spectroscopic measurements is scarce (e.g., Malsy et al 2012). However, only this methodic combination allows relating long-range order-disorder effects as obtained from the average crystal structure based on diffraction data with short-range effects as monitored by Raman or photoluminescence spectroscopy.…”

“…Hoang et al (2001) performed annealing experiments on orange-red spinel from Vietnam and reported that the maximum of the 4 A 2 → 4 T 2 transition shifts from 18,501.4 cm −1 (untreated) to 18,148 cm −1 (annealed at 920 °C). After heat treatment of an orange-red spinel from Lang Chap (Vietnam) at 850 °C (Malsy et al 2012), the orange hue decreased and the color changed to the more desired red.…”

“…Additional distinguishing features of heated stones, observable by microscopy, are altered mineral inclusion and tension cracks around guest minerals and fluid inclusions. However, gem spinels rarely have observable inclusion, and spectroscopic methods are therefore at an advantage (Malsy et al 2012).…”

“…The study of Malsy et al (2012) initiated the present investigation with the aim exploring the effects of variable heat treatment on one gemstone spinel by singlecrystal X-ray diffraction, photoluminescence spectroscopy, and Raman spectroscopy. X-ray diffraction is applied to monitor variations of average bond length upon heat treatment, which have been shown to influence the energy of "chromophoric" absorption bands (Hålenius et al 2010), whereas photoluminescence spectroscopy probes the local Cr 3+ environment.…”

Raman fingerprint of chromate, aluminate and ferrite spinels

“…[13][14][15] Besides geological implications, natural spinels have been appreciated as attractive and brilliant gemstones for centuries for their wide palette of vivid colors, from red to pink, violet to lavender, and light blue to green. [16,17] Raman spectroscopy is a technique widely used for routine identification of materials, and its application to spinels is extremely promising. It may allow distinguishing the spinel species from the characteristic spectral patterns ('fingerprint'), without any preliminary information about composition and structural origin of the individual features.…”