Gemmological and Spectroscopic Characteristics of Purple Sapphire from Southern Madagascar

Sapphire, due to its magnificent colour and special optical effect, is loved by many consumers. However, there are few systematic studies on purple sapphires to date. The gemmological and spectroscopic characteristics of purple sapphire samples from southern Madagascar were investigated using conventional gemmological instruments, photomicrograph, polarizing microscope, Raman spectrometer, Fourier transform infrared spectrometer, laser ablation inductively coupled plasma mass spectrometer, and microscopic UV-Vis-NIR spectrometer. The results showed that the purple sapphires from southern Madagascar have different shades of purple, and there are triangular mounds, pits, quartz crystals and poorly crystallized graphite on the raw stone surface. Magnified observation found that there are lots of zircon inclusions and their combinations with apatite, monazite and ilmenite, and anatase and pyrophyllite are found in individual samples. Combining with the results of LA-ICP-MS and micro-UV-Vis absorption spectra, it showed that: (1)when the Fe content is between 4 000 μg/g and 5 300 μg/g and the Cr content is between 300 μg/g and 500 μg/g, the spectrum at 377, 388 nm and 450 nm reveals clear Fe spectrum and the spectrum near 560 nm reveals clear Cr spectrum, and the samples show light purple; (2)When the Fe content is less than 1 600 μg/g and the Cr content is less than 200 μg/g, the characteristic spectra of Fe and Cr are less obvious, and the samples show light pinkish purple; (3)When the Fe content is lower than 1 600 μg/g and the Cr content is higher than 1 500 μg/g, a clear spectrum of Cr is displayed, and a small amount of Fe²⁺-Ti⁴⁺ ion pairs produce absorption near 560 nm superimposed on the Cr spectrum, and the samples show vivid purple. The samples also have the characteristics of low contents of Ti, V, Mg and Ga, and the Cr/Ga, Fe/Mg and Ga/Mg ratios are more consistent with the geochemical characteristics of metamorphic sapphires. Infrared trans mission spectra show that all the samples had a weak absorption peak at 3 309 cm^-1. The enrichment of elements can be preliminarily identified by UV fluorescence observation.