Gemmological and Mineralogical Characteristics of Lapis Lazuli

Lapis lazuli is a kind of blue mineral with a long history, which occupies a place in the history of human civilization. Study its gemmological and mineralogical characteristics not only helps the identification of lapis lazuli and its imitation products, but also provids technical support for its quality classification. In this paper, 3 lapis lazuli samples with different quality were selected and tested by gem microscope, infrared spectrometer, laser Raman spectrometer, and UV-Vis spectrometer, in order to analyze their mineral components and spectral characteristics comprehensively.The results showed that the mineral compoments of the samples are different obviously: the main mineral components of sample No.1 are lapis lazuli, calcite, pyrite, diopside, and a small amount of sodalite and sphene which indicates average quality. Sample No.2 with relatively poor quality is mainly composed of lapis lazuli, calcite, pyrite, diopside, with a small amount of feldspar. Only lapis lazuli, diopside and a small amount of pyrite are found in sample No.3, which is of high quality. The infrared spectra of the samples show few differences, and are mainly caused by the frame silicate structure of lapis lazuli and the additional anions of its structural channels, which locate around 1 103, 1 014, 969, 921, 624, 549, 517, 460 cm^-1 and 412 cm^-1. The characteristic Raman shifts of lapis lazuli samples are around 257, 544 cm^-1 and 1 092 cm^-1 and the strongest peak appears at 544 cm^-1 which is caused by the symmetric stretching vibration of S₃^-. UV-Vis spectra of the lapis lazuli samples showed the broad band centered at 600 nm (associated with the free radical ion S₃^-) and the weak band around 400 nm (associated with SO₄^2- and S^2-), which give lapis lazuli a blue or bluish purple appearance.