Trace Element and Genesis of Colour Band of Sapphire from Changle, Shandong Province, China

The development of colour band is one of the main features of the sapphire in basalt in Changle, Shandong Province. The trace elements not only affect the colour of the gems, but also play an important role in understanding the genesis of sapphire and the mother source magma. This study takes the sapphires from Changle, Shandong Province as the research objects, and their spectrum characteristics and chemical compositions were tested by UV-visible absorption spectrometer, Fourier transformed infrared spectrometer and laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). The results show that the sapphire samples from Changle have the characteristics of high Fe and Ga, low Mg and Cr, which is in line with the characteristics of sapphire in magma; the Fe content changes regularly with the shades of colour bands, indicating the intability of the sapphire crystallization process. The 3 310 cm^-1 absorption peak related to -OH can be seen in the infrared absorption spectrum. The -OH content and the shade of the colour band are positively correlated. The darker band has the highest-OH content, and the -OH content of light band is relatively low. In the ultraviolet spectrum, the yellow core part has obvious relationship with the 450 nm absorption peak related to Fe³⁺ -Fe³⁺, indicating that the yellow core part is produced under partial oxidizing conditions. The blue colour zone has obvious relationship with the 450 nm absorption peak related to Fe²⁺-Ti⁴⁺, indicating that the blue colour zone is produced under partial reducting conditions.