Gemmological Characteristic and Colour of Zircon under Heat Treatment in Reducing Condition

The colour center caused by radioactive decay in zircons can be removed by heat treatment. This behavior also may modify their colour and transparency to improve the ornamental and commercial value. A large majority of blue zircons in the market are obtained from reddish-brown zircon by heat treatment under high temperature. However, the colour mechanism of which have long been controversial.Reddish-brown and yellowish-brown zircons were applied in this study, and heated in reducing atmosphere in order to confirm the optimum temperature of blue colour modification. The mechanism of altering colour in heat treatment was also investigated in this study by analyzing their standard gemmological properties, infrared absorption spectroscopy (IR) and ultraviolet-visible spectrophotometry (UV-Vis). The results of heat treatment showed that reddish-brown zircons could be changed into light blue zircons by heat treatment under 950-1 000 ℃ in reducing atmosphere.The IR spectra of the samples at 434 cm-1 and 610 cm-1 before heat treatment show their low metamictization.After heat treatment, the absorption of 438/436 cm-1 (peak) get strengthened and the 1 100-900 cm-1 (band) narrowed down, which indicates that the crystalline state of the samples restored. The UV-Vis spectra of the samples showed that the absorption band around 510 nm was generated by Y3+substituting with Zr4+, leading to radiation and turned colourless crystalline zircons into reddish-brown metamict ones. After heat treatment, the vanishment of 510 nm band was caused by the breaking down of colour center generated by Y3+substitution. The 800 nm band showed weak intensity, with the emergence of absorption around 640 nm. The strong absorption peak of 653 nm and 690 nm and a series of weak absorption peaks were caused by U4+. The 640 nm band was presumed to be the main cause of the blue colour of the treated samples, and the 653 nm with its surrounding absorptions might relevant to it.