Paramagnetic Hole Centres in Natural Zircon and Zircon Colouration

This article explores atomic defect centres (colour centres) of natural zircon from three localities (North Carolina, the Massif Central in the south of France and the Ural Mountains of Russia). The origin of zircon colouration is little discussed in the spectroscopic literature. This study further explores paramagnetic properties and optical absorption, and also suggests additional constraints in the nature of some of the colour centres occurring in natural zircon.

Electron paramagnetic resonance (EPR, an absorption technique using magnetic fields to measure transition energies in atomic defect centres) and optical absorption spectroscopy are used on a heterogeneous sample group of eleven small untreated zircon crystals. The findings indicate an obvious correlation between colours and certain paramagnetic defect centres in the crystal structure. However, the EPR studies also show that the presence of tetragonal centres of trivalent rare-earth elements and niobium is not directly related to any specific zircon colouration.

The paper discusses the highly technical EPR spectral indications for electron and hole defect centres (e.g., angular dependencies and crystal axes; principal g-values, etc.), as well as the intricate substitutional charge patterns which reflect the isomorphism of zircon.

The optical absorption spectra in samples from the US and France (from colourless through pinkish to reddish and brownish) show one type of spectra, two broad bands, 340–350 nm and 510–515 nm. The Russian samples (from colourless through yellowish-brownish) are different, showing a more complicated spectra of three dominant bands at 320, 390, and 420 nm and a weak band at 510.

The results confirm that the subdivision of these two common colour series is meaningful.

Abstracted by Edward R. Blomgren