Paramagnetic Hole Centres in Natural Zircon and Zircon Colouration

This article explores atomic defect centers (color centers) 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 coloration 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 color centers occurring in natural zircon.

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

The paper discusses the highly technical EPR spectral indications for electron and hole defect centers (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 colorless 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 colorless 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 color series is meaningful.

Abstracted by Edward R. Blomgren