Abstract Gems & Gemology, Spring 2013, Vol. 49, No. 1

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 southern 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 color centers occurring in natural zircon.

Electron paramagnetic resonance (EPR) and optical absorption spectroscopy are used on a heterogeneous sample group of 11 small untreated zircon crystals. The findings indicate an obvious correlation between color and certain paramagnetic defect centers in the crystal structure. But 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), as well as the intricate substitutional charge patterns that reflect the isomorphism of zircon.

The optical absorption spectra in samples from the United States and France (colorless through pinkish to reddish and brownish) show similar spectra with two broad bands at 340–350 and 510–515 nm. The Russian samples (colorless through yellowish-brownish) show a more complicated spectra of three dominant bands at 320, 390, and 420 nm, plus a weak band at 510.

The results confirm that these two common color series subdivisions are meaningful.

Abstracted by Edward R. Blomgren