Blue Zircon from Ratanakiri, Cambodia

Abstract

Zircon from Ratanakiri Province, northeastern Cambodia, is well known in the gem trade for its vivid blue colour that results from heat treatment. The untreated brown material turns blue under reducing conditions at ~900-1,000°C. Ratanakiri zircon is characterised by remarkably low contents of trace elements. In particular, the actinides have low concentrations (e.g. approximately 120 ppm U and 95 ppm Th). Together with the very young age of the zircon (<1 million years [Ma]), this results in an extremely low self-irradiation dose, which in turn is in agreement with its non-radiation-damaged, nearly perfectly crystalline state. The heat treatment, therefore, does not result in detectable changes in the zircon's structural state. The cause of the blue colour, presumably related to a valence change upon heating in the reducing environment, is still under debate. The absorption of the treated Ratanakiri zircon is decidedly different from that of blue U 4+-doped and blue V 4+-doped synthetic ZrSiO 4. Absorption spectra show a strongly pleochroic band at 18,200-13,000 cm-1 (corresponding to ~550-770 nm wavelength) that is clearly responsible for the treated blue colour; however, its assignment remains unresolved.

GZ7 and GZ8 – Two Zircon Reference Materials for SIMS U‐Pb Geochronology

“…Unpolarised optical absorption spectra of GZ 7 and GZ 8 in comparison with reference spectra of synthetic ZrSiO 4 doped with U 4+ (see Chase and Osmer ) and U 5+ ‐bearing natural zircon (Ratanakiri sample after oxidising heating; see Zeug et al . ). Spectral ranges that are invisible to the human eye are indicated by the grey background shade.…”

“…As references for crystalline zircon, we use the Ratanakiri, Cambodia, zircon [ 206 Pb/ 238 U age 0.92 ± 0.07 Ma (95% confidence uncertainty); calculated alpha dose 0.0004 × 10 18 g −1 ; Zeug et al . ] and synthetic undoped ZrSiO 4 (van Westrenen et al . ).…”

“…Reference measurements were done on U 4+ ‐doped ZrSiO 4 (for details see Chase and Osmer ) and U 5+ ‐containing zircon, produced by oxidised heating of crystals from Ratanakiri, Cambodia (Zeug et al . ). Spectra were measured at room temperature in transmission geometry, by means of a Bruker IFS66v/S FTIR spectrometer equipped with a mirror‐optics IR‐scope II microscope.…”

“… Mass density, alpha dose and unit‐cell parameters for the well‐crystallised Ratanakiri, Cambodia, zircon are from Zeug et al . (); PL FWHM values are from Lenz and Nasdala (). …”

“…Note again that slicing of the stones was done independent from the 4 3 5 crystallographic orientation, and subjecting large fragments of the samples to the preparation of oriented slabs, for obtaining polarised spectra, would have consumed too much material. Reference measurements were done on U 4+ -doped ZrSiO 4 (for details see Chase and Osmer 1966) and U 5+ -containing zircon, produced by oxidised heating of crystals from Ratanakiri, Cambodia (Zeug et al 2018). Spectra were measured at room temperature in transmission geometry, by means of a Bruker IFS66v/S FTIR spectrometer equipped with a mirror-optics IR-scope II microscope.…”