DIFFUSION MECHANISM OF CU IN THE GEM-QUALITY LABRADORITE FELDSPAR

Cu has been known to diffuse easily into labradorite feldspars, and could produce beautiful red or green colors or aventurescence effect in the feldspar crystals, such as the Oregon sunstone, depending on the sizes of the metallic particles exsolved. The fast diffusion rate of Cu in framework silicate like feldspar is very unusual, especially when considering the relatively heavy atomic mass of Cu compared to the other elements in feldspar.

Diffusion experiments quantifying the Cu diffusion process in labradorite were conducted under various temperatures and fO₂, in order to understand the transportation mechanism. Two diffusion pathways with dramatically different diffusion coefficients but similar activation energies were identified in the diffusion profile, corresponding to the diffusion of Cu(I) and Cu(0) respectively. No obvious anisotropy is observed for the Cu diffusion in labradorite. The fO₂ does not affect the diffusion coefficient noticeably, but significantly changes the solubility of Cu in feldspar. The Cu solubility is <100 ppm at 800°C when diffused in air or H₂/N₂ mixed gas, but >1000 ppm when diffused in N₂ at the same temperature. The dramatic increase in solubility above 1000°C in air also suggests that Cu is most soluble when Cu₂O is stable, rather than Cu or CuO.

The X-ray absorption near edge structure (XANES) of the Cu K edge shows that most of the Cu are dissolved as linear 2-coordinated Cu(I) in the feldspar lattice. The UV-Vis-NIR spectra of the labradorite before and after Cu diffusion show no change in the intensities of the Fe(II) and Fe(III) absorption bands, indicating that Fe in the crystal is not directly involved in balancing the charge of Cu(I). The dissolved Cu(I) shows no observable effect on the UV-Vis-NIR spectra, therefore has no contribution to the color of the labradorite gemstones.