High-Temperature Diffusion of Cu²⁺ Ions into Natural Plagioclase Crystals: The Role of Li₂O

Abstract

Plagioclase crystals are earth-abundant and low-cost aluminosilicate materials with a high diffusivity of Cu at high temperatures over 1050 °C. The exploration of copper (Cu 2+ )doped plagioclase crystals is hindered by the thermodynamic instability of Cu 2+ at high temperatures over 1050 °C. In this paper, we developed the high-temperature copper−lithium diffusion method for the preparation of blue−green plagioclase crystals doped with Cu 2+ ions. By combining quantitative element distribution characterization and spectroscopic measurement, we illustrate the thermodynamic mechanism of stabilizing Cu 2+ at high temperatures up to 1150 °C by adding Li 2 O in the diffusant and the diffusion process of Cu 2+ from the diffusant to the plagioclase crystals. Moreover, the high-temperature copper diffusion in the form of Cu + and lithium diffusion in the form of Li + are separated to further verify the diffusion process of Cu 2+ . The as-fabricated fascinating blue−green plagioclase crystals doped with chromogenic Cu 2+ ions have high application prospects in the fields of jewelry decoration materials and mineral pigments.

Fluorescence and Phosphorescence Spectroscopies and Their Applications in Gem Characterization

“…Natural red and greenish sunstone were obtained from Oregon, USA. Zhou et al [44][45][46] found that copper-diffused plagioclase crystals exhibited red, light red, and blue-green colors under different conditions. Cu diffusion in both red and light red sunstones results in orange fluorescence when illuminated by ultraviolet light at 320 nm, while the Cu-Li diffusion-treated blue-green plagioclase crystal only exhibits weak fluorescence similar to natural sunstone [45,46].…”

“…Zhou et al [44][45][46] found that copper-diffused plagioclase crystals exhibited red, light red, and blue-green colors under different conditions. Cu diffusion in both red and light red sunstones results in orange fluorescence when illuminated by ultraviolet light at 320 nm, while the Cu-Li diffusion-treated blue-green plagioclase crystal only exhibits weak fluorescence similar to natural sunstone [45,46]. Compared with natural Oregon sunstone, which only has a weak emission at ~395 nm, the Cu-diffused sunstone with orange fluorescence has a stronger emission band at ~394 nm and an additional emission band at ~555 nm (Figure 10).…”