CAUSE OF RED COLORING IN GEM PLAGIOCLASE FELDSPAR

Abstract

Andesine and labradorite, members of plagioclase feldspar, may have gemological properties. In the gem market, red gem plagioclases have been sold but they may contain natural and treated stones. Consequently, confusion has been made to both trader and customer. Mechanism and cause of red coloring in gem plagioclase feldspar were then investigated in this study. Two sample groups, natural red plagioclase and red copper-diffused plagioclase were collected for this study. Microscopic examination revealed some typical internal features such as oriented and irregular red tubes, colorless rim, mottled colors of red and green, milky cloud along red zone and glistering cloud were found in natural group while red concentric color layer is typically observed in copper-diffused samples. EPMA analyses revealed that all samples have their end-member composition in the range of andesine–labradorite. UV-Vis spectra of all red samples show distinct absorption band at 568-599 nm probably due to Cu0 colloid. LA-ICP-MS analyses supported that copper content is related to red cloud zone, red tubes or twinning plane. In conclusion, the red coloring in gem plagioclase feldspar is probably due to copper nanoparticle that could be diffused into the stones by natural process as well as artificial process. The copper diffusion mechanism could have been taken place along twinning plane, or dislocation tubes as pipe diffusion in both natural and treated samples. Moreover, the surface diffusion maybe caused clearly by diffusion treatment leading to red color intense from outer rim inward core.